Design of an adjustable wheelchair for table tennis participation

Abstract

Purpose:

Physical activity and recreation are very important for people with disabilities and provide benefits for self-esteem, social relationships, employment, rehabilitation, and education. Para Table Tennis is an adaptive sport where participants compete in table tennis while sitting in a wheelchair. However, athletes generally use their everyday wheelchair instead of a device specifically designed for the sport. The specific support, movements, and posture needed to participate in table tennis at the highest level are different than general day-to-day mobility and a device could be optimized for the sport. This research describes the development of a wheelchair specifically designed for para table tennis.

Materials and methods:

The design followed a participatory action design approach which identified the specific needs for a wheelchair to be used during para table tennis.

Results:

Three design needs were identified which included 1) locking the casters in the forward direction, 2) ability to raise the seat height as high as possible while allowing the user’s knees to fit under the table, and 3) adjustable seat angle which will allow some users to have anterior tilt to get their trunk even higher while other users could have posterior tilt for stability. A new chair meeting these needs was designed and prototyped.

Conclusions:

Para table tennis has some specific requirements related to movements and posture which were improved by a new wheelchair design specific for the sport.

Introduction

Participating in society and pursuing healthy lifestyles is important for people with disabilities [1–4]. Participation in society requires proper technology, appropriate supports and an accessible physical environment [5–10]. Sports and recreation are important to wellbeing, to establish a positive perception of self, and to form healthy relationships [11–14]. Several studies have considered the benefits of adaptive sport competitions and recreation activities [15–17]. Adaptive sports and recreation are effective strategies for greater rehabilitation outcomes, for improving perceptions of self among people with disabilities, promoting and demonstrating greater inclusion of people with disabilities in society, and increasing employment, education, and training [1, 18–26]. Technology plays an important role in creating and optimizing sports and recreation opportunities [27,28].

Many Paralympic Sports are “technical” and rely on specialized sports equipment such as wheelchair basketball, wheelchair rugby, wheelchair tennis, and wheelchair racing [28–32]. The ability of the equipment to be customized for each sport has a strong influence on performance [15, 33–36]. Well-designed adaptive sports equipment not only optimize the athlete’s performance, but it also reduces the risk of repetitive injury and pressure injury [32, 37].

Para Table Tennis is a Paralympic sport (as well as a sport in many other adaptive sport competitions) that consists of athletes competing in table tennis while using a wheelchair. Currently, most para table tennis players use their day-to-day wheelchair during their matches. Based on our research, there are no commercially available table tennis wheelchairs on the market. Other adaptive wheelchairs, including rugby wheelchairs, basketball wheelchairs, and tennis wheelchairs are not suitable to be table tennis wheelchairs, due to the unique needs and skills required [29]. Therefore, a new chair was developed to optimize performance during para table tennis.

Methods

A participatory action and design engineering (PADE) approach [37–40] was used to identify the specific design criteria that are needed to participate in para table tennis. An experienced para table tennis athlete was interviewed and used as a model client to determine what functions the athlete needs to perform and how the wheelchair can facilitate those functions. Three primary design characteristics were identified that are needed for the para table tennis wheelchair. These design characteristics and the reasoning why they are important are presented in Table 1.

Table 1.

Critical design characteristics for Table Tennis Wheelchair

Design characteristic	Reasoning
1. Lock the casters in the forward direction	The athlete needs to move the chair forward to defend lob shots and backward to defend hard shots. Side to side movement is not needed. It also allows the athlete to move forward and backward using their one hand without the paddle without the chair turning sideways.
2. Raise seat height with adjustability	It’s better for the athlete to be as high as possible to be able to hit the ball harder and still have it hit the table on the other side of the net. However, the athlete’s knees still need to fit under the table.
3. Have seat angle adjustable	Some athletes may want anterior tilt to allow their truck to be even high while still allowing their knees to fit under the table. Other athletes may want posterior tilt for stability.

These design characteristics were converted to design specifications and included with other design specifications that were based on the measurements of the model client. A complete list of design specifications is presented in Table 2.

Table 2.

Design Specifications based on model client

User Preference
Seat Width	30.5 cm (12 in)
Seat Depth	48.3 cm (19 in)
Back Rest	ADI AL series 40.5 cm (16 in) height, with backrest tubes
Seat Upholstery	Tension adjustable by straps
Cushion	Varilite Zoid Cushion
Front Frame Angle	90°
Push Wheel Camber	5°
Design Constraints
Front Casters	Lock and unlock
Footrests	Length adjustable footrests, with footrest straps
Product Weight	<11.4 kgs (25 lbs.)
Front seat height	51 cm (20 in); with cushion 56 cm (22 in)
Seat angle	Adjustable from 5-degree anterior tilt to 5-degree posterior tilt
Frame	Rigid, 6061 Aluminum Alloy, 1” diameter tubes

The overall assembly was created from three subassemblies that could be designed separately and then combined including the lower frame assembly (wheels and axel), upper frame assembly (seat and backrest), and caster assembly. All subassemblies were iteratively designed, modeled, and modified in Solidworks based on input from the model client. Once the designs were complete, a physical prototype was created.

