Locomotor activity, growth hormones, and systemic robusticity: An investigation of cranial vault thickness in mouse lines bred for high endurance running.

OBJECTIVES: To use a mouse model to investigate the relationships among the components of the systemic robusticity hypothesis (SRH): voluntary exercise on wheels, spontaneous physical activity (SPA) in cages, growth hormones, and skeletal robusticity, especially cranial vault thickness (CVT). MATERIALS AND METHODS: Fifty female mice from lines artificially selected for high running (HR) and 50 from nonselected control (C) lines were housed in cages with (Active) or without wheels (Sedentary). Wheel running and SPA were monitored daily. The experiment began at 24-27 days of age and lasted 12 weeks. Food consumption was measured weekly. Mice were skeletonized and their interparietal, parietal, humerus, and femur were µCT scanned. Mean total thickness of the parietal and interparietal bones was determined, along with thickness of the cortical and diploe layers individually. Geometric cross-sectional indicators of strength were calculated for the long bones. Blood samples were assayed for IGF-1 and IGFBP-3. RESULTS: Physical activity differed significantly among groups, based both on linetype (C vs. HR) and activity (A vs. S). However, contrary to our predictions, the ratio of IGF-1 to IGFBP-3 was higher in C mice than in HR mice. Neither CVT nor postcranial robusticity was affected by linetype or activity, nor were most measures of CVT and postcranial robusticity significantly associated with one another. DISCUSSION: Our results fail to provide support for the systemic robusticity hypothesis, suggesting it is important to rethink the long-standing theory that increased CVT in Homo erectus reflects increased physical activity compared other hominin species.

Genetic and tissue level muscle-bone interactions during unloading and reambulation.

Little is known about interactions between muscle and bone during the removal and application of mechanical signals. Here, we applied 3wk of hindlimb unloading followed by 3wk of reambulation to a genetically heterogeneous population of 352 adult mice and tested the hypothesis that changes in muscle are associated with changes in bone at the level of the tissue and the genome. During unloading and relative to normally ambulating control mice, most mice lost muscle and cortical bone with large variability across the population. During reambulation, individual mice regained bone and muscle at different rates. Across mice, changes in muscle and trabecular/cortical bone were not correlated to each other during unloading or reambulation. For unloading, we found one significant quantitative trait locus (QTL) for muscle area and five QTLs for cortical bone without overlap between mechano-sensitive muscle and cortical bone QTLs (but some overlap between muscle and trabecular QTLs). The low correlations between morphological changes in muscle and bone, together with the largely distinct genetic regulation of the response indicate that the premise of a muscle-bone unit that co-adjusts its size during (un)loading may need to be reassessed.

Modulation of bone's sensitivity to low-intensity vibrations by acceleration magnitude, vibration duration, and number of bouts.

UNLABELLED: Variables defining vibration-based biomechanical treatments were tested by their ability to affect the musculoskeleton in the growing mouse. Duration of a vibration bout, but not variations in vibration intensity or number of vibration bouts per day, was identified as modulator of trabecular bone formation rates. INTRODUCTION: Low-intensity vibrations (LIV) may enhance musculoskeletal properties, but little is known regarding the role that individual LIV variables play. We determined whether acceleration magnitude and/or the number and duration of daily loading bouts may modulate LIV efficacy. METHODS: LIV was applied to 8-week-old mice at either 0.3 g or 0.6 g for three weeks; the number of daily bouts was one, two, or four, and the duration of a single bout was 15, 30, or 60 min. A frequency of 45 Hz was used throughout. RESULTS: LIV induced tibial cortical surface strains in 4-month-old mice of approximately 10 µÎµ at 0.3 g and 30 µÎµ at 0.6 g. In trabecular bone of the proximal tibial metaphysis, all single daily bout signal combinations with the exception of a single 15 min daily bout at 0.3 g (i.e., single bouts of 30 and 60 min at 0.3 g and 15 and 30 min at 0.6 g) produced greater bone formation rates (BFR/BS) than in controls. Across all signal combinations, 30 and 60 min bouts were significantly more effective than 15 min bouts in raising BFR/BS above control levels. Increasing the number of daily bouts or partitioning a single daily bout into several shorter bouts did not potentiate efficacy and in some instances led to BFR/BS that was not significantly different from those in controls. Bone chemical and muscle properties were similar across all groups. CONCLUSIONS: These data may provide a basis towards optimization of LIV efficacy and indicate that in the growing mouse skeleton, increasing bout duration from 15 to 30 or 60 min positively influences BFR/BS.

