ADVERSE-EFFECTS OF STRENUOUS EXERCISE - A DENSITOMETRIC AND HISTOMORPHOMETRIC STUDY IN THE RAT

To investigate the manner in which cancellous bone in different skelet al sites and within a bone site adapts to strenuous training, 5-wk-old male rats were subjected to intensive treadmill running [80% of maxim al O-2 consumption (Vo(2 max))] for 11 wk. Vo(2 max), tibia length, an d bone mineral density were measured. Histomorphometric analysis was p erformed in the epiphysis, primary spongiosa (1 degrees sp) and second ary spongiosa (2 degrees sp) of the contralateral proximal tibia, and the 2 degrees sp of thoracic and lumbar vertebrae. Vo(2 max) was incre ased by 39%. No changes were observed in vertebrae. Tibia length, 1 de grees sp bone volume, and number of trabeculae were significantly decr eased, indicating a retarded longitudinal bone growth. Bone mineral de nsity in the proximal tibia was significantly decreased. In the epiphy sis, a trabecular thinning and an increase of trabecular number were s hown. In the 2 degrees sp, bone volume and number of trabeculae were s ignificantly decreased. The increased total eroded surfaces could indi cate an early but transient increase in bone resorption activity. Oste oid thickness was reduced, whereas osteoclast number and osteoid surfa ces were unchanged, suggesting that the observed bone loss was mostly due to an impaired osteoblastic activity. In conclusion, 1) strenous t raining in young rats reduces longitudinal bone growth and induces bon e loss, 2) the cancellous bone adaptation is site specific, and 3) the bone loss is mainly due to decreased osteoblastic activity rather tha n a global adaptation of bone remodeling.