The timing of the electromyogram in the lateral myotomes of mackerel and saithe at different swimming speeds

Electromyogram (EMG) signals from two points at about 40% L and 65% L ( L = length) in the left latera1 muscle of mackerel ( Scomber scombrus L.) L = 28–33 cm a nd saithe ( Pollachius virens L.) L = 42–50 cm were recorded synchronously with films of steady straight swimming motions. In both species, the duration of EMG activity at both electrodes, remains a constant proportion of the tail cycle period Tat all the tail beat frequencies between 1–8 and 13 Hz. In mackerel and saithe respectively: onset of EMG activity at the front was 74% T and 77% T before the left‐most tail blade position and fronl EMG‐onset occurred 15% T and 18% T before rear onset. The duration of the EMG burst is longer at the front position (41% T and 47% T ) than at the rear (25% T and 27% T ), At all swimming speeds the wave of electrical activation of the muscle travelled between the two electrodes 25% L apart at a velocity between 1.5 and 1.6 L T −1 . Frequencies of spikes within the burst of EMG activity rose from 30–40 Hz at 2 T s −1 to 50–80 Hz at 8 T s −1 . In muscle at 40%L EMG‐onset happens at phase 30° just after muscle strain at this point reaches its resting length while lengthening (360°). At 65% L EMG‐onset occurs earlier in the strain cycle‐350° just before the muscle reaches it resting length while lengthening (360°). This could represent within the length of the fish, a phase shift of up to 90° in the EMG‐onset in relation to the muscle strain cycle. These timings are discussed in relation to optimized work output and a single instant of maximum bending moment all along the left side of the body.