Chronic low-frequency stimulation is by now a well-established method for inducing specific changes in muscle properties. Over the past decade several reviews have been published which summarized the major effects of chronic stimulation (Jolesz and Sr6ter 1981; Salmons and Henriksson 1981; Pette 1984, 1985, 1991; Pette and Vrbovfi 1985; Lieber 1986; Pette and Dfisterh6ft 1992). The increased use of this model has led to an expansion of knowledge in this field and a large body of information has accumulated on the numerous phenomena of the activity-induced transformation of muscle fiber phenotype. These data emphasize that fully differentiated muscle fibers retain the potential to respond to altered functional demands with specific adaptive changes.

Chronic low-frequency stimulation represents an approach that allows researchers to relate functional changes to specific molecular events in the stimulated muscle. Unlike exercise, the activity is restricted to the stimulated muscle and, thus, the muscle is less influenced by other changes that can occur in the body during training. Artificial stimulation bypasses the central nervous system and activates all motor units equally, whereas during exercise individual motor units are activated in a graded and hierarchical manner. Therefore, chronic stimulation can provide information as to the extent of the muscle plasticity. It has also become clear that chronic electrical stimulation of a skeletal muscle may evoke changes which exceed those induced by any other form of increased contractile activity. It has, thus, provided information as to the limits of muscle adaptation. In addition, the effects of chronic stimulation on skeletal muscle have inspired clinical investigators to use this method for medical purposes.