Results

Frame Design

The lower and upper frame designs concepts went through over ten iterations in sketches and software solid modeling with multiple approaches for adjusting the upper and lower fames to achieve the height and angle range requirements. The final design included two parallel tubes for the upper and lower frames that are connected vertically by threaded tie-rods shown in Figures 1 and 2. The lower frame has clamps that can slide forward and backward which connect to the eye end of the threaded tie-rods. The upper frame has threaded inserts welded to the frame which receive the threaded ends of the threaded tie-rods. Jam nuts were added to control unwanted movements of the system. This design allows the distance between the upper and lower frames to be adjusted by turning the threaded tie-rods in the welded insert. The angle of the upper frame can also be adjusted by turning the front two tie-rods and rear two tie-rods to different lengths. The tie-rods are attached to the lower frame by clamps because as the angle between the two frames is altered, the contact point with the lower frame must change. Crossbars were also included in the design of both the upper frame and the lower frame to keep the left and right sides aligned.

Figure 1.

Model of Table Tennis Wheelchair showing the posterior and anterior angle adjustments (top two images) and the height adjustments (bottom two images).

Figure 2.

Image of chair adjustability components with labels.

Wheel Lock Design

Locking of the casters was achieved with a system that rests on the outside of the lower frame tubes and connects to the caster as shown in Figure 3. The base of the caster lock is press fitted to the caster fork and rotates with it when the lock is not engaged. The top piece of the caster lock connects directly to the lower frame with a set screw and contains magnets to hold up the locking piece. The top piece and the base of the caster lock have identical extruding channels while the locking piece has a mirrored internal cutout channel. When the locking piece is dropped into the locked position, the internal channels contain both the extruding channels of the top piece and the base which prevents the top piece or base from rotating with respect to one another. Magnets are connected to the locking piece and the top piece to hold the locking piece up in the unlocked position until the user pushes the locking piece down into the locked position.

Figure 3.

Images of caster lock mechanism. Figure on the left shows the system in the unlocked position. The image on the right shows the system in the locked position.

Physical Prototype

A physical prototype was created based on the final Solidworks model (Figure 4). A bill of materials is provided in Table 3. The upper and lower frames of the wheelchair were made with 25.4 mm (1-inch) outer diameter, 2.1 mm (0.083 in) walled, 6061 aluminum tube to be lightweight and durable. The rear axle, chamber inserts, axle attachment clamps, backrest brackets, and footplate bar were adopted from a Quickie Q7 (Sunrise Medical, Ltd.) frame. The axle attachment clamps were turned on a lathe to fit into the cylindrical lower frame tubes since a Quickie Q7 frame has ovular tubes. The tie-rod threaded inserts and crossbars were welded to the frames. The tie-rod clamps were slid on the lower frame and then are bolted closed in the desired position. The tie-rods are bolted to the clamps and threaded into the inserts. The footplate bar was slid into the ends of the upper frames and clamped in place. The vertical bars for the backrest are attached using the backrest attachment bracket adapted from a Quickie Q7. An ADI backrest (Stealth Products, LLC) was attached to the vertical tubes using a custom 3-D printed bracket which bolts to the backrest and has a bolt through a hole in the vertical tubes to put the backrest at the desired height. The caster assembly was slid into the ends of the lower frame and attached with a pin through the lower frame tubes and caster bolt. The caster fork was custom made and the lower portion of the caster lock assembly was press fitted to the top of the caster fork. The top portion of the caster lock assembly was attached to the lower frame with a set screw.

Figure 4.

Images of physical prototype. Left image shows the entire chair in a heightened position. Middle image shows the castor locks engaged. The right image shows the threaded rods and clamps which allow the height and angle adjustments.

Table 3.

Bill of Materials

Bill of materials
Part	Number	Origin
Rear axle	1	Quickie Q7
Chamber inserts	2	Quickie Q7
Axle clamps	2	Quickie Q7 - modified
Rear wheels	2	66 cm (26 in) Spinergy wheels (SPOX Everyday) with NatrualFit pushrims
Upper frame tubes	2	Custom bent 25.4 mm, 2.1 mm wall thickness, 6061 Aluminum
Lower frame tubes	2	Custom bent 25.4 mm, 2.1 mm wall thickness, 6061 Aluminum
Tie-rods	4	Mcmaster-Carr part # 6066k440
Tie-rod clamps	4	Custom 6061 Aluminum cut on a waterjet and milled to specifications
Threaded inserts	4	Custom bent 25.4 mm, 2.1 mm wall thickness 6061 Aluminum tube with a 6061 Aluminum bar welded inside with a tapped hole for the threaded tie-rod
Jam Nuts	4	Mcmaster-Carr part # 95505A603
Casters	2	Quickie Q7
Caster forks	2	Custom waterjet and milled
Caster lock base	2	Custom 3-D printed on a Selective Laser Sintering machine
Caster lock top	2	Custom 3-D printed on a Selective Laser Sintering machine
Moveable caster lock	2	Custom 3-D printed on a Selective Laser Sintering machine
Magnets	8	Mcmaster-Carr part # 5862K116
Backrest tubes	2	Custom bent 25.4 mm, 2.1 mm wall thickness, 6061 Aluminum
Backrest	1	ADI AL series standard 40.5 cm (16 in) tall
Backrest clamps	2	Custom 3-D printed on a Selective Laser Sintering machine
Backrest attachment brackets	2	Quickie Q7

