New transducers for measuring muscle length in unrestrained animals

Muscle length in unrestrained, chronically implanted animals is conventionally measured with gauges consisting of a compliant silicone rubber tube filled with either hypertonic saline or mercury, the measurement principle being a continuous change in the electrical resistance of the fluid column inside the tubing with stretch. These gauges have two major disadvantages: (1) changes in resistance that are not related to changes in length, such as those produced by changes in temperature or osmotic dilution of the hypertonic saline, cause the measurements to drift, and (2) there is no direct and accurate way to calibrate the measurements. In this communication two new types of muscle length gauge are described that eliminate both problems. Both types make use of the principle of sonomicrometry, i.e., the measurement of distances with pulsed ultrasound. Both types have been successfully used to measure the length of the medial gastrocnemius muscle in chronically implanted cats during treadmill locomotion.