Combining the Athlete and the Wheelchair
This is an excerpt from Adapted Physical Education & Sport 6th Edition With Web Resource by Joseph Winnick & David Porretta.
The process of combining the athlete and the wheelchair into a sport system varies depending on the specific sport. However, some general principles can be applied with regard to fitting the wheelchair itself. Additionally, there are some specific performance considerations for racing wheelchairs and court chairs.
Fitting the Wheelchair to the Athlete
Proper fitting of the wheelchair to the athlete is critical for high levels of athletic performance. Most manufacturers provide retail experts who are experienced in measuring athletes for performance wheelchairs.
In fitting the frame, the two most critical considerations are the dimensions of the seat (width, length, and backrest height) and the position of the seat in relation to the main wheels. Both these considerations serve to ensure that the wheelchair fits the athlete perfectly and that she is in an optimal position to apply force and maneuver the wheelchair. Refer to the application example for a list of considerations to keep in mind while helping athletes find the chair that is best for them.
Helping a Wheelchair Athlete Find the Right Sport and Chair
A community-based junior wheelchair sport program
A 16-year-old junior wheelchair basketball player with a spinal cord injury needs recommendations to refine his individualized transition program to incorporate adult wheelchair sports. The player is tall, has played the center and forward positions, and wishes to purchase his own wheelchair.
What considerations should be taken into account in making recommendations to this athlete?
Considerations for this athlete center on equipment, physical fitness, and individual skills.
- Athlete's height
- Desire to play a certain position
- Need to establish athlete's physical impairment, sport classification level, and trunk stability when seated
- Adjustability for height and point of balance (being able to maximize the seat height to about 21 inches [53 centimeters] for the center and forward positions)
- System considerations such as strapping and mobility in the wheelchair
- Reputable manufacturer
Individual physical fitness
- Strength training program that targets the upper body muscles in paired groups (e.g., biceps and triceps)
- Cardiorespiratory conditioning program that uses an arm crank ergometer or, preferably, a training roller
Individual skills targeted
- Wheelchair mobility skills both with and without the basketball
- Shooting skills both stationary and moving
- Passing skills both stationary and moving
- Studying the sophisticated strategies involved in the adult game
System Considerations for Racing Wheelchairs
A number of system considerations apply to racing wheelchairs. The following section identifies propulsion techniques and how to overcome negative forces as important considerations in developing an athlete's wheelchair racing system.
Propulsion Techniques in Track and Road Racing
Coupled with the evolution of the racing wheelchair has been the development of ever more efficient propulsion techniques. A six-phase technique (see figure 29.7) is most frequently used, although not all athletes use each phase with the same degree of effectiveness. An analysis by O'Connor and colleagues (1998) led the authors to conclude that there is a need for coaches to become more knowledgeable concerning appropriate wheelchair propulsion techniques.
Six-phase propulsion cycle.
The propulsion cycle starts with the hands drawn up as far above and behind the push rim as possible given the seating position and flexibility of the athlete. The hands are then accelerated as rapidly and forcefully as possible (acceleration phase) until they strike the push rim (see point A on figure 29.7). The moment of contact is the impact energy transfer phase (point B on figure 29.7), during which the kinetic energy stored in the fast-moving hand is transferred to the slower-moving push rim. With the hand in contact with the push rim, there is a force application, or push, phase (point C on figure 29.7), and this continues until the hands reach almost to the bottom of the push rim. During the force application phase, most of the propulsion comes from the muscles acting around the elbow and shoulder.
As the hands reach the bottom of the push rim, the powerful muscles of the forearm are used to pronate the hand, which allows the thumb to be used to give a last, powerful flick to the push rim. This last flicking action is reversed by a few athletes who use supination in the rotational energy transfer phase (point D on figure 29.7) to flick the push rim with the fingers rather than the thumb; and research indicates that this type of backhand technique may be more efficient in endurance races (Chow et al., 2001).
Immediately following the rotational energy transfer, the hands leave the push rim during the castoff phase (see point E on figure 29.7). Here it is important that the hand be moving faster than the push rim as it pulls away, since a slower hand will act as a brake on the wheelchair. Often the athlete will use the pronation or supination of the rotational energy transfer phase to accelerate the hands and arms and thus allow them to be carried up and back under ballistic motion. This upward and backward motion is called the backswing phase (point F on figure 29.7) and is used to get the hands far enough away from the push rim to allow them to accelerate forward to strike the push rim at high speed at the start of the next stroke. Goosey-Tolfrey and colleagues (2000) reported that no single identifiable stroke frequency could be recommended as best for wheelchair racing, but the athlete's own freely chosen frequency was the most economical in laboratory conditions.
