Differential effects of jump versus running exercise on trabecular architecture during remobilization after suspension-induced osteopenia in growing rats

Computational modeling for osteogenic potential assessment of physical exercises based on loading-induced mechanobiological environments in cortical bone remodeling

Osteoporosis and disuse can cause bone loss which reduces the weight-bearing strength of long bones. Physical exercise or mechanical loading prevents bone loss as it promotes bone modeling through osteogenesis, i.e., new bone formation. Several studies have observed distinct bone remodeling responses to physical exercises; nevertheless, the underlying mechanism behind such responses is not well established. Loading-induced pore-pressure and fluid motion act as mechanobiological stimuli to bone cells namely osteocytes which further initiate osteoactivities. The shape of loading waveforms also affects the poromechanical environment of bone. Accordingly, the present study hypothesizes that loading waveforms associated with physiological exercises may expose the bone to different mechanobiological stimuli resulting in distinct bone remodeling. A poromechanical finite element model is developed to compute pore-pressure and interstitial fluid velocity in femoral cortical bone tissue (healthy and osteoporotic) subjected to loading waveforms of three physiological exercises namely walking, running, and jumping. The model also computes the mechanobiological stimulus as a function of fluid velocity. The outcomes indicate that pore-pressure and fluid velocity decrease significantly in osteoporotic bone tissue in comparison with healthy tissue. Jumping and running both improve pore-pressure and fluid velocity in healthy and osteoporotic tissues, whereas running significantly enhances mechanobiological stimulus in both the tissues which indicates a possible explanation for distinct bone remodeling to different physical exercises. The present work also suggests that running may be recommended as a potential biomechanical therapeutic to prevent bone loss. Overall, the present work contributes to the area of orthopedic research to develop effective designs of prophylactic exercises to improve bone health.

Progression of microstructural deterioration in load-bearing immobilization osteopenia

Purpose

Immobilization osteopenia is a major healthcare problem in clinical and social medicine. However, the mechanisms underlying this bone pathology caused by immobilization under load-bearing conditions are not yet fully understood. This study aimed to evaluate sequential changes to the three-dimensional microstructure of bone in load-bearing immobilization osteopenia using a fixed-limb rat model.

Materials and method

Eight-week-old specific-pathogen-free male Wistar rats were divided into an immobilized group and a control group (n = 60 each). Hind limbs in the immobilized group were fixed using orthopedic casts with fixation periods of 1, 2, 4, 8, and 12 weeks. Feeding and weight-bearing were freely permitted. Length of the right femur was measured after each fixation period and bone microstructure was analyzed by micro-computed tomography. The architectural parameters of cortical and cancellous bone were analyzed statistically.

Results

Femoral length was significantly shorter in the immobilized group than in the control group after 2 weeks. Total area and marrow area were significantly lower in the immobilized group than in the control group from 1 to 12 weeks. Cortical bone area, cortical thickness, and polar moment of inertia decreased significantly after 2 weeks. Some cancellous bone parameters showed osteoporotic changes at 2 weeks after immobilization and the gap with the control group widened as the fixation period extended (P < 0.05).

Conclusion

The present results indicate that load-bearing immobilization triggers early deterioration of microstructure in both cortical and cancellous bone after 2 weeks.

Effects of Korean red ginseng on three-dimensional trabecular bone microarchitecture and strength in growing rats: Comparison with changes due to jump exercise

Objectives

The preventive effects of Korean red ginseng (KRG) on bone loss and microarchitectural deterioration have been extensively studied in animal models. However, few results have been reported for the effects of KRG on the trabecular microarchitecture as compared to changes resulting from physiological stimuli such as exercise load. We compared the effects of KRG and jump exercise on improvements in trabecular microarchitecture and strength of the distal femoral metaphysis in rats.

Methods and materials

Eleven-week-old male Wistar rats were divided into sedentary (CON), KRG-administered (KRG), and jump-exercised (JUM) groups. Rats were orally administered KRG extract (200 mg/kg body weight/day) once a day for 6 weeks. The jump exercise protocol comprised 10 jumps/day, 5 days/week at a jump height of 40 cm. We used microcomputed tomography to assess the microarchitecture, volumetric bone mineral density (vBMD), and fracture load as predicted by finite element analysis at the right distal femoral metaphysis. The left femur was used for the quantitative bone histomorphometry measurements.

Results

Although KRG produced significantly higher trabecular bone volume (BV/TV) than CON, BV/TV was even higher in JUM than in KRG, and differences in vBMD and fracture load were only significant between JUM and CON. In terms of trabecular microarchitecture, KRG increased trabecular number and connectivity, whereas the JUM group showed increased trabecular thickness. Bone resorption showed significant decrease by JUM and KRG group. In contrast, bone formation showed significant increase by JUM group.

Conclusions

These data show that KRG has weak but significant positive effects on bone mass and suggest that the effects on trabecular microarchitecture differ from those of jump exercise. The effects of combined KRG and jump exercise on trabecular bone mass and strength should be investigated.