Physical activity engendering loads from diverse directions augments the growing skeleton.

OBJECTIVE: An experiment was conducted to determine if modifying habitual activities to involve mechanical loading from more diverse directions can enhance the growing skeleton. METHODS: Growing female C57BL/6J mice were housed individually for 3 months in enclosures designed to accentuate either non-linear locomotion (diverse-orientation loading) or linear locomotion (stereotypic-orientation loading) (n=10/cage type). Behavioral assessments were performed daily to quantify cage activity level. Following the experiment, trabecular and cortical bone structure in the humeral head and distal femoral metaphysis were analyzed with µCT. RESULTS: Throughout the experiment, groups did not differ in cage activity level. Yet, following the experiment, the proximal humeri of mice that experienced increased diverse-orientation loading had significantly greater trabecular bone volume fraction (p=0.004), greater cortical bone area (p=0.005), greater cortical area fraction (p=0.0007), and thicker cortices (p=0.002). No significant group differences were detected in the distal femoral metaphysis. CONCLUSIONS: Diverting habitual activities to entail loading from more diverse orientations can augment the growing mouse skeleton. This study suggests that low-intensity activities that produce loads from diverse directions may represent a viable alternative to vigorous, high-impact exercise as a means of benefiting skeletal health during growth.

Quantification of adiposity in small rodents using micro-CT.

Non-invasive three-dimensional imaging of live rodents is a powerful research tool that has become critical for advances in many biomedical fields. For investigations into adipose development, obesity, or diabetes, accurate and precise techniques that quantify adiposity in vivo are critical. Because total body fat mass does not accurately predict health risks associated with the metabolic syndrome, imaging modalities should be able to stratify total adiposity into subcutaneous and visceral adiposity. Micro-computed tomography (micro-CT) acquires high-resolution images based on the physical density of the material and can readily discriminate between subcutaneous and visceral fat. Here, a micro-CT based method to image the adiposity of live rodents is described. An automated and validated algorithm to quantify the volume of discrete fat deposits from the computed tomography is available. Data indicate that scanning the abdomen provides sufficient information to estimate total body fat. Very high correlations between micro-CT determined adipose volumes and the weight of explanted fat pads demonstrate that micro-CT can accurately monitor site-specific changes in adiposity. Taken together, in vivo micro-CT is a non-invasive, highly quantitative imaging modality with greater resolution and selectivity, but potentially lower throughput, than many other methods to precisely determine total and regional adipose volumes and fat infiltration in live rodents.

Development of diet-induced fatty liver disease in the aging mouse is suppressed by brief daily exposure to low-magnitude mechanical signals.

The age-induced decline in the body's ability to fight disease is exacerbated by obesity and metabolic disease. Using a mouse model of diet-induced obesity, the combined challenge of a high-fat diet and age on liver morphology and biochemistry was characterized, while evaluating the potential of 15 min per day of high frequency (90 Hz), extremely low-magnitude (0.2 G) mechanical signals (LMMS) to suppress lipid accumulation in the liver. Following a 36-week protocol (animals 43 weeks of age), suppression of hepatomegaly and steatosis was reflected by a 29% lower liver mass in LMMS animals as compared with controls. Average triglyceride content was 101.7+/-19.4 microg mg(-1) tissue in the livers of high-fat diet control (HFD) animals, whereas HFD+LMMS animals realized a 27% reduction to 73.8+/-22.8 microg mg(-1) tissue. In HFD+LMMS animals, liver free fatty acids were also reduced to 0.026+/-0.009 microEq mg(-1) tissue from 0.035+/-0.005 microEq mg(-1) tissue in HFD. Moderate to severe micro- and macrovesicular steatosis in HFD was contrasted to a 49% reduction in area covered by the vacuoles of at least 15 microm(2) in size in HFD+LMMS animals. These data provide preliminary evidence of the ability of LMMS to attenuate the progression of fatty liver disease, most likely achieved indirectly by suppressing adipogenesis and thus the total adipose burden through life, thereby reducing a downstream challenge to liver morphology and function.

