Plasticity of skeletal muscle phenotype: Mechanical consequences

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Abstract
Muscles are complex biological machines that perform a wide variety of mechanical activities. Over the past 30 to 40 years, a large amount of effort has been devoted to understanding cellular/molecular responses of skeletal muscle to various altered physiological states (e.g., altered loading state induced via immobilization/spaceflight, resistance training). Many cellular/molecular adaptations brought about by such interventions act on underlying processes that regulate activation, force and velocity of shortening/lengthening, and relaxation. In this context, measurements of mechanical properties (e.g., force–velocity relationship) are important, because they can provide insight into the physiological consequences of such adaptations. During the course of the past 10 to 15 years, a number of investigators have employed the work‐loop technique to provide a more realistic approach toward understanding muscle function. Additionally, the work‐loop technique provides a unique conceptual perspective that integrates: (1) the length–tension relationship, (2) activation kinetics, (3) the force–velocity relationship in the shortening domain, (4) relaxation kinetics, (5) the force–velocity relationship in the lengthening domain, and (6) the compliance of the passive elastic elements. A discussion of those factors (i.e., factors 2–5) that appear to be highly malleable forms the basis of this paper. © 2002 Wiley Periodicals, Inc. Muscle Nerve 26: 740–768, 2002