Fluid mechanics of muscle vibrations.

The pressure field produced by an isometrically contracting frog gastrocnemius muscle is described by the fluid mechanics equations for a vibrating sphere. The equations predict a pressure amplitude that is proportional to the lateral acceleration of the muscle, inversely proportional to the square of the distance from the muscle, and cosinusoidally related to the major axis of lateral movement. The predictions are confirmed by experiments that measure the pressure amplitude distribution and by photographs of muscle movement during contraction. The lateral movement of muscle has the appearance of an oscillating system response to a step function input--the oscillation may be at the resonant frequency of the muscle and therefore may provide a means to measure muscle stiffness without actually touching the muscle.