This basic propulsion stroke is modified by the terrain over which the athlete is wheeling, by the tactics of the race, and by the athlete's level of disability. On uphill parts of a course, the athlete shortens the backswing and acceleration phases so as to minimize the time during which force is not applied to the push rim and during which the chair could roll backward. Tactically, the athlete is either wheeling at constant speed or is making an attack and needs to accelerate. The basic stroke described previously is used at steady speed; during bursts of acceleration, the major change in stroke takes place during the backswing. At steady speeds, the backswing is a relatively relaxed ballistic movement in which the velocity at castoff is used to raise the hand to its highest and most rearward position. This relaxed backswing is efficient and allows a brief moment of rest during each stroke. During acceleration, however, the major change in stroke dynamics is to increase the number of strokes from approximately 80 per minute to more than 120 per minute. This is achieved by a rapid reduction in the time taken for a more restricted backswing.
The stroke is modified during the start of a race. Because the wheelchair is stationary, the hands should grip the push rim (rather than striking it), and for the first few strokes the arc of pushing will be more restricted with as rapid a recovery as possible. The various approaches that have been adopted are dependent on the athlete's preference. Some athletes attempt to make longer, more forceful pushes to get the wheels going, whereas others make shorter, sharper pushes to get the hands moving fast as early as possible.
Retarding Forces and Overcoming Them
While the athlete provides the energy to drive the wheelchair forward, the twin retarding forces of rolling resistance and aerodynamic drag act to slow it down. When propulsive forces are greater than resistance, the wheelchair accelerates, and when the retarding forces are greater, the chair is slowed. Obviously, reductions in rolling resistance and aerodynamic drag translate directly into higher wheeling speeds and improved athletic performance.
On a hard, smooth surface, the majority of the rolling resistance of the wheel occurs at the point where the tire is in contact with the ground. As the tire rotates, each part is compressed as it passes under the hub and is in contact with the surface; then it rebounds as it begins to rise again and contact with the surface is broken. Not all the energy used to compress the tire is recovered on the rebound, and the energy loss (called hysteresis) is the major determinant of rolling resistance.
Rolling resistance of racing wheelchairs is also affected by the camber angle of the main wheel, which increases with camber (Faupin et al., 2004; Mason, van der Woude, de Groot, & Goosey-Tolfrey, 2011) and wheel alignment, referred to as toe-in or toe-out. Wheels that are not toed correctly dramatically increase the rolling resistance of a wheelchair. Athletes should do everything in their power to check and adjust alignment before every important race.
The problem of aerodynamic drag of racing wheelchairs and athletes is unique in sport because of the relatively low speeds at which events take place. Races (10,000 meters) on the track take place at average speeds between 6.84 and 8.40 meters per second (female and males, respectively). Although the race times of wheelchairs have dramatically improved over the last decade, the times are still considerably slower than the speeds found in cycling. This creates special low-speed aerodynamic conditions.
Aerodynamic drag is caused by two separate but interrelated forces called surface drag and form drag. Surface drag is caused by the adhesion of air molecules to the surface of an object passing through it, and it is very powerful at low speeds. Form drag, on the other hand, is caused by the difference in air pressure between the front and the back of an object, which in turn is created by the swirls and eddy currents formed as the wheelchair and athlete pass through the air.
For wheelchair racers, the problem is that smooth surfaces increase surface drag while decreasing form drag. Some aspects of aerodynamic drag reduction are beyond doubt; these are the importance of reducing both surface and form drag by minimizing the drag-producing areas of the wheelchair and the athlete's clothing.
Because aerodynamic drag represents approximately 40 percent of the force acting to slow down a wheelchair racer, methods of minimizing this can pay considerable dividends. The single most effective way in which drag can be reduced is the process of drafting. Drafting occurs when one wheelchair follows closely behind another wheelchair that acts as a wind deflector. At the end of long races, the energy saved by drafting can be a critical determinant of race outcome. Frequently teams work together, taking turns at both leading and drafting so that their overall performance will be increased.
System Considerations for Court Wheelchairs
This section does not include information on propulsion techniques in court sports. There is less research on propulsion techniques for court sports, presumably because of the wide variability in the propulsion techniques as compared to those in racing; however, Vanlandewijck and colleagues (2001) conducted a review of propulsion biomechanics that included not only wheelchair racing but also basketball and rugby. For those interested in increasing wheelchair sport performance, it is recommended reading.
As mentioned previously, the two fundamental features of a sport wheelchair are the dimensions of the seat and its positioning in relation to the wheels, although there are differences in the reasoning behind both of these features in relation to racing wheelchairs. In wheelchair racing, the key performance indicator is speed or endurance (or both) in a predominantly linear direction. However, in court sports, maneuverability is also a key area of performance. Therefore, whereas wheelchair racers require a perfectly fitting seat so that no energy is lost during propulsion, court sport athletes desire a seat customized to their anthropometrics to facilitate their agility. If a seat is too wide, the athlete can slide around in the chair, which equates to a loss of energy during turning; the body has to then catch up before being in a position whereby force can be applied to the wheels. When the seat is the correct width, the wheelchair should be able to respond more effectively to the athlete. This enables those athletes with sufficient trunk function to be able to maneuver their chair without necessarily having to touch their wheels. This feature of performance can also be facilitated by strapping around the knees or lap, which further secures the athlete to the chair, making movements such as tilting in wheelchair basketball possible.
