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Comparison of Maximum Tolerated Muscle Torques Produced by 2 Pulse Durations

W.B. Scott, PT, PhD, is Assistant Professor, Department of Physical Therapy, East Carolina University, Mail Stop 668, Greenville, NC 27858 (USA).
J.B. Causey, PT, DPT, was a student in the Doctor of Physical Therapy program, Department of Physical Therapy, East Carolina University, at the time of the study.
T.L. Marshall, PT, DPT, was a student in the Doctor of Physical Therapy program, Department of Physical Therapy, East Carolina University, at the time of the study.

scottw{at}ecu.edu

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 maximally 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.

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