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 040 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 1070 + 100. 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.
