Differential modulation of spinal and corticospinal excitability during drop jumps

Previously it was shown that spinal excitability during hopping and drop jumping is high in the initial phase of ground contact when the muscle is stretched but decreases toward takeoff. To further understand motor control of stretch-shortening cycle, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases following ground contact in drop jump. Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)-, and long (LLR, LLR2)-latency responses of the soleus muscle (SOL) after jumps from 31 cm height. MEPs and H-reflexes were expressed relative to the background electromyographic (EMG) activity. H-reflexes were highly facilitated at SLR (172%) and then progressively decreased (MLR = 133%; LLR = 123%; LLR2 = 110%). TMS showed no effect at SLR, MLR, and LLR, whereas MEPs were significantly facilitated at the LLR2 (122%; P = 0.003). Background EMG was highest at LLR and lowest at LLR2. Strong H-reflex facilitation at the beginning of the stance phase indicated significant contribution of Ia-afferent input to the α-motoneurons during this phase that then progressively declined toward takeoff. Conversely, corticospinal excitability was exclusively increased at the phase of push off (LLR2, ∼120 ms). It is argued that corticomotoneurons increased their excitability at LLR2. At LLR (∼90 ms), Ia-afferent transmission as well as corticospinal excitability was low, whereas background EMG was high. Therefore it is speculated that other sources, presumably subcortical in origin, contributed to the EMG activity at LLR in drop jumps.