Bone is exquisitely sensitive to dynamic strain magnitudes and distributions. Strain changes stimulate bone remodeling that returns strain to previous, site-specific levels. Despite the current ability to accurately measure bone strains, the exercise-related strains in mature bone and the exercise-induced skeletal remodeling have not been well studied, and thus the current study described exercise-related remodeling of mature bone in terms of in vivo strains and mechanical and geometrical properties.

Remodeling was evaluated in the tarsometatarsus (scTMT) of the three groups of adult roosters:​ basal, control, and exercise. Strains were recorded from three birds/group, while mechanical and geometrical data were collected from all birds. Basal birds provided baseline scTMT characteristics. Control animals were allowed ad libitum activity in caged runs, while exercise birds ran 1 h/d, 5 d/wk for 9 wk at 70-75% predicted maximum aerobic capacity. Control and exercise birds were killed after 9 wk. Strains were recorded during treadmill walking from miniature rosette strain gauges implanted on anterior, medial, and lateral scTMT cortices.

Bone mechanical properties measured by loading scTMT to failure in 3-point bending included loads at the proportional limit, maximum, and failure;​ flexural rigidity;​ tensile stress at the proportional limit;​ and elastic modulus. Cortical morphometry was digitized from photographic slides of a 1-mm thick middiaphysial cross section of each bone. Morphological data included total and cortical cross-section;​ endosteal and periosteal diameters;​ regional cortical thicknesses;​ second moment of area about bending and nonbending axes;​ and centroid location.

Exercise scTMT had less cortical area than controls, but exercise maximum load exceeded control by 6% and basal by 12%. Control and exercise flexural rigidity exceeded basal by 16%. Transverse strains did not differ among groups, but exercise axial and shear strains significantly exceeded control and basal on most cortices. Control anterior axial strain was two times greater than basal.

The results uniquely described mature bone remodeling, suggesting that the structural properties optimized by exercise-induced remodeling differ from those optimized by age-related remodeling. Furthermore, findings support strain's osteoregulatory role but refute previous hypotheses that strain does not change with exercise.