|Resistance Training and Bone Mass
Heidi M. Weingart, M.A. and Len Kravitz, Ph.D.
Maddalozzo, G.F., and Snow, C.M. 2000. High intensity resistance training: Effects on bone in older men and women. Calcified Tissue International, 66, 399-404.
There is increasing emphasis on the potential benefits of adding resistance training to an exercise regimen, especially for the elderly. These benefits may include increased bone mineral density, increased strength, and an increased ability to perform activities of daily living. For the elderly, these benefits of resistance training may, more importantly, result in additional advantages such as increased independence and decreased risk of falls and injuries. Although research has demonstrated the possible benefits of resistance training, there is still much controversy over which training method is better for elderly men and women. The research study reviewed in this article attempts to answer this controversy by comparing high-intensity standing free-weights resistance training to moderate-intensity seated machine-based resistance training. The study also compared elderly men versus elderly women on the effects the two training regimens had on bone mineral density, strength, and body composition.
Morphology of Bone Mineral Density
Bone is a complex tissue that provides lifelong structural support for muscles, protection of vital organs, and stores calcium, which is essential for bone density (Neiman, 1998). All living bones are in a constant stage of breaking down and building up, referred to as remodeling. Specific chemical signals direct some bones cells, referred to as osteoclasts, to break down and remove bone (referred to as bone resorption, which means the act of absorbing). Different chemical messages tell other bone cells, know as osteoblasts, to deposit new bone. Shortly after puberty, bone mineral density reaches its peak (Robergs & Roberts, 1997). The process of remodeling is regulated by a variety of factors, including physical stress and a multiple-hormone system (Robergs & Roberts, 1997).
The multiple-hormone system is sensitive to blood calcium levels. When these levels drop, the parathyroid hormone is released, which activates the osteoclasts. And when the blood calcium levels are high, the thyroid gland releases the hormone calcitonin, which inhibits the osteoclasts and activates the osteoblasts, resulting in bone deposition (Robergs & Roberts, 1997). In females, estrogen tends to inhibit the activity of the osteoclasts and therefore helps retain bone mineral (Robergs & Roberts, 1997). The maintenance of bone mineral density levels is an important concern for postmenopausal women because they no longer have the protective effects of estrogen. Interestingly, although elderly women are more prone to bone loss, testosterone in men tends to have similar effects on bone remodeling as does estrogen (Robergs & Roberts, 1997). Therefore, it is also important for elderly men to maintain their current levels of bone mineral density as testosterone levels decline with increasing age.
One of the best methods to maintain current bone mineral density is through physical activity. Activity increases the physical stresses on bone. These stresses help activate the osteoblasts and favor bone deposition (Robergs & Roberts, 1997). Resistance training is one such physical activity that is increasingly being advocated as a means of retaining and even possibly increasing bone mineral density. However, the question remains, what type of resistance training is best? The research study by Maddalozzo and Snow (2000) examined this question in relation to elderly men and women.
Twenty-four elderly men and 18 elderly women participated in this 24-week study. The mean average age of the men was about 54 years. The women were all postmenopausal and not on estrogen therapy. Their mean age was about 52 years. None of the subjects had participated in an exercise or resistance training program for the two years preceding the study. Baseline measurements were taken for bone mass, body composition, Insulin-like Growth Factor (IGF-I) (the bone deposition favoring hormone), and one repetition maximum (1-RM). Then the subjects were randomly assigned to a high-intensity or a moderate-intensity resistance training program (Maddalozzo & Snow, 2000).
Moderate-Intensity Resistance Training
The moderate-intensity resistance training program was a seated, machine-based program. The subjects in this group trained three times per week for about 75 minutes per supervised session. This group performed 13 exercises each session on weight machines. The exercises included; leg extension, leg press, hamstring curls, arm curls, triceps press, chest press, pec deck, shoulder press, side lateral raise, lat pull-down, seated row, abdominal crunch, and calf raise. Exercise intensities ranged from 40-60% 1-RM for three sets of 10-13 repetitions.
This resistance training program resulted in a significant increase in bone mineral density of the greater trochanter for both males and females. Moderate-intensity resistance training also significantly increased lean mass (3.4% for women and 1.9% for men), peak force, and strength for the men as well as the women. However, there was no significant change in IGF-I for either group. This lack of IGF-I indicates that the increases in bone mineral density were more than likely a result of physical stresses on the bone and not hormonal effects.
High-Intensity Resistance Training
The high-intensity resistance training groups also trained three times per week for about 75 minutes per supervised session. However, these participants used free-weights in a functional standing program. This program involved 12 exercises; free weight back squat, deadlift, biceps curls, sit-ups, triceps extensions, chest press, incline chest press, shoulder press, high lat pull-down, leg curl, gripper, and calf raise. The exercise intensity was set at 70% 1-RM. Subjects performed three sets of eight repetitions. One key difference of this resistance program was that it was periodized. There were two 12-week sessions separated by one transitional (nonresistance fitness activities) week to allow recovery from the high exercise intensity and prevent injuries.
High intensity resistance exercise also resulted in a significant increase in bone mineral density of the greater trochanter for both men and women. However, only men achieved the added result of a significant increase in lumbar spine bone mineral density. High-intensity resistance training resulted in significant increases in lean mass (8.1% for women and 3.2% for men), strength, and peak force for both men and women as well. Again, there were no significant changes in IGF-I for either gender which reemphasizes the probability that physical stress was responsible for the increases in bone mass.
The results of this study demonstrate that both males and females benefit from resistance exercise three times per week of both moderate- and high-intensities. None of the participants were injured during this study indicating that intensities as high as 70% 1-RM are safe for most healthy elderly persons. However, there are always individual conditions to consider. People interested in starting a resistance-training program should consult with their physician first, especially if they are elderly, hypertensive, or have musculoskeletal injuries or diseases. It is suggested that if appropriate for the individual, high-intensity resistance training with the proper recovery periods is more beneficial. High-intensity resistance exercise favored bone formation slightly better than moderate-intensity resistance exercise, especially for elderly men. If it is not safe to have an individual elderly client perform the high-intensity exercises, then the moderate- to low-intensities may more appropriate. Even consider the possibility of combining intensities. Each resistance training program must be tailored for the specific individual and each provides important benefits. The added benefits of increased lean mass and muscle strength are especially important for elderly clients as they may help prevent fall and injuries and preserve independence.
High-intensity and moderate-intensity resistance exercises, when properly progressed and supervised, are safe and beneficial activities for many elderly clients that improve musculoskeletal health and may reduce the likelihood of falling. From the study reviewed in this article, it appears high-intensity resistance exercise yielded slightly greater increases in bone mineral density, lean mass, and muscle strength. However, progression needs to be emphasized in order to prevent overtraining and possible injuries. But most of all, personal trainers should enthusiastically encourage their elderly clients to include resistance training into their lifestyles.
D.Nieman. (1998). The Exercise-Health Connection. Human Kinetics.
R.A. Robergs, and S.O. Roberts. (1997). Exercise Physiology: Exercise, Performance and Clinical Applications. Mosby.
Table 1. Summary of Past Research on Bone Mineral Density (BMD)