Is bone formation induced by high-frequency mechanical signals modulated by muscle activity?

Bone formation and resorption are sensitive to both external loads arising from gravitational loading as well to internal loads generated by muscular activity. The question as to which of the two sources provides the dominant stimulus for bone homeostasis and new bone accretion is arguably tied to the specific type of activity and anatomical site but it is often assumed that, because of their purportedly greater magnitude, muscle loads modulate changes in bone morphology. High-frequency mechanical signals may provide benefits at low- (<1g) and high- (>1g) acceleration magnitudes. While the mechanisms by which cells perceive high-frequency signals are largely unknown, higher magnitude vibrations can cause large muscle loads and may therefore be sensed by pathways similar to those associated with exercise. Here, we review experimental data to examine whether vibrations applied at very low magnitudes may be sensed directly by transmittance of the signal through the skeleton or whether muscle activity modulates, and perhaps amplifies, the externally applied mechanical stimulus. Current data indicate that the anabolic and anti-catabolic effects of whole body vibrations on the skeleton are unlikely to require muscular activity to become effective. Even high-frequency signals that induce bone matrix deformations of far less than five microstrain can promote bone formation in the absence of muscular activity. This independence of cells on large strains suggests that mechanical interventions can be designed that are both safe and effective.

Devastation of bone tissue in the appendicular skeleton parallels the progression of neuromuscular disease.

A mouse model of spinal muscular atrophy with respiratory distress (SMARD1) was used to study the consequences of neuromuscular degenerative disease on bone quantity and morphology. Histomorphometry and micro-computed tomography were used to assess the cortical and cancellous bone in the tibia, femur and humerus of adult neuromuscular degeneration (nmd) mice (up to 21w) and age-matched wild-type controls (WT). At 21w, the average lengths of the humerus, tibia and femur were 15%, 10%, and 10% shorter in the nmd mice, respectively. The midshaft of the humerus, tibia and femur of nmd mice had 41%, 47% and 34% less cortical bone than the WT. In the humeral, tibial, and femoral metaphyses of the nmd mice, there was 50%, 78%, and 85% less trabecular bone volume, and 58%, 92%, and 94% less trabecular connectivity than the WT. NMD cortical bone had less than half of the 42% active surface measured in the WT, yet the mineral apposition rate of those surfaces were similar between strains (nmd: 1.80 microm x day(-1); WT: 2.05 microm x day(-1)). Osteoclast number and activity levels did not differ across strains. These data emphasize that neuromuscular degeneration as a result of immunoglobulin S-mu binding protein-2 (Ighmbp2) mutation will compromise several critical parameters of bone quantity and architecture, the most severe occurring in the trabecular compartment.

Regulation of mechanical signals in bone.

OBJECTIVES: Response of the skeleton to application and removal of specific mechanical signals is discussed. Anabolic effects of high-frequency, low-magnitude vibrations, a mechanical intervention with a favorable safety profile, as well as the modulation of bone loss by genetic and epigenetic factors during disuse are highlighted. METHODS: Review. RESULTS: Bone responds to a great variety of mechanical signals and both high- and low-magnitude stimuli can be sensed by the skeleton. The ability of physical signals to influence bone morphology is strongly dependent on the signal's magnitude, frequency, and duration. Loading protocols at high signal frequencies (vibrations) allow a dramatic reduction in the magnitude of the signal. In the axial skeleton, these signals can be anabolic and anti-catabolic and increase the structural strength of the tissue. They further have shown potential in maxillofacial applications to accelerate the regeneration of bone within defects. Bone's sensitivity to the application and removal of mechanical signals is heavily under the control of the genome. Bone loss modulated by the removal of weight-bearing from the skeleton is profoundly influenced by factors such as genetics, gender, and baseline morphology. CONCLUSIONS: Adaptation of bone to functional challenges is complex but it is clear that more is not necessarily better and that even very low-magnitude mechanical signals can be anabolic. The development of effective biomechanical interventions in areas such as orthodontics, craniofacial repair, or osteoporosis will require the identification of the specific components of bone's mechanical environment that are anabolic, catabolic, or anti-catabolic.

