The efficiency of FES Cycling
In this article, we look at the efficiency of FES Cycling and see that there is actually lots of room for improvement when it comes to metabolic efficiency and power output.
Of course that does not mean that they should not be used. Much research over decades shows the benefits for users from regular training. FES Cycling performance is good enough to produce results if people use these products. The benefits are greater than those via passive cycling alone.
However, any engineer who looks closely at the technology will realise that there is much that can be improved when it coms to FES cycling exercise. This is what engineering aims to do - refine and improve things for the benefit of society. Improving the technology could produce a greater health benefit for the same or less effort so it's worth striving for. It should also be said that this is no trivial challenge. All electro-mechanical systems that interact with the human body tend to offer design difficulties that are not obvious to the casual observer.
FES Cycling's pedigree
Cycling utilising functional electrical stimulation (FES) was first demonstrated over 40 years ago. The intention then as now, was to allow persons with paralysed or weak muscles to power an exercise bike even though they could not achieve this by conscious means.
RehaMove 2 based on the MOTOMed Muvi
This technology achieved a training effect by repeatedly triggering a periodic muscle contraction in suitable muscle groups. Muscles that contract can move limbs, generate work and power and we have long understood that functional electrical stimulation can trigger such stimulation patterns. Users training with an FES bike could exercise with their own paralysed muscles and increase their strength and fitness over time.
If you imagine for a moment how you pedal any bike, you will realise that you have to generate power with the right muscles at the right time.
A normal bike proved hard to power from a standstill with FES applied to the muscles. It's a question of timing the muscle contractions. You would need to get the pedals in the correct position to produce an effect.
The early developers solved this problem by using a passive/active bike. This is one with a motor built in that would move the pedals and hence the legs at a particular speed. Sensors could then be used to determine the position of the pedals moment by moment and a computer could decide when to trigger the muscle group activations.
By using a passive/active bike and sensing the instantaneous position and velocity of the pedals, the fes cycle stimulator could synchronise the contraction of the quadriceps, hamstrings and other muscle groups to power the bike. The system could be smart enough to recognise when power was being generated and the motor would then not need to contribute any power.
Extensive research on this technique of FES cycle training revealed a number of clinical benefits following spinal cord injury, stroke and other neurological conditions. We, and others, have written many articles on these benefits. Today, commercial FES Cycle options such as the Hasomed RehaMove system are available around the world. The cost of these systems unfortunately does not allow all who could benefit to use them and indeed many of our clients are home-based users who have been able to justify the use of these following a catastrophic injury.
Efficiency and power
The developers of these FES Cycling systems have long realised that the efficiency and power output is typically low. One of the researchers with a long interest in this area is Ken Hunt (1) who described two contributory factors to relatively poor efficiency:-
Unfavourable biomechanics
Non-physiological recruitment of muscle-fibres
Hunt and colleagues provide a detailed discussion and review of this topic which is complex. Unfavourable biomechanics stems from a number of factors including the relatively crude way in which skeletal muscle groups are recruited and errors due to the timing of muscle activation and how the user is positioned relative to the bike.
Crank position input to controller
The FES Controller senses the position of the pedals at each instant and uses this information to decide which muscles to stimulate. Control stategies need to take account of how muscle contraction builds during stimulation and how this is affected by pedalling speed. Fatigue and the user’s body position relative to the bike can influence the efficiency and power generated.
In FES Cycling systems that are using the pedal crank position as an input to the control system decisions have to made about when to stimulate the muscle groups. If this is slightly out - perhaps because the user’s position is not optimal - then the cycling effort will be inefficient. In order to get smooth and efficient cycling, the timing of muscle activation will likely need to change as the speed of cycling varies.
In the RehaMove 2 system we can accept the default angles for typical leg cycling muscles or we can choose to vary these based on what we see happening. In a sense, the operator becomes part of the control loop. Researchers are exploring alternative strategies that would automatically adjust the activation angles based on some optimisation approach. Optimisation approaches can also potentially help with the effects of muscle fatigue.
There is also the effect of fatigue and how muscle fibres are recruited with the pattern of FES. We know that muscles activated by FES do not behave in the same way as muscles contracted by conscious, voluntary effort. They tend to fatigue faster when subjected to FES for example.
Muscles contract when subjected to patterns of energy sufficient to generate muscle fibre contractions. For a contraction to be strong enough to pedal a bike, sufficient muscle fibres need to be activated and fibres cannot be part on/part off. They are either on or not and exhibit different properties based on their fibre type. The classification of muscle unit type is often described as being based on the speed of contraction and the resistance to fatigue. Many clinicians recognise this as muscle fibre types I, IIa and IIb. FES activated muscles use a lot more glycogen in type I and type II fibres compared with muscles activated normally.
Overall, research shows that the metabolic efficiency of FES Cycling is at best one half of the efficiency of volitional cycling.
What we know from experience in working with hundreds of clients is pretty obvious really - and it’s that paying attention to the actual trainin gprotocols being used. The duration, frequency and intensity of effort made over a period of time will produce results and improve efficiency.
Engineers know that varying the stimulation waveform shape, and the frequency, pulse width and current intensity can have an impact on power generation, muscle fatigue and efficiency. We just need control systems that can modulate these parameters automatically and perhaps future generations of products can deliver these benefits.
Further reading
Hunt KJ, Fang J, Saengsuwan J, Grob M, Laubacher M. On the efficiency of FES cycling: a framework and systematic review. Technol Health Care. 2012;20(5):395-422. doi: 10.3233/THC-2012-0689. PMID: 23079945.