The movement occurring in whole muscle during contraction is divided between the muscle fibres, the tendon and the longitudinal change in length due to a change in pennation angle. The relative importance of each of these components varies between muscles and with how they are being used. To date it has not been possible to measure each of these movements. By suturing piezoelectric crystals to each end of a small group of muscle fibres in cat medial gastrocnemius muscle, the transit time of ultrasound between the crystals was measured. Assuming a constant velocity of sound in muscle of 1580 m/s, the transit time was used to calculate the muscle fibre length. In "isometric" tetanic contractions the muscle fibres shortened by up to 25% at the expense of the tendons. Lengthening the muscle by 4 mm between each two tetanii, produced less than 4 mm stretch of the muscle fibres as some of the movement was taken up by a change in pennation angle. Except at long lengths, when passive tensions were present, none of this movement between tetanii could be accounted for by stretch of the tendon.