Skeletal Mechanoresponsiveness: Effects of Sex Hormones - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 219

by exercise [ 17 , 26 ]. Exercise plus OVX also increased osteoblast activity com-
pared to OVX alone [ 26 ]. In growing male rats, ORX reduced osteoblast-lined
surface and osteoclast number after eight weeks, and exercise returned these
indices to sham levels, normalizing bone turnover with ORX [ 27 ].
The majority of exercise studies have examined OVX; fewer studies have
examined the role of exercise in bone loss following ORX in male rodents. In
growing male rats, exercise did not protect against cancellous bone loss following
ORX. When young (3 month) male rats were exercised on a treadmill, cancellous
bone volume was decreased at both time points in ORX rats with and without
exercise [ 27 ]. ORX also decreased the mechanical strength of the femoral neck,
and exercise did not prevent this reduction. Thus, for growing rats exercised at a
similar speed, duration and intensity, the skeletons of OVX female rats may be
more sensitive to the anabolic effects of exercise than those of ORX male rats.
Shorter daily exercise durations and moderate intensity may be most beneficial
in counteracting bone loss following estrogen deficiency [ 26 , 17 , 52 ]. Moderate
exercise attenuated OVX-induced cancellous bone loss and increased cortical bone
mass in adult (6-months) rats [ 26 ]. Exercise for 30 min/day increased cancellous
bone volume, with decreased resorption and increased osteoblastic activity,
compared to the sedentary OVX group but was unable to return to the level of
sham controls [ 26 ]. Cortical bone area was not different between the OVX and
sham groups, but increased with 30 min of daily exercise compared to OVX alone.
Bone measures were not different in OVX rats with 60 min/day of exercise
compared to OVX without exercise. When multiple intensities were compared,
running at moderate intensity (10-12 m/min) was more effective at restoring bone
to sham levels than faster speeds (18 m/min) for the same duration [ 17 , 53 ].
Normal, growth-related gains in ash weight were lower in OVX alone and vig-
orously exercised OVX animals, but similar in moderately exercised OVX and
sham animals. Femoral neck strength decreased with OVX and returned to control
levels with moderate intensity running. OVX decreased trabecular bone volume by
52% compared to sham, and moderate exercise reduced this loss to 30%, while
more intense exercise reduced the loss to 40%. The exercise-induced increases in
cancellous bone and bending strength of the humerus differed with exercise
intensity and were greatest with moderate intensity running. Clearly, intensity and
periodicity of exercise can have major impacts on how bone responds to exercise.
Animal studies have focused on administering estrogen to counteract hormonal
loss seen with OVX. The administration of estrogen or phytoestrogens prevented
bone loss associated with OVX and had an additive effect with exercise [ 10 , 28 ,
49 , 54 ]. Bone loss was prevented with pharmacological replacement of estrogen
(17b-estradiol) in growing rats that underwent OVX and subsequent sciatic
neurectomy [ 10 ]. Similar results were found in adult OVX rats when 17b-estradiol
was combined with treadmill exercise [ 49 ]. While combined treatment did not
restore tibial cancellous bone mass to control levels, exercise and estrogen
replacement together were additive and significantly more effective than either
treatment alone. Similarly, combining Genistein, an isoflavone that interacts with
estrogen receptors, with moderate treadmill exercise was additive in preventing