Bone adaptation to a mechanical loading program significantly increases skeletal fatigue resistance.
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.


BACKGROUND
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.


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.
Warden
Stuart J
SJ
Department of Physical Therapy, School of Health and Rehabilitation Sciences, Indiana University, Indianapolis, Indiana 46202, USA. stwarden@iupui.edu
Hurst
Julie A
JA
Sanders
Megan S
MS
Turner
Charles H
CH
Burr
David B
DB
Li
Jiliang
J
D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D001842:Bone and Bones / Q000473:pathology*; D005260:Female; D015775:Fractures, Stress / Q000378:metabolism / Q000517:prevention & control*; D008953:Models, Anatomic; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000473:pathology; D010805:Physical Conditioning, Animal; D051381:Rats; D017207:Rats, Sprague-Dawley; D012306:Risk; D013314:Stress, Mechanical; D013718:Tensile Strength; D014457:Ulna / Q000473:pathology