Effects of Different Types of Mechanical Loading on Trabecular Bone Microarchitecture in Rats

Mechanical loading is generally considered to have a positive impact on the skeleton; however, not all types of mechanical loading have the same beneficial effect. Many researchers have investigated which types of mechanical loading are more effective for improving bone mass and strength. Among the various mechanical loads, high-impact loading, such as jumping, appears to be more beneficial for bones than low-impact loadings such as walking, running, or swimming. Therefore, the different forms of mechanical loading exerted by running, swimming, and jumping exercises may have different effects on bone adaptations. However, little is known about the relationships between the types of mechanical loading and their effects on trabecular bone structure. The purpose of this article is to review the recent reports on the effects of treadmill running, jumping, and swimming on the trabecular bone microarchitecture in small animals. The effects of loading on trabecular bone architecture appear to differ among these different exercises, as several reports have shown that jumping increases the trabecular bone mass by thickening the trabeculae, whereas treadmill running and swimming add to the trabecular bone mass by increasing the trabecular number, rather than the thickness. This suggests that different types of exercise promote gains in trabecular bone mass through different architectural patterns in small animals.

Free-fall landing and interval running have different effects on trabecular bone mass and microarchitecture, serum osteocalcin, biomechanical properties, SOST expression and on osteocyte-related characteristics

The effects of treadmill interval training (IT) and free-fall exercise were evaluated on bone parameters including osteocyte related characteristics. Thirty-eight 4-month-old male Wistar rats were randomly divided into a control (C) group and exercise groups: IT, 10 free-fall impacts/day with a 10-s (FF10) or 20-s interval between drops (FF20), 5 days/week, for 9 weeks. We assessed bone mineral density (BMD); microarchitecture by µCT; mechanical strength by a 3-point bending test; density and occupancy of the osteocyte lacunae by toluidine blue staining; osteocalcin and NTx systemic levels by ELISA; and bone tissue Sost messenger RNA (mRNA) expression by RT-PCR. NTx levels were significantly lower in exercise groups as compared with the C group. In exercise groups the Sost mRNA expression was significantly lower than in C. Tb.N was significantly higher for IT and FF20 compared with the C group. Tb.Sp was significantly lower in FF10 compared with the C group. Both IT and FF20 were associated with higher tibial lacunar density as compared with FF10. compared with FF10, IT fat mass was lower, while tibial osteocyte lacunae occupancy and systemic osteocalcin level were higher. All exercise modes were efficient in reducing bone resorption. Both IT and free-fall impact with appropriate recovery periods, which may be beneficial for bone health and osteocyte-related characteristics. Novelty: Interval training is beneficial for bone mineral density. Exercises decreased both bone resorption and inhibition of bone formation (Sost mRNA). Longer interval recovery time favors osteocyte lacunae density.

Effect of Various Types of Muscle Contraction with Different Running Conditions on Mouse Humerus Morphology

How various types of muscle contraction during exercises affect bone formation remains unclear. This study aimed to determine how exercises with different muscle contraction types affect bone morphology. In total, 20 mice were used and divided into four groups: Control, Level, Down Slow, and Down. Different types of muscle contraction were induced by changing the running angle of the treadmill. After the intervention, micro-computed tomography (Micro-CT), tartrate-resistant acid phosphatase/alkaline phosphatase (ALP) staining, and immunohistochemical staining were used to analyze the humerus head, tendon-to-bone attachment, and humerus diaphyseal region. Micro-CT found that the volume ratio of the humeral head, the volume of the tendon-to-bone attachment region, and the area of the humeral diaphyseal region increased in the Down group. However, no difference was detected in bone morphology between the Level and Down Slow groups. In addition, histology showed activation of ALP in the subarticular subchondral region in the Down Slow and Down groups and the fibrocartilage region in the tendon-to-bone attachment. Moreover, Osterix increased predominantly in the Down Slow and Down groups.Overall bone morphological changes in the humerus occur only when overuse is added to EC-dominant activity. Furthermore, different type of muscle contractile activities might promote bone formation in a site-specific manner.

Does Physical Exercise Always Improve Bone Quality in Rats?

For decades, the osteogenic effect from different physical activities on bone in rodents remained uncertain. This literature review presents for the first time the effects on five exercise models (treadmill running, wheel running, swimming, resistance training and vibration modes) in three different experimental rat groups (males, females, osteopenic) on bone quality. The bone parameters presented are bone mineral density, micro-architectural and mechanical properties, and osteoblast/osteocyte and osteoclast parameters. This review shows that physical activities have a positive effect (65% of the results) on bone status, but we clearly observed a difference amongst the different protocols. Even if treadmill running is the most used protocol, the resistance training constitutes the first exercise model in term of osteogenic effects (87% of the whole results obtained on this model). The less osteogenic model is the vibration mode procedure (31%). It clearly appears that the gender plays a role on the bone response to swimming and wheel running exercises. Besides, we did not observe negative results in the osteopenic population with impact training, wheel running and vibration activities. Moreover, about osteoblast/osteocyte parameters, we conclude that high impact and resistance exercise (such jumps and tower climbing) seems to increase bone formation more than running or aerobic exercise. Among the different protocols, literature has shown that the treadmill running procedure mainly induces osteogenic effects on the viability of the osteocyte lineage in both males and females or ovariectomized rats; running in voluntary wheels contributes to a negative effect on bone metabolism in older male models; whole-body vertical vibration is not an osteogenic exercise in female and ovariectomized rats; whereas swimming provides controversial results in female models. For osteoclast parameters only, running in a voluntary wheel for old males, the treadmill running program at high intensity in ovariectomized rats, and the swimming program in a specific ovariectomy condition have detrimental consequences.

Differential effects of jump versus running exercise on trabecular bone architecture and strength in rats

PURPOSE

This study compared differences in trabecular bone architecture and strength caused by jump and running exercises in rats.

