Neural control of leg stiffness during hopping in boys and men

The purpose of the study was to investigate whether boys and men utilise different control strategies whilst hopping. Eleven boys (11-12yr old) and ten men completed hopping at 1.5Hz, 3.0Hz and at their preferred frequency. A footswitch measured contact and flight times, from which leg stiffness was calculated. Simultaneously, surface electromyograms (EMGs) of selected lower limb muscles were recorded and quantified for each 30ms period during the first 120ms post-ground contact. At 1.5Hz there were no differences between the groups in relative stiffness or muscle activity. At 3.0Hz men had significantly shorter contact times (P=0.013), longer flight times (P=0.002), greater relative stiffness (P=0.01) and significantly greater soleus (P=0.012) and vastus lateralis (P<0.001) activity during the initial 30ms post-ground contact. At the preferred frequency men hopped significantly faster than the boys (P=0.007), with greater leg stiffness (P<0.01) and with more extensor activity in most time periods. Boys and men demonstrated similar control strategies when hopping at a slow frequency, but when hopping frequency increased men were able to better increase feedforward and reflex muscle activity to hop with greater relative stiffness.