Anatomical changes in chronically stimulated skeletal muscles.

Abstract

c hronic electrical stimulation has been used since the last century in an attempt to prevent or slow down muscle atrophy.’ However, studies of its role in transformation of muscle characteristics began much later. Although it has been known for a long time* that nerves supplying muscles involved in various types of movements have distinctly different specific activity patterns (continuous low frequency in slow postural muscles and phasic frequency in fast muscles), they were used for the first time to alter fast muscle properties by Salmons and Vrbova in 1969.3 It was established that low frequency (10 Hz) continuous stimulation not only transforms fast muscles into slow in terms of contractile properties and fatigue resistance, but also alters the pattern of enzyme activities, fiber composition, ultrastructure and vascular s~pply.~ Further experiments with chronic electrical stimulation have demonstrated that a similar conversion of fast to slow muscles can also be achieved using bursts of high frequency pulses, provided the total amount of activity per day is similar to that applied in low-frequency experiments and the duration of stimulation is long enough (minimum 14 days).5,b These experiments are referred to here as long-term stimulation. The pattern of stimulation had a markedly different effect when applied for only 7 days (shortterm stimulation). This article summarizes the findings on the effects of different patterns of long-term electrical stimulation on muscle weight, fiber areas and composition, changes in ultrastructure and vascular (mainly capillary) supply in rats, rabbits, and cats.