The Open Sports Sciences Journal




ISSN: 1875-399X ― Volume 14, 2021
RESEARCH ARTICLE

Changes in Isokinetic Muscle Strength in Adolescent Soccer Players after 10 Weeks of Pre-Season Training



Michal Lehnert1, Petr Stastny2, James J. Tufano2, *, Pavel Stolfa1
1 Palacky University in Olomouc, Faculty of Physical Culture, Tr. Miru 116, Olomouc, Czech Republic, 77111
2 Charles University in Prague, Faculty of Physical Education and Sport, José Martiho 31, Prague, Czech Republic, 16252

Abstract

Background:

During soccer-specific movements, the strength of knee extensors and flexors is of great importance and achieving certain strength ratios between the two has been identified as an important parameter for reducing the risk of soft tissue injuries around the knee.

Objective:

The aim of the study was to evaluate changes in isokinetic strength of the knee flexors and extensors and their strength ratios in elite adolescent soccer players.

Methods:

Before and after 10 weeks of standard pre-season soccer training with progressive eccentric hamstring exercises, the players (n=18; age 17.1±0.4 years) participated in isokinetic testing to assess concentric and eccentric peak torque at 60°·s-1.

Results:

After 10 weeks of training, the peak eccentric torque of the non-dominant quadriceps increased (p=0.018; ω=0.24). Additionally, the average eccentric work increased in the dominant hamstrings (p=0.007; ω=0.23), dominant quadriceps (p=0.02; ω=0.31), non-dominant hamstrings (p=0.003; ω=0.25 and non-dominant quadriceps (p=0.01; ω=0.37). Lastly, the isokinetic functional ratio (eccentric hamstrings-to-concentric quadriceps) increased in favor of eccentric hamstring strength in the non-dominant limb (p=0.04; ω=0.31).

Conclusion:

The results of the study indicate that pre-season training induced suboptimal changes in the isokinetic strength of the knee flexors and extensors in elite adolescent soccer players. However, the lack of injuries combined with an apparent lack of preparedness explained by slow velocity isokinetic testing indicates that future research should investigate other forms of strength testing to determine soccer-specific preparedness such as isokinetic dynamometry at higher speeds (i.e. 180°.s-1 or 240°.s-1) and traditional weight-room testing such as 1RM tests.

Keywords: Conditioning, Football, H:Q ratio, Peak torque, Training, Youth.


Article Information


Identifiers and Pagination:

Year: 2017
Volume: 10
Issue: Suppl 1: M3
First Page: 27
Last Page: 36
Publisher Id: TOSSJ-10-27
DOI: 10.2174/1875399X01710010027

Article History:

Received Date: 06/06/2016
Revision Received Date: 19/08/2016
Acceptance Date: 10/09/2016
Electronic publication date: 28/04/2017
Collection year: 2017

© 2017 Lehnert et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Charles University in Prague Faculty of Physical Education and Sport José Martiho 31 Prague, Czech Republic, 16252; Tel: +420 777 144 962; E-mail: James.J.Tufano@gmail.com





INTRODUCTION

Strength is a component of fitness that is not only crucial for athletes in terms of sports performance, but also in terms of injury prevention [1Cheung RT, Smith AW, Wong P. H:q ratios and bilateral leg strength in college field and court sports players. J Hum Kinet 2012; 33: 63-71.
[http://dx.doi.org/10.2478/v10078-012-0045-1] [PMID: 23487043]
, 2Lehance C, Binet J, Bury T, Croisier J-L. Muscular strength, functional performances and injury risk in professional and junior elite soccer players. Scand J Med Sci Sports 2009; 19(2): 243-51.
[http://dx.doi.org/10.1111/j.1600-0838.2008.00780.x] [PMID: 18384493]
]. The main components of soccer-specific fitness include acceleration, anaerobic repeated sprint ability, and explosive power of the lower extremities [3Bradley PS, Sheldon W, Wooster B, Olsen P, Boanas P, Krustrup P. High-intensity running in English FA Premier League soccer matches. J Sports Sci 2009; 27(2): 159-68.
[http://dx.doi.org/10.1080/02640410802512775] [PMID: 19153866]
-5Verheijen R. The complete handbook of conditioning for soccer:. Reedswain Inc 1998.], indicating that strength, speed, and endurance are all important while attacking and defending in soccer [6Tomáš M, František Z, Lucia M, Jaroslav T. Profile, correlation and structure of speed in youth elite soccer players. J Hum Kinet 2014; 40(1): 149-59.
[http://dx.doi.org/10.2478/hukin-2014-0017] [PMID: 25031683]
, 7Malý T, Zahálka F, Hráský P, Mala L, Izovská J, Bujnovský D, et al. Age-related differences in linear sprint and power characteristics in youth elite soccer players. Journal of Physical Education and Sport 2015; 15(4): 857.]. Specifically, during specific movements like sprinting, jumping, turning, kicking, tackling, and maintaining balance, the strength of the knee extensors and flexors is of great importance [5Verheijen R. The complete handbook of conditioning for soccer:. Reedswain Inc 1998., 7Malý T, Zahálka F, Hráský P, Mala L, Izovská J, Bujnovský D, et al. Age-related differences in linear sprint and power characteristics in youth elite soccer players. Journal of Physical Education and Sport 2015; 15(4): 857., 8Psotta R, Bunc V, Hendl J, Tenney D, Heller J. Is repeated-sprint ability of soccer players predictable from field-based or laboratory physiological tests? J Sports Med Phys Fitness 2011; 51(1): 18-25.
[PMID: 21297559]
].

Although absolute knee flexor and extensor strength play an important role in performance and injury prevention, the relationship between hamstring and quadriceps strength is also an important factor to consider when aiming to reduce the risk of soft tissue injuries around the knee [9Hughes G, Watkins J. A risk-factor model for anterior cruciate ligament injury. Sports Med 2006; 36(5): 411-28.
[http://dx.doi.org/10.2165/00007256-200636050-00004] [PMID: 16646629]
-11Proske U, Morgan DL, Brockett CL, Percival P. Identifying athletes at risk of hamstring strains and how to protect them. Clin Exp Pharmacol Physiol 2004; 31(8): 546-50.
[http://dx.doi.org/10.1111/j.1440-1681.2004.04028.x] [PMID: 15298549]
]. A lack of hamstring strength and an overwhelming dominance of quadriceps strength have been shown to be a predisposing factor for hamstring injuries in semi-pro soccer players [12Sangnier S, Tourny-Chollet C. Comparison of the decrease in strength between hamstrings and quadriceps during isokinetic fatigue testing in semiprofessional soccer players. Int J Sports Med 2007; 28(11): 952-7.
[http://dx.doi.org/10.1055/s-2007-964981] [PMID: 17497573]
]. Therefore, hamstring-to-quadricep (H:Q) strength ratios are often used to describe the dynamic stability of the knee, to identify potential risk factors for the occurrence or recurrence of hamstring and knee injury, to monitor rehabilitation programs, and to make decisions about when an athlete can safely return to training [13Croisier J-L. Factors associated with recurrent hamstring injuries. Sports Med 2004; 34(10): 681-95.
[http://dx.doi.org/10.2165/00007256-200434100-00005] [PMID: 15335244]
].

