What You Will Learn
This chapter provides the essential anatomy and physiology you need to understand why FES cycling works. You do not need a medical background to follow it. We explain how your muscles contract, the difference between upper and lower motor neurons and why that distinction matters, how electrical stimulation triggers a muscle contraction, and why your muscles may fatigue quickly when you first start FES cycling. By the end, you will have a clearer picture of what is happening inside your body when you use an FES cycling system.
The Motor Unit: How Your Muscles Contract
Every voluntary movement you make, from lifting a cup to pedalling a bike, obviously starts with your intention to create the movement. This manifests as a 'signal' of intention from your brain. That signal travels down the spinal cord and ultimately along a nerve to reach the muscles. When it arrives, the muscle fibres contract, producing movement. Although this was simple to describe, it covers a lot of complexity.
The basic building block of this system is the motor unit. A motor unit consists of a single nerve cell (called a motor neuron) and the group of muscle fibres it controls. When the motor neuron 'fires', all the muscle fibres in its motor unit contract together. A large muscle, such as the quadriceps in your thigh, contains thousands of motor units, and the nervous system coordinates them to produce smooth, controlled movement.
These signals, as referred to above, propagate along nerve fibres of which there are different types with different functions. Signals can flow from the brain and towards the brain. For example, the so-called alpha motor neurons innervate muscle fibres and generate force in the muscle. These have the largest diameter but are still only 12 to 20 micrometres. All we need to know for now is that various motor and sensory neurons are necessary for normal function.
Think of it like a set of light switches controlling the lights in a building. Each switch (motor neuron) might control a bank of lights (muscle fibres). To illuminate the whole building, the system turns switches on and off in a coordinated pattern. The more switches that are turned on, the brighter the output. Similarly, recruiting more motor units produces a stronger muscle contraction.
In normal circumstances, the brain manages this process with remarkable precision. It recruits motor units in a specific order, adjusts their firing rate, and coordinates different muscle groups so that movement is smooth and efficient. When injury or disease disrupts the connection between the brain and the muscles, this coordination is lost or at least disturbed. The electrical stimulation provided via FES cycling offers an external way to switch those motor units back on and make muscles contract in a useful way. It's not as efficient as the body's natural method of generating muscle contractions, but it is effective enough to produce an effective exercise effect.
Our companion resource on denervated muscle, or the clinical companion to this guide, covers the motor unit and related topics in greater detail for those who wish to explore this further.
Muscle fibre types: the mixed mosaic
Not all muscle fibres are the same. Human skeletal muscle contains a mixture of fibre types, broadly classified into three categories.
Type I (slow oxidative) fibres are designed for endurance. They contract relatively slowly, resist fatigue, and rely on aerobic metabolism. These are the fibres that maintain posture and sustain low-intensity activity over long periods. They appear red under the microscope because of their rich blood supply and high myoglobin content.
Type IIa (fast oxidative-glycolytic) fibres represent an intermediate type. They offer a useful compromise between endurance and power.
Type IIx (fast glycolytic) fibres are built for power. They contract quickly and forcefully but fatigue rapidly, relying primarily on anaerobic glycolysis. They appear pale, white muscle, in everyday terms.
In healthy, innervated muscle, these fibre types are distributed in a mixed pattern we could describe as a mosaic. This mosaic shows that fibres from different motor units are physically intermingled throughout the muscle. Each motor unit consists of a single fibre type, but the units themselves are scattered, producing the characteristic mixed appearance.
This distribution matters because it gives the muscle versatility. A healthy quadriceps can sustain posture, absorb shock during walking, and produce rapid force during a stumble recovery, all because the mix of fibre types is maintained by normal neural activity.
Upper and Lower Motor Neurons: Why the Distinction Matters
When clinicians discuss neurological injuries, you will hear the terms "upper motor neuron" and "lower motor neuron." Understanding the difference between these two types is important because it determines whether FES cycling can work for you.
The nervous system can be viewed as a two-stage relay. As we learned above, the upper motor neurons transmit signals from the brain down the spinal cord, from where they branch out at various levels of the spine. The lower motor neurons then carry those signals from the spinal cord out to the muscles themselves. You can think of the upper motor neurons as the major 'motorway' connecting the brain to the different levels of the spine. The lower motor neurons are like the local roads linking the spine to each individual muscle.
When the upper motor neurons are damaged, as in most spinal cord injuries, many strokes, and conditions such as multiple sclerosis, signals from the brain can no longer reach the muscles to cause them to contract. However, the lower motor neurons are still intact. The muscles themselves are still capable of contracting; they simply are not receiving the instruction to do so.
This is why FES cycling works. The electrical stimulator bypasses the blocked motorway entirely and delivers its signal directly to the local road, the lower motor neuron. The lower motor neuron then activates the muscle in exactly the same way it would if the signal had come from the brain. The resulting contraction is a genuine, natural muscle contraction.
When the lower motor neurons are damaged, the situation is different. This can happen when the spinal cord is injured quite low in the back, at the lumbar or sacral levels, or when peripheral nerves are directly injured. Here, the local roads themselves are disrupted. The muscle fibres are still present (at least initially), but the nerves that would normally activate them are no longer functioning. In this case, the FES cycling stimulation we use cannot elicit a contraction because there is no intact nerve to stimulate. A different and more specialised form of electrical stimulation is needed, one that works directly on the muscle fibres rather than through the nerve. This is the subject of our companion resource on denervated muscle.
The consequences of these two types of damage also look and feel quite different. Upper motor neuron damage can typically cause muscles to feel stiff and tight, with increased reflexes and spasticity. Lower motor neuron damage produces muscles that are floppy and soft, with reduced or absent reflexes, and that waste away more rapidly. Some people, particularly those with injuries at certain spinal levels, may have a mixture of both.
