Doctor of Philosophy (PhD)



Document Type



It is well documented that the stretch-shortening cycle (SSC), the most common muscle behavior, enhances muscle mechanical output. Stretch load and muscle preactivation level have been suggested as the two important factors regulating mechanical output. The purpose of this series studies is to systematically examine influences of the preactivation and the stretch load on muscle mechanical output during SSC. In the First study, a two dimensional lower extremity dynamic model was used to evaluate the influence of the approach on mechanical output. The peak summed power during the push-off phase demonstrated a quadratic trend across heights and appeared to be driven primarily by the ankle joint response. When an approach was used summed peak power was approximately 10% greater, regardless of the number of steps. In the Second study, we investigated muscle activity of seven major low extremity muscles during drop jumps. The surface EMG activities were full-wave rectified and averaged (aEMG) during the pre-activation (50ms before touchdown), downward and pushoff phases. The results showed that the aEMG of most tested muscles during the preactivation phase and the downward phase increased with more steps of the approach. This increase did not change the antagonist-agonist coactivation ratio, therefore would not attribute to knee joint injury. On the other hand, no aEMG changes were found with different drop heights. In the Third study, stretch load and preactivation were used as inputs for a muscle model to calculate muscle force, muscle velocity and muscle power. This model quantified how the different preactivation level and stretch load (velocity) affect the muscle mechanical output. Results showed that for low preactivation levels, increasing preactivation level could significantly increase gain in height for all stretch velocities we tested, but increasing stretch velocity may decrease the gain in height; for high preactivation levels, further increasing preactivation level may not increase gain in height. Over all, increasing preactivation enhances mechanical output due to increased active state level during SSCs; when preactivation is high, increasing stretch load enhances mechanical output due to increased positive work. Stretch load needs a high preactivation level to maximize the mechanical output.



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Committee Chair

Li Li



Included in

Kinesiology Commons