In the first of a series of blog articles, we are going to look at the factors that impact the effectiveness of FES. This one covers depolarisation and neural recruitment.
Depolarisation is when a nerve undergoes a shift in electric charge distribution and is essential to the function of many cells within the body. Depolarisation within the nerve generates, with sufficient stimulus to achieve threshold, an action potential. This takes place physiologically via the opening and closing of ion channels in response to changes in ligand-gated ion channels or voltage-gated ion channels.
Electrical stimulation causes activation of the voltage-gated ion channels, causing entry of sodium ions into the nerve tissue, resulting in a less negative charge within, until threshold is reached, and an action potential is generated.
It is important to recognise the difference between physiological and FES-evoked muscle activity. Physiologically normal muscle activity is efficient and based upon need. It follows the “size-principle” in that small diameter muscle fibres (Type I) are activated first. As the demand increases, larger diameter muscle fibres are recruited to increase force.
FES evoked muscle activity is not efficient, being involuntary, and uses randomisation of the “size-principle”. As a result, whichever nerves fibres receive sufficient stimulus to reach threshold are recruited, which in practice results in large diameter motor neurons, which recruit fast twitch muscle fibres being activated most readily, but these tire quickly!
Therefore, it is important that you do not stop the stimulation increasing at the very first moment you see a visual muscle response to ensure that sufficient current is applied to activate as many muscle fibres as possible.
This article is taken from our white paper “The integration of Functional Electrical Stimulation (FES) technology and neurorehabilitation”.