Dynamic gearing is a mechanism that has been suggested to enhance the performance of skeletal muscles by maintaining them at the shortening velocities that maximize their power or efficiency. We investigated this hypothesis in three domestic dogs during trotting and galloping. We used ground force recordings and kinematic analysis to calculate the changes in gear ratio that occur during the production of the external work of locomotion. We also monitored length changes of the vastus lateralis muscle, an extensor muscle of the knee, using sonomicrometry in four additional dogs to determine the nature and rate of active shortening of this muscle. During both trotting and galloping, the gear ratios of the extensor muscles of the elbow, wrist and ankle joints were relatively constant early in limb support, but decreased rapidly during the second half of support. The gear ratio at the hip exerted an extensor moment initially, but decreased throughout limb support and became negative midway through support. This pattern of decreasing gear ratio during the second half of support indicates that dynamic gearing does not maximize muscle power or efficiency at the elbow, wrist, hip and ankle joints. In contrast,the extensor muscles of the shoulder and knee joints exhibited an increase in gear ratio during limb support. In two dogs, the vastus lateralis muscle shortened at a relatively constant rate of 3.7-4 lengths s-1 during intermediate-speed galloping. This pattern of increasing gear ratio and constant velocity of muscle shortening at the knee joint is consistent with the hypothesis of dynamic gearing. Given the amount of work done at the knee and shoulder joints of running dogs, dynamic gearing may contribute to the economy of constant-speed running and may be important to integrated limb function.
The storage and recovery of elastic strain energy in muscles and tendons increases the economy of locomotion in running vertebrates. In this investigation, we compared the negative and positive external work produced at individual limb joints of running dogs to evaluate which muscle-tendon systems contribute to elastic storage and to determine the extent to which the external work of locomotion is produced by muscles that shorten actively rather than by muscles that function as springs. We found that the negative and positive external work of the extensor muscles is not allocated equally among the different joints and limbs. During both trotting and galloping, the vast majority of the negative work was produced by the two distal joints, the wrist and ankle. The forelimb produced most of the negative work in both the trot and the gallop. The hindlimb produced most of the positive work during galloping, but not during trotting. With regards to elastic storage, our results indicate that the forelimb of dogs displays a greater potential for storage and recovery of elastic energy than does the hindlimb. Elastic storage appears to be more important during trotting than during galloping, and elastic storage appears to be more pronounced in the extensor muscles of the distal joints than in the extensor muscles of the proximal joints. Furthermore, our analysis indicates that a significant portion of the external work of locomotion, 26% during trotting and 56 % during galloping, is produced by actively shortening muscles. We conclude that, although elastic storage of energy is extremely important to the economy of running gaits, actively shortening muscles do make an important contribution to the work of locomotion.