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.
