Most individuals with spinal cord injuries (SCIs) use manual wheelchairs for mobility at home, school, work, and play . Long-term wheelchair use and its consequences on the musculoskeletal system have become an important issue in manual wheelchair research. Since the physiology of the shoulder is not well adapted to the monotonous nature and peak-force requirements of wheelchair use, the necessarily prolonged and often excessive use of the upper limbs leads to muscle imbalance and attrition injuries . Many manual wheelchair users (MWUs) experience upper-limb pain and injuries that interfere with essential activities of daily living involving wheelchair propulsion and transfer . It is important to develop effective strategies to minimize the destructive effects of shoulder pain and injuries. This is particularly true for senior citizens who are MWUs, in whom problems with upper-limb pain often result in progressively increased dependency.
Muscle imbalance, defined as predominance of one of a synergist pair of muscles during a movement , has become an important topic in the etiology of many musculoskeletal disorders . The shoulder muscles do not gain equally in strength during wheelchair propulsion; only those active in the push phase become stronger, whereas those active in the recovery phase remain at the same level of strength. The repetitive movements required to achieve locomotion will, over time, widen the strength discrepancy between push muscles and recovery muscles [5-7]. Previous studies have suggested muscular imbalance in the shoulder as a source of pain and injury in MWUs [5-6]. Ambrosio et al. have shown that weakness exists in the shoulder adductors in people with paraplegia . This weakness could be a cause for rotator-cuff impingement syndrome [5,8-9]. People with paraplegia also demonstrated a significant weakness during external and internal rotation . Strengthening of shoulder internal and external rotators, as well as adductors, has been recommended in clinical guidelines . Successful rehabilitation processes related to muscle imbalance are often accomplished by addressing the cause of the problem rather than symptomatic treatment of the pain . By understanding muscle imbalances associated with wheelchair propulsion, physical therapists can prescribe appropriate exercises for both treatment and prevention.
The present study identified muscle imbalance associated with fatigue and activity of selected upper-limb muscles during wheelchair propulsion. In general, muscle fatigue plays a critical role in the development of muscle imbalance and overuse . According to the published literature on what precipitates musculoskeletal injuries, injuries around a joint may be caused by uneven fatigue among the different joint muscles . The diverse demands on the muscles surrounding a joint may cause them to fatigue at different rates and to different degrees . Rodgers et al. reported that muscle fatigue in MWUs may be responsible for stress and harmful changes in shoulder joints . Since muscle fatigue leads to decrements in muscle force production , it is probable that during strenuous activity, each of the muscles around a joint reaches a point of fatigue at a different time; the ensuing imbalance in force production may lead to unnatural joint motions, abnormal joint stresses, and, ultimately, injuries. Rodgers et al. showed that joint power shifts from the shoulder joint to the elbow and wrist joints with the onset of fatigue during wheelchair propulsion .
Continuous monitoring of local muscle fatigue during locomotion is possible by measuring myoelectric activity of particular muscles using surface electromyography (sEMG) . Electromyography (EMG) has been widely used in the assessment of musculoskeletal disorders. The myoelectrical manifestations of muscle fatigue can be observed by a decrease in the mean power frequency (MPF) of the power spectrum , whereas EMG amplitude can be used to measure muscular activity . …