- Measurements of the chord length (∝ M0.31) and diameter (∝ M0.35) of the femora, tibiae, humeri and radii from 32 species of mammals, ranging in approximate body mass from 0.020-3500 kg, support previous data which show that mammalian long bones scale close to geometric similarity.
- Scaling of peak stresses based on these measurements of limb bone geometry predicts that peak stress increases ∝ M0.28, assuming that the forces acting on a bone are directly proportional to an animal's weight.
- Peak locomotory stresses measured in small and large quadrupeds contradict this scaling prediction, however, showing that the magnitude of peak bone stress is similar over a range of size. Consequently, a uniform safety factor is maintained.
- Bone curvature (∝ M-0.09) and limb bone angle relative to the direction of ground force (∝ M-0.07) exhibit a slight, but significant, decrease with increasing body mass.
- Duty factor measured at the animal's trot--gallop transition speed does not change significantly with body size.
- The moment arm ratio of ground force to muscular force exerted about a joint was found to decrease dramatically for horses as compared to ground squirrels and chipmunks. This six-fold decrease (∝ M-0.23) provides preliminary data which appear to explain, along with the decrease in bone curvature and angle, the similar magnitudes of peak bone stress developed during locomotion in different sized animals.
- The crouched posture adopted by small quadrupeds while running may allow greater changes in momentum (when accelerating or decelerating) or a decrease in the forces exerted on their limbs.
Keywords:
Peak stress; body size; duty factor; bone curvature; limb angles