The present study was undertaken to investigate in a randomized controlled trial the effects of chronic (10 weeks, 4 h per day, 7 days per week) low-frequency (15 Hz) stimulation (CLFS) of the knee extensor and hamstring muscles of both legs in healthy volunteers via surface electrodes. A control group (n=10) underwent the same treatment (sham stimulation) as the CLFS-treated group (n=10), except that stimulation intensity was kept at a level which did not evoke contractions. Biopsy samples were taken before the onset and after cessation of stimulation from the right vastus lateralis muscle of all subjects. The biopsy samples were analyzed for changes in myosin heavy chain (MHC) isoforms and activities of citrate synthase (CS) and glyceraldehyde phosphate dehydrogenase (GAPDH) as markers of aerobic-oxidative and anaerobic pathways of energy metabolism, respectively. In addition, functional properties, i.e., oxygen consumption (VO(2)) and work capacity, were assessed. Sham stimulation did not affect the functional properties and had no detectable effect on MHC isoform and enzyme activity patterns. Conversely, CLFS induced changes in the MHC isoform pattern in the fast-to-slow direction with an approximately 20% decrease in the relative concentration of MHCIId/x (from 28% to 22%) and an approximately 10% increase in the relative concentration of MHCI (from 30% to 34%). In addition, CLFS led to a approximately 9% increase in the activity of CS concomitant with an approximatley 7% decrease in the activity of GAPDH. This increase in aerobic-oxidative capacity was accompanied by improved work capacity and VO(2) at the anaerobic threshold by 26% and 20%, respectively.