Effect of deconditioning on cortical and cancellous bone growth in the exercise trained young rats

Rearing in female-only groups and dietary mineral supplementation improves sow welfare in the early parities and lifetime performance

The lifetime performance of commercial sows relies on longevity, which is dependent on good health and reproductive performance. However, there is a high rate of wastage of sows in the early parities, which is influenced by the way they are managed and housed during rearing. This study investigated the carry-over effect of gilt rearing strategy on the measures of welfare and performance. Eighty sows were reared using a two by two factorial design: rearing group composition [GC; female-only (FEM) or mixed-sex (MIX) from weaning] with or without supplementary minerals (CON = control diet; SUPP = control + Cu, Zn, and Mn) from 5 wk into the finisher stage. Once served, gilts were managed in a dynamic group gestation pen and fed a standard gestating sow diet. Locomotory ability was scored (0 to 5) and salivary cortisol measured five times during the first gestation, and human approach tests were carried out on day 108. Hooves were scored for injuries and legs for bursas at day 70 of the first gestation, at first weaning, and at the second farrowing. Sow behavior in the hoof scoring crate (movement, vocalization, and handling ease) was also recorded. The number of piglets born alive and dead during the first five parities was recorded as was the performance of the first litter to finish. Data were analyzed using general or generalized linear mixed models, as appropriate, using SAS (v 9.4). There was no effect (P > 0.05) of rearing treatment on locomotory ability, bursa score, the total number of piglets born, or on offspring growth. However, there was an interaction between GC and supplementary minerals (P < 0.05) on salivary cortisol levels with MIX × SUPP sows having the highest levels. Total hoof scores and heel erosion scores were higher in sows reared in MIX groups (P < 0.01), and CON sows tended to have higher horizontal crack scores (P = 0.06). Sows from MIX kicked more at weaning than FEM (P < 0.05) and tended to be more fearful in the forced human approach test (P = 0.1) where they are scored on their reaction to being approached. They also had more stillborn piglets across all five parities than FEM (P < 0.05). Overall, rearing replacement sows in FEM groups and dietary mineral supplementation had minimal but beneficial effects on their subsequent welfare and performance.

Isolated Cyclic Loading During Adolescence Improves Tibial Bone Microstructure and Strength at Adulthood

Bone is a unique living tissue, which responds to the mechanical stimuli regularly imposed on it. Adolescence facilitates a favorable condition for the skeleton that enables the exercise to positively influence bone architecture and overall strength. However, it is still dubious for how long the skeletal benefits gained in adolescence is preserved at adulthood. The current study aims to use a rat model to investigate the effects of in vivo low- (LI), medium- (MI), and high- (HI) intensity cyclic loadings applied during puberty on longitudinal bone development, morphometry, and biomechanics during adolescence as well as at adulthood. Forty-two young (4-week-old) male rats were randomized into control, sham, LI, MI, and HI groups. After a 5 day/week for 8 weeks cyclic loading regime applied on the right tibia, loaded rats underwent a subsequent 41-week, normal cage activity period. Right tibias were removed at 52 weeks of age, and a comprehensive assessment was performed using μCT, mechanical testing, and finite element analysis. HI and MI groups exhibited reduced body weight and food intake at the end of the loading period compared with shams, but these effects disappeared afterward. HI cyclic loading increased BMD, bone volume fraction, trabecular thickness, trabecular number, and decreased trabecular spacing after loading. All loading-induced benefits, except BMD, persisted until the end of the normal cage activity period. Moreover, HI loading induced enhanced bone area, periosteal perimeter, and moment of inertia, which remained up to the 52nd week. After the normal cage activity at adulthood, the HI group showed increased ultimate force and stress, stiffness, postyield displacement and energy, and toughness compared with the sham group. Overall, our findings suggest that even though both trabecular and cortical bone drifted through age-related changes during aging, HI cyclic loading performed during adolescence can render lifelong benefits in bone microstructure and biomechanics. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

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.

Lasting organ-level bone mechanoadaptation is unrelated to local strain

Bones adapt to mechanical forces according to strict principles predicting straight shape. Most bones are, however, paradoxically curved. To solve this paradox, we used computed tomography-based, four-dimensional imaging methods and computational analysis to monitor acute and chronic whole-bone shape adaptation and remodeling in vivo. We first confirmed that some acute load-induced structural changes are reversible, adhere to the linear strain magnitude regulation of remodeling activities, and are restricted to bone regions in which marked antiresorptive actions are evident. We make the novel observation that loading exerts significant lasting modifications in tibial shape and mass across extensive bone regions, underpinned by (re)modeling independent of local strain magnitude, occurring at sites where the initial response to load is principally osteogenic. This is the first report to demonstrate that bone loading stimulates nonlinear remodeling responses to strain that culminate in greater curvature adjusted for load predictability without sacrificing strength.

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.

Young and healthy C57BL/6 J mice performing sprint interval training reveal gender- and site-specific changes to the cortical bone

Physical exercise is considered to impede the bone loss associated with physiological ageing however, a training program that efficiently leads to bone accrual in the healthy does not yet exist. We turned to the C57BL/6 J mouse and designed a sprint interval training for treadmill that was tailored to the individual performance limits. It consisted of four weeks with five training sessions each, followed by another four weeks with three. After completion of the training period, mice were sacrificed and the hind legs were analyzed via µCT and MRI for changes in bone parameters and muscle volume, respectively. Increased performance limits in both sexes confirmed an effect of the treadmill training. However, while male tibiae after eight weeks revealed a significant reduction of cortical bone mass at the distal metaphysis, the cross sectional analysis of female tibiae showed a transient decrease of cortical bone mass after four weeks that was reversed into a significant accrual after eight weeks of training and occurred over the entire length of the tibia. The observed net reduction of female bone mass after four weeks of training is suggestive of a remodelling process which may be delayed in the males.

Morphological and Microstructural Alterations of the Articular Cartilage and Bones during Treadmill Exercises with Different Additional Weight-Bearing Levels

The aim of this study was to investigate the morphological and microstructural alterations of the articular cartilage and bones during treadmill exercises with different exercise intensities. Sixty 5-week-old female rats were randomly divided into 10 groups: five additional weight-bearing groups (WBx) and five additional weight-bearing with treadmill exercise groups (EBx), which were subjected to additional weight bearing of x% (x = 0, 5, 12, 19, and 26) of the corresponding body weight of each rat for 15 min/day. After 8 weeks of experiment, the rats were humanely sacrificed and their bilateral intact knee joints were harvested. Morphological analysis of the cartilages and microcomputed tomography evaluation of bones were subsequently performed. Results showed that increased additional weight bearing may lead to cartilage damage. No significant difference was observed among the subchondral cortical thicknesses of the groups. The microstructure of subchondral trabecular bone of 12% and 19% additional weight-bearing groups was significantly improved; however, the WB26 and EB26 groups showed low bone mineral density and bone volume fraction as well as high structure model index. In conclusion, effects of treadmill exercise on joints may be associated with different additional weight-bearing levels, and exercise intensities during joint growth and maturation should be selected reasonably.

