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Powering Up Paralyzed Muscles - Functional Electrical Stimulation
Nur Azah Hamzaid
PhD, Senior Lecturer, Department of Biomedical Engineering, Faculty of Engineering,
University of Malaya, 50603 Kuala Lumpur, Malaysia

Everything in your body, when injured, can be healed or repaired, except your spinal cord”. Imagine the cable that connects your brain to your muscles is broken – either cut or damaged from an accident - your brain won’t be able to send through its signals to and from the muscles below the lesion thus no movement can happen, at the very least. This condition is usually permanent, as the injured spinal cord cannot be healed, at least for now. There are other complications that come along with spinal cord injury (SCI), but the most obvious are paralysis, with different degrees of sensory and movement ability.

As paralysed muscles require signals from the brain to make contractions, that ability is lost with paralysis. In the long run, the muscles will be atrophied, in other words they shrink and become weak. Thus it is important for the muscles fibres to be kept active so that it retains the mass, endurance and strength. Keeping the muscles working, especially the lower limb muscles, ensures greater physical activity can be achieved, which in turn maintains whole body blood circulation, heart fitness, and potentially the bone strength (1). The best output, even if the mentioned benefits are minimal, is having a healthy psychosocial outlook amongst the spinal injured participants, as the general comment given was “… knew your muscles actually move, which you cannot possibly achieve otherwise, is a great feeling by itself, let alone the fitness benefits that comes with it”.

How is this artificial signal being conveyed to the muscles? In its simplest form, not from the brain, but through an external controller via a technology known as Functional Electrical Stimulation (FES). FES-evoked exercise and other activities involving artificial stimulation of the muscles started in the 1960s when researchers started to contract and move paralysed muscles through electrical stimulation to perform standing and upright stepping. FES exercise systems are now being integrated into their rehabilitation to optimize training (2). A pair of electrode is placed on the skin surface at opposite ends of the muscle, so that when current is sent as signal, the current will flow through the muscle fibres as if it were signals from the brain. This electrical activity would evoke the actin and myosin activity of the muscles, producing real contractions which can produce force and power as normal contractions would.

However, as it is not originated from the brain, the characteristics of the signal might not be optimally similar with the natural signal and pattern our brain would send, thus the duration and smoothness of the contraction would be less than natural contractions. Well-conditioned paralyzed leg muscles that perform evoked cycling could only normally produce significant force and power for about 30 minutes at maximum allowable current of 140mA before the muscles get fatigue and no longer contracts, even with electrical stimulation. This is the main problem of electrical stimulation that hinders the achievement of its great potential benefits to SCI individuals.

searchers have investigated the optimal electrode placement and current stimulation parameters – its frequency, amplitude, duty cycle, monophasic or biphasic current, and the like. They also studied the duration and frequency of training that would deliver optimum benefits to the SCI users. Nevertheless, the gained end benefit so far is still not significant enough for a daily activity level, as the muscles still gets fatigue very easily and quickly. Some group of researchers also performed implanted FES where the electrodes are inserted through the skin directly at the nerve or muscle fibres, to increase specificity and potentially minimize the fatigue effect. This method is more promising, with better functional outcomes such as stepping, walking and grasping.

However, some SCI users prefer to have FES as an optional treatment for muscle and body conditioning only, thus might shy away from implanted FES and are more comfortable with surface stimulation and regular series of evoked FES based training such as cycling exercise. Three main muscles of the legs, i.e. the quadriceps, hamstrings and in some studies, the gluteal are the most commonly reported to being stimulated.

Cycling based FES activity comes in a variety of nature, and has developed over the years. An indoor gym-based ergometer developed by Fornusek and colleagues of the University of Sydney, Australia allows SCI users to perform FES cycling with power performance indicator calculated in real time (3). Of outdoor nature, one example is the Berkelbike, developed in the Netherlands, which allows hand and leg cycling with built-in electrode cables for the leg muscles and encoders at the bicycle crank. Virtual reality (VR) has also been embedded into an FES-based cycling system to allow indoor activity with an “outdoorsy” feel, complete with performance indicators such as power, duration, and slope degree amongst others. A study on VR based FES cycling amongst SCI users were jointly conducted in the University of Sydney and the University of Malaya, Malaysia. These tackle primarily the ‘motivational’ factor amongst its users amongst other reasons.

Another innovation introduced by Glen Davis and Che Fornusek is to use an isokinetic mode of cycling, where the speed of cycling is set constant (3). This enables assisted movement even though the muscles are still very weak and unable to produce movement by itself, which also allows a good range of motion exercise for the leg joints. At the same time, if the muscles are stronger and able to push hard against the foot pedals, isokinetic mode provides instantaneous resistance adaptive to the leg muscle strength, as it tries to keep the rotational speed constant at any point in time. This method of training is especially useful in low speed, or cadence, as the gain in muscle strength and muscle mass is significantly greater at speed 20 rpm and lower. Also, it is worth to note that cycling at lower speed prominently builds the muscles as compared to higher speed which contributes better to heart and lungs training.

Apart from modification and optimization of stimulation current parameters, frequency and duration of exercise, and the mode of cycling, one other dimension to add to the variable is movement pattern. We have established that an elliptical stepping pattern, in a seated body position suitable for SCI individuals who cannot bear their body weight, would enhance the cardiorespiratory responses and muscle power production when compared to the normal ‘circular’ cycling pattern (4). This is due to the longer forward linear component of the motion, which allowed for effective, or should I say - efficient - quadriceps contractions. In normal cycling, the path is just too short for the muscle contractions to contribute. In short, the elliptical path provided greater dose-potency in the domain of FES-evoked cycling effectiveness.

Nevertheless, while all those factors would enhance the biomechanics and physiological responses of the SCI users, independently or in combination with each other, a crucial element that makes the system more ‘human’ is the feedback component. As in how our body works, our body always knows how much it has done, or the current state of itself in any domain of its working, in order to provide consequent signalling so that the final action is achieved or stability is maintained. The same principle is found in a common robotics or mechatronics system, where feedback is always present to provide optimal feedback control. In the case of FES, to date the most common mode of feedback reported is the electromyogram signal, or EMG, of the muscles (5). But you see, FES is electrical based, and so is EMG, and they are of the same muscle – they would ‘clash’ if simply used together concurrently. Of course, with proper signal processing these two signals can easily be split, and a lot of information can be extracted, especially muscle fatigue profile which is highly sought after to optimize performance.

dicator, simply the product of force production and speed, as the feedback parameter (6). In our developed FES ergometer the force and power are read from the end effector, which is through the foot pedals. This method provides a quantitative performance measure, and eliminates the need for extra wiring at the stimulated muscles – FES cables are messy enough for a fast, simple, day-to-day exercise activity. It also works on a primary domain other than electricity, thus careful signal processing and sensory attachment is less crucial. One possible drawback is the readings are at the end effector, and does not ‘look at’ or ‘reads’ the muscles directly, where the actual fatigue originates. One might think that to minimize fatigue, monitoring the power production might be too late or too far from the source of the problem and you cannot pinpoint which particular muscle requires attention most in terms of appropriate signal adjustment.

If there are other ways of directly monitoring the muscle contraction, without having the problems mentioned before, we would have an alternative feedback parameter to overcome or minimize fatigue optimally. When successful, the FES training device and the SCI user would be one complete system working in sync. The major problem of externally-stimulated muscle fatigue would be significantly delayed and the SCI person can gain hopefully maximum benefits of surface FES-evoked activity.

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