Movement timing and reach to reach variability during a repetitive reaching task in persons with chronic neck/shoulder pain and healthy subjects

Neck/shoulder pain is linked to movement repetition, awkward postures, and muscular fatigue. Studies have examined the influence of pain and fatigue on movement characteristics, but few report spatial and temporal characteristics within- and between-dynamic movements. The combined influences of fatigue and pain on these parameters are examined here. A shoulder-injured group (PAIN) (intensity > 3/10, duration > 3 consecutive months) and an age-sex-matched control group (CTRL) (n = 16 in each) performed a repetitive reaching task to voluntary termination. Kinematics, heart rate, and muscle activity were recorded. Group comparisons were made at the beginning and end of task. Both pain and fatigue changed movement parameters with CTRL subjects adapting to fatigue with increased arm movement, while the PAIN group increased center of mass (CoM) and minimized shoulder movement. Fatigue was associated with decreased arm relative variability, while pain caused increased arm and decreased CoM relative variability. Time to peak velocity tended to indicate increased joint coupling in the CTRL group only. Together, this suggests that despite initial differences in movement strategies, both groups tend to move toward more fixed movement strategies at the end of the task. Those of CTRL are more fixed temporally and spatially (in the arm), whereas the PAIN group adaptations are primarily spatial in nature and appear to focus on controlling CoM.