Skeletal site-specific effects of endurance running on structure and strength of tibia, lumbar vertebrae, and mandible in male Sprague–Dawley rats

Bone microarchitecture, bone mineral density (BMD), and bone strength are affected positively by impact activities such as running; however, there are discrepancies in the magnitude of these effects. These inconsistencies are mainly a result of varying training protocols, analysis techniques, and whether or not the skeletal sites measured are weight bearing. This study’s purpose was to determine the effects of endurance running on sites that experience different weight bearing and load. Eight-week-old male Sprague–Dawley rats (n = 20) were randomly assigned to either a group with a progressive treadmill running protocol (25 m/min for 1 h, incline of 10%) or a nontrained control group for 8 weeks. The trabecular structure of the tibia, lumbar vertebra (L3), and mandible and the cortical structure at the tibia midpoint were measured using microcomputed tomography to quantify bone volume fraction (i.e., bone volume divided by total volume (BV/TV)), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), and cortical thickness. BMD at the proximal tibia, lumbar vertebrae (L1–L3), and mandible was measured using dual energy X-ray absorptiometry. The tibia midpoint strength was measured by 3-point bending using a materials testing system. Endurance running resulted in superior bone structure at the proximal tibia (12% greater BV/TV (p = 0.03), 14% greater Tb.N (p = 0.01), and 19% lower Tb.Sp (p = 0.05)) but not at other sites. Contrary to our hypothesis, mandible bone structure was altered after endurance training (8% lower BV/TV (p < 0.01) and 15% lower Tb.Th (p < 0.01)), which may be explained by a lower food intake, resulting in less mechanical loading from chewing. These results highlight the site-specific effects of loading on the skeleton.

The myokine irisin increases cortical bone mass

It is unclear how physical activity stimulates new bone synthesis. We explored whether irisin, a newly discovered myokine released upon physical activity, displays anabolic actions on the skeleton. Young male mice were injected with vehicle or recombinant irisin (r-irisin) at a low cumulative weekly dose of 100 µg kg(-1). We observed significant increases in cortical bone mass and strength, notably in cortical tissue mineral density, periosteal circumference, polar moment of inertia, and bending strength. This anabolic action was mediated primarily through the stimulation of bone formation, but with parallel notable reductions in osteoclast numbers. The trabecular compartment of the same bones was spared, as were vertebrae from the same mice. Higher irisin doses (3,500 µg kg(-1) per week) cause browning of adipose tissue; this was not seen with low-dose r-irisin. Expectedly, low-dose r-irisin modulated the skeletal genes, Opn and Sost, but not Ucp1 or Pparγ expression in white adipose tissue. In bone marrow stromal cell cultures, r-irisin rapidly phosphorylated Erk, and up-regulated Atf4, Runx2, Osx, Lrp5, β-catenin, Alp, and Col1a1; this is consistent with a direct receptor-mediated action to stimulate osteogenesis. We also noted that, although the irisin precursor Fndc5 was expressed abundantly in skeletal muscle, other sites, such as bone and brain, also expressed Fndc5, albeit at low levels. Furthermore, muscle fibers from r-irisin-injected mice displayed enhanced Fndc5 positivity, and irisin induced Fdnc5 mRNA expression in cultured myoblasts. Our data therefore highlight a previously unknown action of the myokine irisin, which may be the molecular entity responsible for muscle-bone connectivity.

Bone Mechanical Properties and Mineral Density in Response to Cessation of Jumping Exercise and Honey Supplementation in Young Female Rats

This study investigated effects of cessation of exercise and honey supplementation on bone properties in young female rats. Eighty-four 12-week-old Sprague-Dawley female rats were divided into 7 groups: 16S, 16J, 16H, 16JH, 8J8S, 8H8S, and 8JH8S (8 = 8 weeks, 16 = 16 weeks, S = sedentary without honey supplementation, H = honey supplementation, and J = jumping exercise). Jumping exercise consisted of 40 jumps/day for 5 days/week. Honey was given to the rats at a dosage of 1 g/kg body weight/rat/day via force feeding for 7 days/week. Jumping exercise and honey supplementation were terminated for 8 weeks in 8J8S, 8H8S, and 8JH8S groups. After 8 weeks of cessation of exercise and honey supplementation, tibial energy, proximal total bone density, midshaft cortical moment of inertia, and cortical area were significantly higher in 8JH8S as compared to 16S. Continuous sixteen weeks of combined jumping and honey resulted in significant greater tibial maximum force, energy, proximal total bone density, proximal trabecular bone density, midshaft cortical bone density, cortical area, and midshaft cortical moment of inertia in 16JH as compared to 16S. These findings showed that the beneficial effects of 8 weeks of combined exercise and honey supplementation still can be observed after 8 weeks of the cessation and exercise and supplementation.

Synchronization of calcium sulphate cement degradation and new bone formation is improved by external mechanical regulation

A major challenge faced in the bone materials of weight-bearing without internal fixture support is the mismatch of material degradation and new bone formation, leading to weakening or even failure of the overall bony structure. This study demonstrated in the rat femur model that calcium sulphate cement degradation and new bone formation could be better synchronized by external mechanical force. An ascending force in line with calcium sulphate cement degradation could achieve bone healing in 37 days with ultimate load to failure of 87.00 ± 7.30 N, similar to that of intact femur (80.46 ± 2.79 N, p = 0.369). In contrast, the healing process under either a constant force or no force illustrated significant residual defect volumes of 1.47 ± 0.44 and 4.08 ± 0.89 mm(3) (p < 0.001), and weaker ultimate loads to failure of 69.56 ± 4.74 and 59.17 ± 7.48 N, respectively (p < 0.001). Our results suggest that the mechanical regulation approach deserves further investigation and may potentially offer a clinical strategy to improve synchronization.

Different short-term mild exercise modalities lead to differential effects on body composition in healthy prepubertal male rats

Physical activity has a vital role in regulating and improving bone strength. Responsiveness of bone mass to exercise is age dependent with the prepubertal period suggested to be the most effective stage for interventions. There is a paucity of data on the effects of exercise on bone architecture and body composition when studied within the prepubertal period. We examined the effect of two forms of low-impact exercise on prepubertal changes in body composition and bone architecture. Weanling male rats were assigned to control (CON), bipedal stance (BPS), or wheel exercise (WEX) groups for 15 days until the onset of puberty. Distance travelled via WEX was recorded, food intake measured, and body composition quantified. Trabecular and cortical microarchitecture of the femur were determined by microcomputed tomography. WEX led to a higher lean mass and reduced fat mass compared to CON. WEX animals had greater femoral cortical cross-sectional thickness and closed porosity compared to CON. The different exercise modalities had no effect on body weight or food intake, but WEX significantly altered body composition and femoral microarchitecture. These data suggest that short-term mild voluntary exercise in normal prepubertal rats can alter body composition dependent upon the exercise modality.