METHODS

Ten-week-old male Wistar rats (n=45) were randomly assigned to three body weight-matched groups: a sedentary control group (CON, n=15); a treadmill running group (RUN, n=15); and a jump exercise group (JUM, n=15). Treadmill running was performed at 25 m/min without inclination, 1 h/day, 5 days/week for 8 weeks. The jump exercise protocol comprised 10 jumps/day, 5 days/week for 8 weeks, with a jump height of 40 cm. We used microcomputed tomography to assess microarchitecture, mineralization density, and fracture load as predicted by finite element analysis (FEA) at the distal femoral metaphysis.

RESULTS

Both jump and running exercises produced significantly higher trabecular bone mass, thickness, number, and fracture load compared to the sedentary control group. The jump and running exercises, however, showed different results in terms of the structural characteristics of trabecular bone. Jump exercises enhanced trabecular bone mass by thickening the trabeculae, while running exercises did so by increasing the trabecular number. FEA-estimated fracture load did not differ significantly between the exercise groups.

CONCLUSION

This study elucidated the differential effects of jump and running exercise on trabecular bone architecture in rats. The different structural changes in the trabecular bone, however, had no significant impact on trabecular bone strength.

Effects of treadmill with different intensities on bone quality and muscle properties in adult rats

BACKGROUND

Bone is a dynamically hierarchical material that can be divided into length scales of several orders of magnitude. Exercise can cause bone deformation, which in turn affects bone mass and structure. This study aimed to study the effects of treadmill running with different intensities on the long bone integrity and muscle biomechanical properties of adult male rats.

METHODS

Forty-eight 5-month-old male SD rats were randomly divided into 4 groups: i.e., sedentary group (SED), exercise with speed of 12 m/min group (EX12), 16 m/min group (EX16), and 20 m/min group (EX20). The exercise was carried out for 30 min every day, 5 days a week for 4 weeks. The femurs were examined using three-point bending test, microcomputer tomography scanning and nanoindentation test; the soleus muscle was dissected for tensile test; ALP and TRACP concentrations were measured by serum analysis.

RESULTS

The failure load was significantly increased by the EX12 group, whereas the elastic modulus was not significantly changed. The microstructure and mineral densities of the trabecular and cortical bone were significantly improved by the EX12 group. The mechanical properties of the soleus muscle were significantly increased by treadmill exercise. Bone formation showed significant increase by the EX12 group. Statistically higher nanomechanical properties of cortical bone were detected in the EX12 group.

CONCLUSION

The speed of 12 m/min resulted in significant changes in the microstructure and biomechanical properties of bone; besides, it significantly increased the ultimate load of the soleus muscle. The different intensities of treadmill running in this study provide an experimental basis for the selection of exercise intensity for adult male rats.

Multiscale experimental study on the effects of different weight-bearing levels during moderate treadmill exercise on bone quality in growing female rats

Background

Bone tissue displays a hierarchical organization. Mechanical environments influence bone mass and structure. This study aimed to explore the effects of different mechanical stimuli on growing bone properties at macro–micro–nano scales.

Methods

Sixty five-week-old female Wistar rats were treadmill exercised at moderate intensity with the speed of 12 m/min, and then randomly divided into five groups according to weight-bearing level. After 8 weeks of experiment, femurs were harvested to perform multiscale tests.

Results

Bone formation was significantly increased by weight-bearing exercise, whereas bone resorption was not significantly inhibited. Trabecular and cortical bone mineral densities showed no significant increase by weight-bearing exercise. The microstructure of trabecular bone was significantly improved by 12% weight-bearing exercise. However, similar positive effects were not observed with further increase in weight-bearing levels. The nanomechanical properties of trabecular bone were not significantly changed by weight-bearing exercise. The macrostrength of whole femur and the nanomechanical properties of cortical bone significantly decreased in the 19% and 26% weight-bearing exercise groups.

Conclusion

When rats ran on the treadmill at moderate intensity during growth period, additional 12% weight-bearing level could significantly increase bone formation, improve microstructure of trabecular bone, as well as maintain the structure and mechanical properties of cortical bone. Excessive weight-bearing level caused no positive effects on the trabecular bone microstructure and properties of cortical bone at all scales. In addition, increased weight-bearing level exerted no significant influence on trabecular and cortical bone mineral densities.

Treadmill running exercise prevents senile osteoporosis and upregulates the Wnt signaling pathway in SAMP6 mice

This study examined the effects of different exercise intensities and durations on bone mineral density (BMD) and bone strength in senescence-accelerated mouse prone 6 (SAMP6) and determined the involvement of the Wnt signaling pathway in exercise-induced osteogenesis. Three-month-old male SAMP6 mice were randomly assigned to different speeds of treadmill running exercise representing low, medium and high intensity, with the duration of five and nine weeks, respectively. We showed that medium-intensity exercise had positive effects on skeletal health, including BMD and bone strength, and the efficacy was higher than that of low-intensity exercise. Interestingly, high-intensity exercise can maintain or even increase bone strength, despite its negative effects on bone mass. Nine weeks of exercise was superior to 5 weeks of exercise, particularly for low-intensity exercise. Furthermore, these effects of exercise-induced osteogenesis are accompanied by activation of the Wnt signaling pathway. Taken together, these results suggest that the positive effects of exercise on osteoporosis prevention are intensity and duration-dependent, and may involve the regulation of Wnt signaling pathways.

