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 100ms pre-ground contact, and then four subsequent stretch reflex phases: background muscle activity (0-30ms), short-latency stretch reflex (31-60ms), intermediate15 latency stretch reflex 61-90ms and long-latency stretch reflex (91-120ms). 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 100ms 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 31-60ms 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.