Cortical and trabecular bone benefits of mechanical loading are maintained long term in mice independent of ovariectomy

Skeletal loading enhances cortical and trabecular bone properties. How long these benefits last after loading cessation remains an unresolved, clinically relevant question. This study investigated long-term maintenance of loading-induced cortical and trabecular bone benefits in female C57BL/6 mice and the influence of a surgically induced menopause on the maintenance. Sixteen-week-old animals had their right tibia extrinsically loaded 3 days/week for 4 weeks using the mouse tibial axial compression loading model. Left tibias were not loaded and served as internal controls. Animals were subsequently detrained (restricted to cage activities) for 0, 4, 8, 26, or 52 weeks, with ovariectomy (OVX) or sham-OVX surgery being performed at 0 weeks detraining. Loading increased midshaft tibia cortical bone mass, size, and strength, and proximal tibia bone volume fraction. The cortical bone mass, area, and thickness benefits of loading were lost by 26 weeks of detraining because of heightened medullary expansion. However, loading-induced benefits on bone total area and strength were maintained at each detraining time point. Similarly, the benefits of loading on bone volume fraction persisted at all detraining time points. The long-term benefits of loading on both cortical and trabecular bone were not influenced by a surgically induced menopause because there were no interactions between loading and surgery. However, OVX had independent effects on cortical bone properties at early (4 and 8 weeks) detraining time points and trabecular bone properties at all detraining time points. These cumulative data indicate loading has long-term benefits on cortical bone size and strength (but not mass) and trabecular bone morphology, which are not influenced by a surgically induced menopause. This suggests skeletal loading associated with physical activity may provide long-term benefits by preparing the skeleton to offset both the cortical and trabecular bone changes associated with aging and menopause.

Elevated mechanical loading when young provides lifelong benefits to cortical bone properties in female rats independent of a surgically induced menopause

Exercise that mechanically loads the skeleton is advocated when young to enhance lifelong bone health. Whether the skeletal benefits of elevated loading when young persist into adulthood and after menopause are important questions. This study investigated the influence of a surgically induced menopause in female Sprague-Dawley rats on the lifelong maintenance of the cortical bone benefits of skeletal loading when young. Animals had their right forearm extrinsically loaded 3 d/wk between 4 and 10 weeks of age using the forearm axial compression loading model. Left forearms were internal controls and not loaded. Animals were subsequently detrained (restricted to cage activities) for 94 weeks (until age 2 years), with ovariectomy (OVX) or sham-OVX surgery being performed at 24 weeks of age. Loading enhanced midshaft ulna cortical bone mass, structure, and estimated strength. These benefits persisted lifelong and contributed to loaded ulnas having greater strength after detraining. Loading also had effects on cortical bone quality. The benefits of loading when young were not influenced by a surgically induced menopause because there were no interactions between loading and surgery. However, OVX had independent effects on cortical bone mass, structure, and estimated strength at early postsurgery time points (up to age 58 weeks) and bone quality measures. These data indicate skeletal loading when young had lifelong benefits on cortical bone properties that persisted independent of a surgically induced menopause. This suggests that skeletal loading associated with exercise when young may provide lifelong antifracture benefits by priming the skeleton to offset the cortical bone changes associated with aging and menopause.

Restriction of dietary energy intake has a greater impact on bone integrity than does restriction of calcium in exercising female rats

We sought to elucidate the effects of restricting calcium, energy, or food on the skeletal integrity of exercising female rats. Female Sprague-Dawley rats (4 mo old) were randomly assigned to 5 groups (n = 10/group): ad libitum intake of an AIN-93M diet (Research Diets D10012M, Research Diets, Inc.) with no exercise (AL-S) or with exercise (AL-EX) or to 1 of 3 exercising restriction groups [40% restriction of calcium only (CAR-EX), energy only (ER-EX), or food (FR-EX)]. All EX rats were treadmill trained 3 d/wk, 45 min/d for 12 wk at ~60% maximal oxygen consumption. After 12 wk, total body bone mineral content (by DXA) and body mass, but not lean mass, were lower in ER-EX (-17%) and FR-EX rats (-13%) compared with the AL-EX group. CAR-EX had few negative effects on bone geometry (by peripheral quantitative computed tomography) or histomorphometry. However, declines in total volumetric bone mineral density at the proximal tibia metaphysic (PTM) were observed in ER-EX (-6%) and FR-EX (-8%) groups; only FR-EX rats exhibited increased osteoclast surface and decreased mineral apposition rate in PTM cancellous bone. Decrements in serum estradiol, uterine weights, or both in these 2 groups implicate altered estrogen status as contributory. Urine pH declined significantly by 12 wk in all restricted groups, but net acid excretion increased only in CAR-EX rats. These findings, when compared with published data on sedentary rats, suggest that treadmill running exercise may mitigate some, but not all, deleterious effects on bone after chronic energy or food restriction but is more protective during calcium restriction.

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

High-impact exercise is considered to be very beneficial for bones. We investigated the ability of jump exercise to restore bone mass and structure after the deterioration induced by tail suspension in growing rats and made comparisons with treadmill running exercise. Five-week-old male Wistar rats (n = 28) were randomly assigned to four body weight-matched groups: a spontaneous recovery group after tail suspension (n = 7), a jump exercise group after tail suspension (n = 7), a treadmill running group after tail suspension (n = 7), and age-matched controls without tail suspension or exercise (n = 7). Treadmill running was performed at 25 m/min, 1 h/day, 5 days/wk. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, with a jump height of 40 cm. Bone mineral density (BMD) of the total right femur was measured by dual-energy X-ray absorptiometry. Three-dimensional trabecular bone architecture at the distal femoral metaphysis was evaluated using microcomputed tomography. After 5 wk of free remobilization, right femoral BMD, right hindlimb muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. Although both jump and running exercises during the remobilization period increased trabecular bone mass, jump exercise increased trabecular thickness, whereas running exercise increased trabecular number. These results indicate that restoration of trabecular bone architecture induced by jump exercise during remobilization is predominantly attributable to increased trabecular thickness, whereas running adds trabecular bone mass through increasing trabecular number, and suggest that jumping and running exercises have different mechanisms of action on structural characteristics of trabecular bone.

Non-uniform decay in jumping exercise-induced bone gains following 12 and 24 weeks of cessation of exercise in rats

The effects of deconditioning on exercise-induced bone gains in rats were investigated in 12-week-old female WKY rats performing a standard jumping exercise regimen for either 8, 12 or 24 weeks, followed by sedentary periods of either 24, 12 or 0 weeks, respectively. Age-matched controls received no exercise over the same period. At the end of the training/sedentary period, the tibiae were harvested for analyses of bone parameters. Gains in tibial fat-free dry weight decayed within 12 weeks of deconditioning, but gains in tibial ultimate bending force (strength), maximum diameter and cortical area were still present at 12 weeks of deconditioning. With the exception of cortical area, all other exercise-induced bone gains decayed by the 24th week of deconditioning. It appears that the decay in exercise-induced bone gains in strength, physical and morphological properties is not uniform, and that gains in fat-free dry weight seem to decay earlier.

