Skeletal strain patterns in relation to exercise training during growth.

Bones are believed to alter their shape in response to changes in tissue strains produced by physical activity and the goal of this study is to examine whether modeling responses of a growing bone to changes in physical exercise are adjusted to maintain a uniform distribution of functional strains. We test this idea by comparing in vivo strains recorded in the tibiotarsus of white leghorn chicks during 'intensive' treadmill exercise (60% of maximum speed, carrying a weight equal to 20% body weight on the trunk: 60%/L) with strains that had been recorded previously during 'moderate' treadmill exercise (35% of maximum speed, unloaded: 35%/UNL) at similar bone sites. Our hypothesis is that modeling adjustments of bones subjected to the intensive load-carrying exercise should re-establish strains recorded in the bones subjected to moderate exercise. At each exercise level, the animals were exercised for 5 days per week (2500 loading cycles per day) from 2 to 12 weeks of age. As in the moderate exercise group studied earlier, strains measured at six functionally equivalent sites on the tibiotarsus of the 60%/L group were consistently maintained during growth from 4 to 12 weeks of age. In addition, the pattern of strain recorded at these sites was uniformly maintained over the full range of speeds recorded (from 0.48 to 2.70 m s-1 at 12 weeks of age). Peak strains measured at 4 weeks of age in the load-carrying exercise group were initially elevated by 57% overall compared with peak strains recorded in the moderate exercise group. At 8 weeks of age, strain levels in the 60%/L group differed by only 4% overall compared with those recorded in the 35%/UNL group. The nature of strain (tensile versus compressive) and the orientation of principal strain at corresponding sites were also similar in the two groups. At 12 weeks of age, however, bone strain levels in the 60%/L group were again elevated (47% overall) compared with those recorded in the 35%/UNL group, although the general pattern and orientation of strains remained similar. This finding suggests a transient modeling response of the bone to the onset of exercise training, which was lost during subsequent growth, possibly because the normal pattern of functional strain was not altered significantly by the faster load-carrying exercise.