SKELETAL STRAIN PATTERNS IN RELATION TO EXERCISE TRAINING DURING GROWTH

Bones are believed to alter their shape in response to changes in tiss ue 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 rec orded in the tibiotarsus of white leghorn chicks during 'intensive' tr eadmill exercise (60 % of maximum speed, carrying a weight equal to 20 % body weight on the trunk: 60 %/L) with strains that had been record ed previously during 'moderate' treadmill exercise (35 % of maximum sp eed, unloaded: 35 %/UNL) at similar bone sites. Our hypothesis is that modeling adjustments of bones subjected to the intensive load-carryin g exercise should re-establish strains recorded in the bones subjected to moderate exercise. At each exercise level, the animals were exerci sed for 5 days per week (2500 loading cycles per day) from 2 to 12 wee ks of age. As in the moderate exercise group studied earlier, strains measured at six functionally equivalent sites on the tibiotarsus of th e 60%/L group were consistently maintained during growth from 4 to 12 weeks of age. In addition, the pattern of strain recorded at these sit es was uniformly maintained over the full range of speeds recorded (fr om 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 elevat ed 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 o rientation of principal strain at corresponding sites were also simila r in the two groups. At 12 weeks of age, however, bone strain levels i n the 60 %/L group were again elevated (47 % overall) compared with th ose recorded in the 35 %/UNL group, although the general pattern and o rientation of strains remained similar. This finding suggests a transi ent modeling response of the bone to the onset of exercise training, w hich was lost during subsequent growth, possibly because the normal pa ttern of functional strain was not altered significantly by the faster load-carrying exercise.