Velocity recovery cycles of human muscle action potentials: repeatability and variability

Velocity recovery cycles (VRCs) of human muscle action potentials have been proposed as a new technique for assessing muscle membrane function in myopathies. This study was undertaken to determine the variability and repeatability of VRC measures such as supernormality, to help guide future clinical use of the method.

To assess repeatability, VRCs with one and two conditioning stimuli were recorded from brachioradialis muscle by direct muscle stimulation in 20 normal volunteers, and the measurements repeated 1 week later. To further assess variability and dependence on electrode separation, age and sex, recordings from an additional 20 normal volunteers were added.

There was a high intraclass correlation between repeated recordings of early supernormality, indicating excellent reliability of this VRC measure. However, relative refractory period had a smaller coefficient of repeatability in relation to the changes previously described during ischemia. We found no evidence that any of the excitability measures depended on electrode separation, conduction time or apparent velocity. There were also no significant differences between the recordings from men and women, or between the recordings from older (mean 44.9 y) and younger (26.5 y) subjects.

VRC measures are sufficiently consistent to be suitable for comparing muscle membrane function both within subjects and between groups. Early supernormality measurements benefit most from within subject comparisons.

These normative data sets provide a firm basis for planning clinical studies.