Surface mechanomyogram reflects muscle fibres twitches summation

Area of Science:

• Biomechanics
• Neuroscience
• Physiology

Background:

• The mechanomyogram (MMG) reflects muscle activity through mechanical vibrations.
• Understanding the summation patterns of muscle fiber twitches is crucial for accurate MMG interpretation.
• Previous models often assume linear summation, which may not hold true under all physiological conditions.

Purpose of the Study:

• To investigate the summation patterns of muscle fiber twitches in the generation of the surface mechanomyogram (MMG).
• To determine the linearity of MMG signal summation under varying stimulation frequencies.
• To identify non-linearities in MMG generation during simultaneous stimulation of different motor units.

Main Methods:

• Stimulated two distinct motor unit (MU) groups (MU1 and MU2) in the extensor digitorum communis (EDC) muscle using needle electrodes.
• Applied separate and simultaneous stimulation protocols at frequencies ranging from 3 Hz to 20 Hz.
• Analyzed mechanomyograms (MMGs) using bispectrum and bicoherence to detect quadratic non-linearities.

Main Results:

• Linear summation of mechanomyograms (MMGs) was observed when individual motor units (MUs) were stimulated separately.
• Differences in bispectrum and bicoherence were detected between generated (MMGg) and simultaneously recorded (MMGs) signals specifically at higher stimulation frequencies (9 Hz MU1 + 20 Hz MU2).
• The study found that MMG generation is linear only at very low firing rates.

Conclusions:

• Mechanomyogram (MMG) signals are generated by the summation of active muscle fiber twitches.
• The summation process is linear only for very low firing rates of muscle fibers.
• Non-linearities become apparent at higher stimulation frequencies, which must be considered in future MMG modeling studies.