1. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Matching initial torque with different stimulation parameters influences skeletal muscle fatigue C. Scott Bickel, PT, PhD; Chris M. Gregory, PT, PhD; Andres Azuero, PhD

2. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Aim – Investigate effect of stimulation frequency, pulse duration, and voltage on skeletal muscle fatigue during repeated contractions started at same relative torque. Relevance – Muscle fatigue is barrier to routine use of neuromuscular electrical stimulation for functional activities. – Systematic study of relative contributions of stimulation frequency, pulse duration, and voltage to muscle fatigue has been limited.

3. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Methods Measured torque in quadriceps of 13 subjects tested on 2 separate occasions using 3 fatigue protocols (initial starting force = 25% maximum voluntary isometric force): – Low frequency (lowHz). – Low pulse duration (lowPD). – Low voltage (lowV). 48 h later, subjects rated quadriceps soreness on Present Pain Intensity (visual analog scale from Short-Form McGill Pain Questionnaire).

4. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Results Relative starting torque for each protocol: – lowHz = 25.7 ± 0.06. – lowPD = 25.5 ± 0.06. – lowV = 25.3 ± 0.03. Fatigue – lowHz significantly less fatigue than lowPD and lowV. – lowPD and lowV not significantly different. Muscle Soreness – lowHz significantly less soreness than protocols that used 60 Hz.

5. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Results Normalized torque values. Obtained during fatigue test (60 contractions; 1 s on, 1 s off) under 3 conditions: ● = low frequency (lowHz), ○ = low pulse duration (lowPD), and ▼ = low voltage (lowV).

6. This article and any supplementary material should be cited as follows: Bickel CS, Gregory CM, Azuero A. Matching initial torque with different stimulation parameters influences skeletal muscle fatigue. J Rehabil Res Dev. 2012;49(2):323–32. http://dx.doi.org/10.1682/JRRD.2011.02.0030 Slideshow Project DOI:10.1682/JRRD.2011.02.0030.jsp Conclusions Muscle fatigue is a primary limitation to widespread use of electrical stimulation. Frequency is key regulator of muscle fatigue, but other parameters can enhance muscle torque output without increased fatigue. Future protocols should combine: – Low frequencies. – Longer pulse durations. – Higher voltages.