Does the mechanical milieu associated with high-speed running lead to adaptive changes in diaphyseal growing bone?

Exercise during growth can be important for attaining optimal bone mass. Hi gh-intensity long-duration protocols, however, can have detrimental effects on immature bone morphology and mechanics. The underlying mechanisms are p oorly understood. Here, we quantified the mechanical environment of the mid diaphyseal rooster tarsometatarsus during high-speed running and examined w hether short bouts of this exercise-related mechanical milieu can induce po sitive changes in cortical bone morphology, mechanics, and mineral ash cont ent. At 9 weeks of age, roosters were assigned to controls (n = 9) and runn ers (n = 8), Treadmill running was applied in loading sessions of 5 min, th ree times per day (approximate to 2600 cycles/day) for 8 weeks, Both contro ls and runners received double-fluorochrome labels during weeks 3 and 8 of the protocol. Middiaphyseal distributions of tarsometatarsal longitudinal n ormal strain, strain rate, and strain gradients engendered by walking and r unning were determined via in vivo strain gauges. Compared with walking, ru nning elevated mean peak strain magnitude by 19%, peak strain rates by 136% , and peak strain gradients by approximately 18%. After 8 weeks of running, middiaphyseal areal and mechanical properties and normalized ash weight we re no different between runners and controls. Transient and focal reduction s in periosteal mineral apposition rates occurred during the exercise proto col, Our current data suggest that reducing the number of loading cycles ca n mitigate the adverse response previously observed in this model with long -duration running, This study also supports the tenet that the exercise-gen erated mechanical milieu must differ substantially from the habitual milieu to induce significant adaptations. (C) 2000 by Elsevier Science Inc. All r ights reserved.