Acute effects of passive stretching and vibration on the electromechanical delay and musculotendinous stiffness of the plantar flexors.

The purpose of the present study was to examine the acute effects of passives stretching versus prolonged vibration on the active and passive properties of voluntary and evoked muscle actions of the plantar flexors. Eleven healthy men performed the isometric maximal voluntary contractions (MVCs) and passive range of motion (PROM) assessments before and after 20 min of passive stretching (PS), vibration (VIB), and control (CON) conditions. In addition, percent voluntary activation was calculated from superimposed and potentiated doublets during the MVCs. Voluntary peak torque (PT) decreased by 11% and 4%, while surface electromyographic (EMG) amplitude decreased by 8% and 16% for the PS and VIB, respectively, with no changes during the CON The electromechanical delay (EMD) decreased and PROM increased following the PS, but was unchanged during the VIB and CON conditions. Musculotendinous stiffness (MTS) decreased at all joint angles following the PS, but decreased only at the furthest joint angle following the VIB. There were no changes in peak twitch torque (PTT), M-wave amplitude, and EMG amplitude during the PROM assessments for all conditions. Both PS and VIB elicited similar decreases in muscle activation, which may be the same centrally-mediated mechanism (i.e., y loop impairment). Changes in the EMD were inversely proportional to the changes in MTS, which occurred only following PS. The present findings indicated that the stretching- and vibration-induced force deficits may have resulted in part from similar centrally-mediated neural deficits, while an elongation of the series elastic component may also have affected the stretching-induced force deficit.