In vivo quantification of subcutaneous and visceral adiposity by micro-computed tomography in a small animal model.

Accurate and precise techniques that identify the quantity and distribution of adipose tissue in vivo are critical for investigations of adipose development, obesity, or diabetes. Here, we tested whether in vivo micro-computed tomography (microCT) can be used to provide information on the distribution of total, subcutaneous and visceral fat volume in the mouse. Ninety C57BL/6J mice (weight range: 15.7-46.5 g) were microCT scanned in vivo at 5 months of age and subsequently sacrificed. Whole body fat volume (base of skull to distal tibia) derived from in vivo microCT was significantly (p<0.001) correlated with the ex vivo tissue weight of discrete perigonadal (R(2)=0.94), and subcutaneous (R(2)=0.91) fat pads. Restricting the analysis of tissue composition to the abdominal mid-section between L1 and L5 lumbar vertebrae did not alter the correlations between total adiposity and explanted fat pad weight. Segmentation allowed for the precise discrimination between visceral and subcutaneous fat as well as the quantification of adipose tissue within specific anatomical regions. Both the correlations between visceral fat pad weight and microCT determined visceral fat volume (R(2)=0.95, p<0.001) as well as subcutaneous fat pad weight and microCT determined subcutaneous fat volume (R(2)=0.91, p<0.001) were excellent. Data from these studies establish in vivo microCT as a non-invasive, quantitative tool that can provide an in vivo surrogate measure of total, visceral, and subcutaneous adiposity during longitudinal studies. Compared to current imaging techniques with similar capabilities, such as microMRI or the combination of DEXA with NMR, it may also be more cost-effective and offer higher spatial resolutions.

The anabolic activity of bone tissue, suppressed by disuse, is normalized by brief exposure to extremely low-magnitude mechanical stimuli.

It is generally believed that mechanical signals must be large in order to be anabolic to bone tissue. Recent evidence indicates, however, that extremely low-magnitude (<10 microstrain) mechanical signals readily stimulate bone formation if induced at a high frequency. We examined the ability of extremely low-magnitude, high-frequency mechanical signals to restore anabolic bone cell activity inhibited by disuse. Adult female rats were randomly assigned to six groups: baseline control, age-matched control, mechanically stimulated for 10 min/day, disuse (hind limb suspension), disuse interrupted by 10 min/day of weight bearing, and disuse interrupted by 10 min/day of mechanical stimulation. After a 28 day protocol, bone formation rates (BFR) in the proximal tibia of mechanically stimulated rats increased compared with age-matched control (+97%). Disuse alone reduced BFR (-92%), a suppression only slightly curbed when disuse was interrupted by 10 min of weight bearing (-61%). In contrast, disuse interrupted by 10 min per day of low-level mechanical intervention normalized BFR to values seen in age-matched controls. This work indicates that this noninvasive, extremely low-level stimulus may provide an effective biomechanical intervention for the bone loss that plagues long-term space flight, bed rest, or immobilization caused by paralysis.

Strain gradients correlate with sites of exercise-induced bone-forming surfaces in the adult skeleton.