Unilateral and bilateral strength asymmetries of the knee flexors and extensors are considered to be the major causes of hamstring injuries and soft tissue injuries to the knee joint in soccer players [13Croisier J-L. Factors associated with recurrent hamstring injuries. Sports Med 2004; 34(10): 681-95.
[http://dx.doi.org/10.2165/00007256-200434100-00005] [PMID: 15335244]
-15Söderman K, Alfredson H, Pietilä T, Werner S. Risk factors for leg injuries in female soccer players: a prospective investigation during one out-door season. Knee Surg Sports Traumatol Arthrosc 2001; 9(5): 313-21.
[http://dx.doi.org/10.1007/s001670100228] [PMID: 11685365]
]. Specifically, unilateral H:Q comparisons are used to assess and identify athletes with a risk of injury and to assess a player’s readiness to return to competition after a knee injury [2Lehance C, Binet J, Bury T, Croisier J-L. Muscular strength, functional performances and injury risk in professional and junior elite soccer players. Scand J Med Sci Sports 2009; 19(2): 243-51.
[http://dx.doi.org/10.1111/j.1600-0838.2008.00780.x] [PMID: 18384493]
, 16Ayala F, De Ste Croix M, Sainz de Baranda P, Santonja F. Absolute reliability of hamstring to quadriceps strength imbalance ratios calculated using peak torque, joint angle-specific torque and joint ROM-specific torque values. Int J Sports Med 2012; 33(11): 909-16.
[http://dx.doi.org/10.1055/s-0032-1311586] [PMID: 22706945]
, 17Colak S. Effects of dynamic stretches on Isokinetic hamstring and Quadriceps femoris muscle strength in elite female soccer players. South African Journal for Research in Sport, Physical Education & Recreation (SAJR SPER) 2012; 34-2.]. Most authors agree that when the conventional H:Q (H:QCONV) strength ratio (i.e. the ratio between concentric knee flexion and concentric knee extension) is less than 0.6 (60%) at a velocity of 60°·s-1, the probability of hamstring injury increases in elite soccer players [18Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci 2003; 11(3): 139-44.] and competitive sprinters [19Yeung SS, Suen AM, Yeung EW. A prospective cohort study of hamstring injuries in competitive sprinters: preseason muscle imbalance as a possible risk factor. Br J Sports Med 2009; 43(8): 589-94.
[http://dx.doi.org/10.1136/bjsm.2008.056283] [PMID: 19174411]
], as the low H:QCONV ratio indicates that the knee musculature may not be able to optimally position the knee joint during movements, increasing the stress placed on intra-articular structures, possibly resulting in biomechanical changes and soft tissue injury [18Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci 2003; 11(3): 139-44., 20Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004.].

While some authors paid attention to the H:QCONV ratio, other authors highlighted the importance of eccentric hamstring actions in dynamically maintaining the stability of the knee joint during active knee extensions [18Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci 2003; 11(3): 139-44., 20Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004.]. This relationship has led to the development of the so-called functional strength ratio (H:QFUNC) (i.e. the ratio between eccentric hamstring strength and concentric quadriceps strength) [21Dvir Z, Eger G, Halperin N, Shklar A. Thigh muscle activity and anterior cruciate ligament insufficiency. Clin Biomech (Bristol, Avon) 1989; 4(2): 87-91.
[http://dx.doi.org/10.1016/0268-0033(89)90044-2] [PMID: 23915999]
]. While the H:QCONV ratio has the ability to identify a muscle imbalance, the H:QFUNC ratio specifically describes the ability of the knee flexors to decelerate rapid knee extension that is initiated by the quadriceps [20Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004.]. Therefore, an H:QFUNC ratio of 1.0 (100%) at an angular velocity of 60°·s-1 expresses the balance between the strength of the knee flexors and extensors, an in turn, the ability to dynamically stabilize the knee joint during activities such as running and kicking [22Aagaard P, Simonsen EB, Andersen JL, Magnusson SP, Bojsen-Møller F, Dyhre-Poulsen P. Antagonist muscle coactivation during isokinetic knee extension. Scand J Med Sci Sports 2000; 10(2): 58-67.
[http://dx.doi.org/10.1034/j.1600-0838.2000.010002058.x] [PMID: 10755275]
].

In accordance with the theory of periodization, the annual training cycles include the stages of strength training aimed at inducing specific adaptations [23Bompa T, Buzzichelli C. Periodization Training for Sports, 3E: Human Kinetics; 2015., 24Wirth K, Schmidtbleicher D. Periodisierung im Schnellkrafttraining. Teil 2: Theoretische Grundlagen der Periodisierung und ihre praktische Anwendung im Schnellkrafttraining. Leistungssport; 2007.]. For example, the post-competitive transition period and the beginning of the pre-season are dominated by general training in order to prepare the muscles, ligaments, tendons, and joints for the sport-specific loads to be experienced in the upcoming season. Hence, the training during the pre-season period aims to increase strength, improve neuro-muscular coordination, and increase skeletal muscle hypertrophy if desired. During the latter stages of this phase, the application of sport-specific movements begins [25Ratamess N. Adaptations to anaerobic training programs. Essentials of Strength Training and Conditioning 2008; 3: 94-119., 26Schmid S, Alejo B. Complete conditioning for soccer: Human Kinetics 1; 2002 ]. For example, it has been shown that a 10-week pre-season eccentric hamstring strength training program significantly reduced the rate of hamstring injury and resulted in increases in maximal running speed and isokinetic hamstring muscle strength in the professional male soccer players during the subsequent 10-month season [27Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 2003; 13(4): 244-50.
[http://dx.doi.org/10.1034/j.1600-0838.2003.00312.x] [PMID: 12859607]
]. Although it is common to strengthen the hamstrings concentrically, it is evident that eccentric strengthening is also essential for both injury prevention and optimal performance. Additionally, Mjolsnes et al. [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
] found that the Nordic hamstring exercise was more effective in developing maximal eccentric hamstring strength than the traditional hamstring curl exercise in well-trained soccer players.

Although studies have focused on isokinetic muscle strength of the lower extremities in multiple periods of the annual training cycle; these studies mostly include adult players [29Eniseler N, Şahan C, Vurgun H, Mavi HF. Isokinetic strength responses to season-long training and competition in turkish elite soccer players. J Hum Kinet 2012; 31: 159-68.
[http://dx.doi.org/10.2478/v10078-012-0017-5] [PMID: 23487507]
, 30Malliou P, Ispirlidis I, Beneka A, Taxildaris K, Godolias G. Vertical jump and knee extensors isokinetic performance in professional soccer players related to the phase of the training period. Isokinet Exerc Sci 2003; 11(3): 165-9.], and only occasionally include elite youth players, specifically under 17 years old (U17) [31Lehnert M, Xaverová Z, De Ste Croix M. Changes in muscle strength in u19 soccer players during an annual training cycle. J Hum Kinet 2014; 42(1): 175-85.
[http://dx.doi.org/10.2478/hukin-2014-0072] [PMID: 25414751]
, 32Lehnert M, Psotta R, Chvojka P, De Ste Croix M. Seasonal variation in isokinetic peak torque in youth soccer players. Kinesiology 2014; 46(1): 79-87.]. Moreover, in studies aimed at U17 players, eccentric strength has rarely been assessed. Therefore, the aim of this study was to evaluate changes in the isokinetic strength of the knee flexors and extensors and H: Q ratios in elite U17 soccer players after the completion of a 10-week pre-season training cycle which included eccentric hamstring training.

