Neuromuscular control in landing from supra-maximal dropping height

The present study utilized high-impact supra-maximal landings to examine the influence of the pre-impact force level on the post-impact electromyographic (EMG) activity and, in particular, on the short latency EMG reflex (SLR) component. Unilateral-leg landings were performed in a sitting position on a sledge apparatus after release from high, but individually constant dropping height. A lower limb guiding device fixed to the front of the sledge seat allowed the subjects to sustain a given pre-set force level up to impact. This force level was either freely chosen or set at 20, 35, and 50% of maximal isometric plantarflexion force. EMG activity was recorded from eight major lower limb muscles. It was expected that the increase in the pre-impact force level would require the intervention of a protective neural strategy during the post-impact phase that would attenuate the SLR amplitude. The ultrasonography recordings confirmed that the soleus fascicles were stretched to induce SLR. The main finding was the similarity across all tested conditions of the impact peak force and post-impact EMG activity, including the SLR response. Both observations are mostly attributed to the similar EMG levels and close force levels reached toward impact. The instruction to maintain a given pre-set force level was indeed overruled when getting close to impact. It is suggested that, in the present supra-maximal landing condition, a protective central neural strategy did occur that took into account the pre-set force level to secure similar impact loads.