Intramuscular pressure changes during contraction in relation to muscle spindles

Abstract

Pressure changes induced by stretch or contraction of the tennuissimus and calf muscles of the cat were studied in relation to postulated mechanism for pressure activation of muscle spindles. Pressure was monitored through tubing filled with low-viscosity silicone oil connecting a hypodermic needle tip to a low-volume displacement transducer. Distribution of changes was detected by moving the needle to different sites in the muscle. Both stretch and total contraction produced essentially uniform pressure increases throughout the muscle. In fractional contractions, including those of single motor units, the center of contraction was marked by strong increases in pressure and the peripheral zones by weaker biphasic and negative changes. Over much of the muscle no change was detectable during contraction of a motor unit. In general the strongest gradient of pressure change seemed directed perpendicular to the axis of the contracting unit. It was concluded that conditions favoring the operation of a pressure mechanism in the spindle might exist in subtotal, but not in total contraction of the muscle.