Age-related differences in the neural regulation of stretch-shortening cycle activities in male youths during maximal and sub-maximal hopping

The aim of the current study was to investigate potential age-related differences in neural regulation strategies during maximal and sub-maximal hopping. Thirty-two boys from three different age groups (9-, 12- and 15-years), completed trials of both maximal and sub maximal hopping, and based on contact and flight times, measures of reactive strength index (RSI = jump height/contact time) and leg stiffness (peak ground reaction force/peak displacement of centre of mass) were collected respectively. During all trials, surface electromyograms (EMG) were recorded from four different muscle sites of the dominant lower limb, during 100. ms pre-ground contact, and then four subsequent stretch reflex phases: background muscle activity (0-30. ms), short-latency stretch reflex (31-60. ms), intermediate15 latency stretch reflex 61-90. ms and long-latency stretch reflex (91-120. ms). Reactive strength index and leg stiffness were measured during the hopping trials. During maximal hopping, both 12- and 15-year olds produced significantly greater RSI (P< 0.02) than 9-year olds, with 15-year olds utilising significantly greater soleus muscle activity during the 100. ms prior to ground contact than the younger age groups (P< 0.01). During sub-maximal hopping, 15-year olds produced significantly greater absolute leg stiffness than both 12- and 9-year olds (P< 0.01), with 9-year olds producing significantly less soleus muscle activity during the 31-60. ms time phase. For all age groups, sub-maximal hopping was associated with significantly greater background muscle activity and short-latency stretch reflex activity in the soleus and vastus lateralis, when compared to maximal hopping (P< 0.001). Results suggest that as children mature, they become more reliant on supra-spinal feed forward input and short latency stretch reflexes to regulate greater levels of leg stiffness and RSI when hopping.