MACHIDA, S., and F. W. BOOTH. Regrowth of Skeletal Muscle Atrophied from Inactivity. Med. Sci. Sports Exerc., Vol. 36, No. 1, pp. 52-59, 2004. The current state of knowledge regarding regrowth of skeletal muscle after inactivity-induced atrophy is reviewed. Muscle regrowth is incomplete after hindlimb suspension in juvenile rats and after limb immobilization in old animals. The process of regrowth from immobilization-induced atrophy likely involves the reversal of directional changes in molecules producing muscle loss while initiating anabolic processes for regrowth of muscle mass. Unfortunately, the molecular mechanisms responsible for successful, or failed, muscle regrowth are not well understood. The purpose of the review is to provide current knowledge about the biology of muscle regrowth from inactivity-induced atrophy.
The contribution of skeletal muscle strength and mass to health is under-recognized, where losses result in an increased incidence of death (45). For example, in men ≥60 yr, lower grip strength values were associated with an increased risk of mortality (45). Muscle mass is lost through physical inactivity. A sedentary lifestyle culminates in premature physical frailty (64). Furthermore, immobilization of limbs produces a rapid loss of muscle mass (13). Molecular links between mechanical unloading/reloading and muscle wasting/regrowth from these inactivity-type conditions are needed for patient care. The speculation is made that such medical evidence would encourage individuals, including patients, to undertake more preventive care, i.e., work either to regain muscle strength after rapid losses of muscle mass, such as in limb immobilization, or work to prevent loss in muscle mass for more prolonged periods (such as sedentary living in nursing homes).
Muscle regrowth after hindlimb unloading and limb immobilization is not complete in animals at certain stages of life (18,51,73). Remarkably, muscles of old animals exhibit little regrowth from atrophy after limb immobilization (18,73). Unfortunately, the molecular mechanisms responsible for successful, partial, and failed muscle regrowth from inactivity-induced atrophy are not well understood. The purpose of this review is to describe the current knowledge regarding regrowth of skeletal muscle atrophied from physical inactivity. To understand which inactivity-induced atrophy processes require reversal to allow muscle regrowth, a brief summary of changes that occur with inactivity-induced atrophy will be reviewed first. [Muscle atrophy in spinal cord injury has been reviewed elsewhere (23) and will not be covered here.]