A Method to Characterize Rat Hindlimb Mechanics Using Dynamic Perturbations

Biomechanical properties, including elasticity, viscosity, and inertia, determine the forces necessary to produce movements. Understanding motor control strategies used by small animals requires knowledge of these properties and their relative importance in motor control. This study established a technique to dynamically perturb the rat hindlimb to determine hindlimb mechanics across a range of configurations. We used a linear motor with high acceleration and precise position servo control to implement fast transient perturbations. A force/torque transducer was mounted on the motor to record force and torque responses from six degrees of freedom during perturbation. A two-camera motion capture system was set up to reconstruct the 3D hindlimb kinematics. A deeply anesthetized animal was placed on a platform, and the hind paw was attached to the transducer. The limb was translated by the motor through a pseudorandom binary sequence of rapid movements with small displacements (2mm). We then fit a second-order linear model to parameterize the elasticity, viscosity, inertia, and background forces of the perturbed system. We obtained mechanical parameters from 197 hindlimb configurations in 3 rats measured across their workspace. The linear model captured R² = 0.93 ± 0.02, 0.95 ± 0.01, and 0.93 ± 0.02 of the dynamic responses from three rats. Parameter values were consistent across repeated trials, demonstrating the reliability of the estimation process. Similarly, analysis of joint kinematics also showed minimal kinematic redundancy of limb joint angles across repeated perturbations. These preliminary results show that this dynamic perturbation platform can reliably characterize the mechanical properties of rat hindlimbs. The hindlimb characteristics measured with these procedures will be critical to understanding the control strategies during locomotion and other behaviors.