Motor unit potential recruitment reference values in common upper and lower extremity muscles

Area of Science:

• Neurology
• Electromyography
• Motor Unit Physiology

Background:

• Accurate assessment of motor unit (MU) recruitment is crucial for diagnosing neuromuscular disorders.
• Establishing normative data for MU electrophysiological parameters is essential for reliable clinical interpretation.

Purpose of the Study:

• To establish reference values for motor unit (MU) recruitment during needle electromyography (EMG) in six common muscles.
• To define upper limits of normal for fastest MU firing rates and recruitment ratios at low to moderate contractions.

Main Methods:

• Needle EMG was performed on 111 healthy subjects across six muscles.
• Fastest MU firing rates and recruitment ratios (RRs) were measured at multiple sites within each muscle.
• The 97th percentile was used to determine upper limits of normal for these parameters.

Main Results:

• Upper limits of normal for fastest MU firing rates ranged from 12-15 Hz.
• Slightly higher fastest firing rates were observed in the deltoid and triceps muscles compared to others.
• Recruitment ratios showed variability, but fastest MU firing rates did not correlate with RR variations.

Conclusions:

• Fastest MU firing rates exceeding 15 Hz at multiple sites suggest reduced MU recruitment.
• Rates above 12 Hz in certain muscles may indicate mildly reduced recruitment.
• Defined reference values for fastest MU firing rates enhance the accuracy of identifying reduced MU recruitment.