Loading and skeletal development and maintenance

Mechanical loading is a major regulator of bone mass and geometry. The osteocytes network is considered the main sensor of loads, through the shear stress generated by strain induced fluid flow in the lacuno-canalicular system. Intracellular transduction implies several kinases and phosphorylation of the estrogen receptor. Several extra-cellular mediators, among which NO and prostaglandins are transducing the signal to the effector cells. Disuse results in osteocytes apoptosis and rapid imbalanced bone resorption, leading to severe osteoporosis. Exercising during growth increases peak bone mass, and could be beneficial with regards to osteoporosis later in life, but the gain could be lost if training is abandoned. Exercise programs in adults and seniors have barely significant effects on bone mass and geometry at least at short term. There are few data on a possible additive effect of exercise and drugs in osteoporosis treatment, but disuse could decrease drugs action. Exercise programs proposed for bone health are tedious and compliance is usually low. The most practical advice for patients is to walk a minimum of 30 to 60 minutes per day. Other exercises like swimming or cycling have less effect on bone, but could reduce fracture risk indirectly by maintaining muscle mass and force.

Minimum level of jumping exercise required to maintain exercise-induced bone gains in female rats

SUMMARY

This study determines the minimum level of exercise required to maintain 8 weeks of jumping exercise-induced bone gains in rats. It was found that the minimum level of exercise required for maintaining the different exercise-induced bone gains varied between 11% and 18% of the initial exercise intensity.

INTRODUCTION

This study ascertains the minimum level of follow-up exercise required to maintain bone gains induced by an 8-week jumping exercise in rats.

METHODS

Twelve groups of 12-week old rats (n = 10 rats per group) were given either no exercise for 8 (8S) or 32 weeks (32S), or received 8 weeks of standard training program (8STP) that consisted of 200 jumps per week, given at 40 jumps per day for 5 days per week, followed by 24 weeks of exercise at loads of either 40 or 20 or 10 jumps per day, for either 5, or 3, or 1 day/week. Bone mass, strength, and morphometric properties were measured in the right tibia. Data were analyzed using one-way analyses of variance.

RESULTS

Bone mass, strength, mid-shaft periosteal perimeter and cortical area were significantly (p < 0.05) higher in the rats given 8STP than that in the 8S group. The minimal level of exercise required to maintain the bone gains was 31, 36, 25, and 21 jumps per week for mass, strength, periosteal perimeter and cortical area, respectively.

CONCLUSIONS

Eight weeks of jumping exercise-induced bone gains could be maintained for a period of 24 weeks with follow-up exercise consisting of 11% to 18% of the initial exercise load.

The bone diagnostic instrument III: testing mouse femora

Here we describe modifications that allow the bone diagnostic instrument (BDI) [P. Hansma et al., Rev. Sci. Instrum. 79, 064303 (2008); Rev. Sci. Instrum. 77, 075105 (2006)], developed to test human bone, to test the femora of mice. These modifications include reducing the effective weight of the instrument on the bone, designing and fabricating new probe assemblies to minimize damage to the small bone, developing new testing protocols that involve smaller testing forces, and fabricating a jig for securing the smaller bones for testing. With these modifications, the BDI was used to test the hypothesis that short-term running has greater benefit on the mechanical properties of the femur for young growing mice compared to older, skeletally mature mice. We measured elastic modulus, hardness, and indentation distance increase (IDI), which had previously been shown to be the best discriminators in model systems known to exhibit differences in mechanical properties at the whole bone level. In the young exercised murine femora, the IDI was significantly lower than in young control femora. Since IDI has a relation to postyield properties, these results suggest that exercise during bone development increases post yield mechanical competence. We were also able to measure effects of aging on bone properties with the BDI. There was a significant increase in the IDI, and a significant decrease in the elastic modulus and hardness between the young and old groups. Thus, with the modifications described here, the BDI can take measurements on mouse bones and obtain statistically significant results.

Short-term exercise in mice increases tibial post-yield mechanical properties while two weeks of latency following exercise increases tissue-level strength

We have previously shown that exercise during growth increases post-yield deformation in C57BL6/129 (B6;129) male tibiae at the expense of reduced pre-yield deformation and structural and tissue strength. Other research in the literature indicates that increased mineral content, cross-sectional geometry and structural strength due to exercise can be maintained or increased after exercise ends for as long as 14 weeks. It was therefore hypothesized that after our exercise protocol ended, effects of exercise on mechanical properties would persist, resulting in increased post-yield behavior and rescued strength versus age-matched control mice. Beginning at 8 weeks of age, exercise consisted of running on a treadmill (30 min/day, 12 m/min, 5 degrees incline) for 21 consecutive days. At the end of running and 2 weeks later, in the cortical bone of the tibial mid-diaphyses of B6;129 male mice, changes due to exercise and latency following exercise were assayed by mechanical tests and analyses of cross-sectional geometry. Exercise increased structural post-yield deformation compared with weight-matched control mice, without changes in bone size or shape, suggesting that exercised-induced changes in pre-existing bone quality were responsible. Over the 2-week latency period, no growth-related changes were noted in control mice, but exercise-induced changes resulted in increased tissue stiffness and strength versus mice sacrificed immediately after exercise ended. Our data indicate that periods of exercise followed by latency can alter strength, stiffness, and ductility of bone independent of changes in size or shape, suggesting that exercise may be a practical way to increase the quality of the bone extracellular matrix.

Sustainability of exercise-induced increases in bone density and skeletal structure

BACKGROUND

The prevalence of osteoporosis with related fragility fractures has increased during the last decades. As physical activity influences the skeleton in a beneficial way, exercise may hypothetically be used as a prophylactic tool against osteoporosis.

OBJECTIVE

This review evaluates if exercise-induced skeletal benefits achieved during growth remain in a long-term perspective.

DESIGN

PUBLICATIONS WITHIN THE FIELD WERE SEARCHED THROUGH MEDLINE (PUBMED) USING THE SEARCH WORDS: exercise, physical activity, bone mass, bone mineral content (BMC), bone mineral density (BMD) and skeletal structure. We based our inferences on publications with the highest level of evidence, particularly randomised controlled trials (RCT).

RESULTS

Benefits in BMD achieved by exercise during growth seem to be eroded at retirement, but benefits in skeletal structure may possibly be retained in a longer perspective. Recreational exercise seems to at least partially maintain exercise-induced skeletal benefits achieved during growth.

