Strategies that improve human skeletal muscle performance during repetitive, non-isometric contractions.

Previous studies have suggested that during sustained maximal voluntary and evoked contractions, decreasing activation rates may minimize fatigue. The idea of gradually decreasing stimulation frequency to preserve force during fatiguing isometric contractions has, however, recently been challenged. The primary purpose of this study was to test the effectiveness of decreasing or increasing electrical stimulation rates during evoked, repetitive, submaximal, non-isometric contractions of healthy human quadriceps femoris muscles. The ability of the muscles to produce a 50 degrees knee excursion repetitively was evaluated using low-frequency trains, high-frequency trains, and a combination of these trains. Results showed that stimulating the muscles with high-frequency trains followed by low-frequency trains produced the worst performance and starting with low-frequency trains followed by high-frequency trains produced the best performance. Present results cast doubt on the applicability of decreasing stimulation rates during non-isometric contractions and suggest that a combination of trains that begin at a low frequency and then switch to a higher-frequency may improve performance during functional electrical stimulation.