MATERIALS & METHODS

The present study was conducted using an intervention study design including pre- and post-testing around a 10-week pre-season training cycle. The pre-test was performed three weeks after the end of the previous season and one day before the first pre-season training session. The post-test occurred at the end of the last week of winter pre-season. On average, the participants performed eight training sessions per week for ten weeks (range of 6-9 training sessions per week).

Participants

Testing was carried out on U17 male soccer players (n = 18; 17.1 ± 0.4 y; 1.78 ± 0.05 m; 70.32 ± 5.51 kg) playing in the Czech first division. Thirteen players had the right leg as the dominant (preferred kicking) leg and five players had the left leg as the dominant leg. The study was approved by the institution’s ethics committee and conformed to the Declaration of Helsinki regarding the use of human subjects. After all players and their legal guardians were fully informed of the aim of the study and of all testing procedures, a written informed consent was obtained. None of the players had acute health problems or previous injuries of the lower extremity and all were familiar with isokinetic testing. During the study period, subjects were not given any dietary or physical activity restrictions other than resting for 24 hours leading up to the pre-and post-testing days.

Measurements

For each leg, isokinetic strength of the knee flexors and extensors was measured using an IsoMed 2000 isokinetic dynamometer (D. & R. Ferstl GmbH, Hemau, Germany). Prior to testing, the players completed a supervised general warm-up that included stationary cycling for 6 minutes at a self-regulated low (80W) to moderate intensity (250W), 5 minutes of dynamic stretching exercises for the quadriceps and hamstrings, and ten vertical jumps with progressive jump effort and 20s rest between jumps.

The players were secured to the dynamometer according to standardized guidelines in a seated position with a hip angle of 100°, and the axis of rotation of the dynamometer was aligned with the axis of rotation of the knee (lateral femoral epicondyle). The arm of the dynamometer lever was fixed to the distal part of the shin and the lower edge of the shin pad was placed 2.5 cm above the medial apex of the malleolus. Individual seat settings were stored in the computer before measuring the dominant leg and were automatically activated in the process of measuring the non-dominant leg and all follow-up testing. The participants were instructed to hold the handgrips located at the side of the seat during all testing efforts. An angular velocity of 60°·s-1 was used, as this speed has been shown to be the most valid speed for exposing strength deficits [33Houweling TA, Head A, Hamzeh MA. Validity of isokinetic testing for previous hamstring injury detection in soccer players. Isokinet Exerc Sci 2009; 17: 213-20.]. Static gravitational correction was applied according to the manufacturer’s guidelines. The testing range of motion was 80° and was set from 10–90° of knee flexion (with 0° = full voluntary extension).

The testing protocol on the dynamometer consisted of one warm-up set and one testing set of concentric and eccentric muscle actions. In the warm-up set, the players performed six reciprocal concentric flexion/concentric extension actions with progressive increases in effort until a maximum action was performed during the sixth repetition [34Brown LE. Isokinetics in human performance: Human Kinetics; 2000.]. After one minute of rest, the players performed a set of six maximal repetitions. The rest time between the measurement of the dominant leg (DL) and the non-dominant leg (NL) was 3 minutes, and the DL was always measured first. During the testing procedure, the players were provided with concurrent visual feedback in the form of an isokinetic strength curve displayed on the dynamometer monitor. Verbal encouragement was not provided. The variables collected from isokinetic testing were absolute peak torque (PT) in Nm and average work (AW) in J, where PT and AW were collected from the trials with the greatest PT and AW, respectively. Lastly, PT was used to determine the H:QCONV and H:QFUNC.

Training Program

The training content, match play, and recovery periods were recorded during the 10-week period. Although the exact durations of training sessions slightly varied throughout the 10-week period, the average session time for each session type is shown in Table 1 (30-50min for main training sessions, plus 5-15 minutes for both warm-up and cool-down). Training took place from Monday to Friday and seven pre-season matches were played on Saturdays, leaving three Saturdays and every Sunday as the rest days. A detailed description of the training schedule is set forth in the supplement of this manuscript.

Table 1
Training schedule during the ten-week pre-season period.


The training intervention consisted of soccer-specific training as well as what can be considered as normal pre-season soccer conditioning. Additionally, the players also participated in supervised strength training with a special focus on eccentric hamstring exercises such as the Nordic curl. This exercise was performed according to the previously established guidelines [27Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 2003; 13(4): 244-50.
[http://dx.doi.org/10.1034/j.1600-0838.2003.00312.x] [PMID: 12859607]
, 28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
, 35Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
, 36Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports 2008; 18(1): 40-8.
[http://dx.doi.org/10.1111/j.1600-0838.2006.00634.x] [PMID: 17355322]
]. The starting position began with the players on their knees, with the knees flexed to 90°, with the hips slightly flexed, and an erect torso. While an exercise partner manually secured the athlete’s ankles to the floor throughout the exercise, the athlete allowed the knees to slowly extend while maintaining slight flexion of the hips, resulting in the hamstrings eccentrically acting against the gravity-induced knee extension. As the athlete’s upper body approached the ground, the hands were used to buffer the fall, letting the chest touch the ground. Upon completion of one repetition, the athletes were told to immediately return to the starting position by thrusting themselves back up using their hands to minimize concentric hamstring loading in the upward phase. The hamstring-focused training was progressively incorporated into the training schedule. In the beginning of the 10-week preseason period, the players performed one set of five repetitions, once per week. The volume of eccentric hamstring training progressively increased each week and by the 5th week of preseason program, the players were performing three sets of 8-12 repetitions three times per week [35Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
] (see supplement 1). During the first four weeks, eccentric hamstring training was completed with 72 hours of rest between the sessions; however, from the 5th week onward, eccentric hamstring sessions were completed on two consecutive days and for the third time after 48 hours of rest. The previous papers [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
, 35Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
, 36Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports 2008; 18(1): 40-8.
[http://dx.doi.org/10.1111/j.1600-0838.2006.00634.x] [PMID: 17355322]
] have recommended to perform eccentric hamstring training up to 3 times per week during pre-season, but did not recommend the specific number of rest days between the sessions. The previous research implemented five repetitions of the Nordic curl during each warm-up training session for six weeks [37Steffen K, Myklebust G, Olsen OE, Holme I, Bahr R. Preventing injuries in female youth footballa cluster-randomized controlled trial. Scand J Med Sci Sports 2008; 18(5): 605-14.
[http://dx.doi.org/10.1111/j.1600-0838.2007.00703.x] [PMID: 18208428]
]. Therefore, it appears as though a minute of daily training allows for daily application of eccentric hamstring exercises, leading to the progressive prescription of three eccentric hamstring sessions in four days in the present study.

