Fig. (3) C2-C7 SVA is the same in A1 B1 and C1. A and A1: when CTL it is close to zero, the contraction of the spinae erector muscles generates a vector (F) too small in the extension of cervical spine. B and B1, C and C1: As Cervicothoracic lordosis magnifies, the resultant vector in the extension of the cervical column magnifies proportionally (F' e F “) improving the effectiveness of the muscular contraction and diminishing the muscular energy expense.