Ontogenetic change of the weight support pattern in growing dogs

Weight support patterns vary widely among mammals. Differences in how much of the body weight is supported by the fore- versus the hind-limbs are well documented among and within species. Intraindividual variation due to ontogenetic processes has been studied in several hindlimb-dominated species and consistently showed a caudal shift in the limbs' support roles. We hypothesized that forelimb-dominated species exhibit a cranial shift in their support pattern and tested this hypothesis by examining the vertical ground reaction forces in growing dogs. Six male Beagle siblings were studied from 9 to 51 postnatal weeks (PW) of age while they trotted on an instrumented treadmill. Ontogenetic shifting in fore-to-hind support was evaluated using vertical force ratios (i.e., peak and impulse) as well as the stance time ratio of the fore- and the hind-limbs. Because morphological and kinematic characteristics influence weight support patterns, changes in body shape (i.e., trunk shape), and average limb position were determined. As in adult dogs, the forelimbs carried a greater proportion of the body weight than the hindlimbs at all ages. When the dogs were younger, peak vertical force occurred earlier during stance in the hindlimbs but not the forelimbs. Both the increasing ratio of the vertical force and the increasing ratio of the stance times indicate an increasing weight support by the forelimbs (i.e., 59% at PW9 vs. 63% at PW51). The observed ontogenetic changes in trunk shape and average limb angle were consistent with this cranial shift in weight support.