Training responses are complex at the level of the whole muscle. One reason for this is the complexity of the exercise being performed during the training—we obtained a glimpse of this complexity in the previous chapters. Which fibers are being recruited to generate what percentage of their maximal force? Are some fibers more overloaded than others are? (Our knowledge from previous chapters tells us that they are.) Do all fibers respond equally to the same degree of stress? Studies with human subjects often measure biochemical muscle responses using the muscle biopsy technique. A biopsy sample taken from vastus lateralis or gastrocnemius is a mixture of fibers of different types and, if taken from trained subjects, of different training states. From the fibers in the sample, we obtain an average of the biochemical or molecular biological property being assayed. This is, of course, assuming that the biopsy technique provides us with a representative sample of the muscle, which is open to argument (see Lexell et al. 1983).
Chronic electrical stimulation of muscles has been used extensively during the past 30 years to demonstrate the limits of adaptability of muscles to increased activity. All indications are that the changes reported with this model may represent the ultimate state of high-intensity and high-volume aerobic training—a state that could never be achieved via voluntary activation. The results from this model have been very instructive with regard to the extent and relative time course of muscle adaptations that occur, against which we can measure changes induced by aerobic-type endurance training. Perhaps more importantly, this model has provided us with information regarding the signals that promote the adaptive changes and regarding the mechanisms of protein metabolism involved (such as gene transcription rates, translation capacity, and posttranslational modifications). This information about protein metabolism especially could not always be provided by the complicated model of voluntary exercise, although for the most part, changes noted after endurance exercise are in the same direction as those reported for electrical stimulation.
For these reasons, considerable attention has been paid to chronic electrical stimulation as an extreme model of muscle adaptation to aerobic-type endurance training. Table 6.1 lists adaptations to chronic electrical stimulation; most of these adaptations have been reported in subjects undergoing endurance training or as differences between endurance-trained and untrained subjects, although the magnitude of the adaptations is less than that observed with electrical stimulation, as is expected.
In this chapter, we will discuss the results emanating from studies using chronic neuromuscular electrical stimulation conducted with animals. The changes reported with this model are in the same direction as the adaptations that have been reported with aerobic-type endurance training, the most evident of which are changes in calcium regulatory proteins, increased activities of mitochondrial enzymes, and changes in the proportions of fiber types from Type IIb toward Type I. In the analysis of muscle changes induced by electrical stimulation, however, we must keep in mind the following differences between electrical stimulation and whole-body endurance training.
- Chronic electrical stimulation typically involves stimulation durations of at least 8 hours and often up to 24 hours per day, a duration obviously longer than that of daily endurance training, even for highly trained athletes.
- Chronic stimulation experiments often do not include rests. Resting is most likely instrumental in determining the final phenotype in response to whole-body endurance training. Resting allows for some recovery from fatigue and thus allows for a higher tension-time index, whereas tension during continuous stimulation falls early during the stimulation and may recover relatively little or not at all (Hicks et al. 1997; Green, Düsterhöft, et al. 1992). In addition, resting might allow changes in protein synthesis that are instrumental in determining the final phenotype. The levels of mRNA most likely decrease during contractile activity, with increased transcription and translation taking place once this activity has ceased (Cameron-Smith 2002). In this chapter, whenever we refer to altered mRNA levels resulting from muscle contractile activity, we refer to analysis of samples taken from resting muscles.
- Chronic stimulation does not produce the patterns of impulse activity that a muscle fiber experiences during whole-body endurance training. For example, recruitment, firing frequency, pattern of firing, and number of impulses per burst should be quite variable among motor units within a muscle, depending on the threshold and the type of motor unit. This activity should also change as fatigue occurs. With chronic stimulation, the same pattern is imposed on all motor units, and the pattern does not change with fatigue.
- Finally, chronic electrical stimulation does not involve voluntary recruitment of motor units. While the significance of this is not fully known at present, it may be that voluntary recruitment involves changes in structure and function at the spinal cord and supraspinal level that are significant contributors to aerobic endurance performance (as we will discuss in the following chapter). Thus, aerobic endurance training might involve muscular changes that are less marked than, and perhaps qualitatively different from, those seen in chronic stimulation, partly because the nervous system becomes trained to alter the way muscle adaptations are translated into better performance.