Statistical Analysis

The mean and standard deviation values were calculated for all PT, AW, and H:Q ratios Tables (2-4). To determine the effectiveness of the intervention, one-sample t-tests were used to compare the absolute differences between the pre-test and the post-test values for each variable. The range of the effect was determined by the ω coefficient (0.00≤ ω ≤0.05 small effect; 0.6≤ ω ≤0.13 medium effect; ω ≥0.14 large effect) [35Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
], and the significance level for all the tests was set at p < 0.05. Statistical analyses were performed using the data analysis software system Statistica, version 12 (StatSoft, Inc., Tulsa, USA). Although differences were not compared between the legs, as the purpose of the training program was not to improve the strength of a specific limb, percentage change values were calculated from the pre-season to the post-season for each variable in order to further describe the results Tables (2-4).

Table 2
Peak torque pre- and post-training.


Table 3
Average work pre and post-training.


RESULTS

Values of the measured variables for the pre- and post-tests are reported in Tables (2-4). A statistically significant increase was observed for PTQ_E_NL (p = 0.018; ω = 0.24), AWH_E_DL (p = 0.007; ω = 0.23), AWQ_E_DL (p = 0.02; ω = 0.31), AWH_E_NL (p = 0.003; ω = 0.25), and AWQ_E_NL (p = 0.01; ω = 0.37). There were no statistically significant changes in the H:QCONV. A significant increase was observed in the H:QFUNC_NL (p = 0.04; ω = 0.31), with no other significant changes in the H:QFUNC.

DISCUSSION

The purpose of the present study was to explore the changes in the knee flexor and extensor PT with respect to the changes in various H:Q ratios after 10 weeks of pre-season training with the inclusion of progressive eccentric hamstring exercises. The main findings of this study were that: 1) PT significantly changed only in the NL knee extensors during eccentric isokinetic actions; 2) eccentric AW increased in the knee extensors and flexors of the DL and NL; and 3) the H:QFUNC ratio increased in the NL but no changes were present for the H:QCONV. Collectively, these data suggest that 10 weeks of pre-season soccer training including eccentric hamstring exercises may not have affected the maximal strength (i.e. no changes in isokinetic PT), but may have increased the ability for the players to maintain the eccentric torque during isokinetic testing evidenced by the greater AW. Therefore, it is probable that eccentric mean force increased in the quadriceps and hamstrings, as the AW increased but the testing range of motion (i.e. distance) did not change.

The PT results of the current study indicate that the pre-training PT values in our group of players were similar to or greater than the PT of other young elite players of the similar age groups [2Lehance C, Binet J, Bury T, Croisier J-L. Muscular strength, functional performances and injury risk in professional and junior elite soccer players. Scand J Med Sci Sports 2009; 19(2): 243-51.
[http://dx.doi.org/10.1111/j.1600-0838.2008.00780.x] [PMID: 18384493]
, 38Forbes H, Sutcliffe S, Lovell A, McNaughton LR, Siegler JC. Isokinetic thigh muscle ratios in youth football: effect of age and dominance. Int J Sports Med 2009; 30(8): 602-6.
[http://dx.doi.org/10.1055/s-0029-1202337] [PMID: 19301218]
-40Maly T, Zahalka F, Mala L, Mala L, Eds. Isokinetic strength characteristics and norms in the best Czech soccer players of U16 category Medicine and Science in Sport and Exercise. Philadelphia, USA: Lippincott Williams & Wilkins 2013.]. Therefore, it is possible that the subjects in this study started the preseason period well prepared in terms of lower limb strength, meaning that any additional strength gains might have been difficult to achieve, especially during a training program including soccer-specific running (See supplement) and might have resulted in an interference effect [41de Souza EO, Tricoli V, Franchini E, Paulo AC, Regazzini M, Ugrinowitsch C. Acute effect of two aerobic exercise modes on maximum strength and strength endurance. J Strength Cond Res 2007; 21(4): 1286-90.
[PMID: 18076237]
]. The PT data from the present study are in accordance with the previously published data [32Lehnert M, Psotta R, Chvojka P, De Ste Croix M. Seasonal variation in isokinetic peak torque in youth soccer players. Kinesiology 2014; 46(1): 79-87.] in which the researchers also assessed the changes in the concentric PT at 60°∙s-1 in a group of young elite soccer players of similar age (age 16.7±0.7 years). After completing the 10-week pre-season period, the authors observed no change in the hamstring or quadriceps PT in both the DL and NL. Similarly, no differences in the PT were present within the DL and NL in a study by Erdemir et al. [42Erdemir I, Ökmen AŞ. Comparative Analysis of Isokinetic Leg Strength in Professional Soccer and Basketball Players. South African Journal for Research in Sport, Physical Education and Recreation 2013; 15(1.03): 15.7-0.9.] employing U17 soccer players. At first glance, the combined results of these studies suggest that pre-season training may not result in any desirable increases in the maximal strength. Although isokinetic testing is a valid and reliable tool for assessing muscular strength [43Caruso JF, Brown LE, Tufano JJ. The reproducibility of isokinetic dynamometry data. Isokinet Exerc Sci 2012; 20(4): 239-53.], it often lacks specificity. Therefore, it is possible that more specific strength tests such as a back squat 1 repetition maximum (1RM) might have indicated that dynamic muscular strength changed during the course of pre-season training. Moreover, the detailed parameters of strength training were not reported in any of the previously mentioned studies, leaving the reader without the knowledge of any strength training program within the training period of players with a similar age and skill level as the ones in the present study. On the other hand, after collegiate soccer players utilized the Nordic hamstring curl for 10 weeks, they experienced an 11% increase in the eccentric hamstring PT measured at 60°s-1, as well as a 7% increase in the isometric hamstring strength at 90°, 60° and 30° of the knee flexion [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
]. Although the authors did not report the loading parameters used during the pre-season training period, it could be assumed that the players in the present study experienced a similar eccentric hamstring training load if similar guidelines were followed. As the players of the present study presented insignificant increases in the hamstring strength, it is possible that increasing the sessions of strength training and decreasing the sessions of soccer-specific training in the present study may result in similar strength increases as those in the previously mentioned study [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
]. However, this interference effect is simply hypothetical, as the lack of training details in another study [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
] does not allow a direct and conclusive comparison between the studies.

Table 4
Peak torque ratios pre and post-training.


