Deconditioning increases bone resorption and decreases bone formation in the rat

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.

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.

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.

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.

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.

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.

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.

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.

Effect of decreased physical activity on bone mass in exercise-trained young rats

The aim of this study was to determine whether decreased physical activity in exercise-trained young rats would result in a lower rate of bone gain or a reversal of the benefits of exercise. Thirty-five female Wistar rats, 6 weeks of age, were randomized into seven groups: 7 weeks of exercise (7EX), 7 weeks of sedentary control (7CN), 11 weeks of exercise (11EX), 7 weeks of exercise followed by 4 weeks of exercise cessation (7EX4C), 7 weeks of exercise followed by 4 weeks of decreased exercise frequency (7EX4F), 7 weeks of exercise followed by 4 weeks of decreased exercise intensity (7EX4I), and 11 weeks of sedentary control (11CN). The running intensity (speed) and duration were 25 m/min for 60 min/day at a frequency of 5 days/week. During the last 4 weeks, exercise frequency was reduced to 1 day/week in the 11EX4F group, and exercise intensity (speed) was reduced to 12 m/min in the 7EX4I group. After each period of exercise, the bone mineral content (BMC) of the proximal, middle, and distal tibiae, determined by dual-energy X-ray absorptiometry (DXA), was significantly greater in the 7EX and 11EX groups than in the 7CN and 11CN groups, respectively, but it was significantly lower in the 7EX4C group than in the 11EX group and did not differ significantly from the values of the 11CN group. Although the BMC of the proximal and middle tibiae did not differ significantly among the 7EX4F, 7EX4I, 7EX4C, and 11CN groups, the BMC of the distal tibia was significantly greater in the 7EX4F and 7EX4I groups than in the 11CN group and tended to be greater than in the 7EX4C group. The results of this study suggest that the effect of decreased exercise intensity and frequency on bone mass appears to be site specific in the tibia of the exercise-trained young rats. This study shows that exercise-trained young rats lose the benefits gained from exercise when exercise is completely ceased, resulting in the reduction of bone mass to levels that do not differ significantly from those of sedentary controls. At least, continuous exercise appears to be necessary for the maintenance of high bone mass.

Effect of exercise training and detraining on bone mineral density in postmenopausal women with osteoporosis

We examined the effect of exercise training and detraining on bone mineral density (BMD) in postmenopausal women with osteoporosis. Thirty-five postmenopausal women with osteoporosis, aged 53-77 years, were randomly assigned to three groups: a control group (n = 20), a 2-year exercise training group (n = 8), and an 1-year exercise training plus 1-year detraining group (n = 7). Exercise training consisted of daily brisk walking and gymnastic training. Calcium lactate, 2.0 g, and 1alpha-hydroxyvitamin D3, 1 microg were supplied daily to all subjects. No significant differences in initial lumbar BMD, measured by dual-energy X-ray absorptiometry (DXA) were found among the three groups. The mean percent change in BMD compared with the baseline was significantly higher at 1 and 2 years in the exercise training group and at 1 year in the detraining group than in the control group, and did not differ significantly at 2 years between the detraining and control groups. These findings indicate that our exercise training program led to a significant increase in lumbar BMD in postmenopausal women with osteoporosis compared with the control, but that the BMD reverted toward a level that was not significantly different from the control with detraining. Continued exercise training is needed to maintain the bone mass gained through exercise training.

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

Exercise enhances bone growth and increases peak bone mass. The aim of this study was to determine whether or not 4 weeks of deconditioning after 8 weeks of exercise in growing rats would result in a decrease in bone gain or reverse the benefits of exercise. Fifty 4-week-old female Sprague-Dawley rats were randomized by a stratified weight method into 5 groups with 10 rats in each group: 8 weeks exercise (8EX), 8 weeks sedentary control (8S), 12 weeks exercise (12EX), 8 weeks exercise followed by 4 weeks sedentary (8EX4S), and 12 weeks sedentary control (12S). The exercise consisted of running on a treadmill with a 5 degrees slope at 24 m/minute for 1 h/day and 5 days/week. After each period of exercise, cancellous and cortical bone histomorphometry were performed on double fluorescent labeled 5-microm-thick sections of the proximal tibia and 40-microm-thick sections of the tibial shaft, respectively. Eight and 12 weeks of exercise resulted in a significant increase in the body weight and gastrocnemius muscle weight by two-way analysis of variance (ANOVA). The femoral wet weight (mg; mean +/- SD; 8EX, 781 +/- 45.1 vs. 8S, 713 +/- 40.5; p < 0.05; 12EX, 892 +/- 41.6 vs. 12S, 807 +/- 19.8; p < 0.05) was significantly higher in the exercise group than that in the respective control groups. The femoral wet weight and bone volume (BV) of the 8EX4S group (818 +/- 46.2 mg and 531 +/- 31.2 microl, respectively) were significantly lower than those of the 12EX group (p < 0.05) and did not differ significantly from those of the 12S groups. The cancellous BV was significantly higher in the 8EX and 12EX groups than that in the respective sedentary groups (p < 0.05). The cortical bone area of the tibial shaft was also significantly higher in the 12EX than that in the 12S group (p < 0.05). The increase in the cancellous BV or cortical bone area was caused by an increase in the mineral apposition rate (MAR), without a significant effect in the labeled perimeter. The bone formation rate (BFR; microm3/microm2 per day) in the cancellous bone (12EX, 27.9 +/- 7.74 vs. 12S, 15.4 +/- 4.56; p < 0.05) or periosteal surface (12EX, 127.6 +/- 27.7 vs. 12S, 79.5 +/- 18.6; p < 0.05) was significantly higher in the exercised groups than that in the respective control group (p < 0.05). Again, deconditioning resulted in a decrease in the cancellous BFR, BV, periosteal BFR, and cortical bone area to levels not significantly different from the 12S group. In conclusion, our findings showed that exercised growing rats, when deconditioned, lost the benefits gained through exercise and their bone parameters were reduced to levels not different from the sedentary control. Thus, continued exercise is required to maintain high bone mass.

