Acute passive stretching alters the mechanical properties of human plantar flexors and the optimal angle for maximal voluntary contraction

The purpose of this study was to investigate whether acute passive stretching (APS) reduced maximal isometric voluntary contraction (MVC) of the plantar flexors (PF) and if so, by what mechanisms. The PF in 15 female volunteers were stretched for 10 min (5×120 s) by a torque motor to within 2° of maximum dorsiflexion (D) range of motion (ROM). MVC with twitch interpolation, maximal Hoffmann reflex (H_max) and compound action potentials (M_max) were recorded at 20° D. Stretch reflexes (SR) were mechanically induced at 200° s⁻¹ between 0° and 10° D and SR torque and EMG amplitude were determined. All tests were assessed pre- (pre) and post-APS (post-test₁). MVC, SR, and M_max were again assessed after additional stretch was applied [mean 26 (1)° D; post-test₂] to test if the optimal angle had been altered. EMG was recorded from soleus (SOL), medial gastrocnemius (MG) and tibialis anterior (TA) using bipolar surface electrodes. APS resulted in a 27% decrease in mean peak passive torque (P<0.05). MVC and SR torque were 7% (P<0.05) and 13% lower at post-test₁ (P<0.05), respectively. SR EMG amplitude of SOL and MG was reduced by 27% (P<0.05) and 22% (P<0.05), respectively. The H_max/M_max EMG and H_max/M_max torque ratios were unchanged at post-test₁. At post-test₂, MVC and SR EMG recovered to pre-APS values, while the SR and M_max torque increased by 19% and 13%, respectively (P<0.05). The decrease in MVC during post-test₁ was attributed to changes in the mechanical properties of PF and not to reduced muscle activation.