D25 Muscle Activity (EMG)

Topic

This electromyography records the final electrical activity of the athlete's muscle as the sum of the action potentials from each active motor unit within that muscle. Consequently, a surface electrode captures the electrical signal from the athlete's superficial muscles—specifically the signal associated with the muscle model in question—at a considerably high sampling frequency.

Standard processing of this muscle electrical signal involves full-wave rectification, followed by band-pass filtering to retain only the relevant frequency band of the muscle activity, and finally, smoothing of the filtered signal using a sliding window; together, these steps transform the raw, noisy electrical signal into a much cleaner, more usable final envelope.

This final muscle signal is normalized against the muscle's maximum voluntary contraction, allowing for a direct comparison of muscle activity between two different subjects or between two different muscles in the same subject.

The onset of muscle activation—referred to as the signal's "onset"—determines the muscle's anticipatory time prior to a given impact; this is conceptually similar to the electromechanical reaction time used in other models within this framework to assess neuromuscular control deficits. Co-activation quantifies the simultaneous activation of an agonist muscle and its antagonist—the same co-activation addressed in other models regarding energy dissipation within a kinetic chain. If the onset of muscle activation occurs later relative to the impact, the muscle's anticipatory time is consequently shortened. Consequently, given the initial muscle electrical activity, the resulting relationship between the signal and the force produced by the muscle is approximately linear during an isometric contraction—even though this relationship varies with both muscle velocity and length, mirroring the Hill curve established by another model in this set regarding muscle force and velocity. Based on this raw electrical signal, the analysis determines the muscle's final activation level relative to its maximum voluntary contraction, as well as the onset and duration of activation, co-activation with the antagonist muscle, and the temporal pattern of the muscle activation that initiated the model.

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