1. Needles inserted into the biceps move with the muscle as the elbow flexes or extends. Pairs of needles were used to indicate changes in length of the muscle fibres. 2. During low-frequency flexion-extension movements the biceps lengthened as the joint extended. 3. When, however, joint movements at greater than 2.2 Hz were maintained by external electrical stimulation of triceps and the long head of biceps, or of biceps alone, the biceps lengthened and shortened in antiphase to the joint movement. The elastic properties of the biceps tendons then combined with the mass of the forearm as a spring-mass system whose natural frequency was about 2.2 Hz. 4. No such phase reversal appeared during voluntary elbow movements at frequencies up to 5 Hz. It was concluded that the combined tendons of biceps, brachialis and brachioradialis made a much less compliant muscle-to-bone coupling. 5. The results are discussed in relation to possible tremor mechanisms.