Background. It is generally agreed that constrained postures during assembly work can lead to musculoskeletal disorders in the neck and shoulders. In a controlled experiment, we investigated the extent to which more variation of upper arm postures in a one-hour repetitive task influences maximal acceptable work pace (MAWP), perceived exertion (RPE), kinematics and muscle activity.

Methods. 13 participants (6 females, 7 males; age 26 (SD 3) years) performed a pick-and-place task for one hour, using their dominant hand to movie pins between two targets. We compared three conditions in which the hand was moved: (1) horizontally, at an intended upper arm elevation of 30°; (2) obliquely, at an upper arm elevation between 20° and 40°; and (3) vertically, at an upper arm elevation between 10° and 50°. Using a psychophysical approach — with imposed work paces changing every two minutes (7-13 cycles/min) — we arrived at the MAWP of each participant. Postures of the arm, trunk and shoulder were recorded throughout, as was the activity of selected muscles (not reported here). Participants reported their RPE (Borg CR-10) at baseline and at MAWP.

Results. The kinematics data confirmed that the conditions had similar average upper arm elevations (32.3° (SD 1.0°) but differed in variation (arm elevation SD: 5.2°, 8.1°, 10.9°). Increased posture variation did not lead to changes in MAWP (10.7, 10.6, 10.8 cycles/min), though it did lead to slightly lower RPE values (average increase from baseline: 5.4, 4.8, 4.7).

Discussion.Increased biomechanical variation has been suggested to reduce the risk of developing musculoskeletal disorders. Even though our data suggest that there may be a trend towards a positive effect of variation on work perception, the increase in posture variation imposed here was not sufficient to influence performance. Further analyses of arm, shoulder and trunk kinematics and muscle activity patterns may reveal biomechani-cal differences of interest between the protocols.