CONCLUSIONS

Exercise during growth may be followed by long-term beneficial skeletal effects, which could possibly reduce the incidence of fractures. Exercise during adulthood seems to partly preserve these benefits and reduce the age-related bone loss.

Bones benefits gained by jump training are preserved after detraining in young and adult rats

We investigated the osteogenic responses to jump training and subsequent detraining in young and adult male rats to test the following hypotheses: 1) jump training has skeletal benefits; 2) these skeletal benefits are preserved with subsequent detraining throughout bone morphometric changes; and 3) there are no differences between young and adult rats during detraining in terms of the maintenance of exercise-induced changes. Twelve-week-old (young) and 44-wk-old (adult) rats were divided into the following four groups: young-sedentary, young-exercised, adult-sedentary, and adult-exercised. The exercised groups performed jump training (height = 40 cm, 10 jumps/day, 5 days/wk) for 8 wk followed by 24 wk of being sedentary. Tibial bone mineral content and bone mineral density in vivo significantly increased with jump training, and the effects were maintained after detraining in both the young and adult exercised groups, although the benefits of training became somewhat diminished. After 24 wk of detraining, the beneficial effects of training on bone mass and strength were preserved and associated with morphometric changes, such as periosteal perimeter, cortical area, and moment of inertia. There were no significant age-exercise interactions in such parameters, except for the periosteal perimeter. These results suggest that there are few differences in bone accommodation and maintenance by training and detraining between young and adult rats.

Exercising stall-housed gestating gilts: effects on lameness, the musculo-skeletal system, production, and behavior

Lameness in breeding-age gilts and sows is a major cause of culling, resulting in increased economic losses and welfare concerns. This study determined if exercise during gestation would affect the musculo-skeletal system, production variables, and behavior. Gilts were blocked by BW and assigned to 1 of 3 treatment groups: control (n = 10; no exercise), low exercise (n = 14; 122 m/d for 5 d/wk), and high exercise (n = 14; 122 m/d for 2 d/wk and 427 m/d for 3 d/wk). All gilts were stall-housed during gestation, and gilts were exercised between d 35 and 110 of gestation. Lameness score, BCS, BW, and blood were taken at multiple points before gestation, and during gestation and lactation. Blood serum was analyzed for carboxy-terminal telopeptide of type I collagen. Sow lying behavior was recorded for 3 d after farrowing. Farrowing data included litter weight and size at birth and weaning, and preweaning mortality. After weaning, 38 sows were slaughtered and muscles and the bones of the left fore- and hind-limbs were harvested. Bone density and quality were determined by computed tomography (CT) scans, dual energy x-ray scans, and bone-breaking force tests. The control group took longer to lie down than both exercise groups, and the low exercise group took longer to lie down than the high exercise group (P < 0.05). The number of pigs weaned was greater in the high exercise group than the control group (P < 0.05). Piglet preweaning mortality was greatest in the control group compared with both exercise groups (P < 0.05). The low exercise treatments exhibited a greater bone density (CT) in the humerus, radius, and tibia compared with that of the control group (P < 0.05). The bone density (CT) of the humerus in the low exercise group was greater than that of the high exercise group (P = 0.03). Breaking force in the humerus and femur was greater (P < 0.05) in the low exercise group than the control group. Breaking force in the tibia of the high exercise group was greater than the control group (P = 0.01). The tibia of both the low and high exercise groups had a greater breaking force (P < 0.05) than the control group. Although there was no benefit of exercise on lameness, differences in bone density and quality, lying behavior, and piglet survivability may provide useful insight into alternative housing for sows.

Effects of jump training on bone are preserved after detraining, regardless of estrogen secretion state in rats

We investigated whether the effects of jump training on bone are preserved after a detraining period in female normal and estrogen-deficient rats. Forty-four 11-wk-old Wistar rats were divided into the following four groups: sham sedentary (n = 12), sham exercised (n = 11), ovariectomized sedentary (n = 10), and ovariectomized exercised (n = 11). An 8-wk exercise period was introduced in which the rats in the exercised groups were jumped 10 times/day, 5 days/wk. This was followed by 24 wk of detraining. At the end of the exercise period, the jump training significantly increased the bone mineral content of the tibia (P < 0.001), measured by dual-energy X-ray absorptiometry. After the detraining period, the bone mineral content (P < 0.01), strength (P < 0.001), and cross-sectional widths (P < 0.001) of the tibia in the exercised groups were still greater than in the sedentary groups, without significant surgery-exercise interactions, although bone stiffness in the fracture test (P < 0.05) and bone area in the center-proximal region, as measured by dual-energy X-ray absorptiometry (P < 0.05), showed significant surgery-exercise interactions. These findings suggest that the exercise effect on bone strength is preserved, accompanied by cross-sectional morphological changes, even under estrogen deficiency.

Effects of isometric strength training followed by no exercise and Humulus lupulus L-enriched diet on bone metabolism in old female rats

We investigated in female rats the effects on bone metabolism of a prolonged no-training period, subsequent to an isometric exercise program, performed during young adulthood and those of a long-term consumption of Humulus lupulus L-enriched diet (genistein 1.92 and daidzein 1.24 mg/kg diet) combined or not with isometric training. Forty-eight rats (4 weeks old) were randomly divided into 4 groups: trained (C-Tr) or nontrained rats (C-NTr) fed with control diet and trained (H-Tr) or nontrained rats (H-NTr) fed with Humulus lupulus L-enriched diet. The diets lasted 100 weeks. Training was followed over a 25-week period. Bone parameters were measured at week 100. Our results showed that no significant difference was observed among the 4 groups in uterine relative weight, calcium (Ca) intake, fecal Ca, urinary Ca excretion, net Ca absorption, plasma Ca, and bone Ca content. Calcium balance was significantly enhanced in H-NTr rats in comparison with C-NTr and C-Tr rats. Isometric strength training led to a significant increase in total bone mineral density (BMD), diaphyseal BMD, and osteocalcin-deoxypyridinoline ratio in C-Tr rats compared with the other groups. The main findings of the present study indicate that in female rats, a 25-week isometric strength training performed during young adulthood followed by a prolonged no-training period increases BMD values and osteocalcin-deoxypyridinoline ratio, whereas long-term consumption of Humulus lupulus L-enriched diet does not improve bone parameters. It suggests that bone gains induced by exercise do not decrease immediately after cessation of training and also confirms the importance of the practice of physical activity during puberty and young adulthood to maximize the achieved peak bone density.

