Advancing femoral nerve stimulation into the stage of science.

TRANSCUTANEOUS ELECTRICAL and, later, magnetic twitch stimulation of the femoral nerve have been introduced to quantify quadriceps muscle function. These techniques were developed in an attempt to overcome the drawbacks of measurements of maximal voluntary contraction force and tetanic stimulations. Maximal voluntary contractions are motivation-dependent maneuvers that can be marred by submaximal activation (1); tetanic stimulations are painful (6), and this is particularly problematic when assessing the elderly and frail. It has been proposed that twitch stimulations of the femoral nerve are suitable for the quantification of quadriceps muscle dysfunction, if present, and for the quantification of quadriceps muscle fatigue. Using paired stimuli, twitch stimulation of the femoral nerve may allow quantifying the relative contribution of low and high frequencies to force output. Single and paired stimuli may also allow quantification of induction and recovery of contractile fatigue. It has also been reasoned that paired stimuli can improve the resolution of twitch interpolation, a technique used to quantify extent of voluntary recruitment and central fatigue (1, 7, 10). Magnetic twitch stimulation of the femoral nerve and its electric counterpart (the less user friendly, less reproducible and the more painful of the two) have been used in many studies (2). Surprisingly, systematic comparison of these two techniques against each other and against a reference method is not available. In this issue of the Journal of Applied Physiology, Vergès and colleagues (11) fill in this important gap in our technical knowledge. For the first time, they painstakingly assess the reliability of magnetic twitch stimulation of the femoral nerve against electrically evoked twitches. In addition, they compare the reliability of magnetic and electrical twitch stimulation against transcutaneous tetanic stimulations of the femoral nerve and of the quadriceps muscle. The reference technique against which all comparisons were performed was the submaximal transcutaneous stimulation of the quadriceps. The assessment was performed in healthy subjects both in the unfatigued and fatigued states. With this investigation, Vergès and colleagues (11) make an important contribution to this challenging field. For the first time they show that single and paired magnetic twitch stimulation of the femoral nerve provide assessment of quadriceps function equivalent to that provided with single and paired electrical twitch stimulation. They also report that the ratio of total twitch force elicited by paired stimulation (electrical or magnetic) at 10 and 100 Hz (Fpaired10:100) provides similar information to the standard lowto high-frequency force ratio elicited by submaximal electrical tetanic stimulation of the muscle (Ftet10:100). Contrary to their expectations, and in a departure from what has been reported for in vitro preparations of rat diaphragm (12), the ratio of the second twitch elicited by paired stimulation (electrical or magnetic) at 10 and 100 Hz (T210:100) was only weakly associated with the Ftet10:100 ratio. The investigators reason that, for the human quadriceps in vivo, the fatigue-associated change in Fpaired10:100 ratio is a better index of lowto high-frequency fatigue than the fatigueassociated change of the T210:100 ratio. Although technical in nature, this investigation raises some interesting biological questions. First, in addition to muscle length, mass, fatigue, and intensity of nerve stimulation, what other factors determine the amplitude of single or paired twitches? A portion of the contraction elicited by nerve stimulation could arise from a central mechanism through an evoked orthodromic sensory volley (3). In leg muscles, evoked sensory volleys require pulse widths lasting more than 0.05 ms (see Figs. 3A and B in Ref. 3), frequency of stimulation greater than 25 Hz (see Fig. 3C and D in Ref. 3), and trains of stimulation lasting more than 1.5–5 s (see Fig. 2 in Ref. 3). The duration of (supramaximal) magnetic twitch stimulation in the protocols of Vergès and colleagues (11) was 0.1 ms, and only paired twitches were used during electrical and magnetic twitch stimulation. Therefore, any central contribution to contractions evoked by (single and paired) twitch stimulations is unlikely to be operative in this study. Second, does coactivation of muscles, other than the quadriceps, determine the amplitude of twitch force elicited by magnetic stimulation? Magnetic stimulation produces a wider field of stimulation than electrical stimulation. When performing magnetic stimulation of the phrenic nerves, particularly with the posterior cervical approach (5), the wider field of stimulation causes coactivation of extradiaphragmatic respiratory muscles (5). In contrast, Vergès and colleagues (11) report nearly absent coactivation of (antagonistic) muscles during magnetic stimulation. Nearly absent coactivation of antagonistic muscles (and supramaximality of the stimulus) during magnetic stimulation underscore the mechanism responsible for the equivalent twitch force and M-wave characteristics with the two twitch stimulating techniques. Third, the investigators report exercise-associated decrease in the area and duration of the M wave, and a (nonsignificant) decrease in M-wave amplitude. These findings substantially confirm the observations of Piitulaien and colleagues (8), who recorded similar changes in M-wave parameters following eccentric exercise on the elbow flexors. Whether decreases in M-wave area, duration, and amplitude purport impaired sarcolemmal excitability, itself due to disturbed ion distribution and/or transport over the plasmalemma, remains to be determined (8). In addition, whether the decreased M-wave duration could also be related to increased muscle fiber conduction velocity secondary to a higher intramuscular temperature remains unknown. Address for reprint requests and other correspondence: Edward Hines, Jr., Div. of Pulmonary and Critical Care Medicine, VA Hospital, 111N, 5th Ave. and Roosevelt Road, Hines, Illinois 60141 (e-mail: [email protected]). J Appl Physiol 106: 356–357, 2009; doi:10.1152/japplphysiol.91155.2008.