Introduction
Various factors of electrical stimulation, such as stimulation frequency, pulse amplitude and width, have been associated with specific neuromuscular responses in paralyzed muscle. Less is known regarding the effect of varying the duration of the interphase interval, between the positive and negative pulses, of an alternating monophasic waveform pattern. It was the purpose of this preliminary study to examine the strength and endurance of electrically stimulated paralyzed muscle using two different interphase intervals (25ms & 40,000ms).
Methods
Four men with complete paraplegia (T3-T11) participated in isokinetic muscle testing and endurance testing of the knee extensors using the programmable Exostim FES system (Neopraxis, AU). Testing was performed with low interphase interval of 25 ms (LPI), and high interphase interval of 40,000 ms (HII) with both conditions applying stimulation frequency 25pps, amplitude of 140ma, and pulse width 200 µs.
The two conditions were both assessed on each of two different days in randomized order, with the mean of the two days serving as the outcome value for each condition. Isokinetic strength testing was performed using a Biodex II system with movement velocity of 60 degrees per second. Endurance was operationally defined as the duration of knee extension (>165degrees) with 3-sec recovery periods at 15-sec intervals.
Results
The results of this assessment series
indicated that the LII produced 8 – 22% more peak torque in the four subjects
while the HII provided considerably more endurance (67 – 144% greater time to
fatigue). These findings suggest that an extended interphase interval may
provide a significantly greater resistance to muscle fatigue and thereby
increase functional capacity during