Exercise-induced changes in the cortical bone of growing mice are bone- and gender-specific

Fracture risk and mechanical competence of bone are functions of bone mass and tissue quality, which in turn are dependent on the bone's mechanical environment. Male mice have a greater response to non-weight-bearing exercise than females, resulting in larger, stronger bones compared with control animals. The aim of this study was to test the hypothesis that short-term weight-bearing running during growth (21 days starting at 8 weeks of age; 30 min/day; 12 m/min; 5 degrees incline; 7 days/week) would similarly have a greater impact on cross-sectional geometry and mechanical competence in the femora and tibiae of male mice versus females. Based on the orientation of the legs during running and the proximity of the tibia to the point of impact, this response was hypothesized to be greatest in the tibia. Exercise-related changes relative to controls were assayed by four-point bending tests, while volumetric bone mineral density and cross-sectional geometry were also assessed. The response to running was bone- and gender-specific, with male tibiae demonstrating the greatest effects. In male tibiae, periosteal perimeter, endocortical perimeter, cortical area, medial-lateral width and bending moment of inertia increased versus control mice suggesting that while growth is occurring in these mice between 8 and 11 weeks of age, exercise accelerated this growth resulting in a greater increase in bone tissue over the 3 weeks of the study. Exercise increased tissue-level strain-to-failure and structural post-yield deformation in the male tibiae, but these post-yield benefits came at the expense of decreased yield deformation, structural and tissue-level yield strength and tissue-level ultimate strength. These results suggest that exercise superimposed upon growth accelerated growth-related increases in tibial cross-sectional dimensions. Exercise also influenced the quality of this forming bone, significantly impacting structural and tissue-level mechanical properties.

Adjuvant arthritis-induced changes on ampicillin binding in serum and tissues under the influence of non-steroidal anti-inflammatory drugs in rats

Adjuvant arthritis, as a model for investigating rheumatoid arthritis (RA), is characterized by reduced plasma albumin levels and interferes with drug binding in the plasma and tissues (liver and bone). Ampicillin interacts with non-steroidal anti-inflammatory drugs (NSAIDs) due to the acidic pk(a). The aim of this study was to investigate in vitro the concentrations of ampicillin in the serum, femur, mandible and liver proteins following the co-administration of ketoprofen, flurbiprofen, ibuprofen, oxyphenbutazone and ASA in adjuvant arthritis versus healthy control rats. Ampicillin binding was found to be reduced in the serum of arthritic rats, and ampicillin binding to serum proteins was also reduced under the influence of NSAIDs in the control animals. Differences in ampicillin binding were observed in the various tissues due to the effect of adjuvant arthritis as well as that due to the co-administration of NSAIDs. In conclusion, this in vitro study may provide a plausible explanation for the ampicillin-NSAIDs interaction and such a finding may be of therapeutic significance in the treatment of painful arthritic disease such as RA.

Bone gained from physical activity and lost through detraining: a longitudinal study in young males

The aim of this study was to investigate the effect of training and detraining on bone mineral density of both weight-bearing and non-weight-bearing bone in a cohort of young males who participated in ice hockey training. Forty-three healthy adolescent ice hockey players (16.7+/-0.6 years) training for a mean of 9.7+/-2.4 h/week and 25 control subjects (16.8+/-0.3 years) training for 2.1+/-2.7 h/week, were included in this longitudinal study. Bone mineral density (BMD, g/cm2) of the arms, the dominant and non-dominant humerus, dominant and non-dominant femur, and the right femoral neck, total hip, and bone area of the femur, humerus and hip were measured at baseline and again after 30 and 70 months using dual-energy X-ray absorptiometry. From baseline to the first follow-up, athletes gained significantly more BMD in the femoral neck (0.07 versus 0.03 g/cm2) and arms (0.09 versus 0.06 g/cm2) compared with the controls (P = 0.04 for both). Between the first and the second follow-up, 21 ice hockey players stopped their active sports career. These men lost significantly more BMD at the femoral neck (-0.02 versus -0.10 g/cm2, P < 0.001), total hip (-0.05 versus -0.09, P = 0.04), dominant (0.02 versus -0.03 g/cm2, P = 0.009) and non-dominant humerus (0.03 versus -0.01 g/cm2, P = 0.03) than the still active ice hockey players (n = 22). At the second follow-up examination, at 22 years of age, the former ice hockey players still had significantly higher BMD at the non-dominant humerus than the controls (P < 0.01). During the total study period, the still active athletes (n = 22) gained significantly more BMD compared with the controls at the femoral neck (0.09 g/cm2; P = 0.008), total hip (0.05 g/cm2, P = 0.04) and arms (0.07 g/cm2; P = 0.01). No differences were seen in bone areas when comparing the different groups. In conclusion, training associated with ice hockey is related to continuous accumulation of BMD after puberty in males. Reduced activity is followed by BMD loss within 3 years of cessation of sports career at predominantly weight-bearing sites. The effects are confined to bone density and not bone size.

Reduced training is associated with increased loss of BMD

This 8-year controlled, follow-up study in 66 Swedish soccer women evaluated the effect of training and reduced training on BMD. The players who retired during the follow-up lost BMD in the femoral neck, whereas the controls did not.

INTRODUCTION

Physical activity during adolescence increases BMD, but whether the benefits are retained with reduced activity is controversial.

MATERIALS AND METHODS

At baseline, DXA evaluated BMD in 48 active female soccer players with a mean age of 18.2 +/- 4.4 (SD) years, in 18 former female soccer players with a mean age of 43.2 +/- 6.2 years and retired for a mean of 9.4 +/- 5.3 years, and in 64 age- and sex-matched controls. The soccer women were remeasured after a mean of 8.0 +/- 0.3 years, when 35 of the players active at baseline had been retired for a mean of 5.3 +/- 1.6 years.

RESULTS AND CONCLUSIONS

The players still active at follow-up had a higher BMD at baseline than the matched controls in the femoral neck (FN; 1.13 +/- 0.19 versus 1.00 +/- 0.13 g/cm2; p = 0.02). The yearly gain in BMD during follow-up was higher in the active players than in the controls in the leg (0.015 +/- 0.006 versus 0.007 +/- 0.012 g/cm2, p = 0.04). The soccer players who retired during follow-up had a higher BMD at baseline than the matched controls in the FN (1.13 +/- 0.13 versus 1.04 +/- 0.13 g/cm2; p = 0.005). The players that retired during follow-up lost BMD, whereas the controls gained BMD during the study period in the FN (-0.007 +/- 0.01 versus 0.003 +/- 0.02 g/cm2 yearly; p = 0.01). The soccer players already retired at baseline had higher BMD at study start than the matched controls in the leg (1.26 +/- 0.09 versus 1.18 +/- 0.10 g/cm2; p = 0.01). The former players who were retired at study start lost BMD, whereas the controls gained BMD during the study period in the trochanter (-0.006 +/- 0.01 versus 0.004 +/- 0.014 g/cm2 yearly; p = 0.01). This study shows that, in girls, intense exercise after puberty is associated with higher accrual of BMD, and decreased physical activity in both the short-term and long-term perspective is associated with higher BMD loss than in controls.

