Red muscle motor patterns during steady swimming in largemouth bass: effects of speed and correlations with axial kinematics

We analyzed midline kinematics and obtained electromyograms (EMGs) from the superficial red muscle at seven longitudinal positions in four largemouth bass swimming steadily at standardized speeds of 0.7, 1.2, 1.6, 2.0 and 2.4 lengths s-1. Analysis of variance was used to test for significant variation attributable to both speed and longitudinal position. EMGs propagated posteriorly were unilateral and alternated between the left and right sides. Despite the propagation of EMGs, all the red muscle along an entire side of the fish was simultaneously active for as much as one-quarter of the locomotor cycle. When expressed as a proportion of the locomotor cycle, EMG durations at a given site did not vary significantly with speed but did vary longitudinally, ranging from values of 0.45 cycles anteriorly to 0.35 cycles posteriorly. The amplitudes of lateral displacement and bending depended on longitudinal position and also increased by a maximum of approximately 50 % with increased swimming speed, whereas for all longitudinal positions the intensity of EMGs increased approximately fourfold with increased swimming speed. Electrical activity of red muscle did not correspond simply to the time of muscle shortening. Instead, the timing of EMG onset and offset relative to both lateral bending and displacement changed significantly with both longitudinal position and increased speed, such that the phase shifts between the EMGs and kinematic values were generally greatest for posterior sites at the fastest speeds. At a single longitudinal position, the phase shift between the EMG and maximal lateral bending could change by more than one-tenth of a cycle from the slowest to the fastest swimming speed. Phase lags per body segment of EMG onset and EMG offset did not vary significantly with either swimming speed or longitudinal position. EMG offset was propagated posteriorly faster than EMG onset, and both EMG onset and EMG offset were generally propagated faster than both lateral bending and displacement. Largemouth bass have a similar number of vertebrae to carp, and these two species also have a very similar pattern of muscle activation that differs substantially from that of the trout, which has nearly twice as many vertebrae.