Site-specific response of bone to exercise in premenopausal women

Abstract

We studied the response of bone at specific skeletal sites to either lower body exercise alone or complemented with upper body exercise in premenopausal women. Thirty-five exercisers and 24 age-matched controls completed the 12-month study. Exercising women (N = 35) were randomly assigned to either lower body resistance plus jump exercise (LOWER) (N = 19) or to lower and upper body resistance plus jump exercise (UPPER + LOWER) (N = 16). Exercisers trained three times per week completing 100 jumps and 100 repetitions of lower body resistance with or without 100 repetitions of upper body resistance exercise at each session. Intensity for lower body exercise was increased using weighted vests for jump and resistance exercises, respectively. Intensity for upper body exercise was increased using greater levels of tautness in elastic bands. Bone mineral density (BMD) at the total hip, greater trochanter, femoral neck, lumbar spine and whole body were measured by dual energy X-ray absorptiometry (Hologic QDR-1000/W) at baseline, 6 and 12 months. Data were analyzed first including all enrolled participants who completed follow-up testing and secondly including only those women whose average attendance was ≥ 60% of prescribed sessions. Group differences in 12-month %change scores for BMD variables were analyzed by univariate ANCOVA adjusted for baseline differences in age. Post hoc tests were performed to determine which groups differed from one another. Initial analysis showed significant differences in greater trochanter BMD between each exercise group and controls, but not between exercise groups (2.7% ± 2.5% and 2.2% ± 2.8% vs. 0.7% ± 1.7%, for LOWER and UPPER + LOWER vs. controls, respectively; p < 0.02) and near significant group differences at the spine (p = 0.06). Excluding exercisers with low compliance, group differences at the greater trochanter remained, while spine BMD in UPPER + LOWER was significantly different from LOWER and controls, who were not significantly different from one another (1.4% ± 3.9% vs. − 0.9% ± 1.7% and − 0.6% ± 1.8%, for UPPER + LOWER vs. LOWER and controls, respectively; p < 0.05). No significant differences among groups were found for femoral neck, total hip or whole body BMD. Our data support the site-specific response of spine and hip bone density to upper and lower body exercise training, respectively. These data could contribute to a site-specific exercise prescription for bone health.

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].