How Long Is Long Enough? Evaluating Sampling Durations for Low Back EMG Assessment

Abstract

Few ergonomic measurement tools explicitly state when and how to sample exposures. Traditional ergonomic sampling has used short, task-based or worst-case measurements, but these may misrepresent exposures, since they neglect the temporal variations throughout the workday. Understanding the representativeness of data from shorter measurement durations compared with full-shift measurements allows for optimization of measurements resources. This study compared a variety of low back electromyography (EMG) exposure metrics measured over a full-shift with the same metrics sampled over shorter durations to identify whether shorter durations provide representative measures of exposure. Portable EMG devices were used to measure low back EMG for 138 full work shifts in a range of jobs in heavy industry. Using a random start time, each full shift of data was resampled for 4 hr, 2 hr, 1 hr, 10 min, and 2 min. Exposure metrics from each duration were compared with the full shift using absolute and percent error, bias, and limits of agreement. Comparisons between one full shift and two full shifts were made for the subset of 35 workers with two measured workdays. Compared with full-shift data, bias is very low at all sampling durations. However, as sampling durations decreased from a full-shift to a few min, the absolute error, percentage error, and limits of agreement for exposure estimates show more deviation from full-day estimates. Estimates of mean and 90th percentile exposure averaged 8% error for 4-hr durations and 14% error for 2-hr durations. The errors for 4-and 2-hr measurement durations may be acceptable for many applications, particularly if the trade-off is measuring more subjects. Sampling durations of 1 hr or less seem likely to produce very large errors over all exposure metrics, particularly for the range and peak exposures. Depending on the purpose of measurement and the detail required, 4 hr or even 2 hr appears to be long enough to reasonably estimate full-shift exposure.

Keywords:

• EMG
• ergonomics
• exposure assessment
• low back disorders
• sampling duration
• sampling strategy

ACKNOWLEDGMENTS

The authors would like to thank all the workers and employers who participated in the study, as well as Kevin Hong, Nancy Luong, Melissa Knott, and James Cooper for their efforts during data collection. This study was funded in part by the WorkSafeBC Research Secretariat, the Canadian Institutes of Health Research Strategic Training Program Bridging Public Health, Engineering and Policy Research, and the University of British Columbia Centre for Health and Environment Research. Dr. Koehoorn was supported in part by a Michael Smith Foundation for Health Research Scholar Award; Ms. Trask was supported by a Michael Smith Foundation for Health Research Junior Research Studentship Training Award and a WorkSafeBC Research Training Award.

Notes

^A EMG in percentage reference contraction.

^B Indicates a significant difference between shorter duration metric and full-shift metric (p < 0.05).

^C The geometric mean (antilog of mean of ln-transformed values).