Viscoelastic Creep In Human Skeletal Muscle

Abstract

INTRODUCTION: Viscoelastic creep can be defined as the increase in tissue length under a constant force or torque. PURPOSE: To examine viscoelastic creep in the plantar flexor muscles. METHODS: Thirteen healthy participants (mean age ± SD = 22 ± 3 yrs; stature = 168 ± 12 cm; mass = 69 ± 17 kg) performed four 30-s constant-torque passive stretches of the plantar flexor muscles on a calibrated Biodex System 3 dynamometer. Each 30-s passive stretch was completed by the dynamometer lever arm by passively dorsiflexing the foot at 5°/s to the point of discomfort, but not pain as acknowledged by the participant. This was quantified as the maximum tolerable passive torque threshold (i.e. 33 N·m), which was held constant for all 4 stretches while the change in dorsiflexion position was monitored under the constant passive torque. Each 30-s passive stretch was separated by 20 s of rest. Position (°) values were sampled from the dynamometer at 1 KHz during the stretches and were quantified at every 5-s period (0, 5, 10, 15, 20, 25, and 30 s). In addition, the percent change in position was quantified for each 5-s epoch (0-5, 5-10, 10-15, 15-20, 20-25, and 25-30 s) relative to the total increase in range of motion (ROM) for each 30-s stretch. A two-way repeated measures ANOVA [stretch × time; 4 × 7] was used to analyze the absolute changes in position, whereas a two-way repeated measures ANOVA [stretch × time; 4 × 6] was used to analyze the percent changes in ROM. RESULTS: For each 30-s stretch, the absolute position increased such that 0 < 5 < 10 < 15 < 20 < 25 < 30 s (P<0.05). At all time points, the position for stretch 1 was less than stretches 2 - 4 (P<0.05), and stretch 2 was less than stretches 3 - 4 (P<0.05). There were no differences at any time points between stretches 3 and 4 (P>0.05). The percent changes in ROM were the same for stretches 1, 2, 3, and 4 during each epoch (P<0.05), however, as the stretch progressed for 30 s, the relative increases in ROM diminished. CONCLUSIONS: During these constant-torque stretches, most of the increases in range of motion occurred as a result of the first two 30-s stretches, since there were no differences in dorsiflexion position between stretches 3 and 4. In addition, the relative changes in ROM that occurred during each of the four 30-s stretches were the same, but the greatest increases (42%) occurred during the first 5-s.