Muscle and joint elastic properties during elbow flexion in Duchenne muscular dystrophy.

1. Maximal voluntary contraction (MVC), series elastic stiffness and total joint stiffness during elbow flexion were investigated in healthy boys and in boys with Duchenne muscular dystrophy (DMD) in order to assess changes in mechanical properties induced by the disease. 2. Two methods were used to perform stiffness measurements: (i) the application of sinusoidal perturbations to the joint during flexion efforts, allowing the calculation of total joint stiffness; (ii) the use of quick-release movements of the elbow, which had previously been maintained in isometric contraction, allowing the calculation of series elastic stiffness. In each case, stiffness was linearly related to torque, leading to the calculation of a normalized stiffness index as the slope of this stiffness-torque relationship. 3. As expected, mean MVC was found to be much higher for healthy boys (20.02 +/- 5.20 N m) than for DMD patients (3.09 +/- 2.44 N m). Furthermore, the results showed that it was possible to characterize healthy and DMD children by virtue of the mechanical properties measured. Mean series elastic stiffness index was higher for DMD children (142.55 +/- 136.58 rad(-1)) than for healthy subjects (4.39 +/- 2.53 rad(-1)). The same holds for mean total joint stiffness index: 43.68 +/- 67.58 rad(-1) for DMD children and 2.26 +/- 0.70 rad(-1) for healthy subjects. In addition, increases in stiffness were more marked in DMD patients exhibiting high levels of muscle weakness. 4. These changes are interpreted in terms of the adaptation of the properties of the muscles and joint involved, i.e. muscle fibres, tendons, peri- and intra-articular structures.