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Background. Neuromuscular electrical stimulation (NMES) is an effective therapeutic technique for strengthening weak muscles. A positive dose-response relationship exists between the elicited muscle forces during training and strength (force-generating capacity) gains. Patient discomfort limits NMES muscle forces, potentially compromising efficacy. Objective. The purpose of this study was to compare the NMES muscle torques produced by stimulation trains consisting of 2 different pulse durations. Design. During a single testing session, the 2 pulse duration conditions (50 and 200 microseconds) were tested on the opposite lower extremities of the participants. Methods. The study participants were 10 adults without remarkable medical histories. The maximum tolerated isometric knee extensor torque was the primary dependent variable. The peak currents and phase charges that produced the maxi-many tolerated torques, as well as the sensory, motor, and pain thresholds for the 2 pulse conditions, were compared. Results. The 200-microsecond pulse duration condition resulted in participants tolerating significantly greater muscle torques; it was associated with significantly greater phase charges but significantly lower peak currents. Limitations. This study only compared muscle torques in response to stimulation trains consisting of pulses with short (50-microsecond) or medium (200-microsecond) durations and did not examine long (~400- to 600-microsecond) durations. Furthermore, the result of this study may not apply to NMES that uses stimulation patterns other than monophasic, square-wave pulsed current. Conclusions. It has been suggested that short pulse durations are most appropriate for NMES because they are less likely to recruit nociceptors. The results of this study, however, support the use of a medium pulse duration rather than a short pulse duration when the goal is to produce a maximum torque response from a muscle. These observations may be related to the currents and phase charges for the pain thresholds for the 2 pulse duration conditions., Neuromuscular electrical stimulation (NMES) is an effective therapeutic technique for strengthening weak muscles in patient populations. (1-4) Furthermore, a positive dose-response relationship exists between NMES-elicited muscle forces during training and [...]

Background and Purpose. Following spinal cord injury (SCI), paralyzed muscles undergo physiological changes that alter their force responses to electrical stimulation. The purpose of this study was to investigate the effects of SCI on the contractile properties and force-frequency relationship (FFR) of the paralyzed human quadriceps femoris muscle of adolescents and young adults. Subjects. Thirteen subjects (11 male, 2 female; age range=11-24 years) with motor complete SCIs and 13 matched control subjects (11 male, 2 female; age range=9-23 years) without SCI participated in the study. Methods. Both groups of subjects underwent the same testing protocol using similar equipment. Results. The paralyzed muscles of the subjects with SCI produced 62% of the peak twitch force and had a fatigue ratio that was 65% of that of the control subjects. The paralyzed muscles contracted 14% and 25% faster and relaxed 38% and 46% faster than the nonparalyzed muscles in nonfatigued and fatigued conditions, respectively. Compared with the control subjects, the subjects with SCI had twitch-to-tetanus ratios that were 84% and 127% greater in nonfatigued and fatigued conditions, respectively. Relative to the control subjects, the FFR of the subjects with SCI was shifted to the left in the fatigued condition. Relative to their respective nonfatigued conditions, the FFR of both groups of subjects shifted to the right with fatigue. Discussion and Conclusion. These findings may have important implications for designing stimulation strategies to reduce the rapid fatigue that limits the clinical efficacy of functional electrical stimulation. [Scott WB, Lee SCK, Johnston TE, et al. Contractile properties and the force-frequency relationship of the paralyzed human quadriceps femoris muscle. Phys Ther. 2006;86:788-799.] Key Words: Electrical stimulation, Muscle fatigue, Muscle performance, Muscle weakness, Muscular atrophy, Paralysis, Spinal cord injuries., Electrical stimulation of paralyzed skeletal muscle can be used to assist people with spinal cord injury (SCI) in performance of functional movements. (1) This therapeutic intervention is called functional electrical [...]