After 10 weeks of pre-season training, the average H:QCONV ratio of the subjects reached from 0.56 to 0.62. Achieving a ratio of 0.60 is considered to be the “injury threshold” meaning that a value below 0.60 when measuring strength at 60°∙s-1 indicates a possible muscle imbalance and may increase the probability of hamstring or ACL injury [20Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004., 34Brown LE. Isokinetics in human performance: Human Kinetics; 2000.]. Although such diagnostics are applicable in the present study (according to previous recommendations set forth using 60°∙s-1), the angular velocity must be considered because the H:QCONV ratios change as the angular velocity changes [44Malý T, Zahálka F, Malá L. Muscular strength and strength asymmetries in elite and sub-elite professional soccer players. Sport Science 2014; 7(1): 26-33.]. There are several ways how to increase the H:QCONV which include: increasing the strength of the knee flexors and extensors, but a greater increase in the knee flexors; increasing strength only in the knee flexors; or maintaining the strength of the knee flexors accompanied by a decrease in the extensor strength. From the aforementioned possibilities, the first could be considered as the most beneficial response to the pre-season training. Although an increase in the knee flexor strength could be considered as beneficial, a significant increase in the extensor strength without an accompanying increase in the flexor strength may be seen as undesirable, decreasing the H:QCONV ratio and possibly resulting in hamstrings injuries. However, none of these cases were observed using isokinetic testing in our study. One possible explanation for the lack of hamstring strength and the resultant lack of increase in the H:QCONV could be an inappropriate application of eccentric hamstring training. As mentioned within the methods, the second half of the training program included eccentric hamstring sessions that were applied three times in four days (Monday, Tuesday, and Thursday), which may not have provided sufficient time for hamstring regeneration. Therefore, it is possible that the hamstring training of the present study did not include an appropriate volume or might have not have been optimally applied. The previous recommendations range from performing five repetitions of the Nordic curl every day [37Steffen K, Myklebust G, Olsen OE, Holme I, Bahr R. Preventing injuries in female youth footballa cluster-randomized controlled trial. Scand J Med Sci Sports 2008; 18(5): 605-14.
[http://dx.doi.org/10.1111/j.1600-0838.2007.00703.x] [PMID: 18208428]
] to multiple sets of multiple repetitions up to three days per week [28Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
, 35Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
, 36Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports 2008; 18(1): 40-8.
[http://dx.doi.org/10.1111/j.1600-0838.2006.00634.x] [PMID: 17355322]
]. Based on the results of the present study, it appears as though combining the previous recommendations of successive days and multiple sets may not result in increases in the eccentric hamstring strength. However, possibly just as important, it should be noted that there were no reported hamstring injuries in the present study. Therefore, it appears that although eccentric hamstring strength did not increase in the present study, the risk of injury did not increase either. Hence, further research should investigate the effects of eccentric hamstring training density on the strength and injury rates.

Similar to the H:QCONV, an H:QFUNC ratio below 0.6 indicates an increased likelihood of knee injury [18Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci 2003; 11(3): 139-44.]. According to other authors, H:QFUNC values ranging from 0.7–1.0 indicate insufficient dynamic stabilization of the knee [45Aagaard P, Simonsen EB, Magnusson SP, Larsson B, Dyhre-Poulsen P. A new concept for isokinetic hamstring: quadriceps muscle strength ratio. Am J Sports Med 1998; 26(2): 231-7.
[PMID: 9548116]
, 46Aagaard P, Simonsen EB, Trolle M, Bangsbo J, Klausen K. Isokinetic hamstring/quadriceps strength ratio: influence from joint angular velocity, gravity correction and contraction mode. Acta Physiol Scand 1995; 154(4): 421-7.
[http://dx.doi.org/10.1111/j.1748-1716.1995.tb09927.x] [PMID: 7484168]
], and a value of 1.0 indicates adequate dynamic joint stability [47Coombs R, Garbutt G, Gulick DT, Yoder HN, Barfield WR, Kirkendall DT, et al. Developments in the use of the hamstring/quadriceps ratio for the assessment of muscle balance. J Sports Sci Med 2002; 1(3): 56-62.
[PMID: 24701125]
]. The average H:QFUNC in the subjects of this study (range of 0.61 to 0.67) was above the “injury threshold,” but was far from the optimal value. In addition, significant increases in the H:QFUNC were generally not present, indicating that the pre-season training program employed in the present study was not successful in increasing the eccentric hamstring strength relative to concentric quadriceps strength. Therefore, the pre-season training of our subjects might not have adequately prepared the muscles to perform in an optimal manner, possibly due to the low volume of lower limb resistance training, infrequent rest days, or too many high-intensity game-like training sessions resulting in an interference effect. As with the H:QCONV, although the players’ H:QFUNC ratio was not ideal, no hamstring or knee injuries occurred during the pre-season program. Additionally, the average H:QFUNC values increased only in the NL, further supported by a large effect size. It is possible that the high volume of soccer-specific training included a lot of kicking exercises and the DL (i.e. kicking leg) was more accustomed to rapid eccentric actions in the hamstrings in order to slow the movement of leg during the follow-through of repetitive kicking actions [20Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004., 48Delextrat A, Gregory J, Cohen D. The use of the functional H:Q ratio to assess fatigue in soccer. Int J Sports Med 2010; 31(3): 192-7.
[http://dx.doi.org/10.1055/s-0029-1243642] [PMID: 20157872]
]. Thus, the increase in the H:QFUNC of the DL seems beneficial, as an increase in the antagonistic eccentric hamstring activation may act as a natural protective mechanism because it reduces unnecessary tension of the anterior cruciate ligament [49Chan K-M, Maffulli N, Korkia P, Li RC. Principles and practice of isokinetics in sports medicine and rehabilitation. Hong Kong: Williams & Wilkins 1996., 50Tourny-Chollet C, Leroy D. Conventional vs. dynamic hamstring-quadriceps strength ratios: a comparison between players and sedentary subjects. Isokinet Exerc Sci 2002; 10(4): 183-92.]. Although previous studies and the present study used slow-velocity isokinetic testing to identify functional muscle imbalances, it must be remembered that angular velocities of the knee observed during the soccer were much faster than 60° per second, so it is possible that a more sport-specific H:QFUNC test should be implemented in the future.

