Effect of relaxation time on hysteresis of human tendon in vivo

OBJECTIVES: The purpose of this study was to investigate the effect of relaxation time on tendon hysteresis. METHODS: Subjects exerted isometric plantar flexion torque from rest to maximal voluntary isometric contractions within around 0.5 s, followed by relaxation with six different times (0.3, 0.5, 0.7, 1, 3, and 5 s). During each trial, tendon elongation in the medial gastrocnemius muscle was measured by ultrasonography. The area within the exerted torque–tendon elongation loop, as a percentage of the area beneath the curve during ascending phase, was calculated as tendon hysteresis. RESULTS: Between the 0.3 and 1 s relaxation time conditions, the hysteresis values were significantly greater for the shorter relaxation time conditions (except between the 0.5 and 0.7 s conditions). In contrast, no significant differences in tendon hysteresis were found between 1 and 5 s of relaxation time conditions. Furthermore, the relationship between relaxation time and tendon hysteresis showed a significantly negative correlation under 1 s or less of relaxation time, but no significant correlation was observed under conditions of 1 s or more. CONCLUSION: These results suggest that relaxation time greatly affects tendon hysteresis under condition that relaxation time was less than 1 s.