Measurement of viscoelastic properties of tendon structures in vivo.

The purpose of this study was to investigate the viscoelastic properties of tendon structures in humans. Elongation of the tendon and aponeurosis of medial gastrocnemius muscle (MG) was directly measured by ultrasonography, while subjects (n=19) performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between tendon elongation (L) and estimated muscle force (Fm) was fitted to a linear regression, the slope of which was defined as compliance of the tendon structures. The hysteresis was calculated as the ratio of the area within the L-Fm loop (elastic energy dissipated) to the area beneath the load portion of the curve (elastic energy input). The resulting L-Fm relationship was non-linear in form, as previously reported on animal and human tendons in vitro. The mean compliance was 4.5+/-1.1. 10-2 mm/N. However, there was a considerable inter-subject variability (2.9 to 7.2. 10-2 mm/N). The Young's modulus, i.e., the slope of the stress-strain curve, was 280 MPa, which tended to be lower than the previously reported values for human tendons. It was also found that the strain of the tendon structures was homogeneously distributed along their length. The mean hysteresis (energy dissipation) was 22.2+/-8.8%. However, again there was a considerable inter-subject variability (9.7 to 37.2%). The present results indicated that the tendon structures of human MG were considerably compliant and their hysteresis was in accordance with previously reported values.