Comparison of force–velocity relationships of vastus lateralis muscle in isokinetic and in stretch‐shortening cycle exercises

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Abstract
Aim: This study investigated the force–velocity characteristics of the vastus lateralis (VL) muscle fascicle and muscle–tendon unit (MTU) in isolated lengthening and shortening actions, and during natural movement. Methods: Four subjects performed maximal eccentric and concentric knee extensions (60, 120 and 180° s−1). Unilateral counter movement jumps and drop jumps in the sledge apparatus served as natural movements. Vastus lateralis fascicle lengths were determined from ultrasonography. In vivo patellar tendon forces (PTF) were measured with an optic fibre technique. Patellar tendon force was derived to VL force according to the cross‐sectional area of the muscle. Force in the direction of fascicle was calculated by dividing the VL force value by cosine of the fascicle angle. Force–velocity curves were constructed using angle specific values from isokinetic knee extensions (classical curve) and using instantaneous values from jumping exercises. Results: In the fascicle level, we did not find an enhanced muscle force in the jumping performances as compared with the classical force–velocity curve. In the muscle–tendon level, the instantaneous force at high muscle–tendon shortening speeds exceeded that extrapolated according to Hill's equation. Conclusion: This difference between fascicle and muscle–tendon behaviour suggests that the neural input in fast stretch‐shortening cycle exercises minimizes the length changes in muscle fascicle and enables storage and recoil of energy from elastic components that contributes to the enhanced mechanical output of the MTU during the push‐off phase.