Bone Adaptation to a Mechanical Loading Program Significantly Increases Skeletal Fatigue Resistance

Abstract

Using a mechanical loading program to induce bone adaptation, we found that small (<2-fold) changes in the structural properties of the rat ulna increased its fatigue resistance >100-fold. This indicates that a moderate exercise program may be an effective preventative strategy for stress fractures.Introduction: There are currently limited preventative strategies for stress fractures. Because stress fracture risk is directly influenced by skeletal properties, it has been hypothesized that modification of these properties using a mechanical loading program may positively influence risk. The aim of this study was to investigate whether the bone changes associated with a mechanical loading program can enhance skeletal fatigue resistance. Materials and Methods: Site-specific mechanical loading was performed on one forearm of adult female Sprague-Dawley rats using the axial compression loading model. Loading was performed 3 days/week for 5 consecutive weeks to induce adaptation. The loaded and nonloaded ulnas in each animal were removed after the loading program, and their material and structural properties were determined. The ulna pairs were subsequently loaded until fatigue failure at the same constant peak axial load. Results: Mechanical loading induced consistent and predictable changes in the structural properties of loaded ulnas, with the largest change being a nearly 2-fold increase in midshaft minimum second moment of area (I MIN ). The mechanical-loading induced bone changes resulted in a >100-fold increase in fatigue resistance in loaded ulnas, with resistance being exponentially related to the structural properties of the ulna. Conclusions: This study found that by enhancing the structural properties of a bone through a mechanical loading program, its fatigue resistance could be significantly improved. This indicates that an exercise program aimed at modifying bone structure may be used as a possible prevention strategy for stress fractures.

Increased density and periosteal expansion of the tibia in young adult men following short-term arduous training

“…The findings from animal models are corroborated, to a certain extent, in human studies that report increased areal bone mineral density following high impact exercise [14][15][16][17] Areal BMD is not a suitable surrogate measure of bone strength with exercise interventions [18] since animal studies report large changes in bone strength despite only modest changes in density [1,19]. This is supported in cross-sectional studies of athletes using peripheral Quantitative Computed Tomography (pQCT), which describe a thicker cortex in the playing arm of tennis players [20], the tibia of triple jumpers [21] and in athletes from impact sports [22] compared with matched controls, with little or no differences in density [23].…”

“…These changes are achieved through the complex orchestration of bone modelling/remodelling [2][3][4], partly mediated by osteocyte signalling [5,6] Studies using the rat ulna have unravelled the osteogenic stimuli of mechanical loading, confirming that bone is most responsive to dynamic loading [1,7], high strain rates [8][9][10] unfamiliar strain distributions [11], and to discrete rather than continuous bouts of loading [12,13].…”

“…Short-term exercise intervention studies examining the human tibia in young adults using pQCT have not detected changes in bone geometry, reporting only modest increases in trabecular density of the distal site following 8 weeks of weight-bearing aerobic exercise training [17], whereas significant improvements in bone structure are observed in the rodent ulnar following only 5 weeks of axial loading [1,9,24]. Differences in the magnitude and pattern of adaptations might be due, in part, to the unnatural loading model of the ulnar or to extrapolation across species.…”

Bone formation after damaging in vivo fatigue loading results in recovery of whole‐bone monotonic strength and increased fatigue life

“…11), a fatigue test might be a more meaningful indicator of whether the bone has recovered its normal function or is susceptible to reinjury. For example, Warden et al 12 used fatigue testing of the rat ulna to demonstrate that bone ''adaptation'' to a 5 week mechanical loading program improved ulnar function by increasing its fatigue life by &100-fold. To our knowledge, there has been no comparable assessment of fatigue behavior after bone ''repair'' following a stress fracture.…”

Is bone loss the reversal of bone accrual? evidence from a cross-sectional study in daughter-mother-grandmother trios

“…(20,21) Such mechanical change can be sensed by osteocytes and osteoblasts, (22,23) which initiate periosteal apposition preferentially where the highest strains exist. (24,25) Accordingly, in this analysis the most significant difference in bone mass between 18-year-old girls and their mothers was found in the anterior region. This indicated that bone mass accrual from early adulthood to middle age is direction specific, corresponding with the change of strains.…”