Effects of high-impact exercise on the physical properties of bones of ovariectomized rats fed to a high-protein diet

The aim of this study was to evaluate the effects of high-impact physical exercise as a prophylactic and therapeutic means in osteopenic bones of rats submitted to ovariectomy and protein diet intake. A total of 64 Wistar rats were divided into eight groups (n = 8 each), being: OVX, ovx, standard diet and sedentary; OVXE, ovx, standard diet and jump; OVXP, ovx, high-protein diet and sedentary; and OVXEP, ovx, high-protein diet and jump; SH, sham, standard diet and sedentary; SHE, sham, standard diet and jump; SHP, sham, high-protein diet and sedentary; and SHEP, sham, high-protein diet and jump. OVX surgery consists of ovariectomy, and sham was the control surgery. The jumping protocol consisted of 20 jumps/day, 5 days/week. The bone structure was evaluated by densitometry, mechanical tests, histomorphometric, and immunohistochemical analyses. A high-protein diet resulted in increased bone mineral density (P = .049), but decreased maximal load (P = .026) and bone volume fraction (P = .023). The benefits of physical exercise were demonstrated by higher values of the maximal load in the trained groups compared to the sedentary groups (P < .001). The sham groups had decreased immunostaining of osteocalcin (P = .004) and osteopontin (P = .010) compared to ovx groups. However, the high-protein diet (P = .005) and jump exercise (P = .017) resulted in lower immunostaining of osteopontin compared to the standard diet and sedentary groups, respectively. In this experimental model, it was concluded that ovariectomy and a high-fat diet can negatively affect bone tissue and the high-impact exercise was not enough to suppress the deleterious effects caused by the protein diet and ovariectomy.

Ground reaction forces during walking with different load and slope combinations in rats

Background

Treadmill animal models are commonly used to study effects of exercise on bone. Since mechanical loading induces bone strain, resulting in bone formation, exercise that induces higher strains is likely to cause more bone formation. Our aim was to investigate the effect of slope and additional load on limb bone strain.

Methods

Horizontal and vertical ground reaction forces on left fore-limb (FL) and hind-limb (HL) of twenty 23-week old female Wistar rats (weight 279 ± 26 g) were measured for six combinations of SLOPE (−10°, 0°, +10°) and LOAD (0 to 23% of body mass). Peak force (Fmax), rate of force rise (RC), stance time (Tstance) and impulse (Fint) on FLs and HLs were analyzed.

Results

For the FL, peak ground reaction forces and rate of force rise were highest when walking downward −10° with load (Fmax = 2.09±0.05 N, FLRC = 34±2 N/s) For the HL, ground reaction forces and rate of force rise were highest when walking upward +10°, without load (Fmax = 2.20±0.05 N, HLRC = 34±1 N/s). Load increased stance time. Without additional load, estimates for the highest FL loading (slope is −10°) were larger than for the highest HL loading (slope is +10°) relative to level walking.

Conclusions

Thus, walking downward has a higher impact on FL bones, while walking upward is a more optimal HL exercise. Additional load may have a small effect on FL loading.

Electronic supplementary material

The online version of this article (doi:10.1186/s40634-017-0102-8) contains supplementary material, which is available to authorized users.

HIGH-IMPACT DROP EXERCISE ALTERS MECHANICAL PROPERTIES IN OSTEOPENIC BONE

ABSTRACT Introduction: Osteopenia is a reversible condition and precedes osteoporosis. Physical activity and mechanical loading appear to play an important role in the regulation of bone homeostasis, without the side effects of targeted drug therapy. However, there is controversy as to which type of stimulus promotes more effective adaptations with respect to mechanical properties of bones. Objective: To investigate the effects of high-impact drop training on bone structure after ovariectomy-induced osteopenia in 40 10-week-old female Wistar rats. Methods: Twenty female rats (prevention program) were randomly assigned into two groups (n=10): Ovariectomized sedentary (OVXs), and OVX trained (OVX+Dropt). OVX+Dropt animals began training 3 days after surgery. Another twenty female rats (treatment program) were randomly assigned to two other groups (n=10): Ovariectomized sedentary (OVXs), and OVX trained (OVX+Dropt). OVX+Dropt animals began training 60 days after surgery. The rats in the trained groups were dropped from 40 cm height 20 times/day, 5 days/week over a period of 12 weeks period. At the end, the biomechanical tests were analyzed. Results: The final load and stiffness of the left tibia in the trained groups were higher than in the sedentary groups (p<0.05). Conclusions: Dropping exercise induced favorable changes in bone mechanical properties. High-impact drop exercise is effective to prevent bone loss after ovariectomy even when osteopenia is already established.

Jumping exercise preserves bone mineral density and mechanical properties in osteopenic ovariectomized rats even following established osteopenia

The effects of jump training on bone structure before and after ovariectomy-induced osteopenia in rats were investigated. Jumping exercise induced favorable changes in bone mineral density, bone mechanical properties, and bone formation/resorption markers. This exercise is effective to prevent bone loss after ovariectomy even when osteopenia is already established.

INTRODUCTION

The present study investigated the effects of jump training on bone structure before and after ovariectomy-induced osteopenia in 80 10-week-old Wistar rats.

