Phase- and task-specific modulation of soleus H-reflexes during drop-jumps and landings.

H-reflexes in the soleus muscle were previously shown to be decreased in drop-jumps at the instant of the short latency response (SLR) of the stretch reflex when falling height was increased. The aim of the present study was to elucidate task-specific modulation of H-reflexes during drop-jumps in more detail. Therefore, soleus H-reflex excitability was compared in drop-jumps from three different falling heights (30, 50, 75 cm) and between drop-jumps and landings. In landings, there is no need for high tendomuscular stiffness like in drop-jumps. Therefore, we assumed reduced spinal excitability of soleus Ia afferents which should be reflected as reduced H-reflexes. H-reflex recruitment curves were recorded in 23 subjects (24 +/- 2 years) at the instant of the SLR during drop-jumps (30 cm-LH, 50 cm-MH, 75 cm-EH) and landings (30 cm falling height). Furthermore, recruitment curves were recorded at ground contact (GND) as a reference to SLR. At SLR, H/M ratios were higher during drop-jumps (LH: 0.49 +/- 0.18) than during landings (0.33 +/- 0.18, P < 0.001). H/M ratios did not differ at GND (LH: 0.46 +/- 0.15; landings: 0.46 +/- 0.14). H/M ratios were progressively decreased at the SLR from LH, to MH, to EH (MH: 0.44 +/- 0.17; EH: 0.40 +/- 0.16, P < 0.001). Again, there were no differences at GND (MH: 0.46 +/- 0.15; EH: 0.44 +/- 0.16). The present study provides further evidence of phase-specific (GND vs. SLR) and task-specific modulation of SOL Ia afferent input. These modulations were thought to be initiated prior to touch down.