Morphological conservation of limb natural pendular period in the domestic dog (Canis familiaris): implications for locomotor energetics.

For better understanding of the links between limb morphology and the metabolic cost of locomotion, we have characterized the relationships between limb length and shape and other functionally important variables in the straightened forelimbs and hindlimbs of a sample of 12 domestic dogs (Canis familiaris). Intra-animal comparisons show that forelimbs and hindlimbs are very similar (not significantly different) in natural pendular period (NPP), center-of-mass, and radius of gyration, even though they differ distinctly in mass, length, moment-of-inertia, and other limb proportions. The conservation of limb NPP, despite pronounced dissimilarity in other limb characteristics, appears to be the result of systematic differences in shape, forelimbs tending to be cylindrical and hindlimbs conical. Estimating limb NPP for other species from data in the literature on segment inertia and total limb length, we present evidence that the similarity between forelimbs and hindlimbs in NPP is generally true for mammals across a large size range. Limbs swinging with or near their natural pendular periods will maximize within-limb pendular exchange of potential and kinetic energy. As all four limbs of moderate- and large-size animals swing with the same period during walking, maximal advantage can be derived from the pendular exchange of energy only if forelimbs and hindlimbs are very similar in NPP. We hypothesize that an important constraint in the evolution of limb length and shape is the locomotor economy derived from forelimbs and hindlimbs of similar natural pendular period.