Portions of this work were presented at the September 1996 meeting of the American Society for Bone and Mineral Research, Seattle, Washington, U.S.A.
We studied the effects of high intensity resistance exercise training on bone metabolism in 17 young adult Oriental males (23–31 years) by measuring sensitive biomarkers of bone formation and resorption. The subjects were assigned to a training group and a sedentary group. The training group followed a weight training program three times per week for 4 months. In the training group, serum osteocalcin concentration and serum bone-specific alkaline phosphatase activity were significantly increased within the first month after the beginning of resistance exercise training, and the elevated levels remained throughout the training period, while there was no significant change in plasma procollagen type-I C-terminal concentration. Urinary deoxypyridinoline excretion was transiently suppressed and returned to the initial value but was never stimulated during the 4 months. These results suggest that the resistance exercise training enhanced bone formation without prior bone resorption. In the sedentary group, there was no significant difference in bone metabolic markers except plasma procollagen type-I C-terminal, which continuously decreased during the experimental period. There were no significant changes in total and regional bone mineral density in either group. In conclusion, (1) resistance exercise training increased markers of bone formation, while it transiently suppressed a marker of bone resorption, and (2) such adaptive changes of bone metabolism to resistance exercise training occurred during the early period of the training, before changes in bone density were observable through densitometry.
It has been suggested that exercises that require heavy loading with a few repetitions,(1) resulting in high strain rates(2) may provide the optimal stimuli for increase in bone mineral density (BMD). Cross-sectional and longitudinal studies have revealed in some cases that resistance exercise training increases BMD.(3-9)
The methodological developments of biomarkers of bone formation and resorption have provided a basis for explaining alterations in BMD. Osteocalcin is a specific product of the osteoblast.(10-12) It is one of the most abundant noncollagenous proteins of bone, and a small fraction of the newly synthesized protein is released into the circulation and can thus be measured by radioimmunoassay (RIA).(13-16) Other markers, such as plasma procollagen type-I C-terminal (PICP) and serum bone-specific alkaline phosphatase (B-ALP) are also considered to be the indicators of bone formation. There seems to be a divergent response among these markers of bone formation in various diseases.(17,18) The various markers may therefore reflect different stages of the osteoblastic development and function.
Most studies of exercise interventions have focused on women and older men, the population at current or future risk for osteoporotic fracture. These studies have yielded equivocal results on the effects of resistance exercise training on bone formation.(5,6,19-21) For example, the serum osteocalcin concentration increased after 12 weeks of resistance exercise training and remained significantly elevated after 16 weeks of training in older men (59 ± 2 years).(6) But a subsequent report from the same laboratory failed to observe the increase in serum osteocalcin concentration in the trained group (61 ± 1 years).(5) The serum osteocalcin concentration increased with 5 months of resistance exercise training, and the elevated level remained for 18 months of training in premenopausal women (34 ± 3 years).(21) One possible means to obtain unequivocal results on the effect of resistance exercise training on bone formation is to increase the intensity of the training by recruiting young males as experimental subjects. In addition, young subjects may be more responsive to resistance exercise training, since bones of young rats require less intense training to increase bone formation than those of aged rats.(22)
Urinary deoxypyridinoline (DPYR) is a promising marker of bone resorption(23,24) but its response to resistance exercise training has not been studied. Since bone gain or loss is related to an uncoupling between these two processes, it is advantageous to measure markers of bone formation and resorption at the same time point. Thus, the purpose of this study was to investigate longitudinally the effects of strenuous resistance exercise training on biomarkers of bone formation and resorption in young males.
MATERIALS AND METHODS
A total of 17 normal, young adult Oriental males 23–31 years of age were recruited for the study, and 15 men completed the program. None of the subjects had participated in a regular exercise program for at least 2 years. They were in good health and taking no medications that would alter calcium or bone homeostasis. To enhance compliance of training, exercise and control groups were formed according to desire rather than random assignment. The two groups contained five smokers (two in the training group and three in the sedentary group). Each subject was given an explanation of the study and signed a consent form (Table 1).
