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Analysis of the results suggested that compensatory load redistribution mechanisms in dogs depend on the gait. All 4 limbs should be evaluated in basic research and clinical studies to determine the effects of lameness on the entire body. Further studies are necessary to elucidate specific mechanisms for unloading of the affected limb and to determine the long-term effects of load changes in animals with chronic lameness.

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.

Alterations in muscle recruitment are key to the functional plasticity of the mammalian locomotor system. One particularly challenging situation quadrupeds may face is when the functionality of a limb is reduced or lost. To better understand how mammals manage in such situations and which muscular adaptations they exhibit when locomoting on three legs, we recorded the activity patterns of two limb and one back extensor muscle in nine dogs using surface electromyography. We compared the timing and the level of recruitment before and after the loss of a hindlimb was simulated. Both the intensity and the timing of the activity changed significantly in the m. vastus lateralis of the remaining hindlimb, consistent with this limb bearing a greater proportion of the body weight as well as with previously reported kinematic changes. In accordance with the greater body weight supported by the forelimbs, the m. triceps brachii showed first and foremost an increased level of excitation. The very asymmetrical changes in the timing and the level of activity in the m. longissimus dorsi reflects the highly asymmetrical functional requirements imposed on the trunk and the pelvis when one hindlimb is no longer involved in the production of locomotor work while the other hindlimb partially compensates the loss. Integration of our electromyographical findings with kinetic and kinematic results illustrates that dogs exhibited a well-coordinated response to the functional requirements of tripedalism and underlines the importance of moment-to-moment modulation in muscular recruitment for the functional plasticity of the mammalian locomotor system.

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