The science behind the design

In Tomcat’s early days, I was asked by an influential Speech and Language therapist, to re-design a wheelchair she’d designed with a very unusual seat.  It exactly resembled a horses saddle with the addition of a back support and it was striking if not very elegant.

I asked her why the unusual seat, to which she replied, ‘Do you know what happens to someone who lolls in a wheelchair, when they are put on the back of a horse?’  I confessed that I did not!

‘They sit up much better!  They are more balanced and alert, and are far more spatially aware!’

‘How does that work?’ says I.

‘It’s all to do with the somatosensory system.  When the neurological pathways are impaired by brain damage for example, there is less information than normal reaching the brain about balance among other things, so people with brain damage will tend to seek out a reliable support and lean against it before switching off.  However, when they find themselves on a live animal that is moving predictably and unpredictably, concern about their safety and vulnerability inevitably kick in and they start to pay much more attention, with balance, alertness and spatial awareness all improving.

I was starting to get very interested.

‘As well as the movement stimulation, the value of being on the horse is that a horses saddle creates an enormous amount of flexing body contact with the thighs and buttocks that greatly intensifies the amount of information reaching the brain, and with more information to process, the brain performs better.  Thus you see a different person on a horse as opposed to a conventional seat’.

I thought this was fascinating, groundbreaking stuff, and though the re-design never materialised for lack of sponsorship, I never forgot the insight into stimulation of the nervous system I’d received from this remarkable lady, and looked for ways to bring her ideas into my own work with trikes.

Willing Guinea Pigs

At that time, Carer Control was turning the industry on its head, and more disabled kids than I could cope with was coming to Tomcat for help, so I had a plethora of willing guinea pigs to experiment with.  I experimented with hard as opposed to soft support cushions and where to position them on the body to have the most effect, and had to conclude at the end of it that I was able to prove my Speech and Language therapist right in everything she said.  I could, to a very large degree, both predict, modify, and in most cases, correct the (lateral) sitting posture of the kids that came my way.  This was particularly true of hemiplegia, which I found particularly interesting as the improvements that can be achieved are more obvious.

Getting balance and posture under control.

My first conclusion was that lateral support spacing must be such that they contact both sides of the body, because if not, the rider will lean towards one support or the other.  With hemiplegia that will always be to the weak side.  It’s obvious when you think about it, but all too often, carers and manufacturers alike, leave a gap that results in poor posture.

The next important consideration is the construction of the lateral pad and its size.  Size is obviously dictated to a degree by the riders physique, but the firmness of the pad is crucial and within the manufacturers control. Too hard and the pad will not make global contact with the body like the saddle, but too soft will not be sufficiently supportive.  For that reason, we always use upholstered cushion pads on our laterals, which though more costly and difficult to make, do give good support together with the comfort and contact area I was looking for.  This factor is another significant posture improver.

Lastly with laterals is their position in relation to the body.  By definition, hemiplegia is not a symmetrical condition and lateral supports cannot be symmetrical either, if good, upright posture is to be achieved.  As a rule of thumb, raising the position of the lateral on the weak side and lowering it on the strong side will correct or at least improve the leaning tendency.  As a result, I designed the tomcat laterals to have significant vertical adjustment as well as lateral adjustment.   If you have a Tomcat, you will be able to try these things for yourself, and you will notice that as you incrementally increase the vertical offset between the pads, so will the spine become more upright.

With significant weakness, I would expect the pad on the weak side to be in contact with the lower or mid thorax with the strong side pad in the waist, but on the hip.  In exceptional cases, improvement can also result from offsetting the laterals laterally, and as you would expect, the lateral on the weak side may need to be positioned closer to the centreline and the strong side further away.

It has always been a great reward to me to begin with a rider with terrible posture at the start, then by making progressive tweaks, bring them upright!  Importantly, you have not just given them support, but you’ve given them confidence in their safety too, and as soon as they stop thinking safety, and start thinking fun and movement they will be off like a rocket.

Getting Propulsion under control

That brings me to the pedalling issues which are also very interesting.

If you think of a pedal as the minute hand of a clock, the effective power stroke is from about one ‘o’ clock to five ‘o’ clock.  Then comes the backward drag to seven ‘o’ clock where the other foot takes over and returns the first foot to ‘one’ so the process can repeat.  The problem with hemiplegia is that there may be little or no help coming from the affected foot, so the better leg may have to do the work of both, but it still only has that ‘one’ till ‘five’ window to provide the propulsion needed to complete a full circle.

So what moves the trike from ‘five’ to ‘one’ when nothing is pushing the pedals?  Clearly it is inertia.  Think of inertia as a flywheel, storing up energy on the power stroke to release it during the dead stroke.  When a trike and rider are in motion, they, too, are like a flywheel, releasing energy to carry the trike forward from one power stroke to another.  It stands to reason, therefore, that the more energy and inertia you can put into the moving trike on that single stroke, the further it will travel and the more likely the rider will be to complete the circle.

Obviously, the faster the trike can be made to move, the further it is likely to travel, thus two trike design factors are crucial to success with hemiplegia.

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

When you have a sensibly high drive ratio, propelling a lightweight trike with good wheel bearings, it will be far easier for the hemiplegic rider to produce sufficient acceleration to keep the trike moving forward to the next push of the pedal.

We often hear some very nice comments at Tomcat about how certain children can ride a Tomcat but nothing else, and that is very rewarding to hear, but it’s engineering not rocket science.

What to look for in a trike.

In an industry where weight is often ignored and gear ratios are standardised, it is little wonder that success is sometimes hit and miss.  Think of it this way: if you struggle to lift a heavy weight yourself, how can a rider with physiological impairment be expected to propel it?

Riders with hemiplegia can, and do, succeed with a trike, but they are a very special case and their unique abilities and difficulties need to be understood and allowed for.  I have only touched on the basic principles in this blog, but if you have any other questions I will certainly try to answer them.  One thing you can be sure of, however, is that success with hemiplegia – as with so many complex conditions – is achieved by design and understanding, never by good fortune.