Physical activity is capable of increasing adult bone mass. The specific osteogenic component of the mechanical stimulus is, however, unknown. Using an exogenous loading model, it was recently reported that circumferential gradients of longitudinal normal strain are strongly associated with the specific sites of periosteal bone formation. Here, we used high-speed running to test this proposed relation in an exercise model of bone adaptation. The strain environment generated during running in a mid-diaphyseal tarsometatarsal section was determined from triple-rosette strain gages in six adult roosters (>1 year). A second group of roosters was run at a high speed (1500 loading cycles/day) on a treadmill for 3 weeks. Periosteal surfaces were activated in five out of eight animals. Mechanical parameters as well as periosteal activation (as measured by incorporated fluorescent labels) were quantified site-specifically in 12 30 degrees sectors subdividing a mid-diaphyseal section. The amount of periosteal mineralizing surface per sector correlated strongly (R2 = 0.63) with the induced peak circumferential strain gradients. Conversely, peak strain magnitude and peak strain rate were only weakly associated with the sites of periosteal activation. The unique feature of this study is that a specific mechanical stimulus (peak circumferential strain gradients) was successfully correlated with specific sites of periosteal bone activation induced in a noninvasive bone adaptation model. The knowledge of this mechanical parameter may help to design exercise regimens that are able to deposit bone at sites where increased structural strength is most needed.

Does the mechanical milieu associated with high-speed running lead to adaptive changes in diaphyseal growing bone?

Exercise during growth can be important for attaining optimal bone mass. High-intensity long-duration protocols, however, can have detrimental effects on immature bone morphology and mechanics. The underlying mechanisms are poorly understood. Here, we quantified the mechanical environment of the middiaphyseal rooster tarsometatarsus during high-speed running and examined whether short bouts of this exercise-related mechanical milieu can induce positive changes in cortical bone morphology, mechanics, and mineral ash content. At 9 weeks of age, roosters were assigned to controls (n = 9) and runners (n = 8). Treadmill running was applied in loading sessions of 5 min, three times per day (approximately 2600 cycles/day) for 8 weeks. Both controls and runners received double-fluorochrome labels during weeks 3 and 8 of the protocol. Middiaphyseal distributions of tarsometatarsal longitudinal normal strain, strain rate, and strain gradients engendered by walking and running were determined via in vivo strain gauges. Compared with walking, running elevated mean peak strain magnitude by 19%, peak strain rates by 136%, and peak strain gradients by approximately 18%. After 8 weeks of running, middiaphyseal areal and mechanical properties and normalized ash weight were no different between runners and controls. Transient and focal reductions in periosteal mineral apposition rates occurred during the exercise protocol. Our current data suggest that reducing the number of loading cycles can mitigate the adverse response previously observed in this model with long-duration running. This study also supports the tenet that the exercise-generated mechanical milieu must differ substantially from the habitual milieu to induce significant adaptations.

Aging-induced osteopenia in avian cortical bone.

Cortical bone loss contributes substantially to the degradation of skeletal integrity associated with aging. However, animal models that closely mimic age-related alterations in cortical bone are limited. The objective of this study was to determine if aged rooster cortical bone demonstrates phenotypic alterations similar to those observed in aged human cortical bone (i.e., expansion of the endocortical and periosteal envelopes and elevated cortical porosity). When compared with young adult roosters, aged roosters demonstrated significant expansion of the endocortical (16%) and periosteal (10%) envelopes, resulting in significantly increased cross-sectional moments of inertia. In addition, aged rooster bone demonstrated significantly elevated cortical porosity (51%) and average area of porosity (83%). We conclude that rooster bone demonstrates age-related adaptations similar to those of humans at both tissue and cellular levels, and may therefore represent a relatively useful, inexpensive animal model for investigating the mechanisms of age-related bone loss.

High-impact exercise and growing bone: relation between high strain rates and enhanced bone formation.