The physical prototype that was created met all the design criteria. The range of seat to floor height is 45.7–50.8 cm (18–20 inches) excluding the cushion or 50.8–55.9 cm (20–22 inches) with the cushion. The lower frame has roughly a 5-degree posterior tilt which made the upper frame, and therefore the seat, adjustable from 15 degrees posterior to 5 degrees anterior. The upper frame can move fore and aft with regards to the lower frame with a 40 mm (1.6 inch) range which can change the stability or propensity to tip fore/aft of the wheelchair. The casters were able to be locked in the forward position and could not be unlocked by jerking the wheelchair to the side. There is roughly 5-degrees lateral movement in the caster lock when engaged. The footrest height is adjustable from 310 to 400 mm (12.2–15.7 in) below the seat. The mounted backrest height is 370 mm (14.5 in). The backrest angle can be adjusted from −16.6 to 13.9 degrees with 4 discrete adjustment positions on the backrest bracket adapted from the Quickie Q7 wheelchair. The weight of the final chair was 11.1 kgs (24.5 lbs).

Discussion

The design process resulted in a wheelchair that has both the required height and angle adjustments and the ability to lock the casters from rotating to optimize performance for wheelchair table tennis. The chair allowed the model client to be as high as possible while still fitting his knees under table-tennis tables. The caster locks allowed the client to move forward and backward without the chair rotating. The client used the prototype in competition and reported to be satisfied with the chair’s performance (Figure 5).

Figure 5.

Images of a user participating in a table tennis match

The caster directional locking system allows the user the option to lock the direction of travel or allow the caster 360-degree rotation. Magnets were implemented to prevent the caster from locking without the user actively engaging the locking mechanism.

The final design simplified the implementation significantly and allowed manufacturing to be streamlined. This implemented structure reduced the hardware complexity over earlier design iterations to a simple threaded vertical support that provides vertical adjustment via a tapped insert welded to the upper frame. Anterior and posterior tilt in this design could be achieved by adjusting the anterior or posterior pair of threaded rods creating seat tilt as desired by the user. The increased stability provided by this configuration was endorsed by the client and production stakeholders as a workable design solution.

The design of the chair could be useful for other adaptive sports and recreation activities. Other table sports, such as billiards, could be more easily played by someone in a wheelchair if the wheelchair were higher so the user could reach over the table more. Additionally, the adjustable frame could be used on basketball wheelchairs so that one chair could be adjusted to be a guard’s chair (needing to be quick, stable, and agile) and a forward’s chair (needing to be higher to collect rebounds) for teams or people being introduced to the sport.

Future Work

Currently, there are some gaps between the threaded tie-rods and clamps due to the size of the eye on the threaded tie-rods (Figure 6 left). Redesigning a bigger or longer clamp would eliminate this gap. During trial, the client had some difficulty reaching and engaging the caster directional locking mechanism. Changing the shape of the locking piece to an oval may make it easier to grip and push down to lock the casters. Additionally, adding a rubber or ribbed surface may increase friction and make it easier to engage the lock (Figure 6 right). Another option is to reduce the magnet force. Currently there are two magnets which have a combined pull force of 3.7 kg (8.2 lbs). Removing one of the magnets or using weaker magnets would reduce the force needed to break the magnet connection to lock the casters. Another potential modification is to have optional / removable anti-tippers that the user can choose to use to increase stability.

Figure 6.

Closeup views showing items for future work

Conclusion

The table tennis chair that was developed met the goals of the design criteria obtained from the model client. The chair is easily adjustable in both height and anterior/posterior tilt allowing the user to be as high as possible while still fitting under the table. The caster wheels were able to be locked allowing the user to use his or her non-paddle hand to move the chair forwards and backwards without the chair turning. The features of this chair are beneficial for table tennis but could also be useful for other sports and recreational activities.

Implications for Rehabilitation:

• Many adaptive sports are improved with sport specific technology

• A model client was used to develop a wheelchair specifically for adaptive table tennis

• The height and angle of the seat of the new wheelchair can be adjusted for optimal stability and reach

• The casters of the new wheelchair can be locked to allow only forward and backward motion