Muscle denervation occurs when the peripheral motor nerve innervation of a muscle is lost. Loss of innervation can occur at the lower motor neuron (LMN) due to trauma, which can lead to irreversible damage or at the upper motor neuron that can result from progressive conditions such as multiple sclerosis or non- progressive conditions such as stroke (Stein, Everaert, Thompson, Chong, Whittaker, Robertson & Kuether, 2010). This blog is going to focus on traumatic injuries to LMN that occur near the spinal level or anywhere along its path to the site of muscle innervation (Kern et al., 2010).
Without a functional nerve supply, muscle can only be stimulated to contract by direct stimulation of the muscle fibre (Kern et al., 2010). Therefore, there are differences between stimulation of muscle via its nerve and direct stimulation of denervated muscle (Low & Reed, 2000). The name of the electrical stimulation used for denervated muscles is called Functional Electrical Stimulation (FES). FES is similar to neuromuscular electrical stimulation (NMES), but has the specific purpose to produce functional movements for those who are limited by denervation (Low & Reed, 2000).
The most common functional deficit experienced in the population of people with muscle denervation is foot drop (Everaert et al., 2010). Foot drop occurs due to damage of the LMN that innervates the dorsiflexor muscles of the ankle, causing a reduction in walking speed, increase in physiological costs of gait and increased risk of falls (Everaert, et al., 2010). Furthermore, a loss of innervation to the flexors of the forearm can be treated with FES to perform grasping actions. There are many components to the application of such a device, which has been shown in empirical research to offer many functional and physiological benefits. However, the application of FES is not widely known due to the niche market it is used to treat. The main aim of this blog is to bring an understanding of its application in relation to the internal physiological changes of denervated muscle and highlight contraindications and precautions when using FES.
Everaert, D. G., Thompson, A. K., Chong, S. L., & Stein, B. R., (2010). Does functional electrical stimulation for foot drop strengthen corticospinal connections. Neurorebilitation and Neural Repair, 24(2), 168 -177.
Stein, R.B., Everaert, D.G., Thompson, A.K., Chong, S.L., Whittaker, M., Robertson, J. & Kuether, G. (2010). Long-Term Therapeutic and Orthotic Effects of a Foot Drop Stimulator on Walking Performance in Progressive and Nonprogressive Neurological Disorders. Neurorehabilitation and Neural Repair 24(2) 152– 167.
Kern, H., Carraro, U., Adami, N., Biral, D., Hofer, C., Forstner, C., Modlin, M., Vogelauer, M., Pond, A., Boncompagni, S., Paolini, C., Mayr, W., Protasi, F. & Zampieri, S. (2010). Home-Based Functional Electrical Stimulation Rescues Permanently Denervated Muscles in Paraplegic Patients With Complete Lower Motor Neuron Lesion. Neurorehabilitation and Neural Repair, 24(8): 709-721
Low, J., & Reed, A. (2000). Electrotherapy explained: principles and practice. Oxford: Butterworth-Heinemann