Adjustment type psychophysical methodology has been used to develop guidelines for repetitive upper limb movements. The acceptability of adjustment type psychophysical methodology for upper limb tasks has not been examined both in terms of the biomechanical loading of the tissues at risk and in terms of the ability of workers to determine loads presented. There is a growing recognition that repetitiveness includes not only frequency of exertion (cycle time), but also the duration of exertion within the cycle time (duty cycle). These two dimensions of repetitiveness are not always controlled for in psychophysical studies. This thesis used adjustment type methodology, under different conditions of cycle time and duty cycle, to determine acceptable torque for an industrial task, in-line screw running. This task was chosen to add to the large library of workplace specific tasks that will be required for psychophysical guidelines to be useful. It also provided a task that requires response to external force generation typical of powered hand tool use as opposed to many existing studies where muscle activity causes the initial activity to occur.

Eight female workers performed a simulated screw running task consisting of grasping a 46 mm diameter handle driven by a computer controlled torque motor, simulating an in-line screw runner. The women worked for 15 days:​ 3 days of training, 11 days of adjustment type psychophysical data collection and 1 day of more detailed biomechanical data collection. The participants were trained to prevent the handle from turning while the motor repetitively applied a torque. During psychophysical data collection different conditions of duty cycle (25, 50, and 83% of time) and cycle time (3, 6, 12 and 20 s) were used and the women were instructed to adjust the torque level such that they were working as hard as they could without undue discomfort. Electromyographic (EMG) signals from extensor carpi radialis brevis (ECRB), flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), and trapezius was recorded every hour during a test contraction to study muscle fatigue. Biomechanical measures taken on the last day for each condition at both the psychophysically determined acceptable torque and a reference torque included EMG, wrist angle, tool angular movement, hand torque, and hand grip force.

Duty cycle was found to significantly affect the amount of torque selected. With duty cycle controlled, cycle time was no longer found to have any significant effect on acceptable torque. Psychophysically determined acceptable torques (PDAT) for 25, 50 and 83% duty cycle were 1.09, 0.9, and 0.73 Nm. Discomfort and stiffness were concentrated on the back of the hand and the thumb web. While there was a significant difference with duty cycle for hand force and EMG during the “operating’' phase of the cycle, there was no difference averaged over the whole cycle. This was due to the biomechanical duty cycles for force and EMG becoming longer than the applied motor duty cycle. At the 83% motor duty cycle the biomechanical duty cycle for EMG was above 90% leading to static muscle loads greater than 1% MVC.

Despite having no effect on the PDAT and discomfort, decreased cycle time was associated with increased flexor muscle fatigue. The results would suggest that muscular fatigue was not a primary sensation used in selection of acceptable torque. The fatigue could be the result of reduced amount of time available for the muscle to completely shut off (i.e. fewer gaps).

This thesis gives psychophysically developed guidelines for a workplace specific task requiring response to an externally generated force typical of powered hand tools. It is concluded that:​ 1) Duty cycle is an important factor in psychophysical adjustment studies of highly repetitive upper limb tasks and needs to be included to generalize results, 2) Participants appear to choose the load based on time weighted hand grip force or muscle activity, 3) Participants are able to estimate the load being adjusted according to Steven’s Law in such a way that they under estimate low loads and over estimate high loads, 4) The PDAT didn’t appear to be based on muscle fatigue, which may limit its use in preventing muscle pain 5) Biomechanical duty cycle is longer than that implied by the task 6) A task duty cycle of 83% with high repetition tasks may lead to static muscle loads of greater than 1% MVC and 7) Based on psychophysical data collection methods, for the monotask of highly repetitive in-line screwrunning, it is recommended that the torque not exceed 1.06 N.m with 25% duty cycle or 0.79 N.m with a 50% duty cycle.