Journal of Neuroscience Methods, 45 (1992) 217-225 217 © 1992 Elsevier Science Publishers B.V. All rights reserved 0165-0270/92/$05.0(I NSM 01437 New transducers for measuring muscle length in unrestrained animals J.L.F. Weytjens, D.A. Viberg, A.A. Caputi 1 K. Kallesoe 2 and J.A. Hoffer : Departments of Clinical Neurosciences and Medical Physiology, Unit~ersity of Calgary, Faculty of Medicine, Calgao', Alberta, Canada (Received 10 January 1992) (Revised version received 25 August 1992) (Accepted 31 August 1992) Key wordY: Muscle length; Length transducers; Chronic recording; Piezoelectricity; Sonomicrometry 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: ( I ) 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. Introduction The conventional implantable muscle length gauge consists of a thin, 4-10 cm-long compliant silicone rubber tube, filled with either mercury or hypertonic saline (45 g of NaCI/1), with either platinum or stainless steel electrodes in contact with the fluid column at each end (Prochazka et al., 1974; Loeb et al., 1980; for reviews, see Proc- hazka, 1984; Loeb and Gans, 1986; Hoffer, 1990; the details of the gauges described in these pa- pers vary considerably). Measurement rests on t Present address: Instituto de Investigaciones Biologicas Clemente Estable, Montevideo, Uruguay. 2 Present address: School of Kinesiology, Simon Fraser Uni- versity Burnaby, British Columbia, Canada. Correspondence: Dr. J.A. Hoffer, School of Kinesiology, Si- mon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. the principle of a change in the electrical resis- tance of the gauge with stretch. These gauges enable one to make measurements that reflect the time course of muscle length change with acceptable error (see Loeb et al., 1980, ['or an error analysis), yet they have 2 major drawbacks: (1) the measurements are sensitive to changes in resistance that are not related to corresponding changes in length, and (2) there is no direct and accurate way to calibrate the measurements. The device is also non-linear; however, as shown be- low, errors due to non-linear behavior are small. Changes in resistance not related to changes in length may be produced by a variety of mecha- nisms, such as changes in temperature (approxi- mately 2% change in conductance/°C for ionic solutions; Fried et al., 1977) and diffusion of interstitial fluid into the length gauge tubing. Whatever the mechanism, the result is drift. Slow changes probably underlie the poor day-to-day reproducibility of the measurements and are dif-