Age-related impact of varied time constraints in an assembly task on biomechanical and physiological exertions

Outline of reasons and objectives
Working under time constraints is particularly difficult for ageing workers. “Leeway” or “room for manoeuvre” may enable regulation strategies to be put in place or production contingencies to be coped with more easily. The objective of this study was to compare, depending on age, the physiological strains recorded during a repetitive assembly task, and to characterise the effects of leeway in time related to a feed system having higher or lower degrees of constraint during assembly cycles that could have contingencies.

Approach
14 junior subjects (25-35 year-olds) and 14 senior subjects (55-65 year-olds) performed an assembly task of the industrial type (cycle time of 22 seconds) under two different workpaces and under two information conditions: the pace could either be forced, with a production penalty if the task was not performed within an imposed cycle time, or allow leeway in time, with a production penalty if 3 consecutive cycle times were longer than 3 theoretical cycle times (66 seconds). Prior information on the occurrence of an assembly cycle with contingencies could be given to or withheld from the subjects. For each of the 4 conditions (for a total length of time of 50 minutes), the subjects performed 136 assembly cycles, 15% of which included contingencies requiring additional operations to be made within the same cycle time. The number of production penalties, the electromyographic activity of 12 muscles, and biomechanical parameters of the movement were analysed. The operatives worked standing up and the parts to be assembled were fed automatically to their work stations.

Main results
The main effects observed concerned age and rate conditions. Thus, the number of production penalties was higher at the forced pace than at the rate with leeway, and this applied more particularly for the senior subjects during the assembly cycles with contingencies. The assembly cycles with contingencies also generated muscular strain that was higher at the forced pace for both classes of age, and at the pace with leeway for the senior subjects only.
For all of the assembly cycles, the pace with leeway made it possible to reduce the muscular strain relative to the strain at the forced pace, and this was more pronounced in the junior subjects. The speeds of movement of the wrist were higher during the assembly cycles with contingencies relative to the speeds during the assembly cycles without contingencies, in particular for the senior subjects and at the forced pace. For the assembly cycles without contingencies, the speeds of the wrist were lower at the pace with leeway than at the forced rate in the junior subjects only. For the assembly cycles with contingencies, the speeds of the wrist were lower at the rate with leeway than at the forced rate for both classes of age. Finally, the condition with prior information about the occurrence of an assembly cycle with contingencies, coupled with the pace with leeway, was accompanied by a slight reduction in the muscular strain, essentially in the senior subjects.

Discussion
The pace allowing leeway in time made it possible to limit the production penalties, the biomechanical stresses and the muscular strains, both for junior and senior subjects, and in more marked manner during assembly cycles with contingencies. Furthermore, although the possibility of leeway in time enabled the junior subjects to perform all of the assembly cycles (with or without contingencies) with the same level of muscular strain, the level of muscular strain in the senior subjects remained slightly higher for assembly cycles with contingencies. Finally, having prior information about the occurrence of contingencies, coupled with the possibilities of leeway in time, made it possible to reduce the strain, in particular for senior subjects. Even though generalisation from a laboratory study should be considered with caution, preferring production systems allowing leeway could have protective effects for operatives, both junior and senior.
These results can enrich INRS training, and will be published in a peer-reviewed journal.