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

Abstract

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 1.6L 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.