Eccentric muscle action increases site-specific osteogenic response

Purpose: Strain magnitude is known to be a primary determinant of the osteo genic response to loading. However, whether bone adaptation to muscle loadi ng is determined primarily by load magnitude is unclear. The purpose of thi s study was to determine the contribution of load magnitude from muscle act ion on the site-specific osteogenic response. Methods: Twenty young women ( 12 exercise, 8 control) served as subjects. Bone mineral density (BMD) of t he whole body and mid-femur segment and body composition were determined by dual-energy x-ray absorptiometry. Knee extension and flexion strengths wer e determined on a KinCom dynamometer, with surface electromyography of the vastus lateralis muscle. Exercise subjects trained three times weekly for 1 8 wk on a KinCom. One leg trained using eccentric knee extension and flexio n. and the opposite leg trained using concentric knee extension and flexion . Results: Eccentric exercise demonstrated greater force production with lo wer integrated electromyographic signal (IEMG) compared with concentric exe rcise. Significant increases in muscle strength occurred in both exercised legs (P < 0.05), which were of similar relative change. However, only the e ccentric trained leg significantly increased mid-femur segment BMD (+3.9%, P < 0.05) and mid-thigh segment lean mass (+5.2%, P < 0.05). Conclusions: T hese results suggest that eccentric muscle training is more osteogenic than concentric muscle training and that eccentric training is more efficient b y attaining higher force production with lower IEMG.