Neuromuscular Correlates of the Contralateral Stretch-induced Strength Loss

The current study investigated the effects of unilateral passive stretching on the neuromuscular mechanisms involved in the force-generating capacity of the contralateral muscle. Twenty-six healthy men underwent unilateral passive stretching of the plantar flexors (5 x 45-s on+15s-off, total stretching time: 225 s). Before and after the stretching protocol, contralateral ankle range of motion (ROM), maximum isometric voluntary contraction (MVC) of the plantar flexors, and electromyographic root mean square (sEMG RMS) of the soleus and the gastrocnemii muscles were determined. Concurrently, V-wave, maximum and superimposed H-reflex, and M-wave were elicited via nerve stimulation to estimate the supraspinal, spinal, and peripheral mechanisms, respectively. The sEMG RMS, V-wave, and H-reflex were normalized to the M-wave. After passive stretching, contralateral ankle ROM was increased [+8% (1%/15%), effect size (ES) 0.43 (0.02/0.84), p<0.001], MVC of the plantar flexors was decreased [-9% (-21%/-2%), ES -0.96 (-1.53/-0.38), p<0.001], and the sEMG RMS/M-wave of the soleus and the gastrocnemii muscles was decreased (≈ -9%, ES ≈ -0.33, p<0.05). Concurrently, the V-wave/M-wave superimposed was decreased in all muscles (≈ -13%, ES -0.81 to -0.52, p<0.05). No change in H-reflex/M-wave and M-wave was observed under both maximum and superimposed condition. The decrease in the MVC and the sEMG RMS of the contralateral muscle was accompanied by a decrease in the V-wave/M-wave but not the H-reflex/M-wave ratios and the M-wave. The present outcomes suggest that only supraspinal mechanisms might be involved in the contralateral decrease in the maximum force-generating capacity.