Effects of Exercise and Physical Interventions on Bone: Clinical Studies - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 242

[ 1 , 17 ]. For example, among 1,171 men aged 65+ years in the Osteoporotic
Fractures in Men (MrOS) study, the pQCT-derived bone strength index of the tibia
was 7% higher in the most physically active quartile than in the least active quartile,
and 5% higher in the quartile with the highest leg muscle power than in the lowest
quartile [ 17 ]. Based on this evidence, the magnitude of the benefit of physical
activity on bone strength in humans is likely considerably less than suggested by
the preclinical studies of mechanical loading [ 81 ]. The effects of physical activity
on fracture risk should reflect the net effects on bone strength and fall risk and can
be best evaluated through a randomized controlled intervention approach.
3 Key Determinants of the Adaptive Response
of Bone to Loading
3.1 Conventional Loading Factors
Preclinical studies of laboratory animals in the 1980s demonstrated that the
adaptive response of bone to mechanical loading is favorably influenced when
strains are dynamic rather than static, are of high peak magnitude and rate, and
present a unique strain distribution [ 91 ]. Further, when these conditions are met,
few loading repetitions are necessary to optimize the adaptive response. In general,
these principles also have been found to be of clinical relevance. Intervention trials
of exercise training indicate that activities that generate relatively high-intensity
bone-loading forces are more likely to result in an increase in BMD than those that
generate low-intensity forces [ 29 , 48 ]. Similarly, cross-sectional comparisons of
adults who participate in different types of sports indicate that activities that
generate high-impact (e.g., volleyball) or odd-impact loading forces (e.g., soccer)
are associated with high BMD whereas activities that involve non-impact loading
(e.g., swimming) are not [ 29 , 48 , 73 ].
Mechanical stresses are introduced to the skeleton through a combination of
external forces (e.g., gravity, ground-reaction, and other external contacts) and
internal forces (e.g., muscle, joint-reaction) [ 6 , 36 , 46 , 78 ]. Any physical activity
will result in some level of muscle force to achieve dynamic equilibrium (i.e.,
produce acceleration and/or provide joint stability). The magnitude of the muscle
forces can range from moderate, such as in endurance weight-bearing activities
(e.g., running), to high, such as in explosive jumping activities (e.g., volleyball)
or activities that involve quick accelerations or directional changes (e.g., soccer).
It has been suggested that muscle forces account for the majority of the adaptive
response of the skeleton to exercise [ 78 ]. Indeed, among 685 women and men aged
20-91 years, total body bone mineral content (BMC) was more strongly associated
with total body fat-free mass (i.e., FFM, a surrogate of muscle mass; r = 0.68 in
women and r = 0.63 in men) than with fat mass (women, r = 0.22; men, r = 0.14)
or total body mass (r = 0.48; men, r = 0.45) [ 46 ]. The strong association of BMC