METHODS

Forty rats (prevention program) were randomly allocated to one of four equal groups (n = 10): sham-operated sedentary (SHAM-SEDp), ovariectomized (OVX) sedentary (OVX-SEDp), sham-operated exercised (SHAM-EXp), and OVX exercised (OVX-EXp). SHAM-EXp and OVX-EXp animals began training 3 days after surgery. Another 40 rats (treatment program) were randomly allocated into another four groups (n = 10): sham-operated sedentary (SHAM-SEDt), OVX sedentary (OVX-SEDt), sham-operated exercised (SHAM-EXt), and OVX exercised (OVX-EXt). SHAM-EXt and OVX-EXt animals began training 60 days after surgery. The rats in the exercised groups jumped 20 times/day, 5 days/week, to a height of 40 cm for 12 weeks. At the end of the experimental period, serum osteocalcin, follicle-stimulating hormone (FSH) dosage, dual X-ray absorptiometry (DXA), histomorphometry, and biomechanical tests were analyzed.

RESULTS

The OVX groups showed higher values of FSH and body weight (p < 0.05). DXA showed that jump training significantly increased bone mineral density of the femur and fifth lumbar vertebra (p < 0.05). The stiffness of the left femur and fifth lumbar vertebra in the exercised groups was greater than that of the sedentary groups (p < 0.05). Ovariectomy induced significant difference in bone volume (BV/TV, percent), trabecular separation (Tb.Sp, micrometer), and trabecular number (Tb.N, per millimeter) (p < 0.05) compared to sham operation. Jump training in the OVX group induced significant differences in BV/TV, Tb.Sp, and Tb.N and decreased osteoblast number per bone perimeter (p < 0.05) compared with OVX nontraining, in the prevention groups. Osteocalcin dosage showed higher values in the exercised groups (p < 0.05).

CONCLUSIONS

Jumping exercise induced favorable changes in bone mineral density, bone mechanical properties, and bone formation/resorption markers. Jump training is effective to prevent bone loss after ovariectomy even when osteopenia is already established.

The effects of exercise load during development on oxidative stress levels and antioxidant potential in adulthood

The objective of this study was to elucidate the impact of physical activity during the growth period as well as on oxidative stress and antioxidative potential in adulthood. The experimental animals used were four-week old male Wistar rats, which were randomly divided into three groups. The exercise loads were as follows: control (CON), treadmill exercise (TE), and jumping exercise (JE). The exercise was performed at the same time of day, at a frequency of five days per week, for eight weeks. Derivatives of reactive oxygen metabolites (d-ROSs) and biological antioxidant potential (BAP) were measured during periods of rest prior to commencement of the experiment and after the experiment. Analysis was conducted using a Wilcoxon signed-rank test and Schaffer's multiple comparison procedure and the significance level was set at p < 0.05. The percent increase in d-ROM levels in the JE group, which experienced short-duration intense exercise loads, was higher than that in the TE group, which experienced moderately intense exercise loads. However, BAP, which is an index of antioxidant potential, markedly decreased in adulthood in the CON group, as compared to that in the developmental period, whereas the exercise groups showed no notable changes in BAP levels. Oxidative stress levels and antioxidant potential are affected differently in adulthood, depending on the intensity of sustained exercise loads experienced during development. Results suggested that in order to increase antioxidant potential, while taking oxidative stress production into account, moderately intense exercise loads are more desirable than highly intense exercise loads.

Microgravity Stress: Bone and Connective Tissue

The major alterations in bone and the dense connective tissues in humans and animals exposed to microgravity illustrate the dependency of these tissues' function on normal gravitational loading. Whether these alterations depend solely on the reduced mechanical loading of zero g or are compounded by fluid shifts, altered tissue blood flow, radiation exposure, and altered nutritional status is not yet well defined. Changes in the dense connective tissues and intervertebral disks are generally smaller in magnitude but occur more rapidly than those in mineralized bone with transitions to 0 g and during recovery once back to the loading provided by 1 g conditions. However, joint injuries are projected to occur much more often than the more catastrophic bone fracture during exploration class missions, so protecting the integrity of both tissues is important. This review focuses on the research performed over the last 20 years in humans and animals exposed to actual spaceflight, as well as on knowledge gained from pertinent ground-based models such as bed rest in humans and hindlimb unloading in rodents. Significant progress has been made in our understanding of the mechanisms for alterations in bone and connective tissues with exposure to microgravity, but intriguing questions remain to be solved, particularly with reference to biomedical risks associated with prolonged exploration missions.

High-impact exercise in rats prior to and during suspension can prevent bone loss

High-impact exercise has been considered an important method for treating bone loss in osteopenic experimental models. In this study, we investigated the effects of osteopenia caused by inactivity in femora and tibiae of rats subjected to jump training using the rat tail suspension model. Eight-week-old female Wistar rats were divided into five groups (n=10 each group): jump training for 2 weeks before suspension and training during 3 weeks of suspension; jump training for 2 weeks before suspension; jump training only during suspension; suspension without any training; and a control group. The exercise protocol consisted of 20 jumps/day, 5 days/week, with a jump height of 40 cm. The bone mineral density of the femora and tibiae was measured by double energy X-ray absorptiometry and the same bones were evaluated by mechanical tests. Bone microarchitecture was evaluated by scanning electron microscopy. One-way ANOVA was used to compare groups. Significance was determined as P<0.05. Regarding bone mineral density, mechanical properties and bone microarchitecture, the beneficial effects were greater in the bones of animals subjected to pre-suspension training and subsequently to training during suspension, compared with the bones of animals subjected to pre-suspension training or to training during suspension. Our results indicate that a period of high impact exercise prior to tail suspension in rats can prevent the installation of osteopenia if there is also training during the tail suspension.

