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INTRODUCTION

Exercise has a positive and significant impact on the human body by improving physiological function, physical fitness, and health. Physical activity can lead to numerous physiological changes that may lower the prevalence of chronic disease and cause significant adaptions in many physiological systems of the body. There are potential benefits of exercise on the cardiovascular, pulmonary, and neuroendocrine systems. Increased physical activity can directly affect energy expenditure, metabolic change, neurobiological effects, and neuroendocrine changes.

BASIC PHYSIOLOGY OF SKELETAL MUSCLE FIBERS

Skeletal muscle fibers are responsible for the generation of force in order to produce movement. Myofibrils are organized to compose skeletal muscle fibers. These myofibrils are surrounded by portions of the sarcoplasmic reticulum and have deep channels (T tubules) (Fig. 25–1).

Figure 25–1

Organization of a skeletal muscle fiber. (Reproduced with permission from Muscle Tissue. In: Mescher AL, eds. Junqueira's Basic Histology, 14e New York, NY: McGraw-Hill; 2016.)

Muscle fibers vary in terms of their mechanical, physiological, and biochemical properties, making skeletal muscle a heterogeneous tissue. Skeletal muscle fibers have been classified by using histochemical techniques (ATPase and oxidative enzymes stains), measurements of contraction (twitch) or fiber shortening velocity, and the identification of myosin heavy chain isoform with use of protein electrophoresis. Skeletal muscle may contain up to three types of fibers in varying proportion: fiber types I, IIa, and IIx.1 It is important to note that skeletal muscle fibers can express more than one type of myosin heavy chain isoform simultaneously.1 Type I myosin heavy chain isoform muscle fibers are the slow, oxidative, fatigue-resistant fibers. Type IIx are very fast contracting, glycolytic, and fatigable fibers. Type IIa have intermediate properties. These fibers are fast contracting but with an oxidative metabolic profile. Fibers that express more than one type of myosin heavy chain isoform are known as hybrid fibers; various combinations (I/IIa, IIa/IIx, I, IIa, IIx) have been reported.1

Force can be generated while skeletal muscle remains static, shortens, or increases in length.2 During most types of movement or exercise, muscles alternate between static and dynamic muscle actions.3

The generation of force by muscle fibers is dependent on a complex series of well-orchestrated events initiated by a nerve impulse, which triggers the release of acetylcholine (ACH) into the synaptic cleft. The ACH initiates muscle contraction by binding to the ACH receptors in the neuromuscular junction. This impulse quickly spreads from the sarcolemma as calcium ions are released into the sarcoplasm. The calcium ions eventually bind to troponin, which causes a change in the morphology of the thin filament. Tropomyosin, which is located on actin (on the thin filament) is shifted, and the shift exposes active sites on the thin filament. Concomitantly, myosin heads located on the thick filaments are also exposed ...

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