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28.06.2012.

Mechanisms of gains of muscle strength


For many years, strength gains were assumed to result directly from increases in muscle size(hypertrophy). This assumption was logical because most who strength trained regularly were men, and they often developed large, bulky muscles. Also, muscles associated with a limb immobilized in a cast for weeks or months start to decrease in size(atrophy) and lose strength almost immediately. Gains in muscle size are generally paralleled by gains in strength, and loses in muscle size correlate highly with losses in strength. Thus, it is tempting to conclude that a direct cause-and-effect relationship exists between muscle size and muscle strength. While there is a relationship between size and strength, muscle strength involves far more than mere muscle size.
Numerous media reports indicate that people can perform superhuman feats of strength during great psychological stress. Straitjackets were designed specifically to control patients in mental hospitals who suddenly go berserk and are impossible to restrain. Even the world of sport boasts isolated examples of superhuman athletic performances, such as Bob Beamon’s long jump of 29ft(2 ½ inches – 8,90m) at the 1968 Olympic Games – a jump that exceeded the previous world record by nearly 2ft(0.6m)! World records are usually broken by inches or centimeters or, more often, mere fractions of inches or centimeters. Beamon’s record stood unbroken till 1991!
Women experience similar, or even greater, percentage increases in strength compared with men who participate in the same training program, but women generally do not experience as much hypertrophy. Similar findings have been reported in children.
This does not mean that muscle size is unimportant in the ultimate strength potential of the muscle. Size is extremely important, as revealed by the existing men’s and women’s world records for competitive weight-lifting.  As weight classification increases(implying increased muscle mass), so does the record for the total weight lifted. However, examples of superhuman strength and studies on women and children indicate that the mechanisms associated with strength gains are very complex and are not completely understood at this time. Obviously, increased muscle size is important, but there is increasing evidence that the neural control of the trained muscle is also altered, allowing a greater force production from the muscle.

Neural control of strength gains

An important neural component explains at least some of the strength gains that result from resistance training. Enoka has made a convincing argument that strength gains can be achieved without structural changes in muscle but not without neural adaptations. Thus, strength is not solely a property of the muscle. Rather, it is a property of the motor system. Motor unit recruitment, stimulation frequency, and other neural factors are also quite important to strength gains. They may well explain most, if not all, strength gains that occur in the absence of hypertrophy, as well as episodic superhuman feats of strength.

Synchronization and recruitment of additional motor units

Motor units are generally recruited asynchronously; they are not all called on at the same instant. They are controlled by a number of different neurons that can transmit either excitatory or inhibitory impulses. Whether the muscle fibers contract or stay relaxed depends on the summation of the many impulses received by the given motor unit at any one time. The motor unit is activated and its muscle fibers contract only when the incoming excitatory impulses exceed the inhibitory impulses and the threshold is met.
Strength gains may result from changes in the connections between motor neurons located in the spinal cord, allowing motor units to act more synchronously, facilitating contraction, and increasing the muscle’s ability to generate force. There is good evidence to support increased motor unit synchronization with resistance training; but there is still controversy as to whether synchronization of motor unit activation produces a more forceful contraction. It is clear, however, that synchronization does improve the rate of force development and the capability to exert steady forces.
An alternate possibility is simply that more motor units are recruited to perform the given task, independent of whether these motor units act in unison. Such improvement in recruitment patterns could result from an increase in neural drive to the alpha-motor neurons during maximal contraction. This increase in neural drive could also increase the frequency of discharge(rate coding) of the motor units. It is also possible that the inhibitory impulses are reduced, allowing more motor untis to be activated, or to be activated at a higher frequency.

Increased rate coding of motor units

The increase in neural drive of alpha-motor neurons could also increase the frequency of discharge, or rate coding, of their motor units. Recall from previous threads that as the frequency of stimulation of a given motor unit is increased, the muscle eventually reaches a state of tetanus, producing the absolute peak force or tension of the muscle fiber or motor unit. There is limited evidence that rate coding is increased with resistance training. Rapid movement or ballistic-type training appears to be particularly effective in stimulating increases in rate coding.

Autogenic inhibition

Inhibitory mechanisms in the neuromuscular system, such as the Golgi tendon organs, might be necessary to prevent the muscles from exerting more force than the bones and connective tissues can tolerate. This control is referred to as autogenic inhibition. During superhuman feats of strength, major damage often occurs to these structures, suggesting that the protective inhibitory mechanisms are overridden.
When the tension on a muscle’s tendons and internal connective tissue structures exceeds the threshold of the embedded Golgi tendon organs, motor neurons to that muscle are inhibited; that is, autogenic inhibition occurs. Both the reticular formation in the brain stem and the cerebral cortex function to initiate and propagate inhibitory impulses.
Training can gradually reduce or counteract these inhibitory impulses, allowing the muscle to reach greater levels of strength. Thus, strength gains may be achieved by reduced neurological inhibition. This theory is attractive because it can at least partially explain superhuman feats of strength and strength gains in the absence of hypertrophy.

Other neural factors

In addition to increasing motor unit recruitment or decreasing neurological inhibition, other neural factors can contribute to strength gains with resistance training. One of these is referred to as coactivation of agonist and antagonist muscles(the agonist muscles are the primary movers, and the antagonist muscles act to impede agonists). If we use forearm flexor concentric contraction as an example, the biceps is the primary agonist and the triceps is the antagonist. If both were contracting with equal force development, no movement would occur. Thus, to maximize the force generated by an agonist, it is necessary to minimize the amount of coactivation. Reduction in coactivation could explain a portion of strength gains attributed to neural factors, but its contribution likely would be small.
Changes also have been noted in the morphology of the neuromuscular junction, with both increased and decreased activity levels that might be directly related to the muscle’s force-producing capacity.



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