Stress Similarity and the Biomechanics of Terrestrial Locomotion in Mammals

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Animals use their limbs to generate and transmit forces associated with movement and suppport against the force of gravity. These forces are an important influence both on the evolution of muscle and bone design, as well as on the adaptive response of these tissues to changes in mechanical loading. Mechanical requirements for locomotor support, in turn, affect the energy expended by an animal during locomotor activity and dictate the limits of its performance. By comparing the mechanics of locomotion of broadly differing sized species, Dr. Biewener seeks to establish general principles that govern how natural selection has favored strategies for effective support of gravitational loads in terrestrial mammals. One such strategy appears to be regular size-dependent change in limb posture. By adopting a more upright posture, larger animals lower the forces that must be transmitted by their muscles and bones when running. This decrease in force offsets the diminished ability of larger animals to support their weight due to the effect of size on the area to volume scaling of their tissues. Underlying this mechanical constraint is the requirement that locomotor stresses (force per unit area) acting in bone and muscle tissue be kept within a safe fraction of each tissue's strength. The "safety factor" of these tissues appears to be roughly one-third of their strength. This research has applied significance relating to the functional mechanism and capacity of bone and muscle to respond adaptively to changes in physical requirements. This is important in terms of physical training associated with sports-related activity, physical therapy programs for recovery from disabling physical injury, for surgical muscle/tendon transplantation strategies to overcome motor deficits, as well as for evaluating the importance of physical exercise in relation to age-related bone loss.