Superimposed single impulse and pulse train electrical stimulation: A quantitative assessment during submaximal isometric knee extension in young, healthy men.

Superimposed electrical stimulation techniques can be used to detect central activation failure (CAF), defined as incomplete central nervous system recruitment, suboptimal activation of motor units, or both. The purpose of this study was to evaluate superimposed electrical stimulation techniques to be used to detect CAF during isometric knee extension. We performed three sets of experiments and compared the torque increments from transcutaneous electrical stimulation with: (i) single impulses of different amplitudes (100 V, 150 V, and 200 V) and a pulse train of 100 Hz (100 V, 100 ms); (ii) pulse trains (100 Hz, 100 V) of different lengths (100 ms, 200 ms, and 300 ms); and (iii) pulse trains (100 Hz, 100 ms) of different amplitudes (50 V, 100 V, 150 V, and 200 V). Stimulation was evaluated at submaximal (80% of MVC) isometric knee extension in 24 healthy young men using a Biodex isokinetic dynamometer. Electrodes were placed over the rectus femoris muscle and all stimulation impulses were monophasic, rectangular waves of 0.2-ms duration. Pulse train stimulation at 100 V always elicited a torque increment, whereas single impulse stimulation, even at 200 V, only caused a torque increment in about half of the trials. For each subject, the pulse train generated a significantly larger torque increment than for any of the three single impulses. There was no significant difference in torque increment between the three pulse trains of different lengths. Pulse trains at 150 V and 200 V generated significantly larger torque increments than at 50 V and 100 V. High-frequency maximal train stimulation may thus improve the detection of CAF during isometric knee extension. Detection of CAF may be important in the clinical assessment of muscle weakness, investigating the mechanisms underlying muscle weakness, and evaluating potential therapeutic strategies.