Background. Relative blood volume (RBV) and muscle oxygenation (TSI) can be measured using near infrared spectroscopy (NIRS), but no studies have used NIRS with workers performing their job in the field. The study aims were (1) to assess day-to-day within-subjects dispersion in NIRS measurements during work, and (2) to determine whether trapezius RBV and TSI differed between office and industrial workers. Electromyography (EMG) measured trapezius muscle activity.

Methods. Portable NIRS and EMG instruments were adhered to the trapezius of healthy female industrial (n = 8) and office (n = 10) workers for approximately four hours on two separate days. Mean and standard deviation (SD) RBV, TSI and 50th percentile EMG were calculated for both days separately. Participants were videotaped to demarcate work and rest periods, and to qualitatively assess ergonomic exposures. Two-way mixed effects models were constructed to examine outcomes, with occupation (office/industrial) and work/rest as fixed effects and subject as a random effect.

Results. Industrial workers appeared to have more variable arm postures and handled heavier loads than office workers. The between-days variability of RBV and TSI indicated that NIRS performed well in an occupational setting. Median trapezius EMG showed an effect of occupation (p < 0.0001), and an interaction between occupation and work/rest (p < 0.0001). As expected, industrial workers had higher median EMG overall and during work, but office workers had a higher EMG during rest. Similar results for EMG SD were found. Mean RBV and RBV SD were greater in industrial workers. No effects on mean TSI were found. However, TSI SD was higher in industrial workers, while TSI SD was greater in office workers during rest.

Discussion. NIRS had a satisfying reliability and showed face validity with respect to expected responses to occupational work. NIRS shows promise as a method for measuring hemodynamics in the field.