Increasing stretch load lowers the mechanical efficiency of the quadriceps femoris muscle-tendon unit

Background The stretch-shortening cycle enhances muscle efficiency and force output by, among others, utilizing elastic energy stored in tendons during muscle stretching, but the level of mechanical efficiency is influenced by the intensity of the stretch. Objective This study aimed to investigate the effects of stretch load and stretching velocity on mechanical efficiency during stretch-shortening cycle contractions. Methods Ten recreationally active young women performed quadriceps SSC contractions under different stretch loads (20 J to 100 J) and knee flexion angular velocities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mn>30</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>⋅</mml:mo> <mml:msup> <mml:mtext>s</mml:mtext> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mspace width="0.25em"/> <mml:mtext>and</mml:mtext> <mml:mspace width="0.25em"/> <mml:mspace width="0.25em"/> <mml:msup> <mml:mn>210</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>⋅</mml:mo> <mml:msup> <mml:mtext>s</mml:mtext> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ) on an isokinetic dynamometer. Muscle activation of the vastus lateralis and rectus femoris was recorded using electromyography (EMG), and mechanical efficiency was calculated as the ratio of positive to total work. Results Results showed that mechanical efficiency decreased with increasing stretch load, with a 17% reduction in mechanical efficiency observed from 20 J to 100 J, accompanied by a threefold increase in knee flexion angle. Although both stretch load and knee flexion angular velocity showed a significant effect on mechanical efficiency ( p &lt; 0.05), stretch load had a more substantial impact than knee flexion angular velocity. EMG analysis showed that vastus lateralis EMG activity was consistently higher during stretching phase of stretch shortening cycle contractions compared to knee extension across all conditions, particularly at lower knee flexion angular velocities. Conclusions These findings highlight the influence of both stretch load and knee flexion angular velocity on stretch-shortening cycle performance, suggesting that optimal stretch load and knee joint angular velocity combinations can enhance mechanical efficiency and muscle-tendon efficiency in stretch-shortening cycle based movements.