Elastic properties of muscles measured at the elbow in man: I. Normal controls.

Passive elastic stiffness of muscle acting at the elbow was assessed in 19 normal subjects by measuring displacements produced by a torque motor acting at the joint. Stiffness ranged from 0.40 to 1.8 Nm/radian and was strongly correlated (r = 0.85) with upper arm volume, allowing us to define a "normal" range for stiffness when corrected for arm volume. In addition, the angle of the elbow with the arm fully relaxed and no external torque applied ("neutral" angle) was found to be 107 degrees +/- 10 degrees. Thus, we have quantified resting stiffness or "tone" in the arm and provided normal data for comparison with patients with pathophysiological conditions such as rigidity or spasticity.