Three-dimensional model of the feline hindlimb.

The production of coordinated movement requires that the nervous system control muscle forces to generate joint torques required for control of the limb endpoint in Euclidean space. The nervous system must be capable of transformation between Euclidean space and joint space and between joint space and muscle space to determine what set of muscle activations is required to generate a desired motion (Lacquaniti and Maioli, 1994). Knowledge of these transformations is required to understand the basis of locomotion (Prochazka, 1996; Rossignol, 1996; Zernicke and Smith, 1996; Duysens et al., 2000; Stein et al., 2000) and other forms of voluntary movement (Mussa-Ivaldi et al., 1985; Flash, 1987; Feldman and Levin, 1995; Ghez et al., 1995; Gordon et al., 1995; Gottlieb et al., 1997; Kargo and Giszter, 2000; Ting et al., 2000). These transformations are based on the architecture and mechanics of the musculoskeletal system. Details of this information are available for humans (Delp et al., 2001), non-human primates (Cheng and Scott, 2000) and other animals, (Kargo et al., 2002; Kargo and Rome, 2002). The cat has been an important experimental subject for studies of normal and abnormal motor control (Macpherson, 1988; Pratt and Loeb, 1991; Prochazka, 1996; Rossignol, 1996; Macpherson and Fung, 1999), but detailed architectural information is not available in comprehensive form. The goal of this paper is to report our studies of the musculoskeletal anatomy of the feline hindlimb and a corresponding mathematical model to predict the transformations described above. Perhaps the first issue that arises in the construction of such a model is interspecimen variability. To generalize from one or a few measurements to a stereotypical or canonical anatomy, the variation between specimens must be small. If interspecimen variation is large, then the description of a sample is less likely to be representative of any individual. Interspecimen variability may also offer a means to understand the observed inter-individual variation in motor control patterns (McCollum et al., 1995). Another concern is the choice of kinematic constraints to be imposed on the model. These have previously ranged from simple hinges in anatomical planes (Mussa-Ivaldi et al., 1985) to screw-displacement axes (Woltring et al., 1985). The former is computationally straightforward, but practically limited. The latter is most useful in describing a specific observed motion for inverse dynamics, and either may introduce non-physiological constraints on the system. A compromise between these extremes has been suggested (Hollister et al., 1992), in which a joint is approximated as a combination of hinges representing physiological planes defined by joint morphology, rather than anatomical planes defined by body posture. This approach allows the computational simplicity of single degree-of-freedom joints, and extends the range over which that approximation closely represents the actual motion. The purpose of the work presented in this paper was to construct a three dimensional model of the feline hindlimb. Specific attention is focused on interspecimen variation in muscle paths and the use of joint axes.