27 mar. 2012


Anne Carrington Gawne, M.D.
edited by Halstead & Grimby, © 1995
Hanley & Belfus, Inc., Philadelphia, PA.
Chapter 9, pp 141-164

Lincolnshire Post-Polio Library copy by kind permission of Dr. Gawne.
Appropriate exercises have been shown to improve muscular strength and endurance, improve range of motion, and reduce functional deficits associated with many disabilities. In dealings with the patient with a history of polio, however, several questions arise: How much exercise is enough, and when is it too much? What kinds of exercise are best? What kinds of exercise may he harmful? And are there any guidelines to prescribe a safe and effective exercise program? To answer these questions, it is helpful to first understand the basic principles of exercise physiology, as well as the pathophysiology involved in post-polio syndrome. Following a discussion of these issues is a review of the literature on the effects of exercise in neurologically intact and post-polio individuals. Finally, a new classification system is presented, which will facilitate the prescription of exercise regimens that are both safe and effective in this population.

The effects of exercise are seen at two levels. The first is the cellular level, or in the muscle fiber. The second is throughout the cardiovascular and respiratory systems to meet the physiologic demands of the muscle fibers. The two are discussed separately, looking first at muscle strength and then at cardiovascular endurance.
Muscular strength is the maximal force that can be exerted by a muscle.[21] An isometric contraction is one in which tension develops but in which the muscle stays the same length.[48] An isotonic contraction, or, more appropriately, a dynamic contraction, is one in which the muscle shortens (concentric) or lengthens (eccentric) against a constant resistance, with the muscle tension varying somewhat throughout the range of motion.[24,48] Anisokinetic contraction is one in which the tension, developed by the muscle as it shortens, is maximal at all joint angles over the full range of motion and in which the velocity remains constant.[56]
When the three types of contractions are compared, the isokinetic one theoretically leads to the greatest improvements in both strength and endurance because it activates a larger number of motor units.[56] However, because it requires special equipment and trained personnel to administer, it is much less readily available. Significant strength gains occur with both eccentric and concentric contractions, and exercise equipment to perform these contractions is easy to find and affordable.[23] The advantages of isometric exercises are limited as the development of strength and endurance is specific to the joint angle at which the muscle is trained. Isometric exercises do have a place with individuals who have joints immobilized secondary to surgery or casting.
The overload principle states that muscles increase in size and strength when forced to contract at tensions close to their maximum.[46] Physiologic changes accompanying increased strength include hypertrophy -- the increase in cross-sectional area of the muscle fibers. Hypertrophy is attributable to one or more of the following changes: increased number of myofibrils per muscle fiber; increased protein, especially in the myosin filament; increased capillary density per fiber; increased amounts and strength of connective tissue; and increased number of fibers from longitudinal fiber splitting.[48] These morphologic changes are seen more frequently with eccentric contractions.
Additional factors responsible for strength gains are central nervous system (CNS) adaptations. These include an increase in the number of motor units activated, an increase in the rate of activation, and increased synchronization of motor unit firing.[56] Such changes occur at the spinal cord level and are responsible for cross-education, a phenomenon seen commonly in a limb contralateral to the trained limb. In general, younger individuals (18 - 26 years old) increase their muscle strength due to hypertrophy, whereas strength gains by older individuals (67 - 72 years old) are predominantly due to CNS adaptations.[49,50]
Atrophy, or the reduction in size of a muscle, occurs with denervation or when an extremity is placed at rest. Morphological changes seen after a muscle has been immobilized include atrophy of type I muscle fibers, whereas atrophy associated with aging is predominantly limited to a decrease in type II fiber area.[56]

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