Table Table 1. Physical Characteristics of the Subjects
Each subject was given a daily supplement of 600 mg of calcium (Ozka Co., Tokyo, Japan) to assure an adequate calcium intake of more than 800 mg/day. Each subject completed dietary records for 3 days before, at the middle, and at the end of the study to ensure that dietary intake did not change throughout the study. Control diets, containing 2843 ± 22 kcal (51 ± 0.5% carbohydrate, 16 ± 0.4% protein, and 33 ± 3% fat) and 840 ± 8 mg of calcium were given for 6 days when blood and urine samples were collected (Fig. 1).
The training group followed a weight training program lasting approximately 45 minutes three times per week for 4 months. Attendance averaged more than 90%. The training program consisted of seven to eight exercises. During the first month of the training, subjects performed two sets of 10 repetitions of each exercise, (60% of one repetition maximum [1 RM] for the first set and 80% of 1 RM for the second set, respectively). From the second month of the training, subjects performed three sets of exercise to further load the skeleton (60% of 1 RM for the first set, 80% of 1 RM for the second and third set, respectively). When the subjects could not complete 10 repetitions at 80% of 1 RM, the load was gradually reduced just before muscular failure so that 10 repetitions could be completed with the heaviest weight. The resistance exercises were performed as follows. Leg extension, leg curl, bench press, sit up, back extension, wrist curl, and leg lunge were performed twice a week. Bench press, lateral pull down, sit up, back extension, arm curl, wrist curl, half squat, and back press were performed once a week. All exercises were performed using free weights except for leg extension, leg curl, bench press, and lateral pull down, which were performed on the weight machine (UESAKA, Tokyo, Japan). Weights were increased on an individual basis as strength improved. Improved strength was measured by 1 RM test and adjusted to accommodate strength gains for each individual at monthly intervals.
Strength performance was determined by 1 RM test, that is, the maximum amount of weight that can be lifted one time with proper technique through the full range of movement, for each exercise except wrist curl and leg lunge. After 5 minutes warming up of fewer than five repetitions at 40–60% of the perceived maximum, single repetitions were performed with increasing loads until the subject was unable to lift additional weight through the full range of motion with correct form. Once the apparent 1 RM was successfully completed, the subject was allowed approximately 5 minutes to recover before attempting heavier weight with 2.5–5 kg increase in mass on the bar, and a final attempt was made to ensure that the subject's maximum had indeed been reached.
In addition to the 1 RM tests, muscle strength was assessed at the knee and elbow on the subject's nondominant side using an isokinetic dynamometer (Biodex System II, Biodex, Shirley, NY, U.S.A.) before and after 4 months of the resistance exercise training. Peak torque productions of knee and elbow extensors and flexors were determined at angular velocities of 60 and 180°/s. Three maximal attempts were made at each speed, and the maximum of these values was used for the analysis.
Assay of biomarkers of bone remodeling
Biomarkers of bone metabolism were measured every month in the training group and every other month in the sedentary group. To evaluate bone metabolism in their actual state, biomarkers in the training group were assessed on both exercise and resting days (Fig. 1).
Fasting blood and 24-h urine samples were stored at −60°C and at −45°C, respectively, to be run simultaneously at the conclusion of the study to eliminate interassay variations. Serum osteocalcin was determined by specific two-site RIA for human osteocalcin (ELSA-OSTEO, CIS BioInternational, Cedex, Saclay, France). Serum B-ALP activity was measured with a colorimetric method that utilizes p-nitrophenyl phosphate as a substrate (Monotest ALPopt and Iso ALP, Boehringer Mannheim, Mannheim, Germany), and plasma PICP was determined by RIA (Procollagen PICP, Orion Diagnostica, Espoo, Finland). Urinary DPYR was measured with an enzyme immunoassay (Pyrilinks-D, Metra Biosystems Inc., Palo Alto, CA, U.S.A.). Duplicate measurements were made of all samples.