This study primarily focused on changes in the PT and the H:Q ratios, whereas other studies also assessed the differences between the Q and H strength and between the DL and NL. In these studies, greater PT values were found in the quadriceps compared to the hamstrings [2Lehance C, Binet J, Bury T, Croisier J-L. Muscular strength, functional performances and injury risk in professional and junior elite soccer players. Scand J Med Sci Sports 2009; 19(2): 243-51.
[http://dx.doi.org/10.1111/j.1600-0838.2008.00780.x] [PMID: 18384493]
, 32Lehnert M, Psotta R, Chvojka P, De Ste Croix M. Seasonal variation in isokinetic peak torque in youth soccer players. Kinesiology 2014; 46(1): 79-87., 51Botek Z, Gába A, Lehnert M, Přidalová M, Vařeková R, Botek M, et al. Condition and body constitution of soccer players in category U19 before and after completing a preparatory period. Acta Gymnica 2010; 40(2): 47-54.] and during eccentric actions compared to the concentric actions [52Daneshjoo A, Rahnama N, Mokhtar AH, Yusof A. Bilateral and unilateral asymmetries of isokinetic strength and flexibility in male young professional soccer players. J Hum Kinet 2013; 36(1): 45-53.
[http://dx.doi.org/10.2478/hukin-2013-0005] [PMID: 23717354]
, 53Cometti G, Maffiuletti NA, Pousson M, Chatard JC, Maffulli N. Isokinetic strength and anaerobic power of elite, subelite and amateur French soccer players. Int J Sports Med 2001; 22(1): 45-51.
[http://dx.doi.org/10.1055/s-2001-11331] [PMID: 11258641]
]. As the present study aimed to quantify changes in strength in response to the same training program (i.e. no control group) throughout a season with subjects, such detailed comparisons were of no interest. However, the absence of H and Q comparisons and laterality analyses could be viewed as a limitation of the present study and should be investigated in future research. Additionally, since the subjects in this study only had a 3-week off-season period before starting their pre-season training associated with this study, it is possible that the 3-week off season served as a taper, actually enabling the players to recover from the high demands of the previous season, resulting in the “optimal” strength levels during the pre-training testing in the present study. Had the off-season period been longer, the players would have become detrained, lowering their pre-season strength levels, possibly allowing greater improvements during the 10-week pre-season period. Unfortunately, in the conclusion, data regarding the player’s strength levels of the previous season were not available, leaving this proposition purely hypothetical. Therefore, although the pre-season program that included mostly game–like training may not have prepared the players in terms of thigh muscle strength development and knee injury prevention when considering hamstring and quadriceps strength ratios, no players experienced a soft tissue lower-limb injury during the study period.

CONCLUSION

The results of this study indicate that the isokinetic strength of the knee flexors and extensors responded differently to 10 weeks of pre-season training in young elite soccer players. Due to sport-specific demands of pre-season soccer training and various recommendations of Nordic curl training that range from five repetitions daily to three days of multiple sets of multiple repetitions, the present study implemented a progressive increase in the eccentric hamstring training’s frequency which ultimately included two consecutive days of eccentric training. Although the pre-season program that included mostly game–like training did not result in inculcating thigh muscle strength and knee injury prevention in the players when considering the isokinetic hamstring and quadriceps strength ratios, no players experienced a soft tissue lower-limb injury during the study period. Therefore, it can be concluded that the training program used in the present study sufficiently prepared the players for the upcoming season. However, the lack of injuries combined with an apparent lack of preparedness explained by slow velocity isokinetic testing indicates that future research should investigate other forms of strength testing to determine sport preparedness, such as isokinetic dynamometry at higher speeds (i.e. 180°.s-1 or 240°.s-1) and traditional weight-room testing such as 1RM tests.

LIST OF ABBREVIATIONS

H:Q  = Hamstrings:Quadriceps ratio Introduction:
H:QCONV  = Conventional Hamstrings:Quadriceps ratio Introduction:
H: QFUNC  = Functional Hamstrings:Quadriceps ratio Introduction:
U17  = Under seventeen years old Introduction:
DL  = Dominant leg Methods:
NL  = Non-dominant leg Methods:
PT  = Peak torque Methods:
AW  = Average work Methods:
H  = Hamstrings
C  = Concentric action
E  = Eccentric action

SUPPLEMENTARY MATERIAL

Supplementary material is available on the publishers Website along with the published article.

Download File


CONFLICT OF INTEREST

The author confirms that this article content has no conflict of interest.

ACKNOWLEDGEMENTS

This study was supported by research grants from the Czech Science Foundation NO. 14-29358P and NO. 16-13750S.

