High-impact exercise and growing bone: relation between high strain rates and enhanced bone formation

We investigated whether high-impact drop jumps could increase bone formatio n in the middiaphyseal tarsometatarsus of growing rooster. Roosters were de signated as sedentary controls (n = 10) or jumpers (n = 10). Jumpers perfor med 200 drop jumps per day for 3 wk. The mechanical milieu of the tarsometa tarsus was quantified via in vivo strain gauges. Indexes of bone formation and mechanical parameters were determined in each of twelve 30 degrees sect ors subdividing the middiaphyseal cortex. Compared with baseline walking, d rop jumping produced large peak strain rates (+ 740%) in the presence of mo derately increased peak strain magnitudes (+ 30%) and unaltered strain dist ributions. Bone formation rates were significantly increased by jump traini ng at periosteal (+ 40%) and endocortical surfaces (+ 370%). Strain rate wa s significantly correlated with the specific sites of increased formation r ates at endocortical but not at periosteal surfaces. Previously, treadmill running did not enhance bone growth in this model. Comparing the mechanical milieus produced by running and drop jumps revealed that jumping significa ntly elevated only peak strain rates. This further emphasized the sensitivi ty of immature bone to high strain rates.