July 2003

Proper Propulsion

By Alicia M. Koontz, PhD, ATP, and Michael L. Boninger, MD

Research has shown that manual wheelchair users are at high risk for developing pain and repetitive strain injuries at the shoulder and wrist.^1-3 Sie et al interviewed 103 subjects with paraplegia and found historical or physical examination evidence of carpal tunnel syndrome (CTS) in 66% of the subjects.¹ More than 50% of the survey respondents with spinal cord injury in a study by Nichols et al reported shoulder pain that was related to wheelchair use and transfers.² A magnetic resonance imaging (MRI) study of the shoulders of 28 individuals with paraplegia revealed a high prevalence of distal clavicle osteolysis and early signs of rotator cuff disease.³

Wheelchair propulsion has been associated with the development of CTS and shoulder abnormalities.^4,5 CTS is thought to occur when there is too much pressure on the median nerve that runs through the carpal tunnel opening in the wrist. There are several tests available to detect the presence of CTS. A nerve conduction test measures how fast electrical impulses travel through the median nerve and compares the results to what is considered to be “normal” nerve conduction.

THE LAWS OF MOTION

Boninger et al performed nerve conduction tests to quantify median nerve function in 34 individuals with paraplegia who used manual wheelchairs on a daily basis.⁴ Researchers also performed a detailed biomechanical assessment of wheelchair propulsion technique at two different speeds common in daily mobility using a force and torque measurement system and a motion analysis system. The results of the study revealed that individuals who weighed more had more impaired functioning of the median nerve. In addition, heavier subjects pushed their wheelchairs with greater propulsive forces.

Because of the relationship between body weight and median nerve injury and body weight and pushrim force, pushrim forces were normalized with respect to the individual’s weight to obtain a more accurate understanding of the relationship between propulsion technique and median nerve injury. The results indicated that higher peak weight-normalized propulsion force and increased rate of applied force were associated with greater injury to the median nerve. In addition, individuals who pushed with a greater number of strokes for a given speed were more likely to have median nerve injury.

These results are not surprising considering that occupational and ergonomics studies have found links between high force, highly repetitive tasks, and risk of CTS.^6,7 Silverstein et al performed a biomechanical investigation on several job types and found that those with high force and high repetition were associated with CTS.⁶ Their definition of high repetition was an average cycle time of less than 30 seconds. The cycle time of a propulsive stroke is approximately 1 second. Therefore, individuals who apply greater forces and more rapidly load the pushrims at higher stroke frequencies are at an increased risk for developing median nerve injury.

A longitudinal study conducted by Boninger and coworkers investigated the progression of shoulder injury as diagnosed using MRI over a period of approximately 2 years.⁵ MRI examinations and a biomechanical analysis of wheelchair propulsion, similar to that described above, were performed on 14 wheelchair users with paraplegia at the beginning and at the end of the 2-year period. Individuals were separated into two groups—those who had increased shoulder pathology after 2 years and those who showed no increase or had slight improvements in shoulder pathology.

The group with advanced shoulder pathologies was found to produce greater weight-normalized forces (>5% of body weight) in the radial direction (downward toward the hub of the wheelchair) than the other group. As Newton’s 3rd Law of Motion states, for every action force, there is an equal and opposite reaction force. The forces exerted on the pushrim (action forces) are transmitted equally and opposite to the upper limb (reaction forces). This would imply that the radial “reaction” forces experienced during wheelchair propulsion drive the head of the humerus bone up into the rotator cuff and overlying coracoacromial arch, which over time can lead to injury. The results of this study suggest that modifying wheelchair propulsion technique to reduce radial forces to less than 5% of body weight could potentially minimize the risk of shoulder injury.

TECHNIQUE AND STROKE PATTERNS

Despite what is known about the relationship between wheelchair propulsion technique and injury, wheelchair users receive little to no information on how best to propel a wheelchair to lessen their risk of injury. It may be possible to modify the way one pushes a wheelchair through training and/or wheelchair setup to reduce propulsive forces and lower stroke frequency.

Until recently, it was thought that wheelchair users pushed with one of two types of stroke patterns: circular and pumping.⁸ The circular pattern followed the path of the pushrim. The pumping pattern had a short and abrupt stroking style that followed the pushrim for only a small arc. As more wheelchair users have undergone a detailed motion analysis of their stroke patterns, at least four distinct patterns have been identified: arc, semicircular, single-looping over, and double-looping over.⁹ Hand motion during the recovery phase of propulsion—the time when the hand is off the rim and preparing for the next stroke—is used to define the differences between the four styles (see figure 1 on page 22):

Figure 1. A graphical illustration of the four propulsion patterns (from left to right): arcing; semicircular; single-looping-overpropulsion; and double-looping-overpropulsion.

1) Semicircular (SC): recognized by the hands falling below the pushrim during the recovery phase.

2) Single-looping-overpropulsion (SLOP): identified by the hands rising above the pushrim during the recovery phase.

3) Double-looping-overpropulsion (DLOP): begins with the hands rising above the pushrim, then crossing over and dropping under the pushrim during the recovery phase.

4) Arcing (ARC): occurs when the hands follow an arc along the path of the pushrim during the recovery phase.

