Effect of stretching training on the viscoelastic properties of human tendon structures in vivo

TLDR

The study investigated whether a 3‑week stretching program changes the viscoelastic properties of human tendons in vivo. Eight men performed 3 weeks of stretching while tendon elongation and aponeurosis were measured by ultrasonography during ramp isometric plantar flexion and relaxation; stiffness was derived from the force–elongation slope, hysteresis from the force–elongation loop area, and flexibility from the passive torque–angle slope. After training, the flexibility index fell by 13.4 % while stiffness remained unchanged and hysteresis decreased from 19.9 % to 12.5 %, indicating that stretching alters tendon viscosity but not elasticity.

Abstract

The purpose of this study was to examine whether stretching training altered the viscoelastic properties of human tendon structures in vivo. Eight men performed the stretching training for 3 wk. Before and after the stretching training, the elongation of the tendon and aponeurosis of medial gastrocnemius muscle was directly measured by ultrasonography while the subjects performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between the estimated muscle force (Fm) and tendon elongation (L) during the ascending phase was fitted to a linear regression, the slope of which was defined as stiffness of tendon structures. The percentage of the area within the Fm-L loop to the area beneath the curve during ascending phase was calculated as an index representing hysteresis. To assess the flexibility, the passive torque of the plantar flexor muscles was measured during the passive stretch from 0 degrees (anatomic position) to 25 degrees of dorsiflexion with a constant velocity of 5 degrees/s. The slope of the linear portion of the passive torque-angle curve during stretching was defined as flexibility index. Flexibility index decreased significantly after stretching training (-13.4 +/- 4.6%). On the other hand, the stretching training produced no significant change in stiffness but significantly decreased hysteresis from 19.9 +/- 11.7 to 12.5 +/- 9.5%. The present results suggested that stretching training affected the viscosity of tendon structures but not the elasticity.