Effect on discomfort of frequency of wrist exertions combined with wrist articulations and forearm rotation

This study investigated the combined effects of forearm rotation, radial/ulnar deviation and flexion/extension on discomfort score for two levels of frequency (10. exertions/min and 20. exertions/min) in a repetitive wrist flexion task with a force of 10. N. There were three levels of wrist deviation (neutral, 35%ROM in radial and ulnar), three levels of wrist flexion/extension (neutral, 35%ROM in flexion and extension) and three levels of forearm rotation (neutral, 60%ROM in prone and supine). The dependent variable was discomfort on a Visual Analogue Scale. ANOVA results showed that there were highly significant effects of all the main factors (p< 0.001) on discomfort. The two-way interaction of forearm rotation with radial/ulnar wrist deviation was highly significant (p= 0.001) as was forearm rotation by participant. Similarly, three of the three-way interactions and one four-way interaction were highly significant (p< 0.001 and p< 0.01), probably due to having participants as one of the factors in them. Posture changes from neutral to 35%ROM increased discomfort by about 20%, but combinations of deviated postures increased discomfort by up to 70%. The higher frequency increased discomfort by 28%. Some of the increase in discomfort appears to have been due to reductions in wrist flexion MVC at non-neutral postures.Relevance to industry: The results of the study will be beneficial for the design of work places, hand tools and task design in repetitive industrial manual work, for example, in assembly work requiring a light force and a frequency of about 10-20 exertions per minute.