Effect of swimming exercise on three-dimensional trabecular bone microarchitecture in ovariectomized rats

Swimming is generally considered ineffective for increasing bone mass in humans, at least compared with weight-bearing sports. However, swimming exercise has sometimes been shown to have a strong positive effect on bone mass in small animals. This study investigated the effects of swimming on bone mass, strength, and microarchitecture in ovariectomized (OVX) rats. OVX or sham operations were performed on 18-wk-old female Fisher 344 rats. Rats were randomly divided into four groups: sham sedentary (Sham-CON), sham swimming exercised (Sham-SWI), OVX sedentary (OVX-CON), and OVX swimming exercised (OVX-SWI). Rats in exercise groups performed swimming in a water bath for 60 min/day, 5 days/wk, for 12 wk. Bone mineral density (BMD) in right femurs was analyzed using dual-energy X-ray absorptiometry. Three-dimensional trabecular architecture at the distal femoral metaphysis was analyzed using microcomputed tomography (μCT). Geometrical properties of diaphyseal cortical bone were evaluated in the midfemoral region using μCT. The biomechanical properties of femurs were analyzed using three-point bending. Femoral BMD was significantly decreased following ovariectomy. This change was suppressed by swimming. Trabecular bone thickness, number, and connectivity were decreased by ovariectomy, whereas structure model index (i.e., ratio of rod-like to plate-like trabeculae) increased. These changes were also suppressed by swimming exercise. Femurs displayed greater cortical width and maximum load in SWI groups than in CON groups. Together, these results demonstrate that swimming exercise drastically alleviated both OVX-induced decreases in bone mass and mechanical strength and the deterioration of trabecular microarchitecture in rat models of osteoporosis.

Pulsed electromagnetic fields partially preserve bone mass, microarchitecture, and strength by promoting bone formation in hindlimb-suspended rats

A large body of evidence indicates that pulsed electromagnetic fields (PEMF), as a safe and noninvasive method, could promote in vivo and in vitro osteogenesis. Thus far, the effects and underlying mechanisms of PEMF on disuse osteopenia and/or osteoporosis remain poorly understood. Herein, the efficiency of PEMF on osteoporotic bone microarchitecture, bone strength, and bone metabolism, together with its associated signaling pathway mechanism, was systematically investigated in hindlimb-unloaded (HU) rats. Thirty young mature (3-month-old), male Sprague-Dawley rats were equally assigned to control, HU, and HU + PEMF groups. The HU + PEMF group was subjected to daily 2-hour PEMF exposure at 15 Hz, 2.4 mT. After 4 weeks, micro-computed tomography (µCT) results showed that PEMF ameliorated the deterioration of trabecular and cortical bone microarchitecture. Three-point bending test showed that PEMF mitigated HU-induced reduction in femoral mechanical properties, including maximum load, stiffness, and elastic modulus. Moreover, PEMF increased serum bone formation markers, including osteocalcin (OC) and N-terminal propeptide of type 1 procollagen (P1NP); nevertheless, PEMF exerted minor inhibitory effects on bone resorption markers, including C-terminal crosslinked telopeptides of type I collagen (CTX-I) and tartrate-resistant acid phosphatase 5b (TRAcP5b). Bone histomorphometric analysis demonstrated that PEMF increased mineral apposition rate, bone formation rate, and osteoblast numbers in cancellous bone, but PEMF caused no obvious changes on osteoclast numbers. Real-time PCR showed that PEMF promoted tibial gene expressions of Wnt1, LRP5, β-catenin, OPG, and OC, but did not alter RANKL, RANK, or Sost mRNA levels. Moreover, the inhibitory effects of PEMF on disuse-induced osteopenia were further confirmed in 8-month-old mature adult HU rats. Together, these results demonstrate that PEMF alleviated disuse-induced bone loss by promoting skeletal anabolic activities, and imply that PEMF might become a potential biophysical treatment modality for disuse osteoporosis.

Effects of different types of jump impact on trabecular bone mass and microarchitecture in growing rats

Substantial evidence from animal studies indicates that jumping increases bone mass and strength. However, most studies have focused on the take-off, rather than the landing phase of jumps. Thus, we compared the effects of landing and upward jump impact on trabecular bone mass and microarchitecture. Male Wistar rats aged 10 weeks were randomly assigned to the following groups: sedentary control (CON), 40-cm upward jumps (40UJ); 40-cm drop jumps (40DJ); and 60-cm drop jumps (60DJ) (n = 10 each). The upward jump protocol comprised 10 upward jumps/day, 5 days/week for 8 weeks to a height of 40 cm. The drop jump protocol comprised dropping rats from a height of 40 or 60 cm at the same frequency and time period as the 40UJ group. Trabecular bone mass, architecture, and mineralization at the distal femoral metaphysis were evaluated using microcomputed tomography. Ground reaction force (GRF) was measured using a force platform. Bone mass was significantly higher in the 40UJ group compared with the DJ groups (+49.1% and +28.3%, respectively), although peak GRF (-57.8% and -122.7%, respectively) and unit time force (-21.6% and -36.2%, respectively) were significantly lower in the 40UJ group. These results showed that trabecular bone mass in growing rats is increased more effectively by the take-off than by the landing phases of jumps and suggest that mechanical stress accompanied by muscle contraction would be more important than GRF as an osteogenic stimulus. However, the relevance of these findings to human bone physiology is unclear and requires further study.

