2.6. Strength-Duration (S-D) Curves

The S-D curve [34] is a traditional test of nerve excitability that is based on the relationship between the intensity of an electric monophasic rectangular pulse and its duration in causing a motor response when a motor nerve is electrically stimulated [35]. The result of this test is a hyperbolic function [35]. For the analysis, the subject lay supine with a pillow under their head. The studied forearm was bare and the skin was shaved and cleaned prior to placement of the electrodes. A blinded and experienced physiotherapist (10 years of experience in classical electro-diagnosis) performed the S-D curves with a constant current stimulator (Myomed 932; Enraf Nonius Co., Rotterdam, The Netherlands). This device is characterized by an intensity of current that can be increased with an accuracy of 0.1 mA from 1 to 15 mA, with an accuracy of 0.5 mA from 15 to 30 mA, and with an accuracy of 1 mA from 30 to 80 mA (maximal intensity). One 24 cm² (6 × 4 cm) ground electrode (anode) was fixed with a Velcro strap on the first third proximal of the forearm, and another 24 cm² (6 × 4 cm) active electrode (cathode) was placed on the second third proximal of the forearm. The first value of S-D curves obtained was the rheobase, with a monophasic rectangular pulse duration of 1000 ms and a between-pulse rest of 1 s. The current intensity was increased until the observation of a minimal visible twitch contraction of the epicondylea musculature (innervated by the radial nerve). Then, the S-D curve was obtained by successively decreasing the durations of the rectangular pulses and observing the current threshold for maintaining a minimal visible twitch contraction of the muscle. When the S-D curve obtained using rectangular pulses was finished, the S-D curve made using triangular pulses was obtained in the same manner. Between the 2 tests, the active electrode was moistened again and replaced in the same position that it was in during the previous curve. The chronaxie and accommodation index (AI) were analyzed [35]. Chronaxie is the impulse time needed to obtain a minimal muscle contraction with a monophasic rectangular impulse and intensity twice the rheobase, measured in milliseconds, being the rheobase the intensity threshold needed to obtain a minimal muscle contraction with a monophasic rectangular impulse and a width of 1000 milliseconds (ms), measured in mA. The normal value of chronaxie is between 0.1 and 1 [35]. The accommodation index is the quotient between the intensity thresholds at 1000 ms triangular and 1000 ms rectangular impulses (adimensional). The normal value of the accommodation index is greater than 3. A minor excitability of the nerve is characterized by values lower than 3 [36].