1. The influence of three mechanical factors, force, muscle length and passive lengthening, on long-lasting changes in voluntary force generation, the force:frequency relationship and the development of tenderness has been studied in healthy human skeletal muscle. The elbow flexors were used in all studies. The effect of muscle length was also investigated in the quadriceps and adductor pollicis muscles. Eighty maximal voluntary contractions (MVCs) were performed: one contraction, lasting approximately 2 s, every 15 s. The MVC and force:frequency relationships were measured before and immediately after the exercise and, together with an assessment of tenderness, at 24 h intervals thereafter. 2. In a series of experiments designed to investigate the effects of force, eccentric (lengthening) contractions were found to cause greater fatique and delayed-onset muscle pain than either isometric or concentric (shortening) contractions. There were, however, no substantial differences between the effects of isometric and concentric contractions. Changes in MVC took 24-48 h to return to normal while the low-frequency fatigue required 3-4 days to recover. 3. Passive lengthening with a comparable number of movements over the full range had no effect on the force generation of the muscle, nor did it cause any muscle pain. 4. In the series of experiments designed to investigate the effects of length, isometric MVCs were performed at either short or long length and the muscles subsequently tested at an intermediate length. The contractions at long length resulted in greater low-frequency fatigue and pain, despite the fact that they generated less force than those at the short length. 5. The results demonstrate that there is no simple relationship between the force generated during exercise and the development of long-lasting muscle fatigue and pain. Furthermore, there is a length-dependent component in the generation of low-frequency fatigue and muscle pain.