Total body, lumber spine, femoral neck, and midradius BMD (g/cm2) were measured in all subjects by dual-energy X-ray absorptiometry (DXA; DCS-3000, Aloka, Tokyo, Japan) at the beginning and at the fourth month of training. Regional scans were made on the subject's nondominant side and were analyzed by a single investigator. Standardization of the densitometer was performed by comparing aluminum phantom measured both at the beginning and at the fourth month of the training period, yielding a 0.89% variation in measurement. The precision for the BMD measurements at our laboratory, determined by double measurements in healthy individuals, was 1–2.5%.
The procedures of the SAS Institute were used for statistical analyses. Comparisons of markers of bone metabolism between time points were made using repeated-measures analysis of variance and post hoc comparisons by Dunnett's test. Comparisons of BMD before and after training in the two groups, and peak torque in the training group were tested using paired t-test. The values given in the text are means ± SEM, and the p < 0.05 level of significance was used.
Physical characteristics of the subjects are shown in Table 1. There were no significant differences between the two groups for any of the variables listed. Maximal muscular strength assessed by the 1 RM test significantly increased by 10.2–32.8% in each exercise during the 4 months of resistance exercise training. Improvement in muscle strength is also reflected by an increase in peak torque for elbow extensors (at 180°/s), elbow flexors (at 60 and 180°/s) and knee flexors (at 60°/s) (Table 2).
Table Table 2. Peak Torque Values for the Training Group
Serum osteocalcin, serum B-ALP, and plasma PICP were measured as biomarkers of bone formation. Both on exercise and resting day during the training, the serum osteocalcin concentration significantly increased. Already at the first month of the training, the difference became statistically significant and this significant increase remained throughout the experimental period (Fig. 2). Similarly, the serum B-ALP activity increased during the training, but the increase on the resting day was not statistically significant (Fig. 3). The plasma PICP concentration slightly increased at the first month of the training, but the change did not reach statistical significance. The plasma PICP concentration returned toward its baseline value thereafter. In the sedentary group, the plasma PICP concentration significantly decreased but the decrease was not observed in the training group (Fig. 4).
Urinary DPYR, an index of bone resorption, decreased during the first 3 months of the training and returned toward the baseline value at the fourth month of the training, although these changes did not reach statistical significance (Fig. 5). There were no significant changes in total body, femoral neck, lumber spine, or midradius BMD after 4 months of the resistance exercise training in either group (Table 3).
Table Table 3. BMD of Selected Regions for Sedentary and Training Groups
The present investigation is the first longitudinal study in young adult males on bone metabolism during resistance exercise training. Total body and regional BMD did not significantly change after 4 months of the training; our longitudinal design therefore enabled us to assess the onset of the changes in bone metabolism before any significant increase in BMD. Since there is a paucity of literature relating the effect of a single bout of resistance exercise on bone metabolism, biomarkers of bone remodeling in the training group were assessed on both the exercise and resting days.
Resistance exercise training increased serum levels of osteocalcin and B-ALP, concordant with previous cross-sectional studies comparing young athletes and sedentary controls.(26-28) The present study showed that the increases in serum osteocalcin concentration and plasma B-ALP activity were already detected within the first month after the beginning of the resistance exercise training. In the present longitudinal study, and a previous cross-sectional study,(29) an effect of resistance exercise training on the plasma PICP concentration was not clearly demonstrated. Furthermore, within a group of actively exercising weight lifters, training intensity positively correlated with circulating level of serum osteocalcin concentration and serum B-ALP activity but not with the plasma PICP concentration.(29) The plasma PICP concentration probably has the least discriminant power of the bone formation because there are other sites of type I collagen synthesis, such as skin, that contribute to the circulating levels of PICP. Taken together, the increases in these indices of bone formation suggest that resistance exercise training may lead to increased bone formation and it begins within the first month of the resistance exercise training.
