The loss of muscle force production after muscle stretching is not accompanied by altered corticospinal excitability.

Abstract

The aim of the present study was to determine whether depression of maximal muscular force and neural drive subsequent to prolonged ( ≥ 60s) passive muscle stretching is associated with altered corticospinal excitability or intracortical (GABAB-mediated) inhibition. Fourteen healthy adult males were tested before and after 5min (5 × 60-s stretches) of intense, passive static stretching of the plantar flexor muscles. Two protocols (A and B) were conducted in a randomized order. Transcranial magnetic stimulation was delivered to the contralateral motor cortex at rest (Protocol A) and during maximal voluntary contractions (Protocol B). Changes in maximal voluntary isometric torque, voluntary surface electromyographic activity of triceps surae muscles (normalized to M-wave; EMG/M), motor-evoked potentials (MEP), and cortical silent period (cSP; Protocol B) in soleus elicited by transcranial magnetic stimulation were examined 10min after stretch. In both protocols A and B, significant decreases were observed immediately after stretching in maximal voluntary plantar flexion torque ( - 20.1 ± 15.9%, P = 0.004; and - 17.2 ± 13.5%, P = 0.006) and EMG/M ( - 18.0 ± 18.2%, P = 0.023; and - 13.0 ± 9.3%, P = 0.003). Decreases in torque and EMG/M were highly correlated (r = 0.67-0.85, P < 0.05). However, no changes were observed in MEP amplitudes during rest ( + 29.3 ± 50.0%) or maximum voluntary contraction ( + 1.9 ± 16.8%), or in cSP ( + 2.1 ± 15.1%). Impaired neural drive contributed to the stretch-induced force loss; however, changes in corticospinal excitability and intracortical inhibition could not explain the phenomenon.