A clinical assessment, including testing how your muscles respond to electrical stimulation, will determine which type of nerve involvement you have and, therefore, which approach is appropriate.
How Electrical Stimulation Activates Your Muscles
The process by which electrical stimulation produces a muscle contraction is straightforward in principle. We're using a method that is quite broadly applicable in electrotherapy and medicine. We are applying energy to the body in the hope that it produces a physiological effect, which in turn results in a therapeutic benefit.
Imagine we have placed two electrodes on a thigh muscle and are using an FES unit. When we use FES, we are 'pushing' electrical energy into the body through one electrode and out through the other. As this pulse of energy passes through the tissues beneath the skin, it reaches the lower motor neuron and triggers it to fire. In fact, our nerves are always in a state ready to fire. The stimulation energy, if sufficiently intense, creates an 'action potential' that propagates along the nerve fibres, triggering muscle contraction.
The important point is that the resulting muscle contraction is identical to a natural one. The stimulator is not doing anything the body does not already know how to do. It is simply providing the trigger that the brain can no longer deliver.
The strength and character of the contraction can be controlled by adjusting several settings on the stimulator. Most of the stimulators used for this application produce what are called biphasic rectangular pulses. This is a fancy way of saying that each pulse consists of a positive-going and a negative-going part. These pulses can be varied in their height, width, and frequency.
Most stimulator designs are so-called current-controlled, meaning it is designed to control the amount of current that flows into the muscle, which corresponds to the height of each pulse in mA. The pulse width (how long each pulse lasts, typically in microseconds) and the frequency (how many pulses are delivered per second) are also controlled. Mostly, the positive and negative phases will be the same height. This reduces the likelihood of skin irritation.
By changing the stimulation settings, your clinician can fine-tune the muscle's contraction strength, smoothness, and fatigue rate. You do not need to understand the technical details of these parameters, but it is useful to know that your FES cycling programme can be adjusted precisely to suit your needs and to change over time as your muscles respond to training.
Why 'Standard Stimulation' Works Here (and When It Does Not)
As we have explained, FES cycling works by stimulating the motor nerves, which in turn activate the muscle. This is the same mechanism used in virtually all standard electrical stimulation devices, whether for cycling, muscle strengthening, or functional movement. Various terms are used to describe the different applications of electrical stimulation, but there is no universally accepted terminology at the moment. We've been using the term FES to reflect functional electrical stimulation. You might have heard the term "Tens" as short for transcutaneous electrical nerve stimulation (a technique for localised pain relief) or NMES (neuromuscular electrical stimulation). Fundamentally, these all have particular stimulation parameters that characterise them, such as frequency, pulse width, and waveform shape. All that matters here is that we understand that changing the stimulation parameters changes the resulting therapeutic effect.
Where the lower motor neurons themselves are damaged, the standard approach will not produce a contraction. The muscle fibres are still present, but they need to be activated directly rather than through the nerve. This requires a fundamentally different type of stimulation: much longer pulses, lower frequencies, and higher energy levels than standard FES devices can deliver. Whilst some FES bikes will offer long pulsewidth waveforms, they are not usually suitable for complete denervation. Specialised equipment such as the RISE Stimulator is designed for this purpose.
If you are unsure whether your lower motor neurons are intact, a simple clinical test called a strength-duration assessment can provide the answer. We discuss this in our companion resource on denervated muscle, and your clinician can arrange this as part of your initial assessment.
Muscle Fibre Types and Why They Matter for Endurance and Fatigue
In Chapter 1, we mentioned that muscles tend to fatigue quickly when you first start FES cycling. To understand why, it helps to know a little more about the different types of muscle fibres we described above.
After a spinal cord injury, something important happens to the mix of muscle fibre types. Without regular use, the muscle fibre types undergo a shift. Type II (a and x) fibres come to dominate. Over time, the muscles of the paralysed limbs become dominated by fast, powerful, but highly fatigable fibres. This is one of the main reasons why, when you first start FES cycling, your muscles may only sustain a contraction for a minute or so before they are exhausted.
The good news is that this process can be influenced by training. Regular FES cycling provides the repeated muscle contractions needed to begin reversing this shift. Over weeks and months of consistent use, the proportion of fatigue-resistant Type I fibres can increase, and your muscles will sustain exercise for longer. This is one of the most important reasons to persist with FES cycling, even when early sessions feel frustratingly short.
There is another factor at work, too. Electrical stimulation recruits muscle fibres in a slightly different order than the body's natural system. Naturally, the nervous system tends to recruit the smaller, fatigue-resistant fibres first and only calls on the larger, fatigable fibres when greater force is needed. With electrical stimulation, the larger fibres tend to be recruited earlier because they are connected to larger nerve fibres that respond more readily to an external electrical pulse. This means that during FES cycling, the most fatigable fibres are often the first to be engaged. It is another reason why early fatigue is expected and not a cause for concern.
Chapter Summary
- A motor unit is the basic building block of muscle contraction: one nerve cell controlling a group of muscle fibres.
- Upper motor neurons carry signals from the brain down the spinal cord; lower motor neurons carry signals from the spinal cord to the muscles. FES cycling works by stimulating the lower motor neurons directly, bypassing damage to the upper motor neuron pathway.
- Where the lower motor neurons are themselves damaged (denervation), standard FES cannot produce a contraction, and a different form of stimulation is required.
- Electrical stimulation produces a natural muscle contraction by triggering the nerve to fire. The strength and character of the contraction can be fine-tuned by adjusting the stimulation settings.
- After a spinal cord injury, muscle fibres shift towards fast, fatigable types, which is why muscles tire quickly at first. Regular FES cycling can begin to reverse this shift over time.
In Chapter 3, we examine how FES cycling has developed, from early experiments in the 1960s to the systems available today.