Wheelchair Biomechanics

Introduction

According to McLaurin & C. E. Brubaker [1] wheelchair biomechanics involves the study of how a wheelchair user imparts power to the wheels to achieve mobility, and helps us understand how the users body interacts with the wheelchair. Because a wheelchair can coast, power input need not be continuous, but each power strike can be followed by a period of recovery, with the stroking frequency depending on user preferences and the coasting characteristics of the wheelchair. The latter is described in terms of rolling resistance, wind resistance and the slope of the surface. From these three factors the power required to propel the wheelchair is determined, and must be matched by the power output of the user. The efficiency of propulsion is the ratio of this power output to the metabolic cost. [1]

Manual wheelchair propulsion, both in daily use and sports use is being increasingly studied, incorporating physiological, engineering and biomechanical perspectives with a focus towards ergonomics and injury mechanisms, especially the phenomena of overuse to the upper extremity. [2][3] Through a synchronised analysis of the movement pattern, Insight into force generation pattern and muscular activity pattern in hand rim wheelchair propulsion dynamics of people with a disability with various levels of physical activity and functional potential have been developed through lab based, synchronised analysis of the movement pattern. [3]  

An important skill related to moving the wheelchair is propulsion. Wheelchair propulsion using both upper extremities is the primary means of maneuvering a manual wheelchair. There are two distinct phase during a wheelchair propulsion cycle; the propulsive or push phase which starts when the hand comes into contact with the pushrim and continues until the point at which contact is removed at the end of the stroke. The recovery phase is the period in which the hand is not directly engaged with the pushrim so involves the motion when the hands disengage from the pushrim until the upper extremities swing back to contact the pushrim once again for the start of the next propulsive phase. In the research four types of propulsion patterns have been identified; arc propulsion, single loop over propulsion, double looping over propulsion, and semicircular which vary based on the trajectory of the hand when the hand is in the recovery period. This variability in propulsion techniques may be due in part to the level and type of impairment but also relate to the level of wheelchair skills training the user has had access to.[4] 

The features required in a wheelchair depend upon user characteristics and intended activities. The ideal wheelchair for an individual will have the features that closely match these characteristics and activities. Thus prescription is not just choosing a wheelchair, but choosing the components of the wheelchair that best serve the intended purpose, which include wheels, tyres, castors, frames, bearings, materials, construction details, seats, backrests, armrests, foot and legrests, headrests, wheel locks, running brakes, handrims, levers, accessories, adjustments and detachable parts. Each component is considered in relation to performance characteristics including rolling resistance, versatility, weight, comfort, stability, maneouvrability, transfer, stowage, durability and maintenance. [1]

Factors That Affect Mobility

Friction

The rolling resistance of the wheelchair will be higher when a higher level of friction is present, therefore the user will require greater energy for its propulsion. But in some cases the factors related to friction on the wheelchair may improve the comfort and stability for the user, so the overall decision taken will be the best compromise according to the user’s profile. In this section we will analyse how the following factors affect the ability of the wheelchair to roll: 

Weight Distribution between Front and Rear Wheels;

Greater weight on the front wheels cause greater friction, but at the same time it makes the chair more stable. A standard wheelchair has a weight distribution of 50/50%, while an adjustable lightweight chair (according to the adjustment) has a distribution of up to 80% weight on the rear wheel and 20% on the front (approximately). This makes it roll better than a standard wheelchair but is less stable, thus requires greater skill to use.

Terrain where Wheelchair Used

Soft ground produces greater friction and therefore requires more effort to propel the wheelchair. Friction is much less on hard terrains or surfaces.

Size and Composition of Rear Wheels

Pneumatic (air filled) wheels are more comfortable as they provide better cushioning, but as they are softer also provide a greater resistance to roll. A correctly inflated pneumatic tyre rolls more easily than an under inflated tyre. Many clients avoid pneumatic castor tyres because they are troublesome and puncture easily. Solids on the other hand are much harder, thus reduced resistance but provide less cushioning to the user. Small wheels have less friction because they have less contact surface with the floor but users feel less control, while larger wheels have better grip, but produce greater friction as they have a larger contact surface.

Size of Caster (Front) Wheels

Large wheels are more recommended for exteriors, and uneven floors, while small wheels are better for indoor use and to practice sports as they turn quicker on smooth and hard surfaces. However, the right size is determined by the combination between the surface on which the wheelchair will be used, the weight distribution in the wheelchair and the type of activities that the wheelchair user is involved in.

Centre of Gravity of Wheechair

Moving the Centre of Gravity backwards and upwards increases the weight on the rear wheels and makes the chair easier to handle but more unstable. Moving the Centre of Gravity down and forward, the chair gains stability but is more difficult to handle. Normally you can reach a compromise according to the needs of the user. It may be necessary to introduce safety devices such as anti-tip wheels

Distance between Axles of Front and Rear Wheels

Long wheelbase tends to keep the course better, goes in a straight line (That's why the racing chairs are very long). Short wheelbase is smoother and easier to turn or handle around obstacles (That's why basketball chairs tend to have this shorter distance).