Increased markers of bone formation are likely explained by reawakening of skeletal modeling. Previous animal studies showed that dynamic loading associated with extracellular fluid flow and the creation of streaming potentials within bone stimulated new bone formation either in trabecular bone(30,31) or cortical bone.(32-35) In fact, exercise intervention increased cross-sectional area.(34,36) Another explanation for the increase in the markers of bone formation is that the surfaces that were already forming bone before the start of the training accelerated apposition rates in response to the loading. In this regard, our data do not allow a choice between those two mechanisms.
Plasma levels of tartrate-resistant acid phosphatase and urinary hydroxyproline excretion measured 3–9 months after the beginning of resistance exercise training in middle-aged and older subjects were similar between control and trained groups,(5,6,20) although those bone resorption markers used in these studies have many short comings.(5,37-39) DPYR, a cross-link of collagen molecules, is present in bone, to a lesser extent in other connective tissues, and is released into the circulation from bone that is undergoing active resorption and is excreted in the urine. Thus it holds considerable promise as a marker of bone resorption.(40-43) In our study, urinary excretion of DPYR was transiently suppressed during the first 3 months of the training and then returned to the initial value. This may suggest that high intensity resistance exercise training reduced activation frequency of new remodeling sites as observed in animal studies(30,35) at least for the first 3 months during the training. Return of DPYR to the initial value at the fourth month may be related to the possibility that newly formed bone began remodeling. Since the level of urinary DPYR did not reach a steady state, a more prolonged experiment should be carried out to evaluate the chronic effect of resistance exercise training on bone resorption.
The increase in bone formation with a decrease in bone resorption during the first 3 months may have initiated an increase in bone mass, although this was not detected as an increase in BMD. This net increase in bone mass may continue after the fourth month, since bone formation remained elevated although bone resorption returned to the initial level. The present results suggest that bone formation in young male subjects was activated directly by stimuli of resistance exercise training independent of bone resorption. It is worth noting that bone adaptation to resistance exercise training may be mediated by somewhat different mechanisms depending upon the gender or age of the subjects, since resistance training increased BMD without a detectable increase in the serum osteocalcin level in postmenopausal women.(44)
To evaluate bone metabolism more precisely, biomarkers of bone metabolism in the training group were assessed on both the exercise and the resting days. Similar increases in markers of bone formation were observed both at 16 and 40 h after a resistance exercise session, confirming a previous report that increases in the serum osteocalcin concentration and B-ALP activity were observed 24 and 48 h after the last training session.(6) Similarly, decreases in a marker of bone resorption in 24-h urine samples were observed on both exercise and rest days in the training group. Thus, the effect of resistance training on bone metabolism was not limited to the day of the exercise, and it remains to be determined how long the altered bone metabolism lasts.
One proposed hypothesis to explain changes in bone metabolism is that the mechanical stress itself is translated into biochemical signals.(45) This theory suggests that bending or loading of a bone, which acts as pulsed electric fields that induce bone cell activity, leads to increased bone deposition at points of compressional stress.(46) Whether the resistance exercise performed in the present study stimulated this process is unknown. Alternatively, the mechanism of the resistance exercise to modify bone metabolism may involve hormonal regulation. Serum concentrations of hormones such as 1,25-dihydroxyvitamin D3 and parathyroid hormone (PTH) known to stimulate bone formation increased after resistance exercise training,(26) whereas immobilization was accompanied by suppression of circulating level of 1,25(OH)2D3 and PTH.(47) It is therefore possible that an increase in bone formation after resistance exercise training is mediated by an increase in these hormonal factors. PTH is unique among the hormonal factors, since it stimulates both bone formation and resorption.(48) But an increase in the plasma PTH concentration after resistance training(43) cannot account for the suppression of bone resorption. The mechanism by which the resistance training modulates bone metabolism remains to be clarified.
In conclusion, our study showed that (1) resistance exercise training increased markers of bone formation, while transiently suppressing a marker of bone resorption, and (2) such adaptive changes of bone metabolism to the resistance training occurred in the early period of the resistance training, even before changes in BMD were detectable with current methodology.
We thank Dr. Maria A. Fiatarone for helpful discussion and comments, Dr. Masakazu Miura for technical advice, and Mr. Takahiko Yasuda for Biodex measurement.