What are the differences between mechanomyography (MMG), accelerography, and electromyography (EMG)?

Comparison of Mechanomyography, Acceleromyography, and Electromyography

Electromyography (EMG) measures the electrical activity of muscle fibers and is the most accurate method for assessing neuromuscular function, while mechanomyography (MMG) measures actual force production and serves as the gold standard for neuromuscular blockade monitoring, and acceleromyography (AMG) measures thumb acceleration but frequently overestimates recovery and produces unreliable values.

Fundamental Differences in Measurement Principles

Electromyography (EMG)

• EMG detects electrical manifestations of action potentials propagating along muscle fiber membranes using electrodes that amplify, filter, and display myoelectric signals 1
• The technique assesses the integrity of the respiratory neuromotor apparatus by evaluating electrical activation from brainstem respiratory neurons through motor nerves, neuromuscular junctions, and muscle membranes 1
• EMG provides information about motor function of nerve roots and spinal cord, detecting both spontaneous (mechanically elicited) and evoked electrical activity 1
• Single fiber action potentials in humans have conduction velocities ranging from 2 to 6 m/second, influenced by fiber diameter, temperature, electrolyte gradients, pH, and fatigue 1

Mechanomyography (MMG)

• MMG is considered the laboratory gold standard for measuring neuromuscular blockade because it directly measures the mechanical force of muscle contraction 2
• The technique quantifies actual muscle performance and preserves muscular neurophysiologic information while assessing function non-invasively 3
• MMG is rarely used in clinical practice despite its gold standard status because it is not commercially available and requires complex setup 2

Acceleromyography (AMG)

• AMG is the most commonly used monitor in clinical settings, measuring thumb acceleration in response to nerve stimulation 2
• This method frequently produces train-of-four (TOF) ratio values greater than 1.0 (occurring in 23% of measurements), which is physiologically impossible and complicates clinical interpretation 2

Clinical Performance Comparisons

Agreement Between Methods for Neuromuscular Blockade Assessment

When comparing TOF ratios, EMG most closely resembles MMG with a mean difference of only 4.7% (95% limits of agreement: -25.2 to 34.6), while AMG shows substantially worse agreement with both MMG and EMG 2:

• AMG vs. EMG mean difference: 14.9% (95% limits of agreement: -13.0 to 42.8) 2
• AMG vs. MMG mean difference: 9.8% (95% limits of agreement: -31.8 to 51.3) 2

Critical Overestimation Problem with Acceleromyography

AMG consistently overestimates neuromuscular recovery compared to EMG, creating a dangerous clinical scenario where residual paralysis may be missed 4:

• When AMG shows a TOF ratio of 0.72, the simultaneously measured EMG value averages only 0.59 (P < 0.001) 4
• When AMG indicates a TOF ratio of 0.90 (traditionally considered adequate recovery), EMG averages only 0.85 4
• Individual patient differences between AMG and EMG can be twice the mean difference, with 95% confidence limits showing very wide variability 4

Posttetanic Count Monitoring Discrepancies

For deep neuromuscular blockade monitoring, AMG and EMG show poor agreement with only 73% concordance in identifying the same blockade status (intense vs. deep block), with a Cohen's kappa of only 0.26 5:

• AMG frequently counts more twitches than EMG, with the mean posttetanic count on EMG being 38% lower than AMG (95% CI: 20-51%, P = 0.0002) 5
• In only 23% of measurements did AMG and EMG record the same posttetanic count 5
• AMG counted more twitches than EMG in 50% of paired measurements 5

Train-of-Four Count Accuracy

When comparing TOF counts to palpation as a reference, EMG shows substantial agreement (kappa = 0.80), while AMG shows only moderate agreement (kappa = 0.63) 6:

• AMG underestimated TOF count in 36% of measurements compared to palpation, with only 3% overestimation 6
• When comparing AMG to EMG directly, agreement was only fair (kappa = 0.38), with AMG underestimating count in 39% of measurements 6

Clinical Applications and Advantages

Electromyography Strengths

• EMG is optimal for analyzing action potentials to assess myopathic changes and comparing motor unit firing frequencies across different respiratory muscles and clinical conditions 1
• The technique helps determine timing and level of muscle activation, identifies which respiratory muscles are active during various respiratory phases, and can diagnose paralysis of specific muscles 1
• EMG coupled with mechanical function tests assesses electromechanical effectiveness of muscle contractile function 1
• Needle EMG of the diaphragm is useful for diagnosing neuromuscular diseases, particularly neuropathic processes like Guillain-Barré syndrome, spinal cord injury, and polyneuropathy of critical illness 1, 7

Mechanomyography Strengths

• MMG has gained significant acceptance in analyzing both isometric and dynamic muscle actions 3
• As the mechanical counterpart to EMG, MMG provides complementary information about actual force production 3
• MMG is effective for assessing muscle performance under both voluntary and evoked muscle contraction 3

Acceleromyography Practical Considerations

• Despite its limitations, AMG remains the most commonly used clinical monitor because it is commercially available and easy to use 2
• However, AMG values less than 0.90 are indicative of incomplete neuromuscular recovery, and clinicians must be aware that AMG overestimates recovery 4

Critical Clinical Pitfalls

Interchangeability Warning

AMG and EMG values cannot be used interchangeably to assess neuromuscular blockade degree, and assuming equivalence creates patient safety risks 5, 4:

• The wide confidence intervals mean individual patients may show dramatically different values between methods 4
• AMG's tendency to overestimate recovery means patients may be extubated with inadequate neuromuscular function 4

Recovery Threshold Differences

With EMG, once the TOF ratio returns to 0.70, first twitch (T1) has returned to 95% of control, but with AMG, return of T1 to 95% of control requires a TOF ratio of almost 0.90 4:

• This fundamental difference means recovery criteria must be method-specific 4
• Using AMG-derived thresholds with EMG monitoring, or vice versa, will lead to incorrect clinical decisions 4

Technical Factors Affecting EMG

EMG signals are influenced by electrode placement, muscle-to-electrode distance, chest wall configuration changes, and muscle temperature 1:

• Cross-talk from adjacent muscles is less problematic with implanted electrodes than surface electrodes but is not completely eliminated 1
• Changes in chest wall configuration systematically affect amplitude and frequency content of evoked compound muscle action potentials with surface or esophageal electrodes 1
• Muscle temperature increases during exercise affect EMG frequency content because propagation velocity correlates with temperature, but no correction method has been described for respiratory muscles 1