In spite of the widespread abuse of androgenic steroids by athletes and recreational body-builders, the effects of these agents on athletic performance and physical function remain poorly understood. Experimentally induced androgen deficiency is associated with a loss of fat-free mass; conversely, physiologic testosterone replacement of healthy, androgen-deficient men increases fat-free mass and muscle protein synthesis. Testosterone supplementation of HIV-infected men with low testosterone levels and of older men with normally low testosterone concentrations also increases muscle mass. However, we do not know whether physiologic testosterone replacement can improve physical function and health-related quality of life, and reduce the risk of falls and disability in older men or those with chronic illness. Testosterone increases maximal voluntary strength in a dose-dependent manner and thus might improve performance in power-lifting events. However, testosterone has not been shown to improve performance in endurance events. The mechanisms by which testosterone increases muscle mass are not known, but probably involve alterations in the expression of multiple muscle growth regulators. Journal of Endocrinology (2001) 170, 27-38The abuse of androgenic steroids by athletes and the proposed anabolic applications of these agents in sarcopenia (loss of muscle mass and strength) associated with aging or chronic illness is based on the premise that these agents increase muscle mass and improve measures of skeletal muscle performance, and that androgen-induced changes in skeletal muscle performance translate into improvements in athletic performance and health-related outcomes (Fig. 1). The premise remains unsubstantiated. There is agreement that testosterone supplementation increases muscle mass and maximal voluntary strength in a variety of clinical and experimental paradigms (Tenover

Total and free testosterone concentrations decline progressively with advancing age because of defects at all levels of the hypothalamic-pituitary-testicular axis. Low total and bioavailable testosterone levels have been associated with decreased skeletal muscle mass, muscle strength, physical function, bone mineral density, and fracture risk, although these associations are weak. The risks and health benefits of long-term testosterone remain poorly understood. Physiologic testosterone replacement of young, androgen-deficient men and older men with low testosterone levels is associated with an increase in fat-free mass, grip strength, and fractional muscle protein synthesis, but we do not know whether testosterone replacement improves quadriceps strength, power, muscle fatigability, and physical function in older men, and whether it can reduce the risk of disability and falls. Testosterone replacement increases vertebral bone mineral density in young hypogonadal men and older men with low testosterone levels, but we do not know whether testosterone reduces fracture risk. Concerns about the potential adverse effects of testosterone on the prostate have encouraged the development of selective androgen receptor modulators that increase muscle mass while sparing the prostate.