We investigated whether high-impact drop jumps could increase bone formation in the middiaphyseal tarsometatarsus of growing rooster. Roosters were designated as sedentary controls (n = 10) or jumpers (n = 10). Jumpers performed 200 drop jumps per day for 3 wk. The mechanical milieu of the tarsometatarsus was quantified via in vivo strain gauges. Indexes of bone formation and mechanical parameters were determined in each of twelve 30 degrees sectors subdividing the middiaphyseal cortex. Compared with baseline walking, drop jumping produced large peak strain rates (+740%) in the presence of moderately increased peak strain magnitudes (+30%) and unaltered strain distributions. Bone formation rates were significantly increased by jump training at periosteal (+40%) and endocortical surfaces (+370%). Strain rate was significantly correlated with the specific sites of increased formation rates at endocortical but not at periosteal surfaces. Previously, treadmill running did not enhance bone growth in this model. Comparing the mechanical milieus produced by running and drop jumps revealed that jumping significantly elevated only peak strain rates. This further emphasized the sensitivity of immature bone to high strain rates.

Bone hyperemia precedes disuse-induced intracortical bone resorption.

An in vivo model was used to determine whether bone hyperemia precedes increased intracortical porosity induced by disuse. Twenty-four adult male roosters (age 1 yr) were randomly assigned to intact-control, 7-days-sham-surgery, 7-days-disuse, and 14-days-disuse groups. Disuse was achieved by isolating the left ulna diaphysis from physical loading via parallel metaphyseal osteotomies. The right ulna served as an intact contralateral control. Colored microspheres were used to assess middiaphyseal bone blood flow. Bone blood flow was symmetric between the left and right ulnae of the intact-control and sham-surgery groups. After 7 days of disuse, median (+/-95% confidence interval) standardized blood flow was significantly elevated compared with the contralateral bone (6.5 +/- 5.2 vs. 1.0 +/- 0.8 ml x min-1 x 100 g-1; P = 0.03). After 14 days of disuse, blood flow was also elevated but to a lesser extent. Intracortical porosity in the sham-surgery and 7-days-disuse bones was not elevated compared with intact-control bones. At 14 days of disuse, the area of intracortical porosity was significantly elevated compared with intact control bones (0.015 +/- 0.02 vs. 0. 002 +/- 0.002 mm2; P = 0.03). We conclude that disuse induces bone hyperemia before an increase in intracortical porosity. The potential interaction between bone vasoregulation and bone cell dynamics remains to be studied.

Adaptation of bone to physiological stimuli.

The ability of bone to alter its morphology in response to local physical stimuli is predicated upon the appropriate recruitment of bone cell populations. In turn, the ability to initiate cellular recruitment is influenced by numerous local and systemic factors. In this paper, we discuss data from three ongoing projects from our laboratory that examine how physiological processes influence adaptation and growth in the skeleton. In the first study, we recorded in vivo strains to quantify the locomotion-induced distribution of two parameters closely related to bone fluid flow strain rate and strain gradients. We found that the magnitude of these parameters (and thus the implied fluid flow) varies substantially within a given cross-section, and that while strain rate magnitude increases uniformly with elevated speed, strain gradients increase focally as gait speed is increased. Secondly, we examined the influence of vascular alterations on bone adaptation by assessing bone blood flow and bone mechanical properties in an in vivo model of trauma-induced joint laxity. A strong negative correlation (r2 = 0.8) was found between increased blood flow (76%) in the primary and secondary spongiosa and decreased stiffness (-34%) following 14 weeks of joint laxity. These data suggest that blood flow and/or vascular adaptation may interact closely with bone adaptation initiated by trauma. Thirdly, we examined the effect of a systemic influence upon skeletal health. After 4 weeks old rats were fed high fat-sucrose diets for 2 yr, their bone mechanical properties were significantly reduced. These changes were primarily due to interference with normal calcium absorption. In the aggregate, these studies emphasize the complexity of bone's normal physical environment, and also illustrate the potential interactions of local and systemic factors upon the process by which bone adapts to physical stimuli.

Skeletal adaptation to mechanical stimuli in the absence of formation or resorption of bone.

