Dynamic activity of the upper limb: effects on shoulder fatigue and discomfort

Musculo-Skeletal Disorders (MSDs) of back pain and upper limbs are the most common occupational injury group in Ireland (HSA, 2007). Across Europe upper limb related MSDs account for over 45% of the total number of reported cases of occupational ill health and injuries (EUROSTAT, 2004). MSD risk factors include poor posture, repetitive or forceful movements, fast paced work, vibration and poor ergonomic conditions, and affect muscles, tendons, ligaments and blood circulation. High task repetition can lead to residual fatigue, which over time can result in damage to tissues. But the role of repetition can often be overly simplified in job evaluation and in the study of its relationship with occupational MSDs. This thesis focuses on the role of dynamic loading of the upper limb (with emphasis on the shoulder) as part of a broader interpretation of repetition as a risk factor for upper limb MSDs. Two case studies and five laboratory experiments were conducted. The case studies demonstrated high levels of muscle activity and movement variability. This thesis proposes that dynamic loading of the upper limb is multifactorial comprising cycle time, movement frequency, muscle activity variability, work/rest regime and posture variability. Quantitative testing (using electromyography and electrogoniometers) as well as qualitative testing (discomfort) methods was used to study musculoskeletal strain. The laboratory experiments tested combinations of these factors at various levels to test for main effects. Taguchi methods were used to model the effects of physical factors on the perception of discomfort in the shoulder region. The studies for the distal part of the upper limb revealed that high movement frequency within the shoulder’s safe range of motion (0-40° flexion, 0-30°) abduction did not increase the average electrical activity of the forearm muscles above the level of 50%MVE, even for the heaviest tasks of load lifting. This suggests that the shoulder muscles may not be affected by heavy loading in the hands as anticipated. Results from collective study of shoulder movement frequencies, work/rest regimes, postures and loads using a Taguchi experiment design and regression analyses were used to test and model shoulder discomfort effects. The findings of low levels of EMG fatigue recorded across many of the experimental conditions indicate that for the Upper Trapezius high movement frequency dynamic contractions may affect the rate of the Motor Unit (MU) recruitment in order to protect the muscle from overloading, at least in short duration treatments. There is a need to further investigate the effects of interactions of repetition with other factors on MU recruitment patterns in the muscles of the upper limb.