Effects of chronic low-frequency electrical stimulation of human knee extensor muscles exposed to static passive stretching

The study investigated the effects of chronic low-frequency electrical stimulation (CLFES) of the stretched knee extensor muscles versus CLFES without a resistance load. A total of 19 male volunteers were randomized into two groups with similar aerobic and force-velocity properties. In group 1, anterior thigh muscles (the knee extensors) of both legs were exposed to CLFES (15 Hz). In group 2, the CLFES of the knee extensors was combined with stretching. The stimulated subjects demonstrated a marked tendency towards increasing endurance and a significant increase (by 10%) in both succinate dehydrogenase activity and the percentage of muscle fibers containing slow myosin heavy chain (MHC) isoforms. At the same time, the proportion of fibers reacting with the antibody against fast MHC isoforms decreased by 12%. In group 1, the maximum voluntary force (MVF) significantly decreased, the volume of m. quadriceps femoris (as measured by magnetic resonance imaging) remained unchanged, and the size of the fast fibers slightly decreased (by 11%). In group 2, no significant decrease in the MVF was observed, while there was a significant increase in muscle volume and the cross-sectional area of muscle fibers.