REFERENCES

[1] Cheung RT, Smith AW, Wong P. H:q ratios and bilateral leg strength in college field and court sports players. J Hum Kinet 2012; 33: 63-71.
[http://dx.doi.org/10.2478/v10078-012-0045-1] [PMID: 23487043]
[2] Lehance C, Binet J, Bury T, Croisier J-L. Muscular strength, functional performances and injury risk in professional and junior elite soccer players. Scand J Med Sci Sports 2009; 19(2): 243-51.
[http://dx.doi.org/10.1111/j.1600-0838.2008.00780.x] [PMID: 18384493]
[3] Bradley PS, Sheldon W, Wooster B, Olsen P, Boanas P, Krustrup P. High-intensity running in English FA Premier League soccer matches. J Sports Sci 2009; 27(2): 159-68.
[http://dx.doi.org/10.1080/02640410802512775] [PMID: 19153866]
[4] Ferrari Bravo D, Impellizzeri FM, Rampinini E, Castagna C, Bishop D, Wisloff U. Sprint vs. interval training in football. Int J Sports Med 2008; 29(8): 668-74.
[http://dx.doi.org/10.1055/s-2007-989371] [PMID: 18080951]
[5] Verheijen R. The complete handbook of conditioning for soccer:. Reedswain Inc 1998.
[6] Tomáš M, František Z, Lucia M, Jaroslav T. Profile, correlation and structure of speed in youth elite soccer players. J Hum Kinet 2014; 40(1): 149-59.
[http://dx.doi.org/10.2478/hukin-2014-0017] [PMID: 25031683]
[7] Malý T, Zahálka F, Hráský P, Mala L, Izovská J, Bujnovský D, et al. Age-related differences in linear sprint and power characteristics in youth elite soccer players. Journal of Physical Education and Sport 2015; 15(4): 857.
[8] Psotta R, Bunc V, Hendl J, Tenney D, Heller J. Is repeated-sprint ability of soccer players predictable from field-based or laboratory physiological tests? J Sports Med Phys Fitness 2011; 51(1): 18-25.
[PMID: 21297559]
[9] Hughes G, Watkins J. A risk-factor model for anterior cruciate ligament injury. Sports Med 2006; 36(5): 411-28.
[http://dx.doi.org/10.2165/00007256-200636050-00004] [PMID: 16646629]
[10] Maly T, Zahalka F, Mala L. Differences between isokinetic strength characteristics of more and less successful professional soccer teams. Journal of Physical Education and Sport 2011; 11(3): 306.
[11] Proske U, Morgan DL, Brockett CL, Percival P. Identifying athletes at risk of hamstring strains and how to protect them. Clin Exp Pharmacol Physiol 2004; 31(8): 546-50.
[http://dx.doi.org/10.1111/j.1440-1681.2004.04028.x] [PMID: 15298549]
[12] Sangnier S, Tourny-Chollet C. Comparison of the decrease in strength between hamstrings and quadriceps during isokinetic fatigue testing in semiprofessional soccer players. Int J Sports Med 2007; 28(11): 952-7.
[http://dx.doi.org/10.1055/s-2007-964981] [PMID: 17497573]
[13] Croisier J-L. Factors associated with recurrent hamstring injuries. Sports Med 2004; 34(10): 681-95.
[http://dx.doi.org/10.2165/00007256-200434100-00005] [PMID: 15335244]
[14] Iga J, George K, Lees A, Reilly T. Reliability of assessing indices of isokinetic leg strength in pubertal soccer players. Pediatr Exerc Sci 2006; 18(4): 436.
[http://dx.doi.org/10.1123/pes.18.4.436]
[15] Söderman K, Alfredson H, Pietilä T, Werner S. Risk factors for leg injuries in female soccer players: a prospective investigation during one out-door season. Knee Surg Sports Traumatol Arthrosc 2001; 9(5): 313-21.
[http://dx.doi.org/10.1007/s001670100228] [PMID: 11685365]
[16] Ayala F, De Ste Croix M, Sainz de Baranda P, Santonja F. Absolute reliability of hamstring to quadriceps strength imbalance ratios calculated using peak torque, joint angle-specific torque and joint ROM-specific torque values. Int J Sports Med 2012; 33(11): 909-16.
[http://dx.doi.org/10.1055/s-0032-1311586] [PMID: 22706945]
[17] Colak S. Effects of dynamic stretches on Isokinetic hamstring and Quadriceps femoris muscle strength in elite female soccer players. South African Journal for Research in Sport, Physical Education & Recreation (SAJR SPER) 2012; 34-2.
[18] Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci 2003; 11(3): 139-44.
[19] Yeung SS, Suen AM, Yeung EW. A prospective cohort study of hamstring injuries in competitive sprinters: preseason muscle imbalance as a possible risk factor. Br J Sports Med 2009; 43(8): 589-94.
[http://dx.doi.org/10.1136/bjsm.2008.056283] [PMID: 19174411]
[20] Dvir Z. Isokinetics: muscle testing, interpretation, and clinical applications: Elsevier Health Sciences 2004.
[21] Dvir Z, Eger G, Halperin N, Shklar A. Thigh muscle activity and anterior cruciate ligament insufficiency. Clin Biomech (Bristol, Avon) 1989; 4(2): 87-91.
[http://dx.doi.org/10.1016/0268-0033(89)90044-2] [PMID: 23915999]
[22] Aagaard P, Simonsen EB, Andersen JL, Magnusson SP, Bojsen-Møller F, Dyhre-Poulsen P. Antagonist muscle coactivation during isokinetic knee extension. Scand J Med Sci Sports 2000; 10(2): 58-67.
[http://dx.doi.org/10.1034/j.1600-0838.2000.010002058.x] [PMID: 10755275]
[23] Bompa T, Buzzichelli C. Periodization Training for Sports, 3E: Human Kinetics; 2015.
[24] Wirth K, Schmidtbleicher D. Periodisierung im Schnellkrafttraining. Teil 2: Theoretische Grundlagen der Periodisierung und ihre praktische Anwendung im Schnellkrafttraining. Leistungssport; 2007.
[25] Ratamess N. Adaptations to anaerobic training programs. Essentials of Strength Training and Conditioning 2008; 3: 94-119.
[26] Schmid S, Alejo B. Complete conditioning for soccer: Human Kinetics 1; 2002
[27] Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 2003; 13(4): 244-50.
[http://dx.doi.org/10.1034/j.1600-0838.2003.00312.x] [PMID: 12859607]
[28] Mjølsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004; 14(5): 311-7.
[http://dx.doi.org/10.1046/j.1600-0838.2003.367.x] [PMID: 15387805]
[29] Eniseler N, Şahan C, Vurgun H, Mavi HF. Isokinetic strength responses to season-long training and competition in turkish elite soccer players. J Hum Kinet 2012; 31: 159-68.
[http://dx.doi.org/10.2478/v10078-012-0017-5] [PMID: 23487507]
[30] Malliou P, Ispirlidis I, Beneka A, Taxildaris K, Godolias G. Vertical jump and knee extensors isokinetic performance in professional soccer players related to the phase of the training period. Isokinet Exerc Sci 2003; 11(3): 165-9.
[31] Lehnert M, Xaverová Z, De Ste Croix M. Changes in muscle strength in u19 soccer players during an annual training cycle. J Hum Kinet 2014; 42(1): 175-85.
[http://dx.doi.org/10.2478/hukin-2014-0072] [PMID: 25414751]
[32] Lehnert M, Psotta R, Chvojka P, De Ste Croix M. Seasonal variation in isokinetic peak torque in youth soccer players. Kinesiology 2014; 46(1): 79-87.
[33] Houweling TA, Head A, Hamzeh MA. Validity of isokinetic testing for previous hamstring injury detection in soccer players. Isokinet Exerc Sci 2009; 17: 213-20.
[34] Brown LE. Isokinetics in human performance: Human Kinetics; 2000.
[35] Sayers B-E. The Nordic eccentric hamstring exercise for injury prevention in soccer players. Strength Condit J 2008; 30(4): 56-8.
[http://dx.doi.org/10.1519/SSC.0b013e31817f985d]
[36] Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports 2008; 18(1): 40-8.
[http://dx.doi.org/10.1111/j.1600-0838.2006.00634.x] [PMID: 17355322]
[37] Steffen K, Myklebust G, Olsen OE, Holme I, Bahr R. Preventing injuries in female youth footballa cluster-randomized controlled trial. Scand J Med Sci Sports 2008; 18(5): 605-14.
[http://dx.doi.org/10.1111/j.1600-0838.2007.00703.x] [PMID: 18208428]
[38] Forbes H, Sutcliffe S, Lovell A, McNaughton LR, Siegler JC. Isokinetic thigh muscle ratios in youth football: effect of age and dominance. Int J Sports Med 2009; 30(8): 602-6.
[http://dx.doi.org/10.1055/s-0029-1202337] [PMID: 19301218]
[39] Lehnert M, Urban J, Procházka JH, Psotta R. Isokinetic strength of knee flexors and extensors of adolescent soccer players and its changes based on movement speed and age: Acta Universitatis Palackianae Olomucensis; 2011.
[40] Maly T, Zahalka F, Mala L, Mala L, Eds. Isokinetic strength characteristics and norms in the best Czech soccer players of U16 category Medicine and Science in Sport and Exercise. Philadelphia, USA: Lippincott Williams & Wilkins 2013.
[41] de Souza EO, Tricoli V, Franchini E, Paulo AC, Regazzini M, Ugrinowitsch C. Acute effect of two aerobic exercise modes on maximum strength and strength endurance. J Strength Cond Res 2007; 21(4): 1286-90.
[PMID: 18076237]
[42] Erdemir I, Ökmen AŞ. Comparative Analysis of Isokinetic Leg Strength in Professional Soccer and Basketball Players. South African Journal for Research in Sport, Physical Education and Recreation 2013; 15(1.03): 15.7-0.9.
[43] Caruso JF, Brown LE, Tufano JJ. The reproducibility of isokinetic dynamometry data. Isokinet Exerc Sci 2012; 20(4): 239-53.
[44] Malý T, Zahálka F, Malá L. Muscular strength and strength asymmetries in elite and sub-elite professional soccer players. Sport Science 2014; 7(1): 26-33.
[45] Aagaard P, Simonsen EB, Magnusson SP, Larsson B, Dyhre-Poulsen P. A new concept for isokinetic hamstring: quadriceps muscle strength ratio. Am J Sports Med 1998; 26(2): 231-7.
[PMID: 9548116]
[46] Aagaard P, Simonsen EB, Trolle M, Bangsbo J, Klausen K. Isokinetic hamstring/quadriceps strength ratio: influence from joint angular velocity, gravity correction and contraction mode. Acta Physiol Scand 1995; 154(4): 421-7.
[http://dx.doi.org/10.1111/j.1748-1716.1995.tb09927.x] [PMID: 7484168]
[47] Coombs R, Garbutt G, Gulick DT, Yoder HN, Barfield WR, Kirkendall DT, et al. Developments in the use of the hamstring/quadriceps ratio for the assessment of muscle balance. J Sports Sci Med 2002; 1(3): 56-62.
[PMID: 24701125]
[48] Delextrat A, Gregory J, Cohen D. The use of the functional H:Q ratio to assess fatigue in soccer. Int J Sports Med 2010; 31(3): 192-7.
[http://dx.doi.org/10.1055/s-0029-1243642] [PMID: 20157872]
[49] Chan K-M, Maffulli N, Korkia P, Li RC. Principles and practice of isokinetics in sports medicine and rehabilitation. Hong Kong: Williams & Wilkins 1996.
[50] Tourny-Chollet C, Leroy D. Conventional vs. dynamic hamstring-quadriceps strength ratios: a comparison between players and sedentary subjects. Isokinet Exerc Sci 2002; 10(4): 183-92.
[51] Botek Z, Gába A, Lehnert M, Přidalová M, Vařeková R, Botek M, et al. Condition and body constitution of soccer players in category U19 before and after completing a preparatory period. Acta Gymnica 2010; 40(2): 47-54.
[52] Daneshjoo A, Rahnama N, Mokhtar AH, Yusof A. Bilateral and unilateral asymmetries of isokinetic strength and flexibility in male young professional soccer players. J Hum Kinet 2013; 36(1): 45-53.
[http://dx.doi.org/10.2478/hukin-2013-0005] [PMID: 23717354]
[53] Cometti G, Maffiuletti NA, Pousson M, Chatard JC, Maffulli N. Isokinetic strength and anaerobic power of elite, subelite and amateur French soccer players. Int J Sports Med 2001; 22(1): 45-51.
[http://dx.doi.org/10.1055/s-2001-11331] [PMID: 11258641]
Track Your Manuscript:


