Acute effects of passive stretching of the plantarflexor muscles on neuromuscular function: The influence of age

The acute effects of stretching on peak force (F_peak), percent voluntary activation (%VA), electromyographic (EMG) amplitude, maximum range of motion (MROM), peak passive torque, the passive resistance to stretch, and the percentage of ROM at EMG onset (%EMGonset) were examined in 18 young and 19 old men. Participants performed a MROM assessment and a maximal voluntary contraction of the plantarflexors before and immediately after 20 min of passive stretching. F_peak (-11%),%VA (-6%), and MG EMG amplitude (-9%) decreased after stretching in the young, but not the old (P>0.05). Changes in F_peak were related to reductions in all muscle activation variables (r=0.56-0.75), but unrelated to changes in the passive resistance to stretch (P≥0.24). Both groups experienced increases in MROM and peak passive torque and decreases in the passive resistance to stretch. However, the old men experienced greater changes in MROM (P<0.001) and passive resistance ( P=0.02-0.06). Changes in MROM were correlated to increases in peak passive torque (r=0.717), and the old men also experienced a nonsignificant greater (P=0.08) increase in peak passive torque. %EMGonset did not change from pre- to post-stretching for both groups (P=0.213), but occurred earlier in the old (P=0.06). The stretching-induced impairments in strength and activation in the young but not the old men may suggest that the neural impairments following stretching are gamma-loop-mediated. In addition, the augmented changes in MROM and passive torque and the lack of change in %EMGonset for the old men may be a result of age-related changes in muscle-tendon behavior.