Strength-duration time constant and rheobase measurements: Comparison of the threshold tracking method and a manual...

To compare the strength-duration time constant (SDTC) and rheobase measurements obtained by the threshold tracking method (TT) and by a non-automated method (MM).

The MM procedure involved measuring, using a routine electrodiagnostic device, the intensity required to evoke a motor response whose amplitude corresponds to 40% of the maximum amplitude for four stimulus duration (1.0, 0.7, 0.5, 0.2 ms), and studying the linear relationship between stimulus charge and stimulus duration (slope = rheobase, intercept on the x-axis = SDTC). Using TT and MM, 30 successive healthy subjects (mean age = 38 years old) underwent a prospective evaluation of SDTC and rheobase of the median nerve motor axons at the wrist. Nerve stimulation and bipolar recording of evoked motor responses were performed with disposable self-adhesive surface electrodes.

The Spearman correlations between the two methods were 0.78 (p < 0.0001) for SDTC and 0.96 (p < 0.0001) for the rheobase. The Bland-Altman analysis did not reveal any systematic bias of MM compared to TT.

The MM procedure was reliable for strength-duration relationship analysis.

We encourage neurophysiologists, who do not have dedicated threshold tracking equipment, not to hesitate to use these simple tools to assess peripheral nerve excitability.