Meeting Abstract

P1.170  Monday, Jan. 4  Contributions of active muscles to joint impedance in rats MORRISON, David*; JINDRICH, Devin L; Arizona State University; Arizona State University djmorri2@asu.edu

During legged locomotion, the mechanical properties of joints and legs (i.e. force, impedance) are modulated to achieve task-level movement dynamics. A ‘tuned’ musculoskeletal system can contribute to stable locomotion. Joint and leg mechanics reflect both passive properties and the properties of active muscle. However, the sensitivity of joint-level impedance to different patterns of muscle activity have not been well characterized in rodents. Therefore, we examined the relationship between muscle activity and ankle joint impedance in the laboratory rat. In deeply anesthetized animals, we stimulated the tibialis anterior (TA) and gastrocnemius (GAS) muscles with acute implanted electrodes, using 50 Hz trains of 0.2 ms square wave pulses while the ankle joint was attached to a position-controlled force transducer. The ankle was subject to sinusoidal angular displacements at 5 frequencies between 1-10 Hz. TA and GAS were stimulated using currents ranging from 0.0-10.0 mA for 3 seconds, with 30 second rest periods between trials. Static forces were estimated by averaging transducer output, and impedance properties calculated from force changes and phase relative to the position changes. Joint mechanics were dominated by position-dependent ‘stiffness’ properties; the contribution of damping was relatively less than stiffness. Whereas mean forces increased with increasing muscle activation, dynamic stiffness remained nearly constant over different levels of muscle force.