Plantar flexor muscle stretching depresses the soleus late response but not tendon tap reflexes

The purpose of this study was to investigate changes in muscle spindle sensitivity with early and late soleus reflex responses via tendon taps and transcranial magnetic stimulation, respectively, after an acute bout of prolonged static plantar flexor muscle stretching. Seventeen healthy males were tested before and after 5 min (5 × 60-s stretches) of passive static stretching of the plantar flexor muscles. Maximal voluntary isometric torque and M wave-normalized triceps surae muscle surface electromyographic activity were recorded. Both soleus tendon reflexes, evoked by percussion of the Achilles tendon during rest and transcranial magnetic stimulation-evoked soleus late responses during submaximal isometric dorsiflexion were also quantified. Significant decreases in maximal voluntary isometric plantar flexion torque (-19.2 ± 13.6%, p = .002) and soleus electromyographic activity (-20.1 ± 11.4%, p < .001) were observed immediately after stretching, and these changes were highly correlated (r = 0.76, p < .001). No changes were observed in tendon reflex amplitude or latency or peak muscle twitch torque (p > .05). Significant reductions in soleus late response amplitudes (-46.9 ± 36.0%, p = .002) were detected, although these changes were not correlated with changes in maximal electromyographic activity, torque or tendon reflex amplitudes. No changes in soleus late response latency were detected. In conclusion, impaired neural drive was implicated in the stretch-induced force loss; however, no evidence was found that this loss was related to changes in muscle spindle sensitivity. We hypothesize that the decrease in soleus late response indicates a stretch-induced reduction in a polysynaptic postural reflex rather than spindle reflex sensitivity.