Moderate intensity resistive exercise improves metaphyseal cancellous bone recovery following an initial disuse period, but does not mitigate decrements during a subsequent disuse period in adult rats

Spaceflight provides a unique environment for skeletal tissue causing decrements in structural and densitometric properties of bone. Previously, we used the adult hindlimb unloaded (HU) rat model to show that previous exposure to HU had minimal effects on bone structure after a second HU exposure followed by recovery. Furthermore, we found that the decrements during second HU exposure were milder than the initial HU cycle. In this study, we used a moderate intensity resistance exercise protocol as an anabolic stimulus during recovery to test the hypothesis that resistance exercise following an exposure to HU will significantly enhance recovery of densitometric, structural, and, more importantly, mechanical properties of trabecular and cortical bone. We also hypothesized that resistance exercise during recovery, and prior to the second unloading period, will mitigate the losses during the second exposure. The hypothesis that exercise during recovery following hindlimb unloading will improve bone quality was supported by our data, as total BMC, total vBMD, and cancellous bone formation at the proximal tibia metaphysis increased significantly during exercise period, and total BMC/vBMD exceeded age-matched control and non-exercised values significantly by the end of recovery. However, our results did not support the hypothesis that resistance exercise prior to a subsequent unloading period will mitigate the detrimental effects of the second exposure, as the losses during the second exposure in total BMC, total vBMD, and cortical area at the proximal tibia metaphysis for the exercised animals were similar to those of the non-exercised group. Therefore, exercise did not mitigate effects of the second HU exposure in terms of pre-to-post HU changes in these variables, but it did produce beneficial effects in a broader sense.

The osteogenic effects of swimming, jumping, and vibration on the protection of bone quality from disuse bone loss

We assessed and compared the effects of swimming, jumping, and vibration therapies on the prevention of bone loss because of unloading. Eighty Wistar rats were randomly divided into eight groups: S, permanent hind limb-suspended rats; CON, control rats; S + Swim, unloading interrupted by swimming exercise; S + C(Swim), suspension interrupted by regular weight-bearing with the same duration as in the S + Swim protocol; S + Jump, unloading interrupted by jumping exercise; S + C(Jump), suspension interrupted for regular weight-bearing as in the S + Jump group; S + Vibr, unloading interrupted by vibration; and S + C(Vibr), suspension with interruptions for regular weight-bearing with the same protocol as that used for the S + Vibr rats. At the end of the experiment, the bone mineral density, bone strength, histomorphometric parameters, and serum levels of the bone markers were analyzed. The hind limb-suspended rats exhibited bone quality loss. In contrast, the trained rats showed a significant increase in bone mass, bone strength, bone formation, and serum levels of bone markers compared with the respective controls. Although we did not find a significant difference among the three physical exercises, the osteogenic effect of vibration was slightly lower than that of swimming and jumping. Thus, all physical exercises were efficient in preventing bone loss because of unloading and preserving bone quality.

The Contribution of Experimental in vivo Models to Understanding the Mechanisms of Adaptation to Mechanical Loading in Bone

Changing loading regimens by natural means such as exercise, with or without interference such as osteotomy, has provided useful information on the structure:function relationship in bone tissue. However, the greatest precision in defining those aspects of the overall strain environment that influence modeling and remodeling behavior has been achieved by relating quantified changes in bone architecture to quantified changes in bones' strain environment produced by direct, controlled artificial bone loading. Jiri Hert introduced the technique of artificial loading of bones in vivo with external devices in the 1960s using an electromechanical device to load rabbit tibiae through transfixing stainless steel pins. Quantifying natural bone strains during locomotion by attaching electrical resistance strain gages to bone surfaces was introduced by Lanyon, also in the 1960s. These studies in a variety of bones in a number of species demonstrated remarkable uniformity in the peak strains and maximum strain rates experienced. Experiments combining strain gage instrumentation with artificial loading in sheep, pigs, roosters, turkeys, rats, and mice has yielded significant insight into the control of strain-related adaptive (re)modeling. This diversity of approach has been largely superseded by non-invasive transcutaneous loading in rats and mice, which is now the model of choice for many studies. Together such studies have demonstrated that over the physiological strain range, bone's mechanically adaptive processes are responsive to dynamic but not static strains; the size and nature of the adaptive response controlling bone mass is linearly related to the peak loads encountered; the strain-related response is preferentially sensitive to high strain rates and unresponsive to static ones; is most responsive to unusual strain distributions; is maximized by remarkably few strain cycles, and that these are most effective when interrupted by short periods of rest between them.

