Altered neuromuscular control of leg stiffness following soccer‑specific exercise

 Abstract Purpose To examine changes to neuromuscular control of leg stiffness following 42 min of soccer-specific exercise. Methods Ten youth soccer players, aged 15.8 ± 0.4 years, stature 1.73 ± 0.06 m and mass 59.8 ± 9.7 kg, hopped on a force plate at a self-selected frequency before and after simulated soccer exercise performed on a non-motorised treadmill. During hopping, muscle activity was measured using surface electromyography from four lower limb muscles and analysed to determine feedforward- and feedback-mediated activity, as well as co-contraction. Results There was a small, non-significant change in stiffness following exercise (26.6 ± 10.6 vs. 24.0 ± 7.0 kN m−1, p > 0.05, ES = 0.25), with half the group increasing and half decreasing their stiffness. Changes in stiffness were significantly related to changes in centre of mass (CoM) displacement (r = 0.90, p < 0.01, extremely large correlation) but not changes in peak ground reaction force (r = 0.58, p > 0.05, large correlation). A number of significant relationships were observed between changes in stiffness and CoM displacement with changes in feedforward, feedback and eccentric muscle activity of the soleus and vastus lateralis muscles following exercise (r = 0.64–0.98, p < 0.05, large–extremely large correlations), but not with changes in co-contraction (r = 0.11–0.55, p > 0.05, small–large correlations). Conclusions Following soccer-specific exercise individual changes in feedforward- and reflex-mediated activity of the soleus and vastus lateralis, and not co-contraction around the knee and ankle, modulate changes in CoM displacement and leg stiffness.