Bone mass and endocrine adaptations to training in spinal cord injured individuals

To investigate whether exercise training can produce increases in bone mass in spinal cord-injured (SCI) individuals with established disuse osteopenia, nine subjects (age 28.2 years, time since injury 6.0 years, level of injury C5-T7) were recruited for a 9-month training program using functional electrical stimulation cycle ergometry (FES-CE), which produces active muscle contractions in the paralyzed limb. After training, bone mineral density (BMD, by X-ray absorptiometry) increased by 0.047 +/- 0.010 g/cm2 at the lumbar spine; changes in BMD at the femoral neck, distal femur, and proximal tibia were not significant for the group as a whole. In a subset of subjects training at > or = 18 W for at least 3 months (n = 4), BMD increased by 0.095 +/- 0.026 g/cm2 (+18%) at the distal femur. By 6 months of training, a 78% increase in serum osteocalcin was observed, indicating an increase in bone turnover. Urinary calcium and hydroxyproline, indicators of resorptive activity, did not change over the same period. Serum PTH increased 75% over baseline values (from 2.98 +/- 0.15 to 5.22 +/- 0.62 pmol/L) after 6 months' training, with several individual values in hyperparathyroid range; PTH declined toward baseline values by 9 months. These data establish the feasibility of stimulating site-specific increases in bone mass in severely osteopenic bone with muscle contractions independent of weight-bearing for those subjects able to achieve a threshold power output of 18 W with FES-CE. Calcium supplementation from the outset of training in osteopenic individuals may be advisable to prevent training-induced increases in PTH.