Parameters of FES for Denervated Muscle

So far we have outline the niche market that FES serves, as well as the degenerative muscle changes that result from denervation and how electrically induced isometrics contractions can help to prevent/slow and help reverse the severity of the dysfunctional tissue. Now we can look at more specifically at parameters used for FES for successful applicaiton.

To overcome the degenerative changes and produce a desirable contraction of a selected muscle/muscle groups (eg. peroneals), a slow-rising electrical pulse is required (Low & Reed, 2000). This slow rising pulse, also known as selective triangular or accommodation pulses, allows the stimulation to accumulate in the denervated muscle while innervated structures accommodate to the slow rising pulse (Kern et al., 2010). Interestingly, a square pulse was originally used for denervated muscle stimulation with long pulse durations of 30ms or more, before investigations found a slow rise time to be more effective.

FES provides a means of selectively stimulating different tissues (Low & Reed, 2000). Research has found that to stimulate muscle fibre but not nerve requires triangular pulses with a leading edge time of 50ms and pulse of 100ms. To stimulate denervated muscle fibre only, a rise time of longer than 100ms is best, with triangular pulses of 300 – 500ms (Low & Reed, 2000). Further research supports the use of long duration pulses that take a long time to reach peak amplitude and also highlighted in their findings that higher amplitudes are more effective as more current is needed (Cakmak, 2004 & Kern et al., 2010).

Comparison of the different parameters for nerve/innervated tissue vs. denervated muscle:

·         Nerve (innervated muscle) stimulation favours square pulses that reaches peak amplitude rapidly.

·         Denervated muscle stimulation prefers long duration, high amplitude electrical pulse that takes a long time to reach peak amplitude.

Figure 3. Comparison of square wave pulses (innervated muscle) and triangular wave pulses (denervated muscle).

Figure 4. Electrode Placement.

Electrode Placement

Optimal positioning of the stimulating electrodes is at each end of the muscle belly, allowing for the charge to travel down the long axis of the muscle fibers and thus an effective contraction. Implanted FES devises are also available, where muscle or nerve-based electrodes are installed surgically and connected to an implanted stimulation device, so no material crosses the skin. Implanted electrodes offer numerous advantages over surface and percutaneous stimulation for long-term clinical use, including improved convenience, cosmesis, reliability, and repeatability, where surface electrodes are more ideal for short-term therapeutic applications (Hardin, et al., 2007).

Figure 4. illustrates the the placement of the elctrodes on the antereior lateral shank. A) represents the electronic stimulator. B) and C) are surface electrode placed over the motor point of the peroneal nerve and the tibialis anterior muscle belly. D) is a sensory insole located within the shoe on the heel to detect impact.

Cakmak, A. (2004). Electrical Stimulation of Denervated Muscles. Disability and Rehabilitation26(7): 432-433

Hardin, E., Kobetic, R., Murray, L., Corado-Ahmed, M., Pinault, G., Sakai, J., Bailey, S.N., Ho, C. & Triolo, R.J. (2007). Walking after incomplete spinal cord injury using an implanted FES system: a case report. Journal of rehabilitation research and development. 44(3), 333-46.

Low, J., & Reed, A. (2000). Electrotherapy explained: principles and practice. Oxford: Butterworth-Heinemann.