Endorsements



"Open access will revolutionize 21st century knowledge work and accelerate the diffusion of ideas and evidence that support just in time learning and the evolution of thinking in a number of disciplines."


Daniel Pesut
(Indiana University School of Nursing, USA)

"It is important that students and researchers from all over the world can have easy access to relevant, high-standard and timely scientific information. This is exactly what Open Access Journals provide and this is the reason why I support this endeavor."


Jacques Descotes
(Centre Antipoison-Centre de Pharmacovigilance, France)

"Publishing research articles is the key for future scientific progress. Open Access publishing is therefore of utmost importance for wider dissemination of information, and will help serving the best interest of the scientific community."


Patrice Talaga
(UCB S.A., Belgium)

"Open access journals are a novel concept in the medical literature. They offer accessible information to a wide variety of individuals, including physicians, medical students, clinical investigators, and the general public. They are an outstanding source of medical and scientific information."


Jeffrey M. Weinberg
(St. Luke's-Roosevelt Hospital Center, USA)

"Open access journals are extremely useful for graduate students, investigators and all other interested persons to read important scientific articles and subscribe scientific journals. Indeed, the research articles span a wide range of area and of high quality. This is specially a must for researchers belonging to institutions with limited library facility and funding to subscribe scientific journals."


Debomoy K. Lahiri
(Indiana University School of Medicine, USA)

"Open access journals represent a major break-through in publishing. They provide easy access to the latest research on a wide variety of issues. Relevant and timely articles are made available in a fraction of the time taken by more conventional publishers. Articles are of uniformly high quality and written by the world's leading authorities."


Robert Looney
(Naval Postgraduate School, USA)

"Open access journals have transformed the way scientific data is published and disseminated: particularly, whilst ensuring a high quality standard and transparency in the editorial process, they have increased the access to the scientific literature by those researchers that have limited library support or that are working on small budgets."


Richard Reithinger
(Westat, USA)

"Not only do open access journals greatly improve the access to high quality information for scientists in the developing world, it also provides extra exposure for our papers."


J. Ferwerda
(University of Oxford, UK)

"Open Access 'Chemistry' Journals allow the dissemination of knowledge at your finger tips without paying for the scientific content."


Sean L. Kitson
(Almac Sciences, Northern Ireland)

"In principle, all scientific journals should have open access, as should be science itself. Open access journals are very helpful for students, researchers and the general public including people from institutions which do not have library or cannot afford to subscribe scientific journals. The articles are high standard and cover a wide area."


Hubert Wolterbeek
(Delft University of Technology, The Netherlands)

"The widest possible diffusion of information is critical for the advancement of science. In this perspective, open access journals are instrumental in fostering researches and achievements."


Alessandro Laviano
(Sapienza - University of Rome, Italy)

"Open access journals are very useful for all scientists as they can have quick information in the different fields of science."


Philippe Hernigou
(Paris University, France)

"There are many scientists who can not afford the rather expensive subscriptions to scientific journals. Open access journals offer a good alternative for free access to good quality scientific information."


Fidel Toldrá
(Instituto de Agroquimica y Tecnologia de Alimentos, Spain)

"Open access journals have become a fundamental tool for students, researchers, patients and the general public. Many people from institutions which do not have library or cannot afford to subscribe scientific journals benefit of them on a daily basis. The articles are among the best and cover most scientific areas."


M. Bendandi
(University Clinic of Navarre, Spain)

"These journals provide researchers with a platform for rapid, open access scientific communication. The articles are of high quality and broad scope."


Peter Chiba
(University of Vienna, Austria)

"Open access journals are probably one of the most important contributions to promote and diffuse science worldwide."


Jaime Sampaio
(University of Trás-os-Montes e Alto Douro, Portugal)

"Open access journals make up a new and rather revolutionary way to scientific publication. This option opens several quite interesting possibilities to disseminate openly and freely new knowledge and even to facilitate interpersonal communication among scientists."


Eduardo A. Castro
(INIFTA, Argentina)

"Open access journals are freely available online throughout the world, for you to read, download, copy, distribute, and use. The articles published in the open access journals are high quality and cover a wide range of fields."


Kenji Hashimoto
(Chiba University, Japan)

"Open Access journals offer an innovative and efficient way of publication for academics and professionals in a wide range of disciplines. The papers published are of high quality after rigorous peer review and they are Indexed in: major international databases. I read Open Access journals to keep abreast of the recent development in my field of study."


Daniel Shek
(Chinese University of Hong Kong, Hong Kong)

"It is a modern trend for publishers to establish open access journals. Researchers, faculty members, and students will be greatly benefited by the new journals of Bentham Science Publishers Ltd. in this category."


Jih Ru Hwu
(National Central University, Taiwan)


Browse Contents




Webmaster Contact: info@benthamopen.net
Copyright © 2021 Bentham Open