This thesis describes the measurement of high g-forces, in the range of 400-800g's occurring in a spinal construct during the breaking of the set-screw head using a manual torquing instrument. The measurements were validated using a high speed camera. The design torque (11 N-m) required for breaking the set-screw was compared with the torque calculated from the distortion energy theory for material fracture (9.9 N-m) and an actual measurement using a torque wrench (11.3 N-m). A comparison between the manual and powered instruments showed a 27.68% reduction in g-forces and a 36.42% reduction in die-down time while using the powered instrument. 8.52% less energy was felt on the adjoining screw. The consistency in the powered instrument is higher because the standard deviation using the manual instrument is 8.46, compared to the powered instrument which was 8.31. Also 25g's was recorded on the surgeon's wrist and elbow. Based on previous work done, the external work done by the surgeon was about 60-120 KJ. The onset of fatigue was apparent in consecutive break events as illustrated by the change in EMG parameters over time.

Repetitive and forceful exertions have been dentified as an important risk factor for occupational injuries. One method used to determine appropriate exposures to these and other risk factors is psychophysics, which is based on individual perceptions of task demands and/or risk. Effects of work exposure have been indicated as of potential importance, but have not been well studied. Indications from an earlier study related to psychophysical limits for a repetitive manual torquing task were that five days of work conditioning had minimal effects on resulting Maximum Acceptable Repetition Rates (MARR). However, it is unknown whether and how longer work exposure durations might influence MARRs. The current study investigated the effects of work exposure on MARR and adjustment time over 10 working days (two weeks) with two days of rest after five days. Ten participants (five males and five females) performed a manual torquing (45 Nm load) task at mid-chest level in the coronal plane for a one hour test session. Starting repetition rate for each participant was set at single high and low rate on alternate days. Temporal (exposure) effects were determined, where day of exposure was the independent variable, and MARR and adjustment time were the dependent variables. Final MARRs were relatively lower during the first few exposure days (14 - 15 repetitions/min) and increased for days 5 - 7 (16 - 18 repetitions/min). On average participants made four adjustments to reach MARR . Day was not found to significantly affect MARR, though week affected both MARR and adjustment time. Thus, an exposure of two weeks may be needed to obtain stable and valid psychophysical limits for manual torquing and, perhaps, related tasks. / Master of Science