John D. Lloyd, PhD, CPE
Board Certified Ergonomist
32824 Michigan Avenue
San Antonio, Florida 33576
Tel: 813-624-8986
Email: DrJohnLloyd@Tampabay.RR.com
Repetitive Wrist Movements: Clinical Implications for Ergonomic Workplace Surveillance
Background - Hand intense occupational activities have been associated with an increase in the incidence of carpal tunnel syndrome (CTS). CTS is characterized by an impairment of median nerve function. To date, a dose-response relationship between wrist activity and median nerve performance has not been documented. Since repetitive hand/wrist activity in the workplace has significant implications, it is important to establish a scientific basis for the etiology of work-related carpal tunnel syndrome.
Methods - Twenty-seven clinically-confirmed asymptomatic female participants were asked to perform prescribed repetitive wrist motion activities, the intensity and duration of which was controlled in a laboratory environment. Wrist motion was recorded using a state-of-the-art 3D electromagnetic tracking system (HumanTRAC). Sensory median nerve response to imposed physical stressors was monitored antidromically and recorded using a clinical electroneurometer every ten minutes throughout the simulated work activities. Near-nerve skin temperature was recorded at three sites along the distal sensory branch of the median nerve every twenty minutes.
Results - After adjusting for changes in near-nerve skin temperature, a significant within-subject effect of duration of exposure (time) was detected. Sensory median nerve conduction velocity differed statistically between the static and high wrist activity conditions after 120 minutes of exposure, signifying adverse effects on nerve conduction that are uniquely attributable to repetitive hand motion. Using regression analysis, a maximum safe wrist-workload exposure limit is proposed. A biomechanical model is presented to calculate the effect of physical risk factors on tendon forces at the wrist, which can be employed for workplace exposure surveillance.
Conclusions - Across the study population of clinically asymptomatic female participants, a change in median nerve performance was observed. This significant effect was evoked due to imposed physical stressors. A dose response relationship between work intensity, exposure time and median nerve conduction velocity was demonstrated.
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