What is conduction velocity and how is it calculated?

Conduction Velocity: Definition and Calculation

Definition

Conduction velocity is the speed at which action potentials propagate along nerve or muscle fiber membranes, measured in meters per second (m/s). 1

In muscle fibers specifically, conduction velocity represents the rate at which depolarization travels along the sarcolemma, ranging from 2 to 6 m/s in humans. 1 For nerve fibers, the velocity varies by fiber type—myelinated A delta fibers conduct at approximately 9 m/s, while conduction velocity in myelinated fibers is nearly proportional to fiber diameter. 2, 3

Physiological Determinants

The conduction velocity depends on multiple structural and physiological factors:

Passive (Cable) Properties

• Membrane capacitance per unit length (proportional to fiber circumference) 1
• Internal resistance (inversely proportional to the square of fiber diameter) 1
• Fiber diameter—larger diameter fibers conduct faster 1, 3

Active (Membrane Excitability) Components

• Ion gradients across the membrane, particularly sodium and potassium 1
• Ion channel gating properties, influenced by electric field strength 1
• Temperature—higher temperatures increase conduction velocity 1
• pH and calcium concentrations—acidosis and altered calcium levels affect velocity 1, 4

Structural Factors in Myelinated Nerves

• Myelin thickness—for fixed axon diameter, velocity increases with myelin thickness 3
• Internode distance—optimal spacing maximizes conduction velocity 3
• Optimal ratio of axon diameter to total fiber diameter exists for maximum velocity 3

Calculation Methods

Direct Measurement (Time-Domain Method)

The fundamental calculation is: Conduction Velocity = Distance / Time 1

For nerve conduction studies:

1. Stimulate the nerve at a known location using electrical or magnetic stimulation 1
2. Record the compound muscle action potential (CMAP) with surface electrodes 1
3. Measure the latency—time from stimulus trigger to CMAP detection 1
4. Calculate velocity by dividing the distance between stimulation and recording sites by the latency 1

Important caveat: For respiratory motor nerves like the phrenic nerve, true conduction velocity cannot be measured because the nerves are not accessible over sufficient length. 1 Instead, motor latency is measured, which includes time for action potential initiation, saltatory conduction through myelinated axons, slow conduction along terminal twigs, and neuromuscular junction transmission. 1

Surface EMG Cross-Correlation Method

For muscle fiber conduction velocity during voluntary contraction:

1. Place bipolar surface electrodes along the muscle fiber direction with known interelectrode distance 5
2. Record EMG signals from both electrode pairs simultaneously 5
3. Calculate the time lag between the two signals using cross-correlation analysis 1, 5
4. Compute velocity = interelectrode distance / time lag 1, 5

This method provides an average estimate of conduction velocity from many fibers and motor units. 5

Frequency-Domain Analysis

Muscle fiber conduction velocity can be estimated indirectly through power spectral analysis of the EMG signal:

• Power spectrum characteristics are related to membrane conduction velocity 1
• Spectral shifts toward lower frequencies indicate reduced conduction velocity, as seen in muscle fatigue 1
• The center frequency (fc) can be calculated from spectral moments and reflects changes in conduction velocity 1

Clinical Applications and Pitfalls

Standardization Requirements

Conduction velocity measurements must be standardized for firing rate, as velocity increases with stimulation frequency. 6 At 5 Hz stimulation, mean velocity was 3.74 m/s, increasing to 4.80 m/s at 40 Hz—a 31% increase. 6

Factors Affecting Accuracy

• Electrode positioning significantly affects measurements—distance from the motor end-plate zone alters recorded signals 1
• Muscle-to-electrode distance reduces signal amplitude with greater attenuation of high frequencies 1
• Temperature variations must be controlled, as conduction velocity is temperature-dependent 1
• Signal-to-noise ratio, electrode position, and muscle cross-talk can confound frequency-domain estimates 1

Clinical Utility

Surface EMG-based conduction velocity measurement is particularly valuable for assessing muscle fatigue, where localized fatigue causes reduced conduction velocity reflected as spectral shifts. 1 This has been demonstrated during inspiratory resistive breathing and in patients with COPD during exertion. 1

For diagnostic purposes, nerve conduction studies focus on latency measurements rather than true velocity calculations for most respiratory nerves, given anatomical constraints. 1