Effect of treadmill exercise on bone mass in female rats

Increasing peak bone mass at skeletal maturity, minimizing bone loss during middle age and after menopause, and increasing bone mass and preventing falls in advanced age are important measures for preventing osteoporotic fractures in women. Exercise has generally been considered to have a positive influence on bone health. This paper reviews the effects of treadmill exercise on bone in young, adult, ovariectomized, and osteopenic female rats. Treadmill exercise increases cortical and cancellous bone mass of the tibia as a result of increased bone formation and decreased bone resorption in young and adult rats. The increase in lumbar bone mass seems to be more significant when long-term exercise is applied. Treadmill exercise prevents cancellous bone loss at the tibia as a result of suppressed bone resorption in ovariectomized rats, and increases bone mass of the tibia and mechanical strength of the femur, as a result of suppressed bone resorption and increased bone formation in osteopenic rats after ovariectomy. Treadmill exercise transiently decreases the serum calcium level as a result of accumulation of calcium in bone, resulting in an increase in serum 1,25-dihydroxyvitamin D(3) level and a decrease in serum parathyroid hormone level. We conclude that treadmill exercise may be useful to increase bone mass in young and adult rats, prevent bone loss in ovariectomized rats, and increase bone mass and bone strength in osteopenic rats, especially in the long bones at weight-bearing sites. Treadmill exercise may have a positive effect on the skeleton in young, and adult, ovariectomized, and osteopenic female rats.

Interventions to prevent bone loss in astronauts during space flight

This paper reviews the interventions to stabilize calcium balance and bone metabolism and prevent bone loss in astronauts during space flight. Weightlessness during space flight results in calcium, vitamin D, and vitamin K deficiency, increases urinary calcium excretion, decreases intestinal calcium absorption, and increases serum calcium level, with decreased levels of serum parathyroid hormone and calcitriol. Bone resorption is increased, whereas bone formation is decreased. The loss of bone mineral density (BMD) in the spine, femoral neck and trochanter, and pelvis is 1.0-1.6% per month. High calcium intake and vitamin D supplementation during space flight does not affect bone metabolism, but prevents an elevation of serum calcium level through increased calcitriol level, while vitamin K counteracts the reduction in bone formation. However, there are no data to show the efficacy of pharmaceutical agents for prevention of development of osteoporosis in astronauts during flight, although the preventative effect of bisphosphonates, testosterone, and vitamin K2 on cancellous bone loss in the tibia or BMD loss in the hindlimb was reported in tail-suspended mature rats. It still remains uncertain whether these agents can prevent cortical bone loss caused by weightlessness in tail-suspended rats. Therefore, in addition to calcium, vitamin D, and vitamin K supplementation, agents that have both potent anti-resorptive and anabolic effects on cancellous and cortical bone may be needed to stabilize calcium balance and bone metabolism and prevent bone loss in astronauts during space flight.

Three-dimensional changes in the condyle during development of an asymmetrical mandible in a rat: A microcomputed tomography study

A rapidly growing postnatal animal model was used to study changes in the calcified tissue of the mandibular condyle during altered muscle function. A maxillary occlusal splint was designed to shift the mandible laterally (left) during closure. Groups of 5 Wistar rats were killed at 5, 9, 15, 21, 30, and 40 weeks (n = 30), with an equal number of controls. The experimental animals developed shorter, asymmetrical mandibles compared with the control animals. The left condyle became larger and thicker than the right condyle. Microcomputed tomography assessment of the left and right condylar trabecular bone indicated that both had less bone volume than the control condyle. The right masseter muscle significantly lost fiber size and type IIA oxidative fibers, suggesting that the right masseter muscle was used with less tension development. In contrast, the left masseter maintained its fiber size and was similar to the control masseter fiber diameters. Comparison in the sequence of changes indicated that the morphologic changes occurred first in the ramus (age, 5 weeks), before the corpus (age, 15 weeks), and before changes in masseter fiber size and composition (age, 9 weeks). This study showed that both the mandible and the condyle modified their shape and size, as well as the trabecular bone of the condyle, during shifting of the mandible to one side as it closed.

Short-duration exercise and confinement alters bone mineral content and shape in weanling horses

The hypothesis that short-duration exercise may ameliorate the decrease in bone mass observed with confinement was investigated with 18 quarter horses (nine colts and nine fillies) weaned at 4 mo of age and placed into box stalls. After a 5-wk adjustment period, individuals were grouped by age and weight, and then divided randomly into three treatment groups: 1) group housed; 2) confined with no exercise; and 3) confined with exercise. The confined and exercised groups were housed in 3.7 m x 3.7 m box stalls for the 56-d duration of the trial. The exercised group was sprinted 82 m/d, 5 d/wk, in a fenced grass alleyway. The weanlings were led down an alleyway, turned loose in a small pen, and then released and allowed to run back down the alley. The group horses were housed together in a 992-m2 drylot with free access to exercise. On d 0, 28, and 56, dorsopalmar and lateromedial radiographs of the left third metacarpal bone were taken to estimate changes in bone mineral content and cortical widths. Mean values of medial, lateral, and total radiographic bone aluminum equivalence increased over time (P < 0.05), whereas dorsal and palmar radiographic bone aluminum equivalence did not change significantly. Dorsal, medial, and total radiographic bone aluminum equivalence tended (P = 0.09) to differ by a treatment x day interaction, with values increasing over time only in the exercised group. Normalized medial and total radiographic bone aluminum equivalence tended (P < 0.1) to differ (P < 0.01) with treatment, with exercised horses having greater bone aluminum equivalence than confined horses. Dorsopalmar cortical width in exercised horses was greater than on d 56 (treatment x day; P = 0.07). The dorsopalmar medullary cavity decreased in exercised vs. group-housed horses (P = 0.027), whereas dorsal and medial cortical width tended to increase only in the exercised horses (treatment x day; P < 0.01). This study indicated that a short-duration exercise protocol might be effective in improving bone mass and therefore skeletal strength in horses.

Combined intervention of soy isoflavone and moderate exercise prevents body fat elevation and bone loss in ovariectomized mice

Body fat accumulation and bone loss are both often associated with estrogen deficiency following menopause. In this study, we examined whether soy isoflavone, one of the phytoestrogens, and moderate exercise interventions exhibit cooperative effects on body composition and bone mass in ovariectomized (OVX) mice. Eight-week-old female mice were assigned to 6 groups: (1) sham-operated (sham); (2) OVX; (3) OVX with received a soy isoflavone diet (OVX+ISO); (4) OVX with exercised on a treadmill (OVX+EX); (5) OVX with given both isoflavone and exercise (OVX+ISO&EX ); and (6) OVX with treated with 17 beta-estradiol subcutaneously (OVX+E2). Body composition and bone mineral density (BMD) were estimated by dual-energy x-ray absorptiometry (DXA). After the 6-week intervention, whole body fat (%) in the OVX group showed significantly higher than that in the sham group. Intervention of exercise and isoflavone alone partially inhibited OVX-induced body fat gain, and the combined intervention as well as E2 treatment completely restored fat mass to the sham level. Lean body mass in the whole body was not different in OVX group compared with that in OVX+ISO, OVX+EX, and OVX+E2 groups, but it was significantly higher in OVX+ISO&EX than in other groups. BMD of the whole body, lumbar spine, or femur showed significantly reduced by OVX, and the bone loss was partially inhibited by intervention of exercise or isoflavone alone. However, the combined intervention completely restored the bone mass to the level of sham, as did E2. Serum total cholesterol was significantly increased by OVX, which was normalized by the combined intervention or E2 treatment. These results demonstrate that combined intervention of soybean isoflavone and exercise prevented body fat accumulation in the whole body with an increase in lean body mass and restoration of bone mass, and reduced high serum cholesterol in OVX mice.

