Muscle architecture and functional anatomy of the pelvic limb of the ostrich (Struthio camelus)

Abstract

The functional anatomy of the pelvic limb of the ostrich (Struthio camelus) was investigated in order to assess musculoskeletal specialization related to locomotor performance. The pelvic limbs of ten ostriches were dissected and detailed measurements of all muscle tendon units of the pelvic limb were made, including muscle mass, muscle length, fascicle length, pennation angle, tendon mass and tendon length. From these measurements other muscle properties such as muscle volume, physiological cross-sectional area (PCSA), tendon cross-sectional area, maximum isometric muscle force and tendon stress were derived, using standard relationships and published muscle data. Larger muscles tended to be located more proximally and had longer fascicle lengths and lower pennation angles. This led to an expected proximal to distal reduction in total muscle mass. An exception to this trend was the gastrocnemius muscle, which was found to have the largest volume and PCSA and also had the highest capacity for both force and power production. Generally high-power muscles were located more proximally in the limb, while some small distal muscles (tibialis cranialis and flexor perforatus digiti III), with short fibres, were found to have very high force generation capacities. The greatest proportion of pelvic muscle volume was for the hip extensors, while the highest capacity for force generation was observed in the extensors of the ankle, many of which were also in series with long tendons and thus were functionally suited to elastic energy storage.

Fig. 1

Scaling of pelvic limb mass was found to be directly proportional to body mass, M^1.06 (n = 10).

Fig. 2

Lateral view of the anatomy of the pelvic limb of the ostrich.

Fig. 3

Mean MTU mass (± SD) as a proportion of total pelvic limb muscle mass for all muscles of the pelvic limb of the ostrich (n = 11).

Fig. 4

MTU length measurements, showing proportion of tendon length to muscle belly length for both proximal (Prox) and distal (Dist) pelvic limb muscles of the ostrich (n = 11).

Fig. 5

Muscle functional distribution at each joint, shown for both the ostrich and the horse, as percentage of total pelvic limb muscle volume. Horse data were taken directly from Payne et al. (2005a).

Fig. 6

Maximum functional force production per kilogram of pelvic limb muscle acting at each joint for the pelvic limb of both the ostrich and the horse.