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This study examined squat jumping (SJ) mechanics following length restricted strength training regimens of the knee extensors and flexors. SJ from a 110 degrees knee joint angle starting position and isometric moment-knee angle relations of knee extensors and flexors were analysed in 16 athletes before and directly after 8 weeks of strength training regimens that were restricted to knee joint angles corresponding to long muscle-tendon unit (MTU) length for the knee extensors and flexors. SJ mechanics were studied using a two-dimensional kinematic model with three rigid bodies (upper leg, lower leg, foot) in combination with force plates measurements of ground reaction force in the right and left lower extremity. Centre of mass jumping height significantly (p<0.05) increased post training, but this was not explained by enhanced absolute power generation in the knee joint. However, post training small but significant (p<0.05) shifts to smaller knee joint angles occurred in the normalized [% Max.] knee joint angle dependent power generation in the right and left knee joint during SJ. The isometric moment-knee angle relation of the knee extensors was also significantly (p<0.05) shifted to longer MTU lengths of knee extensors. Length restricted strength training may alter the mechanical situation during both isometric contractions and dynamic athletic movements.

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Our study showed strength loss in both knee extensors and knee flexors in all patients who had undergone reconstruction surgery after the same ACL-rehabilitation's program. We can state that there is strength loss in hip and ankle muscles in 3 - 6 months post OP patients. A reduction of muscle strength at the hip flexor from all patients was seen after 12 months post-OP. It is suggested that the strength loss in the hip flexors could be due to bi-articular muscles surrounding the knee.This results has to be consider on the planning of strength training after ACL injuries.