Exercise of young thoroughbred horses increases impact strength of the third metacarpal bone.

Exercise can have a profound effect on bone mass, but little is known of its effect on bone's material properties. In this experiment, our hypothesis was that a large difference in the training regimen of young thoroughbreds would produce a measurable difference in the mechanical properties of their bone material. When they were about 19 months old, eight thoroughbred racehorses were given one of two exercise regimens that lasted for 19 weeks: four horses (controls) were walked for 40 minutes a day but had no other exercise, and the remaining four (exercised) were additionally trotted for 20 minutes a day and given progressively intensive exercise on a treadmill. Mechanical testing to failure was performed on longitudinal beam specimens of the mid-diaphysis of the metacarpal. There was no difference in Young's modulus or bending strength between the two groups, although these properties varied somewhat depending on the position within the cortex from which the specimens had come. The specimens from the exercised horses had a slightly higher toughness, as measured by work (area under the load-deformation curve). They had a considerably higher impact strength. The impact strength of specimens from the outer cortex was also higher than that of those from the inner cortex in both groups. Impact strength correlated positively with the amount of microcracking produced during testing. Microcracking is related to structural and microstructural features in the bone. Increased loading caused the bone to respond in a way that enhanced its ability to microcrack and hence its toughness.