Increasing the Number of Unloading/Reambulation Cycles does not Adversely Impact Body Composition and Lumbar Bone Mineral Density but Reduces Tissue Sensitivity

A single exposure to hindlimb unloading leads to changes in body mass, body composition and bone, but the consequences of multiple exposures are not yet understood. Within a 18wk period, adult C57BL/6 male mice were exposed to one (1x-HLU), two (2x-HLU) or three (3x-HLU) cycles of 2 wk of hindlimb unloading (HLU) followed by 4 wk of reambulation (RA), or served as ambulatory age-matched controls. In vivo µCT longitudinally tracked changes in abdominal adipose and lean tissues, lumbar vertebral apparent volumetric bone mineral density (vBMD) and upper hindlimb muscle cross-sectional area before and after the final HLU and RA cycle. Significant decreases in total adipose tissue and vertebral vBMD were observed such that all unloaded animals reached similar values after the final unloading cycle. However, the magnitude of these losses diminished in mice undergoing their 2^nd or 3^rd HLU cycle. Irrespective of the number of HLU/RA cycles, total adipose tissue and vertebral vBMD recovered and were no different from age-matched controls after the final RA period. In contrast, upper hindlimb muscle cross-sectional area was significantly lower than controls in all unloaded groups after the final RA period. These results suggest that tissues in the abdominal region are more resilient to multiple bouts of unloading and more amenable to recovery during reambulation than the peripheral musculoskeletal system.

Jump exercise during hindlimb unloading protect against the deterioration of trabecular bone microarchitecture in growing young rats

Three-dimensional femoral trabecular architecture was investigated in tail-suspended young growing rats and the effects of jump exercise during the period of tail-suspension were also examined. Eight-week-old male Wistar rats (n = 24) were randomly assigned to three body weight-matched groups: a tail suspended group (SUS, n = 8); a sedentary control group (CON, n = 8) and rats primed with jump exercise during the period of tail suspension (JUM, n = 8). The jump exercise protocol consisted of 30 jumps/day, five days/week with a 40 cm jump height. After 3 weeks of jump exercise, bone mineral density (BMD) of the entire right femur was measured using dual energy X-ray absorptiometry. Three-dimensional trabecular bone architecture at the distal femoral metaphysis was evaluated using microcomputed tomography (micro-CT). Tail suspension caused a decrease in femoral BMD (-5%, p < 0.001) and trabecular bone architectural deterioration. Deterioration in the trabecular network during hindlimb unloading was mostly attributed to the reduction of trabecular number (-32%, p < 0.001) in the distal femoral metaphysis. Jump exercise during the tail suspension period increased trabecular thickness (14%, p < 0.001) and the reduction of trabecular number was suppressed. The present data indicate that jump exercise applied during hindlimb unloading could be able to inhibit bone loss and trabecular bone architectural deterioration caused by tail suspension.

Discordant recovery of bone mass and mechanical properties during prolonged recovery from disuse

Profound bone loss at weight bearing sites is a primary effect of long-duration spaceflight. Moreover, a significant increase in estimated fracture risk remains even 1 year after returning to Earth; hence, it is important to define how quickly bone integrity can recover following prolonged disuse. This study characterized the loss and recovery dynamics of bone following a period of rodent hindlimb unloading in three anatomic sites. We hypothesized that the rat femoral neck would exhibit a discordant recovery dynamic most similar to that observed in astronauts' proximal femur; that is, bone mineral content (absolute mass) at this site would recover faster and more completely than would bone density and cortical area, and they will all recover before bone strength does. We characterized loss and long-term recovery of densitometric properties at the femoral neck, proximal tibia metaphysis, and tibia diaphysis, and also mechanical properties at the femoral neck and tibia diaphysis for which mechanical testing is amenable. We assessed the relationship between calculated strength indices and measured mechanical properties. Adult male Sprague-Dawley rats (6 months) were assigned to baseline, age-matched control (AC), and hindlimb unloaded (HU) groups. The HU group was unloaded for 28 days and then returned to normal cage activity for 84 days of weight bearing recovery (3 times the duration of HU). Fifteen animals were euthanized from each of the HU and AC groups on days 28, 56, 84, and 112 of the study. At baseline and then every 28 days in vivo longitudinal pQCT scans were taken at proximal tibia metaphysis (PTM) and tibia diaphysis (TD); ex vivo pQCT scans were taken later at the femoral neck (FN). TD and FN were tested to failure to measure mechanical properties. The hypothesis that the femoral neck in rats will exhibit a discordant recovery dynamic most similar to that observed in astronauts' proximal femurs was not supported by our data. At the femoral neck, densitometric and geometric variables (total BMC, total vBMD, cancellous vBMD, and cortical area) recovered to age-matched control levels after a recovery period twice the duration of unloading. Contrary to our hypothesis, changes in densitometric variables at the PTM provided a better model for changes in the human femoral neck with prolonged weightlessness. Following 28 days of HU, PTM total BMC recovered to age-matched control levels after roughly two times the duration of unloading; however, total vBMD did not recover even after three recovery periods. Cortical thinning occurred at the PTM following HU likely due to inhibition of periosteal growth; cortical shell thickness did not recover even after three recovery periods. Calculated strength indices suggested a loss in strength at the tibial diaphysis, which was not confirmed with direct testing of mechanical properties. HU had no effect on maximum fracture force at mid-tibia diaphysis; however, femoral neck experienced a significant loss of maximum force due to unloading that fully recovered after 28 days. Estimated strength indices for the femoral neck suggested a recovery period of 56 days in contrast to the 28-day recovery that was observed with mechanical testing. However, the inaccuracy of strength indices vs. directly measured mechanical properties highlights the continued importance of ground based animal models and mechanical testing. Our results demonstrate that the PTM in the rat better matches loss and recovery dynamics observed in astronauts' proximal femur than does the rat FN, at least in terms of densitometric variables. More complete utility of the rat PTM as a model in this case, however, depends upon meaningful characterization of changes in mechanical properties as well.