Muscle fibers cannot continually lift a load. After a short time, the muscle will lose its ability to exert tension and will ultimately fail. Due to the accumulation of waste products and the depletion of stored energy materials, a muscle is said to be have lost its contractility and become fatigue. Although a short rest period will usually allow the circulation to remove waste products, a much longer period is required for the resynthesis of energy-producing materials. In general, fatigue occurs earlier with closely spaced stimuli, while fatigue for single twitches occurs only after very long intervals.