\section{Background}\label{background}
Spinal Cord Injury (SCI) occurs due to severe trauma to the spinal cord and has been considered as a catastrophic condition ever since it was first documented by the ancient Egyptians around 5000 years ago (Eltorai, 2003). In the U.K., the annual incidence of SCI is about 20 people per million of the population and in total over 36,000 people suffer from it (Department of Health, 2005). The corresponding statistics in the U.S.A. are 40 and 265,000 people, respectively (National Spinal Cord Injury Statistical Centre, 2010). An almost inevitable consequence of SCI is the loss of motor function of the lower and upper limbs, depending on the site and extent of the lesion along the spinal cord (Sie \& Waters, 2003). Hence mobility of these patients is a major concern for their rehabilitation. The spinal cord may also be damaged due to factors other than trauma such as congenital or systemic disorders, infections, tumours etc., which account for numerous Spinal Cord Disorders (SCDs) such as multiple sclerosis, syringomyelia, arachnoiditis etc. (Woolsey \& Martin, 2003a, 2003b). More than 52,000 people in the UK currently suffer from multiple sclerosis (Department of Health, 2005).

Wheelchairs provide mobility to persons with paraplegia and tetraplegia, i.e., impaired function of lower, and both lower and upper extremities, respectively. A range of manually operated or electric-powered wheelchairs are commercially available and are to be selected depending on the abilities of the user (Cooper et al, 2003). Manual wheelchairs can only be used by persons with functional upper limbs having adequate power to propel the wheelchair. Moreover, long-term use could also lead to fatigue and pain of the upper limbs (Somers, 2010). Electric-powered wheelchairs may be used by people with paraplegia as well as tetraplegia (Ding \& Cooper, 2005).

The second half of the 20th century saw many developments in the fields of emergency medical services, surgical procedures, pharmacology, rehabilitation etc., in addition to technological advancements that improved the treatment and care of patients with SCI and SCD (Eltorai, 2003). Hence there has been a steep rise in the number of patients surviving with disabilities (Cooper, 1995). Since they would require the use of wheelchairs on a long-term basis, it is imperative that we strive to impart greater autonomy to the users and decrease their dependency on care-givers. Moreover the psychological aspects of being in control of one's life and having a sense of self-worth are critical for SCI and SCD patients to re-integrate with the community (Somers, 2010). Hence considerable amount of research and development is being done towards this end, worldwide.