Volume 19 Issue 9 - September 23, 2011 PDF
Swimming training benefits bone material properties in rats
Huang, T. H.*, Hsieh, S. S., Liu, S. H., Chang, F. L., Lin, S. H., Yang, R. S.
Institute of Physical Education, Health and Leisure Studies, National Cheng-Kung University
Huang, T. H., Hsieh, S. S., Liu, S. H., Chang, F. L., Lin, S. H., Yang, R. S. (2010). Swimming Training Increases the Post-Yield Energy of Bone in Young Male Rats. Calcified Tissue International 86: 142-153.
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Generally, exercise without weight-bearing such as swimming showed no significant benefit for bone mineral density (BMD) and bone mineral content (BMC). Therefore, swimming has been frequently concluded to be profitless for bone health. However, the validity for using BMD or BMC to predict bone health remains controversial. In fact, bone fractures occur in people with normal or sub-optimal BMD, which means values of BMD and BMC cannot fully represent bone mechanical properties. Moreover, in a comprehensive viewpoint, swimming exercise has been verified regarding its contributions for health in many other fields, including cardiovascular system function, asthma, injury rehabilitation, etc. Therefore, it seems to be unfair to judge the efficiency of non-weight-bearing exercise (e.g. swimming) on bone just by BMD and BMC.

Recently, “bone quality” has been used rather than “bone density” as another term for representing bone health. Parameters including microarchitecture of trabecular bone, bone turnover, microfracture, and mineralization are important components of bone quality. Moreover, organic bone matrix (e.g. collagen fibers) have been mentioned in terms of their contributions to bone mechanical properties. Thus, it would be valuable to investigate whether swimming exercise could benefit parameters contribute to bone quality as well as tissue mechanical properties.

Unfortunately, measuring parameters of bone quality mentioned above are frequently not feasible for human studies due to the invasion and/or the concern of radiation. Thus, we investigated the effects of non-weight-bearing exercise on growing bone by using male Wistar rats as research subjects. We trained young male Wistar rats at different swimming intensities and assessed the changes using serum marker assays, and measuring tissue size, tissue mass, histomorphometry, densitometry and biomechanical properties. We hypothesized that non-weight-bearing exercise might benefit bone in some biomechanical properties.

Figure 1 Animals (Wistar rats) were subjected to swim with additional loads  attached on tails.
Young male Wistar rats (7 weeks old) were assigned into four groups: 1) the baseline control (BCON) group, sacrificed at the beginning of the exercise training program; 2) the control (CON) group, subjected to normal cage live and sacrificed at the same age of two exercise group; 3) the low-intensity (LOW) group, swimming with a weight attached to the tail equal to 2% body weight (Figure 1); 4) the high-intensity (HIGH) group, swimming with a weight attached to the tail equal to 4% body weight (Figure 1). Exercise training program was maintained for eight weeks (1 hour/day and 5 days/week). Three days after exercise training, animals of the CON, LOW and HIGH groups were sacrificed under deep anesthesia. Blood, muscle and bone samples were collected for serial measurements and analysis. Procedures for animal experimentation throughout this study followed the “Guiding Principles in the Care and Use of Animals” of the American Physiological Society.

After the eight-week training program, either two exercise groups or the CON group showed similar growth pattern and significantly higher in most parameters as compared to the BCON group. Therefore, we omitted the data of the baseline control group and focus on the comparison among the HIGH, LOW and CON groups.

Normally, people would expected that young kids should be bigger or stronger after participating a period of sports training. However, the size-related measurements in the current study seemed not favor this general idea. After an eight-week swimming training, young male rats were lower in body weight, muscle and bone mass, small in bone size, and lower in BMD and BMC values (Table 1).

Table 1. Values of body weight, muscle mass, bone size and bone density
Data were presented as mean ± SEM. BW, body weight; EDL, extensor digitorum longus; WW, bone tissue wet weight; FFDW, bone tissue fat-free dry weight; CSA, tissue cross-sectional area; CSMI, cross-sectional moment of inertia; BMD, bone mineral density; BMC, bone mineral content; *, p<0.05 vs. the CON group.

Table 2. Femoral tissue biomechanical properties estimated by three-point bending test
Data were presented as mean ± SEM. *, p<0.05 vs. the CON group.

Recently, it has been verified that those size-dependant parameters are highly correlated to BW. And, after normalized those parameters by using BW as a covariate (data were shown in original publication), the absolutely higher values in muscle mass, bone mass and bone density were disappeared; implying that exercise groups were just smaller in body size as well as bone size. Furthermore, we investigate the quality of bone by using biomaterial testing methods.

Interestingly, we found several surprising results from biomaterial property analysis. In the present study, we estimated the biomaterial properties of femora by using a three-point bending test (Figure 2A). As shown in Figure 2B, femora of exercise groups revealed a capability to absorb more energy between tissue yield point and fracture point. Up-to-date, it has not been clarified yet regarding the meaning of post-yield behaviors of biomaterial. Several previous studies have mentioned about the contributions from the non-mineralized organic bone matrix on bone toughness. Briefly, a bone tissue with better organized collagen fiber would have a higher capacity to absorb more energy before tissue failure. In clinical practice, bone tissues with higher post-yield energy might just show a smaller damage or a fissure instead of suffering a serious fracture under a sudden impact. Further investigations are needed to verify this idea.
Figure 2 Three-point bending test for femora. A. A draft typical three-point bending test; B. Load-displacement curves of swimming groups and control group.

Experimental animals were not voluntary for the current swimming training program. Mentally, they might suffer some pressure, which could negatively affect the results of the present study. For further excluding the potential adverse effects from swimming, we perform serial analyses of serum bone markers and histomorphometry (data were shown in original publication), which were methods sensitive to the bone turnover as well as status of bone health. Furthermore, bone turnover related markers are correlated to mental as well as physiological stress. No difference was shown in histomorphometry of spongy bone between swimming groups and age-matched control group (the CON group). And, serum bone marker assay even showed that the LOW group had a lower bone resorption activity, suggesting swimming exercise might prohibit osteoclastic activity and subsequently bone loss.

Per the results of our study, endurance swimming training might not be favorable if people care about gaining the absolute or size-dependent components of health. However, there is one point needs to be reminded that the age-matched control group in general animal studies are breed ad libitum, which is more likely a sedentary life style and possibly higher in risk of various chronic diseases. For a development country, there are increasing numbers of people, especially young kids, who are over high in daily caloric consumption. We should be more careful to distinguish the difference between over-weight (obese) and strong for our next generation. Moreover, in view of biological efficiency, an organism that is in optimal physiological condition will not necessarily be absolutely higher in every related parameter. For example, for a distance runner or an endurance athlete, higher in muscle mass or bone size could be more likely to be a burden than a profit.
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