§1 Bone strain in vivo
The skeleton represents about 17% of the weight of the human body, so structural efficiency of bones is relevant to their performance in the body. To ascertain the significance of bone elasticity and strength data in connection with the function of bone in the body, it is desirable to know what levels of stress and strain occur in bones during normal activities and under traumatic conditions. It is possible to make inferences from macroscopic measurements of forces acting on the extremities. The validity of such inferences is rendered uncertain by the fact that muscle forces cannot generally be uniquely determined. One can determine bone strains explicitly in various animals and in man by directly cementing foil strain gages to bone surfaces. In a human volunteer, maximum strain along the tibia axis was about 3.5x10-4 during normal walking at 1.4 m/s and 8x10-4 during running at 2.2 m/s. Strains of similar magnitudes have been observed in animals such as sheep. The largest strain magnitude observed in the normal activity of an animal was 3.2x10-3 in the tibia of a galloping horse. In comparison, in tension in the longitudinal direction human bone yields at a strain of 6.7x10-3 and fractures at a strain of 0.03. The strain levels observed
in vivo are significant in view of the fatigue properties of bone.
§2 In Vivo Bone Strains in the Thoroughbred Racehorse.
[after Dr. Peter Muir, Veterinary Medicine]
Direct measurement of strains on the surface of bones from the limbs of a range of animals of different sizes in-vivo has suggested that peak bone strain is scaled allometrically in the range of -2000 to -3000 microstrain, to maintain a constant safety margin to yield of 2.1 to 3.1, and a low fracture risk. However, in-vivo strain gauge measurements from the dorsolateral mid-diaphyseal surface of the Mc-III bone of the thoroughbred during galloping have been over - 5000 microstrain in some individual horses, suggesting that the fracture risk for this bone in some individual thoroughbred racehorses may be higher than previously thought. These compressive strains are much higher than have been measured in human running athletes, where compressive strains remained below about 2000 microstrain, even during strenuous activity. The alteration in Mc-III bone geometry that occurs during training and racing of young thoroughbreds is thought to explain the smaller compressive strains that have been measured in more highly adapted Mc-III bones. Muscle fatigue during racing also may result in additional increases in bone strain during high speed running. Furthermore, loading of bone at high physiological strain rates, characteristic of vigorous activity, is more fatiguing (in the sense of mechanical damage, not physiological tiring) to compact bone than loading at low strain rates, with a greater increase in resultant bone microdamage. During running, the medial and lateral Mc-III cortices are principally loaded in compression, whilst the dorsal surface is loading in tension during the most stressful part of the gait cycle.