In the world of manual wheelchairs, there are two different and in certain respects, incompatible concepts for creating wheelchair user stability. These two differing ways leads to many disagreements between the proponents, each thinking that their way is better. The concepts are Static Stability and Dynamic Stability, neither of which is better nor worse. They achieve different practical goals. Their individual usage depends upon the physicality of the wheelchair user and the specifics of how the wheelchair is used in the environment.
Stability is the state of being stable. It is steadiness. A stable object is in a state of balance or is acted on by an equilibrium of forces. It is the opposite of instability. For example, despite moving at high speed through space, the Earth has a high degree of stability. When there is an earthquake, a section of the Earth becomes temporarily unstable as irregular forces are applied in that area.
Static Stability is based on the concept of using physical support to make the person stable. Dynamic Stability is based on the concept of using dynamic support and controlled movement to enable the person to be stable.
Stability for a wheelchair user means that (1) the person’s body is stable where it remains in a upright seated position and doesn’t fall forward, backward or to the side, and (2) the wheelchair itself is stable by not tipping forward, flipping over backward, or falling over to the side.
When it comes to Static Stability, if the wheelchair loses stability so does the user. In this case, the wheelchair user is relying primarily on his/her wheelchair for stability. Therefore, he is dependent upon his wheelchair’s stability for his own stability. On the other hand, Dynamic Stability is created by the active interplay between the wheelchair user and his (or her) wheelchair. Dynamic Stability depends upon the wheelchair user controlling stability/instability through physical actions.
The primary method of providing a user with Static Stability is to setup his/her wheelchair in a manner such that it is least likely to tip over forward, flip over backwards, and fall over sideways. These methods limit the degrees of freedom in which the wheelchair can move (fall over). In fact, the greatest degree of Static Stability is created when the person’s wheelchair is completely tied down as is done to wheelchairs on public transportation such as buses. The goal of Static Stability is to eliminate unwanted movement, which as a practical matter, translates to limiting wanted movement too.
Users of Dynamic Stability such as bikers, skiers, and skateboarders depend primarily upon controlled movement to keep from falling. They use their motion, inertia and the force of friction and gravity to create stability. In many situations, going too slow tends to reduce their stability and increases their risk of falling. Dynamic Stability is created by the system which consists of the person and his/her mobility device. These two entities become one. They work seamlessly together to create stability for the overall system.
Static Stability on the other hand is a dependent relationship. The user depends wholly upon his/her wheelchair to provide the desired stability. When the person’s wheelchair becomes unstable due to forces acting on it, so does the user.
The inherent contrasts between the concepts of Static Stability and Dynamic Stability create incompatible methodologies and opinions among wheelchair users, medical professionals, and wheelchair manufacturers. The root of the problem is that the majority of the people involved don’t understand that these two types of stability must be viewed as separate concepts that must work in cooperation with each other. Overall system stability is created by using different degrees of both Static Stability and Dynamic Stability as determined by the user’s unique circumstances. Learning from Jet Fighters and Adaptive Skiing
A jet fighter is the epitome of the high performance result of combining Static and Dynamic Stability. The pilot is strapped tightly to his (or her) seat (Static Stability). Regardless of whether the plane is sitting on the runway (Static Stability) or inverted in flight (Dynamic Stability), his harness keeps his body properly aligned to control his aircraft. If the pilot loses control of his body position in the cockpit, he will likely lose control of his aircraft. When in flight, the airplane is dependent upon the physical principles of movement (Dynamic Stability) to maneuver through the air. If the airplane travels too slow, it will stall and crash to the ground. After the airplane has landed, it will be tied down (Static Stability) on the tarmac to keep it from moving when acted upon by the force of the wind. The pilot and his aircraft are an integrated system that relies upon using the concepts of both Static and Dynamic Stability to create its high level of performance.
Adaptive skiing is another prime example of high level performance that combines static and dynamic stability. This high level is seen both terms of the absolute performance of the top tier athletes and also by the wide inclusion and participation of people with varied disabilities. Generally speaking, the monoski relies more on Dynamic Stability and less on Static Stability. In comparison, the bi-ski relies on more Static Stability and less Dynamic Stability. A single leg amputee skier (three tracker) capitalizes on more Dynamic Stability than a monoskier. Typically, greater absolute skiing performance comes with the use of more Dynamic Stability.
For adaptive sitskiers, Static Stability serves the purpose of enabling the skier to engage in the Dynamic Stability skiing movements. In order for the skier to control his (or her) skiing device, he must be able to control his body in a predictable manner. If his body is being bounced around haphazardly, he will be unable to do so. Therefore, bodily instability is reduced through his seating and strapping system. Outriggers also are used to create Static Stability when they are placed on the snow. They provide physical support to prevent the sitski from tipping over when it is not dynamically stable. When the sitski is in motion, the outriggers play an important role in creating Dynamic Stability.
Each sitskier requires an optimum level of Static Stability to enable peak performance. Too much, and the skier’s body is overly restricted. He is unable to move enough to ski dynamically. Too little, and the skier’s body is unstable. This lack of bodily stability inhibits him from controlling his sitski. Each skier requires a different degree of Static Stability depending upon his or her degree of disability and individual physicality. Generally speaking, the greater a person’s disability, the more Static Stability required to ski effectively. But more use of Static Stability usually translates to limiting his or her absolute skiing performance. In simple terms and all other things being equal, the lessor the disability, the greater the person’s skiing ability and vice-versa.
The Adaptive Skiing Community recognizes the important role of Static Stability in enabling people of various disabilities to ski. The skier’s seating system is a substitution (less effective) for his lack of voluntary body control. It is customized to create the optimum amount of Static Stability while keeping in mind that higher bucket seats and more body straps will restrict the skier from getting the maximum amount of body angulation that is critical to dynamic skiing.
Voluntary body control enables both Static Stability and Dynamic Stability because the person’s body can stiffen and bend as required. Whereas seats and straps only serve to bodily limit movement. Hence, the performance minded adaptive skier will rely on as little artificial Static Stability methods as possible in order to enable the greatest amount of Dynamic Stability.
In skiing, Dynamic Stability is created by the interaction and the balancing of body/device movement, frictional and gravitational forces. In certain situations, Static Stability and Dynamic Stability work together, and in others, they are directly opposed. This fact is an inescapable aspect of skiing. The Adaptive Skiing Community understands that Static Stability is indispensable, but it must be used as sparingly as possible to create the greatest degree of skiing performance. As a result, adaptive skiing has and continues make huge advances in skiing outcomes for all involved.