The Science Behind the Design

Hemiplegia affects movement and balance on one side of the body, which can make riding a conventional bicycle extremely difficult. With the right design, however, a trike can provide stability, improved posture and the ability to pedal more effectively.

In Tomcat’s early days, I was asked by an influential Speech and Language Therapist to redesign a wheelchair she had developed with a very unusual seat. It closely resembled a horse’s saddle, with the addition of a back support. It was striking, if not particularly elegant.

I asked her why she had chosen such an unusual seat.

She replied:

“Do you know what happens to someone who lolls in a wheelchair when they are put on the back of a horse?

They sit up much better. They are more balanced and alert, and far more spatially aware.

It’s all to do with the somatosensory system. When neurological pathways are impaired by brain damage, for example, there is less information than normal reaching the brain about balance and body position. As a result, people will often seek a reliable support and lean against it before effectively switching off.

However, when they find themselves on a live animal that moves both predictably and unpredictably, concern about safety and vulnerability inevitably kicks in. They begin to pay far more attention, and their balance, alertness and spatial awareness all improve.

As well as the movement stimulation, the value of being on a horse is that the saddle creates an enormous amount of flexing body contact with the thighs and buttocks. This greatly intensifies the amount of information reaching the brain. With more information to process, the brain performs better. Thus you see a very different person on a horse compared with someone sitting in a conventional seat.”

I found this fascinating and groundbreaking. Although the redesign never materialised due to lack of sponsorship, I never forgot the insight I had gained into stimulation of the nervous system. From that moment on, I began looking for ways to bring those ideas into my own work with trikes.

Experimenting With Posture and Support

At the time, Carer Control was turning the industry on its head and more disabled children than I could cope with were coming to Tomcat for help. This provided a wealth of opportunity to experiment and learn.

I tried hard and soft support cushions, and experimented with where to position them on the body to have the greatest effect.

By the end of it, I had to conclude that the Speech and Language Therapist had been absolutely right.

With careful positioning of supports, I was able to predict, modify and in most cases correct the lateral sitting posture of many of the children who came my way. This was particularly noticeable with hemiplegia, where improvements in posture can often be very clear.

Improving Balance and Posture

My first conclusion was that lateral supports must contact both sides of the body.

If there is a gap, the rider will inevitably lean toward one support or the other. With hemiplegia, that lean will always be toward the weaker side. It sounds obvious when you think about it, yet carers and manufacturers alike often leave a gap that results in poor posture.

The next important consideration is the construction and firmness of the lateral pads.

Pad size will naturally depend on the rider’s physique, but firmness is entirely within the manufacturer’s control.

• If the pad is too hard, it cannot make global contact with the body.
• If it is too soft, it will not provide adequate support.

For this reason, we use upholstered cushion pads on our laterals. They are more costly and difficult to manufacture, but they provide both good support and a large contact area, which greatly improves posture.

Finally, there is the position of the lateral supports.

Hemiplegia is not a symmetrical condition, so the supports cannot be symmetrical either if upright posture is to be achieved.

A useful rule of thumb is:

• Raise the lateral support on the weak side
• Lower the support on the strong side

This helps correct the natural leaning tendency.

For that reason, Tomcat laterals were designed with significant vertical as well as lateral adjustment.

If you have a Tomcat trike, you can experiment with this yourself. As you gradually increase the vertical offset between the pads, you will often see the spine become more upright.

With significant weakness, the pad on the weak side may contact the lower or mid-thorax, while the strong side pad sits lower at the waist or hip.

In exceptional cases, improvement can also come from offsetting the laterals sideways, bringing the weak side pad closer to the centreline and positioning the strong side pad slightly further away.

It has always been enormously rewarding to begin with a rider who has very poor posture and, through careful adjustments, gradually bring them upright!

More importantly, you have not simply given them support, you have given them confidence in their safety. Once a rider stops worrying about stability and begins thinking about movement and fun, they are often off like a rocket.

Managing Pedalling With Hemiplegia

That brings us to the pedalling issues, which are equally interesting.

Think of a pedal as the minute hand of a clock.

The effective power stroke occurs between about one o’clock and five o’clock.

After that comes the backward drag to around seven o’clock, where the other foot takes over and returns the pedal to one o’clock so the cycle can repeat.

With hemiplegia, the problem is that the affected leg may contribute very little power, or none at all. This means the stronger leg may need to do the work of both.

However, it still only has that one-to-five window to generate the power needed to complete the full rotation.

So what moves the pedal from five o’clock back to one when nothing is pushing? The answer is inertia.

You can think of inertia as a flywheel. Energy created during the power stroke is stored and then released during the dead stroke.

When a trike and rider are in motion, they act like a flywheel themselves. Energy generated during the power stroke helps carry the trike forward until the next push of the pedal.

The more energy and inertia generated in that single power stroke, the further the trike will travel and the more likely the rider is to complete the full pedal cycle.

Why Weight and Gearing Matter

Two design factors become crucial for riders with hemiplegia:

• The trike must be as light as possible
• The drive ratio must be as high as possible

With a suitably high drive ratio and a lightweight trike running on good wheel bearings, it becomes much easier for the rider to generate enough acceleration during that single power stroke to keep the trike moving forward.

We often hear wonderful comments from families telling us that their child can ride a Tomcat but cannot ride anything else.

That is very rewarding to hear, but the explanation is quite simple. It is engineering, not rocket science.