Angle of Rear Wheels (Camber)

Minimum friction is achieved with the rear wheels vertical in neutral position, which is parallel to the wheelchair and perpendicular to the floor. Increasing the camber of the wheels ie.; if the wheels have a Positive Angle so they have a greater width at the base, gives the user better control to direct the wheelchair but increases the resistance to movement. On the other hand decreased camber when the wheels have a Negative Angle so have asmaller width at the base will create more friction, thus create more instability. (Fig.8)

Angle of Caster (Front) Wheels

Caster wheels are at 90º in order to keep the same mechanical balance in all directions. if the angle is more open or less open, the front part of the frame will be higher or lower depending on the direction and will create additional resistance and friction to movement.

Factors That Affect Propulsion

The assembly of the wheelchair must ensure effective propulsion together with minimum energy expenditure. Each user has a different propulsion capacity due to their personal circumstances, which at times can be limited.That is why it is important to keep in mind the following significant factors that will impact on chair composition according to each user needs, in order to optimize the propulsion.

Range of Motion & Muscle Activity

Wheelchair Biomechanics - Fig 1.jpg
The degree of mobility that the user has in the trunk, shoulder, elbow, wrist and fingers can limit the possibility of carrying out the entire optimal propulsion route.

If the user has good mobility in these joints, the most effective pushing technique is indicated in Fig.1 starting from behind the trunk finishing at the level of mid thigh. In this way, the muscles activity of the arm allows the good application of forces.


Posture

Wheelchair Biomechanics - Fig 2.jpg
To be able to propel correctly and take advantage of all the energy of during propulsion, the user must be correctly seated (erect) in a symmetrical sitting position. In this position the user can fully reach the push rims and perform the full movement of the arm, to start the propulsion of the wheel from behind, applying force throughout the full movement.

If the user slides forward in the seat as indicated in Fig.2, the push rims will be too high and it can be uncomfortable to start the propulsion from behind, so the user will tend to start the push more forward on the wheel, thus have a shorter and less efficient push stroke.

Height and Position of the Wheels

To achieve more efficient propulsion, the rear wheels should be located so that the user can touch the axle of the rear wheel with the fingertips with a relaxed shoulder (Fig. 3). If the axle of the wheel is higher than indicated, the push rim will also be high, and the user will have to flex the arms more to propel (Fig. 4), which can make propulsion more inefficient and uncomfortable. The same happens if the axle of the wheel is lower than the tip of the fingers. The user will have to perform the propulsion with the arms extended and will not be able to apply the necessary force for propulsion (Fig. 5). The height of the cushion can also impact and change the height of the centre of gravity so the height of the cushion should be taken into account while preparing the wheelchair.  

Wheelchair Biomechanics - Fig 3 - 5.jpg
 

 This same rule also applies in relation to the optimal position of the wheel. If the wheel is forward and the axle remains in front of the fingers, the user will initiate propulsion too far back and will not be able to complete the entire push stroke (Fig. 6). While if the axle is behind the fingers, the user will start propulsion more forward on the wheel and therefore have a shorter and less efficient push stroke (Fig. 7). 

Wheelchair Biomechanics - Fig 6.jpg
Wheelchair Biomechanics - Fig 7.jpg

The stability of the wheelchair is also affected by the position of the rear wheel. If the wheels are backward the chair will be more stable (case of standard wheelchairs) but also will require more energy for propulsion. The “wheelie” will more difficult or impossible to achieve. 

Light chairs tend to have the rear wheels more forward than the standard wheelchair. In this case, you lose in stability but need less leverage and lower energy for propulsion. The “wheelie” will be also much easier. It is a dynamic position. 

Wheel Size

Wheels smaller than 600 mm (24 ") are usually used for users with difficulty of movement in the shoulders or kyphosis column. Smaller wheels are also used in children's chairs so that the push ring stay at a height more appropriate to the length of kid’s arms.

Distance Between Axles

A long distance between the rear and front axles allows to maintain a more stable and straight course, but more energy is needed to rotate. A short wheelbase rotates easily and is easier to handle because it requires less energy for propulsion. 

Angle of the Wheel (Camber)

Optimal propulsion is carried out with the rear wheels parallel to the seat. In this way the distance of the arms to the body is adequate to apply the necessary energy for the correct propulsion. If the wheels are wider at the base, the chair is more stable, but the arms are closer to the body. A greater abduction of the shoulders is necessary and then the propulsion is more difficult and less effective. If the wheels are closer together at the base, the arms will be very far from the body and it will be difficult to apply the necessary force for propulsion. In addition, the chair is more unstable and that’s why this option is never chosen. (Fig.8) 
Wheelchair Biomechanics - Fig 8.jpg
 

References

  1. 1.0 1.1 1.2 McLaurin CA, Brubaker CE. Biomechanics and the Wheelchair. Prosthetics and Orthotics International. 1991 Jan 1;15(1):24-37.
  2. Van der Woude LH, Veeger HE, Dallmeijer AJ, Janssen TW, Rozendaal LA. Biomechanics and Physiology in Active Manual Wheelchair Propulsion. Medical Engineering and Physics. 2001 Dec 1;23(10):713-33.
  3. 3.0 3.1 Vanlandewijck Y, Theisen D, Daly D. Wheelchair Propulsion Biomechanics. Sports Medicine. 2001 Apr 1;31(5):339-67.
  4. Morgan KA. Wheelchair Training Program for New Manual Wheelchair Users. Washington University. 2015 Available at: https://openscholarship.wustl.edu/cgi/viewcontent.cgi?article=1493&context=art_sci_etds