Muscle hypertrophy is an adaptive response to overload. Progressive resistance exercise (PRE) is thought to be among the best means to achieve hypertrophy in humans. While functional adaptations to PRE in muscles of humans are made in the clinic, it is difficult to evaluate hypertrophic responses and underlying mechanisms because the adaptations require many weeks or months before they become evident and there is a large variability in response to PRE among humans. In contrast, various animal models have been shown to induce rapid and extensive muscle hypertrophy and some models allow precise control of the exercise parameters. By examining the animal models of muscle hypertrophy and understanding the advantages and disadvantages of each, clinicians may be able to evaluate and use relevant data from these models to design new strategies for modification of PRE in humans. The purpose of this article is to review animal models that are currently used in basic research laboratories, discuss the hypertrophic and functional outcomes, and relate these to PRE used in the clinic.

J Orthop Sports Phys Ther. 2002; 32(2):36–43.

Key Words: muscle growth, muscle strength, overload, resistance training, skeletal muscle

One of the remarkable features of skeletal muscle is its adaptability. Skeletal muscle adaptations are characterized by modifications of morphological, biochemical, and molecular variables that alter the functional attributes of specific skeletal muscle fiber types. Skeletal muscle adaptation is diverse and the magnitude of change is dependent on many factors, such as activity pattern, age, and muscle fiber type composition. The adaptation of skeletal muscle in the adult population is well described. In contrast, the adaptation of skeletal muscle in the older population is less documented, especially in the area of inactivity-induced alterations. Age-related changes in skeletal muscle may play a significant role in the magnitude of change with inactivity and influence the rehabilitation process for the older adult. A consistent feature of age and inactivity is limb muscle atrophy and the loss of peak force and power. Differences exist in the rate and mechanisms of muscle wasting and in the susceptibility of a given fiber type to atrophy. Most likely, the rapid muscle wasting might be in part due to a decrease in protein synthesis coupled with an increased degradation. Besides the quantitative change in muscle mass, age and inactivity induce important qualitative changes in the structure of key skeletal muscle proteins that are manifested in alterations in contractile properties. Therefore, the purpose of this clinical commentary is to identify the major effects of age and inactivity on skeletal muscle structure and function, and discuss potential therapeutic interventions. Special emphasis will be placed on how alterations in muscle structure affect function and on the cellular and molecular mechanisms of the age-related and inactivity-induced muscle changes.

J Orthop Sports Phys Ther. 2002; 32(2):44–57.

Key Words: contractile properties, fiber types, hindlimb unweighting, myosin heavy chain isoforms