Inertia, resonant frequency, stiffness and kinetic energy of the human forearm.

The biodynamics of the forearm of a number of relaxed human subjects have been investigated using a printed motor as a torque generator. Oscillation in the horizontal plane at the resonant frequency has been maintained by feed-back of a modified velocity signal. By means of negative position feed-back the stiffness could be artificially increased and the resonant frequency accordingly elevated. From these changes it has been possible to calculate limb inertia and muscle stiffness, treating the limb as a torsion pendulum. The correlation of inertia with the fifth power of limb length has been studied, this relationship being predicted on theoretical grounds. A significant correlation was found between resonant frequency and the square root of the reciprocal of inertia, a relationship also predicted on theoretical grounds. As the limb was kept on resonance the motion was limited solely by the damping. When the rate of oscillation was increased by the use of added stiffness the peak velocity increased, indicating that the damping was reduced. The mean inertia and mean stiffness values for the men were higher than those for the women. The kinetic energy of rapid voluntary movements was greater in men than in women.