Fig. 7. Left figure (A), with the small arrows, shows a milder case of separation of the yearling shin and the newly formed bone that was formed in response to training. Right figure (B), with the large arrows, shows a more severe separation of the yearling shin and the newly formed shin with more periosteal response than in the left figure.
Fig. 6. Radiograph of the dorsal metacarpus shows a stress fracture in the middle of three layers of bone. The horse had a period of rest, and the lameness re-occurred during the second training episode. Black arrow marks the demarcation between the yearling shin and the bone formed after a period of training; white arrow marks the new bone formed after the period of rest. It is likely that the stress fracture in the middle layer of bone was incompletely healed during the period of rest.
There is some individual variation in the ability to respond to training, as with most biologic systems. Some individuals can mount the response at a rate that virtually precluded their bone being overloaded and some individuals cannot respond fast enough for even the most moderate levels of training. The art of understanding this biologic variation is another quality needed to properly dose training. Whereas the addition of bone to the surface of the
bone to add strength and improve resistance to over- load is a workable option for long bones, it is not an option for joint surfaces such as the distal cannon bone.20,24 Because the anatomy must be preserved in the joint, the bone does not have the option to simply hypertrophy the size of the distal metacarpal or metatarsal surface because it would distort the joint surface. Therefore, in its adaptation to train- ing, the only option available for the distal metacar-
