Modeling Individual Muscle Forces In The Lower Extremity During Loaded And Unloaded Hexbar Vertical Jumps

Abstract

Hexagonal barbell (HB) loaded jumps are often used in training to increase lower extremity power. Given the importance of coordinated muscular effort in achieving maximal power output, an understanding of how the lower extremity musculature individually performs during loaded jumps would be advantageous. The purpose of this study is to describe the effect of load on individual muscle forces, muscle torques, and the contribution to the net joint moment (NJM) during the concentric phase of loaded HB jumps.10 male collegiate athletes performed 5 maximal HB jumps at 0%, 20%, 40% and 60% of their HB deadlift 1-repetition maximum. Filtered Ground reaction forces and 3D lower extremity marker trajectories were input into a 23 DOF musculoskeletal model and muscle forces were estimated with static optimization. Peak muscle force (xBW) was calculated for the gluteus maximum (GMAX), biceps femoris – long head (BFL), rectus femoris (RF), vastus intermedius (VAST), gastrocnemius (GAS), and soleus (SOL). RMANOVA and LSD comparisons were used for analysis (p < 0.05). Muscle torque (Nm/kg) and the contribution to the NJM was calculated for each muscle and analyzed qualitatively. A significant increase in peak muscle force across loads existed for VAST (p = 0.009) and GAS (p < 0.001), and significant decreases were noted for RF (p = 0.017). There was no significant difference in peak force of GMAX (p = 0.325), BFL (p = 0.369), or SOL (p = 0.122) across loads. Torque contribution from individual muscles was unaltered at the ankle but shifted towards the vasti at the knee and the extensors at the hip with increasing loads. Loaded hexbar jumping is not simply a higher intensity version of vertical jumping, and the lower extremity joints and corresponding musculature are not impacted equally by the addition of load. The varied effect of load on mechanical demands at the lower extremity joints, and thus force and torque output from individual muscles, is important to consider when using loaded jumps as part of training for athletic performance.