Chronic electrical stimulation provides one of the "cleanest" views of muscle adaptation to increased use. This model has long been used by basic scientists to study skeletal muscle adaptation since it induces a repeatable, quantifiable amount of "exercise." Thus, we have been able to synthesize a collection of such studies in a manner that provides a fairly global view of muscle structural and physiological changes that occur in response to the stimulation.
It is known that muscles which perform different tasks, in addition to having different muscle architecture, respond to different electrical input. For example, skeletal muscles that play a postural role, and thus have a high proportion of slow fibers, are physiologically activated at low frequencies. Conversely, muscles with a very high proportion of fast fibers may be activated only intermittently with high frequency bursts of electrical activity. The fact that electrical activity and muscle properties seem to be intimately interrelated, provides an experimental basis for understanding muscle plasticity. The best documented effects of electrical stimulation on skeletal muscle are those that occur after chronic, low-frequency stimulation (similar to the activity of a "slow" muscle) is imposed upon a predominantly "fast" muscle. If the stimulator is activated at a nominal frequency of about 10 Hz and allowed to operate 8-24 hours per day, a well-defined progression of changes is observed whereby the fast muscle first changes its metabolic and then its contractile properties to completely "transform" into a "slow" muscle.
Based on time-series studies and single fiber biochemistry that the changes that occur result from a true transformation of a single fast fiber into a slow fiber and not from selective loss of fast fibers with subsequent slow fiber regeneration or proliferation. The fast fibers actually become slow fibers. In fact, even the physical appearance of the stimulated muscle approaches that of the more postural muscles by taking on a "deep red" appearance.
Time course of muscle fiber transformation:
Here is an important point: just because activity increases, does not mean that all muscle proteins must also increase. Some proteins increase in amount (i.e., are upregulated) and others decrease in amount (i.e., are downregulated). The "choice" which the muscle makes depends on the goal of the adaptation. As the muscle is receiving information consistent with being chronically active at fairly low levels, it alters its genetic expression pattern to create a structure consistent with this functional requirement. In the case of decreased expression of amount and activity of SR calcium transport proteins, this can be detected physiologically as a prolonged time-to-peak twitch tension and a prolonged relaxation time of a muscle twitch or as a decrease in the fusion frequency.
We conclude two things from this typical time-course of transformation:
The decrease in skeletal muscle mass and fiber area should not be viewed as an atrophic or degenerative response, i.e., an undesirable "overuse" type of injury. Rather, it appears that fiber atrophy represents a deliberate adaptive response of the muscle fiber to chronic stimulation—perhaps to decrease diffusion distances from the muscle fiber to the interstitial spaces which contain the capillaries. As an aside, it is interesting that chronic low frequency electrical stimulation is the best way to make a muscle "weaker". This certainly should raise some eyebrows in the therapeutic community since electrical stimulation is commonly used to treat muscle atrophy.
For a look at an abstract from a 1996 study we did comparing electrical stimulation training to voluntary training, click here.
Last Updated: Friday, 13-Jan-2006 15:56:16 PST
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