Skeletal Mechanoresponsiveness: Effects of Sex Hormones - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 218

by exercise and even increased above sham levels. In adolescent (3-months at time of
OVX) female rats, OVX decreased cancellous bone volume in the distal femur by
40-50% compared to sham-operated animals [ 16 , 17 ]. Treadmill exercise partially
rescued the bone volume lost due to OVX, but the levels remained below sham levels
[ 16 , 17 ]. However, exercise did completely counteract the decreased mechanical
strength of the femoral neck and tibial shaft with OVX, returning strength levels to
those of sham rats [ 16 , 17 ]. Importantly, in these studies measures of mass and
volume were not accurate surrogates for bone strength. The effect on cancellous bone
volume fraction was similar in adult (5-months) rats exercised on a treadmill for
30 mins daily, but whole bone strength was not measured [ 26 ]. Skeletally mature
(C8-months) rats also respond to exercise following OVX, but the effects are more
variable, with both partial and complete rescue of bone loss reported [ 47 - 50 ].
The response to loading following OVX is age-dependent; exercise in juvenile
rats restores bone to equal or greater levels than those of sham animals, but
increases in older rats are insufficient to reach sham levels. In most exercise
experiments, only the OVX group responded to exercise, whereas the sham
animals did not. However, in young rats OVX generally increased body mass over
control levels and partially protected the animals from osteopenia; therefore, the
increased body mass loading with OVX may contribute to the skeletal changes
seen with running [ 51 ].
Other forms of intrinsic skeletal loading by exercise, such as tower or ladder
climbing, can also recover the bone loss associated with hormone deficiency in rats.
Tower climbing for 3-months in mature (12-months) rats recovered bone loss by
thickening the remaining trabeculae, without changing trabecular separation [ 15 ].
OVX in combination with exercise led to similar bone mass and strength as sedentary
sham-surgery rats. Exercise prevented OVX-induced cortical and cancellous bone
loss by depressing the elevated bone turnover following OVX [ 15 ]. Adolescent
(3-months) OVX rats that were trained to climb a ladder for 12 weeks had increased
tibial BMD, tissue mineral density, and bending strength compared to intact
and OVX sedentary controls, and similar values to intact exercised rats [ 32 ].
High impact exercise caused by jumping strengthened bones in OVX rats with
relatively few numbers of jumps. These protocols are effective in both sham and OVX
animals. Jump training for 2-months increased tibial bone mass, cortical area and
whole bone strength in both young (3-months) and adult (9-months) rats with only ten
daily jumps [ 29 , 30 ]. In young rats, jump training significantly increased serum
osteocalcin in both OVX and sham operated animals, hinting at increased osteoblast
activity [ 30 ]. In adult rats, bone mass and strength of OVX rats increased with
exercise to approximately the same levels as sham-exercised rats [ 29 ]. Similar results
have been reported for different jump training protocols. When four bouts of ten
jumps were performed every 48 h, ash weight, BMD, and bone strength increased in
the tibia of OVX rats compared to sedentary controls [ 31 ].
The anabolic effect of exercise with hormone deficiency occurs through both
increased bone formation and decreased bone resorption, as well as combined
effects. In the rat, estrogen deficiency creates increased bone turnover and bone
resorption [ 24 , 42 ]. Increased numbers of osteoclasts following OVX were reduced