EFFECTS OF UNILATERAL STRENGTH TRAINING AND DETRAINING ON BONE-MINERAL MASS AND ESTIMATED MECHANICAL CHARACTERISTICS OF THE UPPER-LIMB BONES IN YOUNG-WOMEN

The aims of this study were to examine the effects of 12 months unilat eral high-resistance strength training and 8-month detraining on bone mineral content (BMC), density (BMD) and estimated mechanical characte ristics of upper limb bones, and also to estimate consequent loading i nduced strains on forearm bone shafts. Thirteen female physiotherapy s tudents (mean 23.8 +/- 5.0 yrs, 166 +/- 7 cm, 64.4 +/- 13.3 kg) traine d their left upper limbs with dumbbells on average 2.8 times per week for 12 months, followed by eight months detraining. Nineteen students served as controls (mean 25.7 +/- 5.2 yrs, 165 +/- 4 cm, 62.1 +/- 7.0 kg). BMC, BMD, and bone width and estimated cortical wall thickness (C WT) were measured at five different sites in both upper extremities (p roximal humerus, humeral shaft, radial shaft, ulnar shaft, and distal forearm) using dual energy x-ray absorptiometry (DXA) scanner. In addi tion, cross-sectional moment of inertia (CSMI) was estimated from DXA data. The maximal isometric strength of the upper extremities was meas ured with an arm flexion-extension dynamometer. The training increased significantly the flexion strength by 14% (p = 0.001). During the det raining period, all measured strength values in the training group dec reased in both limbs with respect to values after training. Despite th e clear effect on muscular strength, no significant intergroup differe nces were observed in BMC, BMD, bone width, CWT, or CSMI values at any measured site after the training or detraining period. The estimated loading-induced strains remained within customary loading, and the cha nge in strain level was only 15%. In conclusion, this study indicated that using high-resistance strength training may not provide an effect ive osteogenic stimulus for bone formation and geometric changes in up per limb bones of young, healthy, adult women. The interaction of bone s and muscles may play an important and relatively unrecognized role i n the development of bone strength, suggesting that the entire biomech anical environment should be carefully considered when evaluating the osteogenic efficiency of physical loading.