Too often, unique loading environments fail to alter bone mass and morphology, calling to question the validity of Wolff's Law; the skeleton's plasticity to mechanical signals(1). We propose that bone can accommodate new loading environments without the need to form or resorb tissue, and that a critical aspect of bone tissue's ability to adapt to mechanical stimuli is first achieved via the plasticity of the osteocyte. We suggest that the osteocyte is capable of "normalizing" its local mechanical environment by modulating its cytoskeletal architecture, attachment to the matrix, configuration of the periosteocytic space, and communication channels to surrounding cells. We believe that through this local adaptive mechanism the osteocyte can accommodate the majority of changes in the mechanical milieu without altering the tissue architecture. It is only when bone tissue is subject to more severe (albeit rare) increases or decreases in the functional environment, the osteocyte participates in the formation and/or resorption of bone by coordinating site-specific recruitment of osteoblasts and/or osteoclasts. In vivo models of bone adaptation, combined with in situ reverse transcriptase-PCR, semi-quantitative RT-PCR, Northern analysis, immuno-cytochemistry and histomorphometry, can demonstrate how distinct mechanical stimuli influence the osteocyte's cytoskeletal and lacunar architecture, coupling (and uncoupling) of the osteocyte to the matrix and neighboring cells, and the osteocyte's participation in the recruitment and differentiation of osteoblasts and osteoclasts. Thus, the osteocyte controls three strategies to modulate its local and global environment in response to three distinct functional stimuli: 1) exogenous mechanical stimuli which are distinct from normal but sufficient to maintain bone mass, 2) mechanical stimuli which are osteogenic, and 3) disuse. If it is true that the resident cell population is capable of accommodating subtle changes in the functional milieu before modification of tissue morphology is deemed necessary, a novel strategy for the development of prophylaxes for osteopenia, osseointegration and fracture healing may become apparent.

Functional adaptation of bone to exercise and injury.

Bone adapts to altered physical stimuli, dietary changes, or injury. Dietary calcium and vitamins play important roles in maintaining skeletal health, but high-fat diets are pervasive in western cultures and may contribute to the increasing prevalence of osteoporosis and incidence of related hip fractures. Exercise helps maintain bone mass and counter osteoporosis, but exercise can also have detrimental effects-particularly for immature bone. Some negative exercise effects may also be linked to diet. For example, insufficient dietary protein during exercise can impair bone development and remodeling. Bone remodeling is a potent example of tissue repair. Chronically altered loading after a joint injury, however, can result in remodeling processes that can be detrimental to the joint. Anterior cruciate ligament injury, for example, commonly leads to osteoarthritis. Early changes in the periarticular cancellous bone may play a role in the development of knee osteoarthritis. Although these factors influence skeletal health, the mechanisms remain unclear by which bone interprets its environment and responds to mechanical stimuli or injury. To understand why different levels of exercise are beneficial or detrimental or why altered joint loading leads to changes in periarticular bone structure, underlying mechanisms must be understood by which bone interprets its mechanical environment.

Adipogenesis is inhibited by brief, daily exposure to high-frequency, extremely low-magnitude mechanical signals.

Obesity, a global pandemic that debilitates millions of people and burdens society with tens of billions of dollars in health care costs, is deterred by exercise. Although it is presumed that the more strenuous a physical challenge the more effective it will be in the suppression of adiposity, here it is shown that 15 weeks of brief, daily exposure to high-frequency mechanical signals, induced at a magnitude well below that which would arise during walking, inhibited adipogenesis by 27% in C57BL/6J mice. The mechanical signal also reduced key risk factors in the onset of type II diabetes, nonesterified free fatty acid and triglyceride content in the liver, by 43% and 39%, respectively. Over 9 weeks, these same signals suppressed fat production by 22% in the C3H.B6-6T congenic mouse strain that exhibits accelerated age-related changes in body composition. In an effort to understand the means by which fat production was inhibited, irradiated mice receiving bone marrow transplants from heterozygous GFP+ mice revealed that 6 weeks of these low-magnitude mechanical signals reduced the commitment of mesenchymal stem cell differentiation into adipocytes by 19%, indicating that formation of adipose tissue in these models was deterred by a marked reduction in stem cell adipogenesis. Translated to the human, this may represent the basis for the nonpharmacologic prevention of obesity and its sequelae, achieved through developmental, rather than metabolic, pathways.