Dynamics of viscoelastic creep during repeated stretches

The present study examined the viscoelastic creep responses in vivo during repeated constant-torque stretches in human skeletal muscle. Twelve healthy participants completed four consecutive 30-s constant-torque passive stretches of the right plantar flexor muscles. Position and surface electromyographic (EMG) amplitude values were quantified at every 5-s period and the percent change in position was quantified for each 5-s epoch relative to the total increase in ankle joint position for each stretch. In addition, the absolute changes in position were plotted on a logarithmic time scale and fit with a linear regression line to examine both the rate of increase (slope) and the overall increase in position over the entire stretch (y-intercept). The percent change and slope were similar (P>0.05) over all four stretches, with the majority of increases in position occurring within the initial 15-20 s of each stretch (84%). Absolute ankle joint position and the y-intercept increased (P<0.05) following both the first and second stretch but plateaued (P>0.05) after the third stretch. In addition, EMG amplitude values did not change (P>0.05) during or between each 30-s stretch. These data indicate that the amount and rate of viscoelastic creep were similar during practical durations of constant-torque stretching despite no change in ankle joint position following three 30-s stretches.