Viscoelastic properties and flexibility of the human muscle-tendon unit in benign joint hypermobility syndrome

Abstract

Objective: To examine the passive energy absorption of the hamstring muscle-tendon unit in 9 women with benign joint hypermobility syndrome (BJHS) and 9 age and sex matched controls.

Methods: Resistance to stretch was measured as knee flexion moment (N.m) with an isokinetic dynamometer during passive knee extension. The angle that induced a stretch sensation without pain was the maximal stretch angle. Force, angle, angular velocity (0.09 rad/s), and electromyograph were simultaneously and continuously recorded during the stretch. Hamstring cross sectional area was obtained with magnetic resonance imaging.

Results: Forearm skin extensibility was greater for BJHS (3.6+/-0.5 cm) than for controls (1.3+/-0.2 cm) (p < 0.01). Similarly, the Beighton score was greater for BJHS (6.6+/-0.8) than for controls (0.4+/-0.2) (p < 0.001). Maximal stretch angle was greater for BJHS (delta1.35+/-0.07 rad) compared to controls (delta0.98+/-0.05 rad) (p < 0.001), and the corresponding peak moment was also greater for BJHS (1.3+/-0.4 Nm/cm2) than for controls (0.6+/-0.1 Nm/cm2) (p < 0.01). For a given mutual angle the passive energy absorption was the same for both groups, but at the maximal stretch angle the total area-normalized energy was greater for BJHS (0.36+/-0.04 J/cm2) than for controls (0.18+/-0.28 J/cm2) (p < 0.001).

Conclusion: The lack of difference in passive energy absorption for a given mutual stretch angle suggests that passive properties of the muscle-tendon unit of BJHS are similar to those of controls. However, the greater maximal stretch angle and corresponding peak moment in BJHS suggests a greater subjective tolerance to passive stretch. That is, increased flexibility in BJHS is not a function of altered passive properties of the muscle-tendon complex. It remains unknown if the enhanced tolerance to passive tension plays a role in the development of musculoskeletal ailment.