Effects of immobilization, three forms of remobilization, and subsequent deconditioning on bone mineral content and density in rat femora

Disuse is associated with bone loss, which may not be recoverable. It is not known whether intensified remobilization is beneficial in restoring disuse-related bone loss nor if any such benefit would depend upon continuing mobilization for its maintenance. After an immobilization period of 3 weeks, the effects of free remobilization (11 weeks), and low-and high-intensity treadmill running (11 weeks) with and without subsequent deconditioning (18 weeks) on the bone mineral content (BMC) and density (BMD) of the hindlimb femora of Sprague-Dawley rats (n = 98) were studied using a dual-energy X-ray absorptiometric (DXA) scanner. Our hypothesis was that intensified remobilization is beneficial in restoring the BMC and BMD from disuse to normal while subsequent deconditioning is deleterious to these parameters. Immobilization for 3 weeks produced a significant BMC and BMD loss in the immobilized left femur (range -4.4 to -12.8%; p < 0.05-0.001). In the groups with free remobilization (free cage activity), the body weight-adjusted BMCs and BMDs always remained below those in the controls (range -2.3 to -12.1%; p values ranging from NS to < 0.01). Both low- and high-intensity running restored BMC and BMD in the immobilized limb, the effect being better in the latter group. In both of these groups, the values of the immobilized left limbs and those of the free right limbs exclusively exceeded the corresponding values of the age-matched control rats (left limb values 3.0-21.1% higher with p values ranging from NS to < 0.01; right limb values 7.9-21.4% higher with p < 0.05-0.01). However, after the deconditioning period of 18 weeks, the above described beneficial effects of low- and high-intensity running were lost, the left and right limb BMC and BMD values being lower than those in the age-matched controls (range -3.8 to -8.7%; p values ranging from NS to < 0.05). In conclusion, this study clearly indicates the need for greater than normal activity to restore the BMC and BMD after disuse to normal levels. However, the benefits of intensified remobilization are lost if the activity is terminated, and therefore, after immobilization and disuse, bone loading activities should be continued, perhaps indefinitely.

Physical activity and bone mass: exercises in futility?

Growing bone responds to low or moderate exercise through significant additions of new bone in both cortical and trabecular moieties and results in adaptation through periosteal expansion and endocortical contraction. Intracortical activation frequency declines in growing bone in response to exercise, reducing porosity and the remodelling space. These adaptations can be maintained into and throughout adulthood. Young bones have a greater potential for periosteal expansion than aging bone, allowing them to adapt more rapidly and efficiently to an acute need for increased strength, but a threshold level of activity exists above which some bones respond negatively by suppressing normal growth and modelling activity, reducing geometric, mechanical and material properties in cortical and trabecular bone. From cross-sectional studies, differences in bone mass between exercising and non-exercising adults are generally less than 10%, but do not account for exercise history which may be very important, and often fail to consider important confounding variables. There is sufficient longitudinal data to demonstrate that moderate to intensive training can bring about modest increases of about 1-3% in bone mineral content (BMC) of men and premenopausal women. In young adults very strenuous training may increase BMC of the tibia up to 11% and its bone density (BD) by 7%, but may represent periosteal woven bone formation in response to excessive strain. Some evidence shows that exercise can also add bone mass to the post-menopausal skeleton, although the amounts are site-specific and relatively modest. Increases as high as 5-8% can be found after 1-2 years of intensive exercise, but additions of bone to the femur and radius are generally less than 2%, well within the range of the remodelling space and measurement precision. Although increases in bone mass of the post-menopausal skeleton may be extremely modest, physical activity is important to preserve bone mass and muscle function. Detraining reduces any bone mass increase to pre-existing values so that long-term benefits are only retained with continuing exercise. Most importantly, the amount of bone gain that can be achieved appears dependent primarily on the initial bone mass suggesting that individuals with extremely low initial bone mass may have more to gain from exercise than those with moderately reduced bone mass.

The effects of exercise mode, swimming vs. running, upon bone growth in the rapidly growing female rat

The purpose of this study was to compare the effects of two programs of endurance training, of equal duration and intensity, on bone development in female rats. Thirty-eight female Wistar rats were randomly assigned to one of three groups: run-trained (RUN), swim-trained (SWIM) or control (CON). The RUN group ran at a speed of 27 m/min up an 8 degrees incline. Swim trained animals swam with 2% of body weight attached to their tails. Training sessions were 2 h/day, 5 days/week and were conducted over a 10-week period. Hindlimb and forelimb muscles were removed upon sacrifice and analyzed for citrate synthase (CS) activity, liver (LG) and muscle (MG) glycogen. The parametrial fat pads were removed, digested with collagenase, and 2-deoxy-D-[3H]glucose uptake measured in isolated cells. Bone weight, length, diameter, ponderal index and bone mineral content (BMC) were measured in the femur and humerus of each animal. The LG, MG, fat cell volume, glucose uptake of the adipocyte and adrenal weight data indicate that the training response was identical. The CS activity of the muscles indicated that mechanical and recruitment patterns of the upper and lower body differ and could be responsible for bone development patterns found in this study. Exercise had a minimal effect on bone growth in the run-trained animals but did stimulate development in the swim-trained animals. The humerus of the SWIM was significantly (P < 0.05) heavier, wider and had a greater BMC when compared with those of the RUN and CON rats. The results of this study indicate that the muscular forces applied by the swim training protocol produced greater bone adaptations than the forces applied by a running protocol of equal duration and intensity.