Resonance properties of the human elbow

In normal human subjects, elbow resonance was measured by driving the forearm over a range of frequencies. The relaxed forearm, in a manipulandum with a low inertial moment, had a resonance of 0.068 +/- 0.06 Hz which was remarkably consistent for either sex and over a wide variety of arm sizes. Elbow stiffness, therefore, appears to be well adapted to the forearm moment of inertia. The resonant frequency increased as the elbow muscles were cocontracted, but the amplitude of resonance dropped exponentially because of increased damping. By 25% of maximum electromyogram levels the elbow was critically damped. Comfortable voluntary motion exhibited an amplitude-period relation very similar to that of forearm resonance, suggesting that significant use was made of resonance properties. It was found that the forearm was voluntarily oscillated at its resonant frequency within a specific range, characteristic for the individual. Depending on movement frequency, resonance can provide anything from a major to a negligible contribution to voluntary oscillatory motion. It would greatly increase efficiency of performance when antagonist muscles are kept as relaxed as possible to minimize damping.