Abstract
To examine changes to neuromuscular control of leg stiffness following 42 min of soccer-specific exercise. 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. 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). 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.
Highlights
Soccer is characterised by the need for players to repeatedly utilise stretch-shortening cycle (SSC) contractions to provide a high rate of force development in rebounding activities, such as sprinting, turning and jumping
This was a result of individual responses to the intervention, with half of the group displaying a decrease in leg stiffness and the other half an increase in leg stiffness post the prolonged exercise
Altered stiffness was mediated by modulating CoMd during ground contact, with both changes in stiffness and CoMd strongly related to changes in feedforward, feedback and total eccentric muscle activity of lower limb extensor muscles
Summary
Soccer is characterised by the need for players to repeatedly utilise stretch-shortening cycle (SSC) contractions to provide a high rate of force development in rebounding activities, such as sprinting, turning and jumping. Fatigue during soccer has been associated with increased injury risk in adult (Hawkins et al 2001) and youth football (Price et al 2004). It has been speculated that altered leg stiffness and movement control during a soccer-match contributes to the increased injury risk associated with fatigue (Cone et al 2012). While previous research has demonstrated both adults (Andersson et al 2008) and youths (Oliver et al 2008) have impaired SSC performance and function following soccer exercise, these studies have only considered slow SSC movements not associated with high levels of leg stiffness.
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