The backrest is another dimension of the seat that warrants consideration when one is configuring a sport wheelchair. The backrest is essentially designed to improve the athlete's stability, which can be impaired if the backrest is too low for the functional capacity of the athlete. Alternatively, if the backrest is too high, movements can be restricted when the athlete is trying to move backward to reach a ball in basketball or rugby or hitting the ball in tennis. Strapping around the trunk can be applied to facilitate stability, although similar precautions must be taken to ensure that strapping is used only if the functional capacity of the athlete requires. If too much strapping is applied too tightly, the athlete's ability to move can be unnecessarily sacrificed at the expense of stability.
To further facilitate the fitting of the athlete to the sport wheelchair and subsequently maximize maneuverability performance, molded seats have recently emerged in wheelchair tennis and wheelchair basketball (figure 29.8). Since a molded seat will mimic the exact dimensions of each individual athlete, previous limitations associated with a conventional seat, such as energy loss during propulsion and impaired maneuverability, should be eradicated.
Example of (a) a conventional sport wheelchair seat and (b) a molded seat to facilitate maneuverability performance.
Photos courtesy of Dr. John Lenton.
Once the seat is successfully designed for the specific athlete, the next thing to consider is where the seat fits in relation to the main wheels in both a horizontal (anterior - posterior) and vertical position (see figure 29.9).
(a) Anterior - posterior and (b) vertical main-wheel adjustments.
Anterior - Posterior Seat Position
Horizontal positioning of the main wheels affects the mobility of the chair. The farther forward the main wheel from a hypothesized neutral position (see figure 29.9a, position A), the more maneuverable the chair (see figure 29.9a, position B). Unfortunately, the farther forward the main wheel relative to the center of gravity, the more likely it is that the chair will tilt up. Although the introduction of the anti-tip castor wheel prevents the athlete from falling backward, it does place a large percentage of body mass over the rear castors. Consequently, athletes need to reposition their body weight forward in order to drive the wheels forward, which will be limited by their trunk function. However, this is a position that many low-point wheelchair rugby players are forced to adopt since they do not have the triceps function or stability to sit above the wheel and drive it down. Alternatively they choose to sit farther back so that they can make the most of their biceps function and "pull" the wheel up and forward.
Vertical Seat Position
Vertical positioning of the main wheel affects the height at which the athlete sits and the center of gravity of the system. This fundamentally affects the handling properties of the chair. Again, using a hypothetical neutral position (figure 29.9b, position A), the lower the athlete sits relative to this neutral position (figure 29.9b, position D), the more maneuverable the wheelchair. Therefore, all other things being equal, the athlete should sit as low as possible. However, performance considerations place a premium on height in all sports. Shooting is easier in basketball when athletes sit high because they are closer to the basket. Likewise, receiving a rugby pass is easier if one sits higher and can reach above the opponent. Finally, a tennis serve is made easier when the athlete is elevated above the height of the net, as there is now a greater margin for error. Given the advantages associated with sitting high, athletes can often forsake the optimal position for pushing the wheelchair, putting their mobility performance at risk. As the height of the seat increases, the athlete effectively moves farther away from the wheels. In order to access enough of the wheels to effectively apply force, athletes (depending on trunk function) will have to lean forward. In order to reduce the distance that athletes have to lean, many have countered this by selecting a larger wheel size to make the wheels more accessible in a higher seat position. However, this can introduce alternative and potentially negative effects on performance, with a larger wheel thought to impair acceleration and maneuverability performance. Mason and colleagues (2012a, 2012b) have provided a more in-depth evaluation of the effects of wheel size on aspects of mobility performance in wheelchair basketball players.
In summary, when enhancing wheelchair sport performance on the court, athletes should identify the functional aspects of the game and their roles or positions coupled with their strengths and weaknesses. This will depend in part on the disability level of the athlete. After identifying these roles, athletes should select the wheelchair setup that will improve functionality within the roles. It is stressed that the positioning of the main wheel will fundamentally affect the performance characteristics of the chair. After the athlete has identified the appropriate wheelchair setup, consideration needs to be given to combining the athlete and the wheelchair into a performance system through the use of appropriate strapping techniques.
Sport-specific skills are critical to the elite athlete's program. Common to skills in court sports are acceleration, speed (which depends on power, which depends on strength), and maneuverability with the target object, whether it be a basketball, volleyball (as used in wheelchair rugby), or tennis racket. Goosey-Tolfrey (2010b) reports other sport-specific skills as described by key sport coaches for the aforementioned sports. Skills tests have been developed for wheelchair basketball, wheelchair rugby, and tennis (Newbery, Richards, Trill, & Whait, 2010; Yilla & Sherrill, 1998), and field-based fitness testing is described in detail in the review article by Goosey-Tolfrey and Leicht (2013). Task analysis of skill performance is also suggested by Davis (2002, 2011).
Instructional materials that focus on the skills and strategies involved in many wheelchair sports are also available (Goosey-Tolfrey, 2010b). Again, the systems approach should be incorporated, with athletes practicing their skills in their competitive system that includes their sport-specific wheelchair and strapping.
More Excerpts From Adapted Physical Education & Sport 6th Edition With Web Resource
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