Wheelchair Biomechanics

Original Editor - Naomi O'Reilly as part of the Wheelchair Service Provision Content Development Project

Top Contributors - Naomi O'Reilly, Kim Jackson, Uchechukwu Chukwuemeka, Lucinda hampton, Simisola Ajeyalemi, Olajumoke Ogunleye, Shaimaa Eldib and Amrita Patro  

Introduction

To study biomechanics of manual wheelchair helps to understand how interact the user’s body parts and functions with her/his wheelchair. It is the first step for choosing the appropriate design, the adapted accessories and adjust them according to individuals needs, profile and anatomy. Together with user’s feelings and feedbacks, biomechanics guides you to reach the optimal posture and efficient propulsion for the realisation of daily life activities.

According to McLaurin & C. E. Brubaker wheelchair biomechanics involves the study of how a wheelchair user imparts power to the wheels to achieve mobility. 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.

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. 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.

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.

Factors That Affect Mobility

Friction

The higher the friction, the rolling resistance of the chair will be higher, and therefore the user will require greater energy for its propulsion. But often the factor that increase the friction, improve the comfort and stability. Therefore the decision taken will be the best compromise according to the user’s profile.

In this section we will analyse how they affect the ability to roll the following factors: 

• Weight distribution between the 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 80% weight on the rear wheel and 20% on the front (approximately). This makes it roll better than a standard but that is less stable.
• The terrain on which the wheelchair will be used. The soft ground produces a greater friction and therefore it requires more effort to propel the wheelchair. The friction is less in hard terrains or surfaces.
• Size and composition of the wheels:The pneumatic wheels are more comfortable to cushion better, but they offer a greater resistance to roll because they are softer. The resistance is lower in wheels with solid covers for being harder. The small wheels have less friction because they have less contact surface with the floor but users feel less control. Larger wheels have better grip for having a larger contact surface but also produces superior friction.
• Size of the front wheels:Large wheels are more recommended for exteriors, and uneven floors. The small wheels are better for indoor use and to practice sports for its greater quickness of turn on smooth and hard surfaces. However, the right size is determined by the combination between the surface on which it will be used and the weight distribution in the wheelchair.
• Centre of gravity of the chair: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. If 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 devices safety as anti-tipping wheels).
• Distance between axles of front and rear wheels:A long wheelbase keeps the course better (That's why the racing chairs are very long). A short wheelbase is smoother and easier to handle (That's why basketball chairs tend to have this shorter distance). 
• Angulation of the rear wheels:The minimum friction is achieved with the rear wheels vertical in neutral position, it means parallel to the wheelchair and perpendicular to the floor. If the wheels have a positive angle (greater width in the base) the user will have a better control to direct the wheelchair but it increases the resistance to movement. A negative angle (smaller width at the base) makes more friction, more instability and the shoulder posture worse for propulsion. 
• Angle of the front wheels:The front 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 a minimum expenditure ofenergy.Each user due to his personal circumstances has a different propulsion capacity andsometimes limited.That is why it is important to keep in mind the following significant factors that will look for the chair composition according to each user needs, in order to optimize the propulsion.

Range of Motion & Muscle Activity

The degree of mobility that the user has in the column, shoulder, elbow, wrist and fingers will delimit the possibility of carrying out the entire optimal propulsion route. 

If the user has good mobility in these joints, the most effective route is indicated starting from behind the trunk until finishing at the level of the thighs. In this way, the muscles activity of the arm allows the proper application of forces.

Posture

To be able to propel correctly and take advantage of all the energy of this propulsion, the user must be correctly seated (erect) in a symmetrical sitting position. Only at this condition the user can properly reach the push rings 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 in the seat, the rings will be too high and it will be very uncomfortable to start propelling from behind, so the user will tend to start it ahead. In this way the propulsion will be shorter and less efficient. 

Height and Position of the Wheels

To achieve more efficient propulsion, the rear wheels must be located so that the user, with relaxed shoulder and letting the stretched arm fall, can touch the axis of the rear wheel with the fingertips (Fig. 3). If the axis of the wheel is higher than indicated, the push ring will also be high, and the user will have to flex the arms too much to propel (Fig. 4). The propulsion will be uncomfortable and inefficient. The same happens if the axis 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 cushion high interferes in the height of the gravity centre and its thickness should be tajken into account while building the wheelchair. 

This same rule also applies about the optimal position of the wheel. If the wheel is forward and the axis remains in front of the fingers, the user will initiate propulsion too far back and will not be able to complete the entire course (Fig. 6). 

If the axis is behind the fingers, the user will start forward propelling and therefore will have a shorter and less efficient movement (Fig. 7). 

The position of the rear wheel affects as well the stability of the chair. 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. 

References