1. Normal subjects performed a step test in which the quadriceps of one leg contracted concentrically while the contralateral muscle contracted eccentrically. 2. Maximal voluntary force and the force: frequency relationship were altered bilaterally as a result of the exercise, the changes being greater in the muscle which had contracted eccentrically. Recovery occurred over 24 h. 3. Electromyographic studies using three sites on each muscle showed an increase in electrical activation during the exercise only in the muscle which was contracting eccentrically. Recovery followed a time course similar to that of the contractile properties. 4. Pain and tenderness developed only in the muscle which had contracted eccentrically. Pain was first noted approximately 8 h after exercise and was maximal at approximately 48 h after exercise, at which time force generation and electrical activation had returned to pre-exercise values. 5. Eccentric contractions cause more profound changes in some aspects of muscle function than concentric contractions. These changes cannot be explained in simple metabolic terms, and it is suggested that they are the result of mechanical trauma caused by the high tension generated in relatively few active fibres during eccentric contractions.