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

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) wi th an isokinetic dynamometer during passive knee extension. The angle that induced a stretch sensation without pain was the maximal stretch angle. For ce, angle, angular velocity (0.09 rad/s), and electromyograph were simultan eously and continuously recorded during the stretch. Hamstring cross sectio nal area was obtained with magnetic resonance imaging. Results. Forearm skin extensibility was greater for BJHS (3.6 +/- 0.5 cm) t han 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) c ompared to controls (Delta0.98 +/- 0.05 rad) (p < 0.001), and the correspon ding peak moment was also greater for BJHS (1.3 +/- 0.4 Nm/cm(2)) than for controls (0.6 +/- 0.1 Nm/cm(2)) (p < 0.01). For a given mutual angle the pa ssive energy absorption was the same for both groups, but at the maximal st retch angle the total area-normalized energy was greater for BJHS (0.36 +/- 0.04 J/cm(2)) than for controls (0.18 +/- 0.28 J/cm(2)) (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 maxima l stretch angle and corresponding peak moment in BJHS suggests a greater su bjective tolerance to passive stretch. That is, increased flexibility in BJ HS is not a function of altered passive properties of the muscle-tendon com plex. It remains unknown if the enhanced tolerance to passive tension plays a role in the development of musculoskeletal ailment.