Motor Nerve Conduction Velocity Formula
Motor nerve conduction velocity (MNCV) is calculated by dividing the distance between two stimulation points by the difference in latency between those points: MNCV = Distance / Time. 1
The Standard Calculation Method
The fundamental formula for calculating motor nerve conduction velocity is straightforward:
- MNCV (m/s) = Distance (mm) / Latency difference (ms) 1
- The nerve is stimulated at two known locations along its course, and compound muscle action potentials (CMAPs) are recorded with surface electrodes placed over the target muscle 2
- The latency (time from stimulus to CMAP onset) is measured at each stimulation site 2
- The distance between the two stimulation points is measured on the skin surface 3
- The conduction velocity is then calculated by dividing this distance by the difference in latencies between the proximal and distal stimulation sites 4
Practical Application
When performing nerve conduction studies:
- Stimulate the nerve supramaximally at a distal point (e.g., wrist for median nerve) and record the distal latency 2
- Stimulate at a proximal point (e.g., elbow) and record the proximal latency 3
- Measure the distance between stimulation sites along the nerve's anatomical course 3
- Calculate velocity using the formula: Distance / (Proximal latency - Distal latency) 1, 4
Critical Technical Considerations
Several factors must be controlled to ensure accurate measurements:
- Temperature significantly affects conduction velocity - the limb must be warmed to at least 32-34°C, as cooler temperatures artificially slow conduction 1
- Electrode positioning matters - electrodes must be properly aligned with muscle fiber direction and placed at sites with low motor end plate density 2
- Distance measurement accuracy is essential - measure along the actual nerve course, not in a straight line 3
- Supramaximal stimulation ensures all motor axons are activated, providing reproducible CMAPs 2
Common Pitfalls to Avoid
- Do not confuse latency with conduction velocity - latency is the time measurement, while velocity requires both distance and time 2, 1
- Avoid measuring over joints where the nerve course is variable or difficult to measure accurately 3
- Account for side-to-side differences - the right nerve may be slightly shorter than the left (e.g., right phrenic nerve), resulting in shorter latencies 2
- Recognize that conventional MNCV measures only the fastest conducting fibers, not the full distribution of fiber velocities 5
Normal Values
For reference, motor nerve conduction velocities in adults typically range:
- Upper extremity motor nerves: 50-70 m/s 6
- Lower extremity motor nerves: 40-60 m/s 4
- Phrenic nerve latencies: 6-8 ms (though this is latency, not velocity) 2
- Accessory nerve: 67.98 m/s average (range 56.95-79.01 m/s) 3
Human muscle fiber conduction velocity ranges from 2 to 6 m/s, which is distinct from nerve conduction velocity 2