The Effect of Aging on Skeletal Mechanoresponsiveness: Animal Studies - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 197

These authors also looked at how aging influences the skeleton's ability to
maintain treadmill running induced gains during de-conditioning (free cage
activity only). Both the age groups lost bone after 14 weeks of de-conditioning. It
was concluded that there is a need of constant exercise for maintenance of bone
gained in response to exercise during growth and maturation.
Hoshi et al. [ 34 ] studied the effect of running exercise during different stages of
life on femoral density and mechanical properties. Ten-week old mice were
subjected to voluntary running in a revolving wheel either at different ages
(10-30, 30-50 and 50-70 weeks) or throughout the duration of the experiment
(10-70 week). It was observed that voluntary running distance decreased with age.
However, compared to sedentary control, running at all ages—with the exception
of the 50-70 week old animals—increased the cortical thickness index, breaking
force, and ultimate stress and elasticity of femurs. Bone density was higher in all
the mice that were subjected to exercise. Based on these observations it can be
concluded that exercise at any age from youth to middle age is beneficial in
arresting age-related bone loss, although starting exercise at middle age may be
less effective. The latter conclusion may be influenced by the reduction in distance
run in the older animals.
3.2 Jumping
Jump training is another physiologically relevant, intrinsic loading modality. The
method involves training the animals to jump up to or down from a platform.
Mosley et al. [ 25 ] reported a strain magnitude of 2300 le for dropping from a
30 cm height (*twofold greater than for running). The strain can be further
increased by attaching additional weight to the animal while the number of loading
cycles is determined by the number of jumps. The rate of loading cannot be easily
controlled for this loading method. We are unaware of studies that have charac-
terized age-related differences in strain magnitude or distribution at relevant
skeletal sites for jumping. A variation of this loading modality is squat-like
exercise [ 35 ], although it is not known if the resulting strain magnitude is similar
to that of jumping.
Umemura et al. subjected female rats of different ages (3-27 months) to
8 weeks of jump training [ 32 ] (same study reviewed for run training in previous
session). The fat-free weight of the tibia and femur of jump-trained rats was
greater than untrained rats for all age groups (except for the 12-months femur), and
the diameter of the tibia (measured at the distal tibio-fibular junction) was also
greater in jump-trained rats for all age groups. However, jump training increased
femoral and tibial length and tibial diameter only in the younger groups (3 and
6 months). Compared with the observations in rats subjected to running, it was
concluded that jump training has a greater effect on bone adaptation than running
despite the briefer training time (jumping 10 min/day; running 60 min/day). It was
speculated that this difference is a result of jump training's ability to induce higher