In the second of a series of blog articles, we are going to look at the factors that impact the effectiveness of FES. This one covers frequency and amplitude.
The effect of frequency
Frequency is the number of pulses delivered each second, measured in Hertz (Hz). At low frequencies (2-10Hz) twitch contractions occur as the individual muscle fibres are allowed to return to their resting length between each pulse. As the frequency increases, the contraction becomes gradually more sustained as the fibres are unable to return to their resting length between pulses until full tetany is achieved. Frequency of the pulses has been studied extensively because of its important role in determining the torque development and controlling muscle fatigue (Gorgey and Dudley, 2008).
Increasing the frequency results in increased torque production but concurrently accelerates muscle fatigue. Also, increasing the frequency from 25 to 100 Hz has been shown to increase the evoked torque without increasing the size of the cross-sectional area (CSA) that was activated. However, muscle fatigue may limit how much a further increase in frequency can further increase torque output. (Gorgey and Dudley 2008).
The effect of amplitude
The effect that current amplitude has on evoked torque and activated muscle CSA has been previously investigated. Adams et al (1993), showed that increasing the current amplitude in a manner to increase stimulation from 25% to 75% of maximum voluntary isometric torque (MVIT) increased the percentage of knee extensor muscle group activated from 18% to 54%.
Gorgey and Dudley (2008) demonstrated that increasing the current amplitude resulted in a proportional increase in the torque produced and the size of the activated CSA of the stimulated muscle and cite work by Levangie and Norkin (2005) stating that the rate of muscle strength recovery following anterior cruciate ligament injuries has been associated with the stimulation at higher percentage of MVIT.
However, increasing current amplitude to maximise torque output and to produce clinically meaningful percentage of MVIT was limited by participants’ tolerance to the stimulation, and not all healthy participants were able to tolerate such a high level of stimulation. Therefore, a patient’s pain tolerance to electrical stimulation is considered another limitation to the process of maximising torque output.
Furthermore, work by Doucet, Lam and Griffin (2012) states that when amplitude is turned too high, it can limit central nervous system (CNS) involvement, which may limit rehabilitation effects by using only peripherally mediated impulses in those with intact CNS input.
This article is taken from our white paper “The integration of Functional Electrical Stimulation (FES) technology and neurorehabilitation”.