Growth-period exercise affects not only bone strength, but also bone shape in juvenile fowl

It is well established that exercise during growth results in increases in bone mass and strength. Here we examine the effect of exercise on not only bone strength, but also overall bone shape, an important determinant of muscle and joint function, using a bipedal animal model (guinea fowl; Numida meleagris). We subjected one group of fowl to a high-acceleration exercise protocol lasting from 4 wks to 14 wks of age (EXE; n=15). A second group of animals were restricted from exercise (SED; n=15). We examined general indices of bone strength and compared bone shape between groups using geometric morphometrics. We specifically asked if the ratio between the tarsometatarsus width at the ankle (affecting the ankle extensors moment arms) and the tarsometatarsus length (affecting the moment arm of the ground reaction force), referred to here as Lever Index, was affected. This ratio influences the ankle extensor muscle mechanical advantage. We found that the tarsometatarsus cross sectional area was 7% greater in EXE birds after training (p < 0.05). The minimum second moment of area (Imin), maximum second moment of area (Imax), and polar moment of inertia (J) were likewise elevated in EXE animals (12-13%; p < 0.05). These data confirm that the training protocol produced a loading effect with an expected increase in bone axial strength and bending rigidity. We also show that the TMET from the EXE group had a statistically different shape from the SED group (p < 0.05), independent of bone size. In absolute terms, the EXE animal tarsometatarsus was shorter and stockier than the SED animals, beneficial to bone strength, while maintaining overall bone volume. Linear bone dimensions confirmed that the Lever Index was higher in EXE birds, especially after normalizing for body weight, reflecting greater ankle muscle joint mechanical advantage. A relatively larger muscle (in-lever) moment arm in acceleration-trained animals, combined with previously observed increases in muscle fiber lengths with the same training, can lead to an elevated power capacity of the muscles independent of muscle mass. Our results suggest that overall bone shape responds plastically to movement history during growth in ways that may alter muscle and locomotor function.