Relationships between surface EMG variables and motor unit firing rates

Area of Science:

• Neuroscience
• Biomedical Engineering
• Kinesiology

Background:

• Surface electromyography (sEMG) is a common non-invasive method to assess muscle electrical activity.
• The physiological interpretation of sEMG signals, particularly their relationship with underlying motor unit behavior, remains a subject of scientific inquiry.
• Understanding the link between sEMG parameters and motor unit firing rates (MUFR) is crucial for accurate clinical and research applications.

Purpose of the Study:

• To investigate the relationship between motor unit firing rates (MUFR) and the root mean square (RMS) amplitude and mean power frequency (MPF) of the surface electromyography (sEMG) signal.
• To determine if MUFR can be accurately predicted from sEMG signal characteristics in the biceps brachii muscle.

Main Methods:

• Simultaneous indwelling and surface electromyography (sEMG) recordings were obtained from the biceps brachii during maximal isometric elbow flexion in eleven subjects.
• Motor unit firing rates (MUFR) were derived from indwelling recordings, while RMS amplitude and MPF were calculated from sEMG signals over 500 ms epochs.
• Statistical analyses, including R-squared values, were performed to assess the strength of the relationships between MUFR and sEMG parameters across group means, all trials, and within individual subjects.

Main Results:

• A strong relationship (r² = 0.91) was observed between MUFR and RMS amplitude at the group mean level.
• However, the relationship between MUFR and MPF was weak (r² = 0.20) at the group mean level.
• When analyzing all trials or within individual subjects, the relationships between MUFR and both RMS amplitude (r² = 0.19 and mean r² = 0.13) and MPF (r² = 0.0037 and mean r² = 0.040) were significantly weaker, indicating poor predictive power.

Conclusions:

• The findings suggest that motor unit firing rates (MUFR) cannot be reliably predicted from the RMS amplitude or mean power frequency (MPF) of the surface electromyography (sEMG) signal.
• The strong group mean relationship between MUFR and RMS amplitude may be an artifact of averaging and does not translate to individual trial or subject predictions.
• This highlights limitations in using sEMG signal characteristics alone to infer detailed motor unit behavior, emphasizing the need for cautious interpretation of sEMG data.