High bone mass gained by exercise in growing male mice is increased by subsequent reduced exercise

Exercise-induced bone gains are lost if exercise ceases. Therefore, continued exercise at a reduced frequency or intensity may be required to maintain these benefits. In this study, we evaluated whether 4 wk of reduced exercise after 4 wk of running exercise in growing male mice results in the maintenance of high bone mass. Five-week-old mice were divided into the following groups: 1) baseline control; 2) 4-wk control; 3) 4-wk exercise; 4) 8-wk control; 5) 4-wk exercise followed by 4-wk cessation of training; and 6) 4-wk exercise followed by reduced exercise at half the frequency. The regimen consisted of exercise 6 days/wk, and the reduced exercise regimen consisted of running 3 days/wk on a treadmill for 30 min/day, at 12 m/min on a 10 degrees uphill slope. Running exercise significantly increased bone mineral density of the femur, periosteal mineral apposition rate, bone formation rate, percent labeled perimeter at the midfemur, and osteogenic activity of bone marrow cells. However, these parameters declined to the age-matched sedentary control after cessation of training. In contrast, the reduced exercise group had significantly higher mineral apposition rate compared with those of the sedentary control and cessation of training groups. Furthermore, bone mineral density for the reduced exercise group was significantly higher than those for the other groups. These results suggest that the high bone formation gained through exercise can be maintained, and bone mass was further increased by subsequent exercise even if the exercise frequency is reduced.

Rapid loss of bone mineral density of the femoral neck after cessation of ice hockey training: a 6-year longitudinal study in males

The aim of this study was to evaluate the effect of training and reduced training on BMD in young ice hockey players during 6 years of follow-up. We found BMD gains in the femoral neck in the ice hockey group compared with controls. However, these gains were lost with reduced activity after cessation of career.

INTRODUCTION

It has been suggested that increasing bone mass by intense physical activity during childhood and adolescence may decrease the risk of osteoporosis later in life.

MATERIALS AND METHODS

In this longitudinal study, 43 ice hockey players (16.7 +/- 0.6 years) and 25 control subjects (16.8 +/- 0.3 years) were studied at baseline and after a mean period of 30 and 70 months. The groups did not differ in weight or height. Bone mineral density (BMD; g/cm2) was measured for total body, femoral neck, and spine using DXA. Volumetric BMD (vBMD; mg/cm3) of the femoral neck was estimated.

RESULTS

The ice hockey players were found to have gained significantly more femoral neck BMD than controls (0.07 versus 0.03 g/cm2, p = 0.04) and to have gained femoral neck vBMD, whereas the controls did not (16 versus 0 mg/cm3, p = 0.049) between baseline and the first follow-up. At the first follow-up, the ice hockey players were found to have significantly higher BMD at the femoral neck and total body versus controls (p < 0.05). Between the first and second follow-ups, 21 ice hockey players stopped their active sports career. During this time period, these subjects lost significantly more femoral neck BMD (0.10 versus 0.02 g/cm2, p < 0.001) and femoral neck vBMD (38 versus 4 mg/cm3, p < 0.001) compared with the 22 ice hockey players who continued training. The former ice hockey players also lost significantly more neck vBMD (38 versus 14 mg/cm3, p = 0.009) compared with the controls during the same period. At the second follow-up, only the 22 ice hockey players who had continued their training were found to have significantly higher BMD at the femoral neck (p = 0.01), total body (p = 0.04), and spine (p = 0.02) compared with the controls. The former athletes were found to have intermediate BMD at all sites.

CONCLUSION

In summary, we have demonstrated fast BMD loss at the femoral neck after decreased physical activity in young men. We conclude that ice hockey training during childhood and adolescence may not prevent the development of osteoporosis of the femoral neck later in life if the activity is not maintained.2003;18:1964-1969

The bone gain induced by exercise in puberty is not preserved through a virtually life-long deconditioning: a randomized controlled experimental study in male rats

To investigate the controversial issue whether exercise-induced positive effects on bone can be maintained after cessation of exercise, 100 5-week-old male Sprague-Dawley rats were used to assess the effects of long-term exercise (EX, treadmill running) and subsequent deconditioning (DC, free cage activity) on the femoral neck and femoral midshaft. At entry, the rats were randomly assigned into eight groups: four control groups (C14, C28, C42, and C56), and four exercise groups (EX, EX + DC14, EX + DC28, and EX + DC42). Rats in the exercise groups were first subjected to a 14-week period of progressively intensifying running, after which the rats of group EX were killed and the remaining exercise groups (EX + DC14, EX + DC28, and EX + DC42) were allowed to move freely in their cages for a subsequent deconditioning period of 14, 28, or 42 weeks, whereas control rats were kept free in their cages for the entire study period (0-56 weeks) and killed with their respective exercise group. At each time point, a comprehensive analysis of the femoral neck and midshaft characteristics (peripheral quantitative computed tomography analysis and fracture load [Fmax]) was performed. In comparison with their age-matched controls, 14 weeks of treadmill training resulted in significant (p < 0.05) increases in all measured femoral neck parameters of the growing male rats (i.e., +25% in total cross-sectional area [tCSA], +28% in total bone mineral content [tBMC], +11% in total bone mineral density [tBMD], and +30% in Fmax). On the contrary, no exercise-induced positive effects were seen in femoral midshaft. The exercise-induced benefits in the femoral neck were partially maintained during the deconditioning period of 14 weeks, the tCSA being + 17%, tBMC + 18% (both p < 0.05), and the Fmax + 11% (p = 0.066) higher in the exercised group than control group. However, after 42 weeks of deconditioning, these benefits were eventually lost. In conclusion, exercise through the period of the fastest skeletal growth results in significant improvements in size, mineral mass, and strength of the femoral neck of male rats. However, these exercise-induced bone benefits are eventually lost if exercise is completely ceased, and thus, continued training is probably needed to maintain the positive effects of youth exercise into adulthood. Further studies should focus on assessing the minimal level of activity needed to maintain the exercise-induced bone gains.

Does exercise during growth have a long-term effect on bone health?

There is increasing evidence that growth is a critical time for altering body tissue composition and fostering either the development or the prevention of disease. The focus of this review is to examine the effect of regular exercise during growth on long-term bone health.