Introduction: Mechanical stresses on the bone are an important aspect of physical activity that promotes bone preservation and increases in bone mass. Exercise intensities leading to bone preservation and accrual have not been adequately defined for humans in general, and postmenopausal women in particular.
Materials and methods: To quantify parameters of effective walking intensity for preservation and accrual of bone mineral, healthy postmenopausal women engaged in 30 weeks of supervised walking, 4.8 km per day, 4 days a week at intensities of 102% or 123% of the ventilatory threshold (VT) equivalent to 67% and 86% of maximal effort (VO2 max). Subjects were matched by age, body mass, hormone replacement status (HRT) and VT. Areal bone mineral density (aBMD) determined by DXA (n=25) and bone formation markers osteocalcin (OC), and bone-specific alkaline phosphatase (bALP) (n=43), were measured at the outset and at 15-week intervals. Peak vertical forces at corresponding intensities were measured (n=9) on a force plate.
Results: aBMD of legs and whole body, but not of other sites, and lean mass of legs, but not of arms, increased after 15 weeks of high intensity, compared to moderate losses for low intensity training. Leg and total body aBMD was preserved and slightly increased with loads greater than 872.3 newtons (N) with a walking intensity above 115% of VT or 74% of VO2 max, speeds above 6.14 km/h, and heart rates above 82.3% of age-specific maximum. OC and bALP did not correlate with training-induced changes in aBMD.
Conclusions: At exercise intensities above 115% of VT or 74% of VO2 max, and walking speeds above 6.14 km/h, mechanical loading of 872.3 N or 1.22 times body weight is sufficient for increases in leg muscle mass and preservation of BMD in postmenopausal women.