D8 Electromyography (EMG) in swimming muscles

Topic

Electromyography implanted in a fish's swimming musculature records the electrical activity of muscle fibers during propulsion, thereby providing information on muscle recruitment patterns, activation intensity, and the timing coordination between different muscle groups along the fish's body. The signal captured by each electrode represents the summation of action potentials from all motor units active directly beneath that electrode at any given moment. Two types of swimming musculature are distinguished in fish: red muscle—slow and fatigue-resistant—used for sustained swimming; and white muscle—fast but easily fatigued—reserved for sprints and the "C-start," a rapid escape maneuver.

The electrical signal recorded by electromyography in either muscle type is processed using the same principles applied to mammals: the signal undergoes full-wave rectification (taking its absolute value) and is then integrated over time to quantify relative muscle activation; alternatively, the root mean square (RMS) of the signal values ​​is calculated within sliding time windows to determine the instantaneous amplitude of muscle activation. As part of this same processing, the center frequency of the signal's power spectrum decreases as the muscle fatigues, serving as an index of muscle fatigue based on the electrical signal's own spectrum.

Beyond the extent of muscle activation, electromyography also reveals the timing of activation by comparing the latency of muscle activity against the fish's body kinematic wave—introduced in the previous model of this series—thereby identifying the specific phase of the beat cycle in which activation occurs. If the muscle activates while shortening, it generates positive work and acts as a propulsive motor; conversely, if it activates while lengthening, it generates negative work and acts as a shock absorber, absorbing energy rather than contributing it.

This analysis of the muscle activation phase relative to the beat cycle makes it possible to calculate the net work produced by each muscle segment along the fish's body during each cycle—information essential for understanding how propulsive mechanical power is distributed across the entire body.

During sustained swimming, only the red muscle tissue introduced earlier in this model remains active, producing a low-amplitude electrical signal; however, during sprints or rapid maneuvers, the fish progressively recruits its white muscle tissue, causing the recorded electrical signal to increase in both amplitude and spectral frequency. This progressive recruitment follows the same principle governing muscle recruitment in other vertebrates—Henneman's principle—whereby slower muscle fibers with lower activation thresholds (the red muscle fibers) are recruited first, and the fast white muscle fibers are engaged only when the propulsive force demand exceeds the output capacity of the red muscle alone.

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