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Static and Dynamic Muscle Contractions

When a muscle contracts concentrically, the contraction can either be sustained or it can be brief and followed by a relaxation period.

During static muscle contractions (static exercise) the active muscles remain contracted for the duration of the activity. An alternative term used to describe this type of contraction is isometric (iso means same; metric means length), meaning that the muscle maintains the same length during the contraction. In reality, few sporting activities involve muscle contractions that are exclusively static. Typical exercises in which static muscular contractions predominate are weight lifting, arm wrestling, pushing in the rugby scrum, and pulling at tug-of-war.

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The important physiological effect of a sustained static muscular contraction is that the pressure inside the muscle rises dramatically. This exerts pressure on the (collapsible) arteries that traverse that muscle and provide oxygen and energy to it. If the pressure becomes sufficient to reduce or completely obstruct the muscle blood flow, the contracted muscle receives insufficient oxygen to sustain the mitochondrial production of ATP. Thus, the muscle cells have to rely on oxygen-independent mechanisms to produce the ATP necessary for contraction. Furthermore, the absence of an adequate blood supply prevents removal of certain toxic metabolic by-products, in particular hydrogen ions (protons) produced by oxygen-independent metabolism. Accumulation of these protons causes the acidity of the muscle to increase. This in turn ultimately inhibits further muscular contraction, in particular by preventing the binding of calcium to troponin-C, thereby preventing the interaction of myosin and actin. These two factorsan inadequate oxygen supply and an inability to remove metabolic by-products explain why static muscle contractions are so painful and why they can be sustained for only relatively short periods of time.

The second important effect of a static muscle contraction is that it evokes powerful reflexes, probably in response to the reduced muscle blood flow (Mitchell et al, 1977), which cause the heart to pump more rapidly and more powerfully and the blood pressure to rise. This effect may be so marked that heart rates as high as 160 to 190 beats per minute and systolic blood pressures in excess of 300 mm Hg may occur.

However, these high heart rates and blood pressures do not mean that the athlete is achieving a training effect equivalent to that which he or she would achieve by running at an equivalent heart rate. For, as we shall see, training is absolutely specific. That is, the body adapts to the specific stress to which it is exposed, and because the physiological stresses of static exercise are exacdy opposite of those of dynamic exercise (see Exercises 1.2), static exercise plays only a small role in improving running ability.

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