Site-specific response of bone to exercise in premenopausal women
Introduction
Hip and spine fractures are two of the most commonly fractured sites related to osteoporosis and bone mineral density (BMD) measured at these sites is an index of bone health and fracture risk. There are common determinants of axial bone mineral density (BMD), such as age; however, the relative influence of other factors may vary from site to site. For example, the spine is more sensitive than the hip to changes in circulating sex hormones because of the higher proportion of trabecular bone [1], [2]. Physical activity is known to influence bone as well though the relative effects of varying modes of activity may be site specific.
Wolffe's law states that bone responds to the forces placed upon it, such that BMD is greatest in regions of high force application. Perhaps the most commonly recognized illustration of Wolffe's law is the consistent difference between the playing and non-playing arm of racquet sports athletes, where the playing arm is not only denser, but structurally stronger than the non-playing arm [3], [4]. Other contralateral differences in BMD related to the asymmetric loading patterns during activity have been reported in several groups of athletes [5], [6], [7].
To date, most exercise interventions aimed at improving bone health in women have been general rather than specific to either the hip or spine. While it is useful to define a general exercise prescription for overall bone health [8], a targeted program may be more effective at improving site-specific BMD. The limited exercise programs specific to the hip or spine have shown that impact (jump) training increased hip, but not spine BMD [9], [10], [11], [12]; and that a 3-year back strength training program, did not increase spine BMD [13]. To our knowledge, no study has been designed to identify site-specific exercises for each clinical site within the same study population.
We previously report that impact plus lower body resistance training improved hip, but not spine, bone density in premenopausal women [12]. To also study the response of the spine to specific training, we added upper body exercise to the lower body program in order to provide a targeted stimulus to improve spine BMD. We hypothesized that women who performed both upper and lower body exercise would increase BMD at both the spine and the hip [14], [15], [16] whereas women who performed lower body exercise only would demonstrate an increase in BMD at the hip but not the spine [9], [10], [11].
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Participants
Women residing in Corvallis, Oregon, and surrounding areas were solicited to participate in a 12-month exercise training program. To be eligible, women had to be premenopausal at study enrollment (9–12 menstrual cycles in the previous 12 months), and were excluded for any of the following: history of chronic disease known to affect bone metabolism or exercise capacity, smoking, breast feeding, intention to become pregnant within the next year, preexisting musculoskeletal condition, or regular
Participants
Thirty-five exercisers (LOWER, n = 19; UPPER + LOWER, n = 16) and 24 controls were tested after 12 months. Seven women in the exercise group (LOWER, n = 2; UPPER + LOWER, n = 5) withdrew within the first month of the exercise program for the following reasons: increased musculoskeletal pain (n = 3), relocation (n = 1); pregnancy (n = 1); disinterest (n = 2). Data for these women were not included in statistical analyses. All controls completed both baseline and 12-month visits. Baseline data on dependent measures
Discussion
Our study demonstrated that bone responds to site-specific exercise. That is, women who added upper body resistance exercise to a routine of lower body resistance plus jump training increased BMD of both the hip and spine. In contrast, women who performed lower body training only, increased hip but not spine BMD. To our knowledge, we are the first to compare exercise that targeted the hip only to exercise that targeted both the hip and spine, sites of great clinical importance, in the same
Acknowledgments
This project was supported by funding from the National Aeronautics and Space Association Graduate Research Program and Life Fitness, Inc.
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