Acute exercise results in impaired muscle function; that is, fatigue develops. The impaired muscle function in fatigue may be due to central (in the central nervous system) and/or peripheral (within the muscle) factors. My research group studies mechanisms of peripheral fatigue with a special interest in changes in sarcoplasmic reticulum (SR) calcium handling. Experiments are performed on intact single muscle fibers obtained from mice and rats. Central fatigue is often assessed with the twitch interpolation technique, where electrical stimulation is superimposed on an ongoing voluntary contraction and the resulting change in force is measured. We have mimicked this stimulation pattern in single fiber experiments and observe a force increase that could be interpreted as central fatigue; the underlying principles will be discussed. Force recovery after fatiguing stimulation may be slow, especially at low stimulation frequencies. Recent data from our laboratory show that two different mechanisms can cause this prolonged force depression: decreased SR calcium release and reduced myofibrillar calcium sensitivity. The relative importance of these two mechanisms appears to depend on the production of different reactive oxygen species.