Effects of Exercise and Physical Interventions on Bone: Clinical Studies - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 243

with FFM seemingly provides compelling support for the notion that muscle forces
are the primary mediators of the effects of mechanical loading on the skeleton. Yet
activities that are effective in building muscle mass are not necessarily more
beneficial than non-muscle building activities. For example, intervention trials that
compared weight-bearing endurance and resistance exercise found that both modes
of training resulted in significant and similar increase in total hip and lumbar spine
BMD, despite the fact that only resistance training generated an increase in FFM
[ 49 , 89 ]. Further, femoral neck BMD increased in response to weight-bearing
endurance exercise but not resistance exercise [ 49 ]. This suggests that resistance
training does not necessarily replicate the local skeletal stimulus produced by
weight-bearing activities. Additional studies are needed to clarify the aspects of
each training mode that are beneficial to the skeleton.
3.2 Additional Loading Factors
3.2.1 Novel Loading Factors
Preclinical research has revealed additional determinants of the skeletal adaptation
to loading that have not yet been extensively evaluated in clinical studies. One
important concept that has emerged is that bones appear to become desensitized to
mechanical stress after only a few loading cycles [ 80 ]. In rats, the mechanosensi-
tivity of bone is reduced by approximately 95% after only 20 loading cycles that
engender a high level of strain. However, interposing rest intervals of at least 4 h
between loading sessions can more than double the bone formation response, as
compared with applying the same number of loading cycles in a single session. This
suggests that multiple short bouts per day of bone-loading exercise would have more
favorable skeletal effects than a single longer bout. There is preliminary support for
this concept in humans. Jumping exercises performed twice per day over 8 weeks
resulted in a significant increase in a biomarker of bone formation whereas per-
forming the same number of jumps in a single session per day did not [ 20 ].
It has also been found in preclinical studies that the bone formation response to
loading can be augmented by interposing brief rest intervals of 10-15 s between
loading cycles [ 80 , 90 ]. The clinical relevance of this for exercise prescription is
questionable. It would be possible to use this approach in an activity such as weight
lifting, but performing repetitions at 15 s intervals may have practical limitations.
However, there may be other approaches for building rest intervals into an exercise
session. For example, if a weight lifting prescription includes two sets of 10 dif-
ferent exercises, it might be more beneficial to do one set of each exercise and then
repeat the circuit, rather than doing two sets of an exercise before moving to the
next station. Rest intervals during a chronic loading intervention may also favorably
influence skeletal adaptations [ 85 ]. In rats, axial loading for 10 weeks out of a
15 week intervention (i.e., no loading during weeks 6-10) resulted in larger
increases in bone strength than axial loading for the entire 15 weeks.