The North American Thoroughbred racehorse was chosen as a model to study the pathogenesis of fatigue failure of bone. This species has a high incidence of spontaneous fatigue failure of bone (bucked shins) during its early training. In vivo strain gauge studies of the third metacarpal bone of four young racehorses running at racing speeds showed high principal compressive strains [-4,841 +/- 572 (SD) microstrain] while two older horses had lower principal compressive strains (-3,317 microstrain measured at racing speed, -3,250 microstrain extrapolated from a slower speed run). Previously reported inertial property measurements of the third metacarpal bone were related to the difference in bone strains seen in young and older horses. The high strains on the surface of the third metacarpal bone associated with young horses in training may lead to high strain, low cycle fatigue. The changing shape of the third metacarpal bone during maturation may be consistent with the lower strains recorded during high speed exercise in the older animals. This phenomenon may allow for the accumulation of additional strain cycles in older animals before failure occurs.