Motor Nerve Conduction Velocity Calculation
Motor nerve conduction velocity is calculated by dividing the distance between two stimulation sites by the difference between proximal and distal latencies: (Distance between stimulation sites) ÷ (Proximal latency - Distal latency). 1
Fundamental Calculation Method
The American Academy of Neurology states that motor nerve studies require stimulating the nerve at two anatomically defined points and recording compound muscle action potentials (CMAPs) at each site. 1
The correct formula is: Conduction Velocity = (Proximal distance - Distal distance) ÷ (Proximal latency - Distal latency), where the numerator represents the distance between the two stimulation sites and the denominator represents the time difference for the action potential to travel that distance. 1
Latency is defined as the time from stimulus delivery to CMAP onset, measured separately at each stimulation site (distal and proximal). 1
Technical Requirements for Accurate Measurement
Supramaximal stimulation must be applied at both the distal point (e.g., wrist for median nerve) and proximal point to ensure activation of all motor axons and produce reproducible CMAP amplitudes. 1
The American Association of Neuromuscular and Electrodiagnostic Medicine recommends placing surface electrodes over the target muscle, with proper alignment along muscle fiber direction at sites with low motor end-plate density. 1
Temperature control is essential, as the American College of Sports Medicine notes that conduction velocity is temperature-dependent, with higher temperatures increasing velocity. 1
Common Pitfalls to Avoid
Do not confuse latency with velocity—latency measures time alone (in milliseconds), whereas velocity requires both the distance between stimulation points and the latency difference. 1
Electrode positioning and muscle-to-electrode distance significantly affect measurements, potentially reducing signal amplitude and attenuating high frequencies. 1
Side-to-side anatomical differences can affect measurements; for example, the right phrenic nerve is often slightly shorter than the left, resulting in marginally shorter latencies. 1
Clinical Context and Normal Values
In healthy adults, motor nerve conduction velocities typically range from 50-70 m/s depending on the nerve tested, with accessory nerve motor conduction averaging 67.98 m/s. 2
The American Academy of Neurology reports that peroneal motor nerve conduction velocity typically ranges 38-41 m/s in patients with peripheral neuropathies such as CIDP. 3
Human skeletal muscle fiber conduction velocity (2-6 m/s) is distinct from and much slower than nerve conduction velocity. 1