Use of near-infrared spectroscopy in evaluating upper extremity muscle hemodynamics during an eight-hour exposure to repetitive hand-wrist motions

We evaluated local hemodynamic responses simultaneously from flexor and extensor muscles in the forearm of 14 male workers while performing repetitive ulnar deviation task for 8 hours. First, a psychophysical approach was utilized to identify participants´ maximum acceptable torque (in Nm) at 15 and 25 repetitions per min (rpm). Second, while performing an ulnar deviation task repetitively for 8 hours at their psychophysically acceptable torque, Tissue Oxygenation Index (TOI) and Tissue Hemoglobin Index (THI) were collected. Workers exerted lower acceptable torque for the task with a repetition rate of 25 rpm as compared to 15 rpm (3.57 ± 2.7 versus 3.97 ± 2.5 Nm, P=0.014). When the TOI responses (representing muscle oxygen saturation) after each hour of effort were matched for the net acceptable torque exerted, the mean TOI-torque ratio was 10.2% higher in the flexor region as compared to the extensor region throughout the workday. Also, when corrected for the torque exerted during both the 15 rpm and 25 rpm tasks, a dynamic adjustment of THI (representing muscle blood volume) to the time course of TOI resulted, suggesting a strong coupling between muscle blood flow and oxygen uptake as the task continued. Although the acceptable torque exerted was lowest for the 25 rpm task, the increased TOI-torque ratio coupled with increased THI per torque generated suggests that working at 25 rpm is metabolically more challenging than at 15 rpm.