Stress fractures are a source of frustration in amateur and professional athletes. They invariably interrupt training due to pain and if untreated can progress to a complete fracture. While microdamage is a normal phenomenon that triggers targeted remodeling and skeletal renewal, microdamage is also hypothesized to be the primary source of stress fractures. For example, an “error” in workload such as too rapid progression of training, can promote microdamage accumulation, microcrack coalescence, and result in the progression to a stress fracture. The response of bone to atypical loading conditions has been less studied and can help identify conditions that may place bone at increased risk of stress fractures. Therefore, the aim of this thesis was to (1) evaluate impact strength following fatigue loading of bone, (2) quantify the strain distribution during basketball maneuvers, and (3) evaluate the effect of musculoskeletal fatigue and bracing on tibial strains.

Cyclic loading of bone below yield limit is associated with microdamage accumulation and can increase injury risk. The recovery of bending strength has been shown to be a function of the rest days allowed after fatigue loading in rodents. Therefore, the first aim of this study was to investigate if similar results would occur under impact conditions. Cyclic axial compression load was applied in vivo on the right forelimbs while left forelimbs served as controls. Two rest groups were used:​ one day of rest and seven days of rest. Afterwards, all ulnae were scanned using micro-Computed Tomography followed by the impact testing.The micro-CT scan confirmed the formation of woven bone on loaded ulnae after seven days rest. The peak impact force was 37.5% higher in the control (mean = 174.96 ± 33.25N) specimens compared to the loaded bones (mean = 130.34 ± 22.37N ). Fourier-transformed infrared spectroscopy analyses showed no change of chemical composition in the cortical region between the loaded and control ulnae, but woven bone had lower carbonate and amide I content than contralateral controls (p < 0.05). Bones that experienced fatigue loading became less stiff, weaker, and more prone to fracture when subjected to impact loading. The formation of woven bone after seven days of rest did not restore the stiffness upon impact and confirms that rest time is crucial to the recovery of fatigue damage. One limitation of this study was the use of simulated impact loading, which is not reflective of the physiological loads expected during intense activities such as those encountered during sports.

The tibia is a common site for stress fractures, and there is a need to understand how the tibia is loaded in vivo to understand how stress fractures develop and design exercises to build a more robust bone. In Aim 2, I used subject-specific, muscle-driven, finite element simulations of 11 collegiate basketball players to calculate strain and strain rate distributions at the midshaft and distal tibia during six activities:​ walking, sprinting, lateral cut, jumping after landing, changing direction from forward-to-backward sprinting, and changing direction while side shuffling. Maximum compressive strains were at least double maximum tensile strains during the stance phase of all activities. Sprinting and lateral cut had the highest compressive (-2773±934 µε and -2697± 815 µε, respectively) and tensile (999±381 µε and 907 ±261 µε, respectively) strains. These activities also had the highest strains rates. Compressive strains principally occurred in the posterior tibia for all activities;​ however, tensile strain location varied. In particular, activities involving a change in direction increased tensile loads in the anterior tibia.

In addition to specific activities, other external factors such as functional fatigue and the use of an ankle brace can potentially influence tibial bone loads. In Aim 3, after the subjects went through a series of fatigue exercises, they repeated sprinting, jumping after landing, and lateral cut. Muscle-driven finite element simulations were created and analyzed. Maximum tensile strain decreased 22.4%, and maximum compressive decreased 8.2% under fatigue compared to normal state. Similarly, in Aim 4, 11 basketball players performed all maneuvers as those in Aim 2 (except walking) with a Bilateral Ankle Stabilizing Orthosis (ASO) brace. Wearing ASO braces had a negligible effect on the magnitude of the strains. However, wearing ASO braces during lateral cut resulted in a greater area with elevated tensile strains compared to normal and fatigue conditions. Whereas the distribution of maximum compressive strains was similar across all three conditions. Wearing ASO braces did not significantly affect the strain and strain rate. Moreover, neuromuscular fatigue showed a greater impact on jumping.

In summary, the prevention of stress fractures requires a balance between excessive microdamage that may place it at risk and beneficial microdamage that may encourage remodeling. Under atypical conditions, bone requires rest in order to recover from fatigue-initiated damage. Computational methods provide a unique non-invasive means for evaluating the physiological loading state of bone. During sports, activities that involve a change in direction may promote remodeling because strains are increased relative to walking. However, care must be taken if there is existing microdamage.