Neuromuscular Strategies in Stretch-Shortening Exercises with Increasing Drop Heights: The Role of Muscle Coactivation in Leg Stiffness and Power Propulsion.

Riccardo Di Giminiani,Aldo Giovannelli,Francesco Masedu,Andrea Di Blasio,Lorenzo Capuano,Pascal Izzicupo

doi:10.3390/ijerph17228647

Riccardo Di Giminiani, Aldo Giovannelli + Show 4 more

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https://doi.org/10.3390/ijerph17228647

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Abstract

When applying drop jump exercises, knowing the magnitude of the stimulus is fundamental to stabilize the leg joints and to generate movements with the highest power. The effects of different drop heights on leg muscles coactivation, leg stiffness and power propulsion were investigated in fifteen sport science students. Drop jumps from heights of 20, 30, 40, 50, and 60 cm in a random order were performed on a force platform. During each drop jump, the ground reaction force, knee angle displacement, and synchronized surface-electromyography root-mean-square (sEMGRMS) activity (vastus lateralis, VL; vastus medialis, VM; rectus femoris, RF; biceps femoris, BF; tibialis anterior, TA and lateral gastrocnemius, LG) were recorded. The coactivation in the pre-contact phase, between VL and BF, VM and BF as well as RF and BF, was dependent on the drop height (p < 0.01; effect size (ES) ranged from 0.45 to 0.90). Leg stiffness was dependent on the drop height (p < 0.001; ES = 0.27–0.28) and was modulated by the coactivation of VM–BF (p = 0.034) and RF–BF (p = 0.046) during the braking phase. Power propulsion was also dependent on the drop height (p < 0.001; ES = 0.34); however, it was primarily modulated by the coactivation of LG–TA during the braking phase (p = 0.002). The coactivation of thigh muscles explains leg stiffness adjustments at different drop heights. On the contrary, the coactivation of shank muscles is mostly responsible for the power propulsion.

Highlights

Plyometric exercises are movements in which the interaction between the human body and the environment is characterized by large transient reaction forces [1]
The differences in knee angle displacement among the several trials performed at different drop heights were not significant (p > 0.05) and the intra-class correlation coefficient (ICC) was equal to 0.89
The main results, in accordance with our hypothesis, were (a) the drop heights modulated differently leg stiffness and power propulsion; (b) the coactivation of the thigh muscles was dependent on drop height only during the pre-contact phase and it increased during the braking and propulsion phases, neither was dependent on drop height; (c) the coactivation of shank muscles was not dependent on drop height; and (d) the coactivation of thigh muscles influenced leg stiffness, whereas the shank muscle influenced the power propulsion

Summary

Introduction

Plyometric exercises (i.e., drop jumps) are movements in which the interaction between the human body and the environment is characterized by large transient reaction forces [1] These exercises involve a muscle action called the stretch–shorten cycle (SSC), where the stretch force is imposed on the neuromuscular system by gravity [2], and when changes in drop height occur, the momentum at impact increases. The amount of energy reused if a muscle stretch is followed by a concentric contraction will change [3,4] The latter biomechanical condition (i.e., the stretch–shorten cycle) strongly stimulates muscular and neurogenic structures [4], even though the more complex and interesting is neurogenic, as the motoneuron activation in the early contact phase is modulated by afferents feedback to increase the impulse through the synchronization of α-motoneurons in the preactivated muscle [5]. Public Health 2020, 17, 8647; doi:10.3390/ijerph17228647 www.mdpi.com/journal/ijerph

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