Three-dimensional model of the feline hindlimb

Abstract

This article describes a three-dimensional musculoskeletal model of the feline hindlimb based on digitized musculoskeletal anatomy. The model consists of seven degrees of freedom: three at the hip and two each at the knee and ankle. Lines of action and via points for 32 major muscles of the limb are described. Interspecimen variability of muscle paths was surprisingly low; most via points displayed a scatter of only a few millimeters. Joint axes identified by mechanical techniques as noncoincident and nonorthogonal were further honed to yield moment arms consistent with previous reports. Interspecimen variability in joint axes was greater than that of muscle paths and highlights the importance of joint axes in kinematic models. The contribution of specific muscles to the direction of endpoint force generation is discussed.

Fig. 1

The device used for mechanical identification of joint axes. A rail was fixed to one limb segment by bone screws, and the guide holder could be freely positioned using a sliding, rotating clamp. The guide tube, fixed to the guide holder by a ball-and-socket joint was able to be arbitrarily oriented to the joint. Proper identification of the axis was indicated when the rail attached segment could be freely rotated without moving the guide tube.

Fig. 2

Conventions used for segmental reference frames and joint angle descriptions.

Fig. 3

Knee flexion axis estimates for each specimen are presented as thin lines with the axis used in the final model presented as a heavy line. The final axis discards the single wild estimate and is slightly reoriented to match the tibial estimate.

Fig. 4

Thin lines represent the axes of rotation of the shank for each specimen, and the axes used in the model are shown as heavy lines. The steep inclination of the proximal axis reflects the strong coupling of abduction and internal rotation at the knee.

Fig. 5

Ankle abduction moment arms (cm). Moment arms and angles(°) are positive in abduction.

Fig. 6

Ankle extension moment arms(cm). Moment arms and angles (°) are positive in extension.

Fig. 7

Horizontal plane endpoint forces predicted by the model (A), or generated by intramuscular stimulation of individual muscles (B, Nichols et al., 2003). Although force directions appear to be rotated between the two cases, the relative orientations of most muscles are maintained.