While the single-looping-over form of propulsion is more widely used among wheelchair users with paraplegia,⁹ the semicircular technique may be more beneficial. In this pattern of propulsion, the user’s hand drops below the pushrim during the recovery phase. The semicircular pattern has been associated with lower stroke frequency,⁹ greater time spent in the push phase relative to the recovery phase,⁹ less angular joint velocity and acceleration,¹⁰ and increased efficiency.¹¹ The semicircular pattern makes sense in that the hand follows an elliptical pattern with no abrupt changes in direction and extra hand movements. It is the same pattern that is used by wheelchair racers.¹²

THE RIGHT POSITION

It is important to consider that wheelchair setup will influence propulsion technique. Brubaker found that a more rearward seat position decreased rolling resistance and increased propulsion efficiency.¹³ Hughes et al studied six individuals with paraplegia during wheelchair propulsion for different seat positions (two vertical and three horizontal: 2 x 3 design).¹⁴ Low seat positions resulted in significantly greater upper extremity motions than the high seat positions. A more rearward seat resulted in greater shoulder motion in the sagittal plane and a larger hand contact angle. Masse and colleagues found that low and rearward seat positions required less muscle effort and the upper extremity joints moved more smoothly (less joint acceleration) than in higher seat positions.¹⁵ Greater push and recovery times and lower stroke frequencies were also observed in the low, rearward seat positions.

Our research showed that wheelchair users with a more rearward seat position pushed with less force and less rate of force loading, used fewer strokes to go a targeted speed, and had greater hand contact angles.¹⁶ Greater distances between the shoulder and the axle resulted in smaller contact angles. A seat height that allows for 100 to 120 degrees of elbow flexion angle has shown the most benefit when considering the biomechanical aspects of propulsion.¹⁷

Unfortunately, there is a trade-off to consider, when either training wheelchair users on technique or setting up the wheelchair to reduce the risk of injury and maximize efficiency. Studies of unimpaired individuals and individuals with spinal cord injury have found that extremes of wrist flexion and extension greatly increase the pressure within the carpal tunnel and likely predispose the individual to median nerve injury.^18,19 With this in mind, a propulsion technique that minimizes wrist range of motion would seem ideal.

However, a recent study showed that wheelchair users who pushed with larger wrist ranges of motion have healthier median nerves than users who pushed with smaller ranges of motion.²⁰ More wrist range of motion occurs because a greater percentage of the pushrim is utilized during each stroke. Greater utilization of the pushrim (larger pushrim contact angles) means fewer strokes are needed to go the desired speed. In addition, forces and the rate of loading tend to be lower when a greater percentage of the pushrim is used. Therefore, minimizing wrist range of motion should be considered to a lesser degree than reducing forces and stroke frequency. It is difficult to achieve all three goals because the hand is constrained to follow along the path of the pushrim when propulsive forces are being imparted.

PROPELLING ADVICE

The following guidelines for wheelchair propulsion technique and setup are based on findings from literature on injury biomechanics and ergonomics.

Wheelchair users should be instructed to:

• Use long and smooth strokes that limit high forces and rate of loading on the pushrim
• Allow the hand to naturally drift down when letting go of the pushrim; make an effort to keep the hand below the pushrim when not in contact with the pushrim

1. Sie IH, Waters RL, Adkins RH, Gellman H. Upper extremity pain in the postrehabilitation spinal cord injured patient. Arch Phys Med Rehabil. 1992;73:44-48.
2. Nichols PJ, Norman PA, Ennis JR. Wheelchair user’s shoulder? Shoulder pain in patients with spinal cord lesions. Scand J Rehabil Med. 1979;11:29-32.
3. Boninger ML, Towers JD, Cooper RA, Dicianno BE, Munin MC. Shoulder imaging abnormalities in individuals with paraplegia. J Rehabil Res Dev. 2001;38(4):401-8.
4. Boninger ML, Cooper RA, Baldwin MA, Shimada SD, Koontz AM. Wheelchair pushrim kinetics: body weight and median nerve function. Arch Phys Med Rehabil. 1999;80:910-915.
5. Boninger ML, Dicianno BE, Cooper RA, Towers JD, Koontz AM, Souza AL. Shoulder injury, wheelchair propulsion, and gender. Arch Phys Med Rehabil. In press.
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15. Masse LC, Lamontagne M, O’Riain MD. Biomechanical analysis of wheelchair propulsion for various seating positions. J Rehabil Res Dev. 1992;29:12-28.
16. Boninger ML, Baldwin MA, Cooper RA, Koontz AM, Chan L. Manual wheelchair pushrim biomechanics and axle position. Arch Phys Med Rehabil. 2000;81:608-613.
17. van der Woude LHV, Veeger DJ, Rozendal RH, Sargeant TJ. Seat height in handrim wheelchair propulsion. J Rehabil Res Dev. 1989;26:31-50.
18. Gelberman RH, Hergenroeder PT, Hargens AR, Lundborg GN, Akeson WH. The carpal tunnel syndrome. A study of carpal canal pressures. J Bone Joint Surg [Am]. 1981;63:380-383.
19. Gellman H, Chandler DR, Petrasek J, Sie I, Adkins R, Waters RL. Carpal tunnel syndrome in paraplegic patients. J Bone Joint Surg [Am]. 1988;70:517-519.
20. Impink B, Boninger ML, Cooper RA, Koontz AM. Median and ulnar nerve function related to wrist range of motion during wheelchair propulsion. Proceedings of the RESNA 2003 Annual Conference, Atlanta. In press.