Background: Symptoms of fatigue and pain are often reported for the trapezius muscle in the shoulder. The present study evaluated endurance in the trapezius muscles of healthy participants using electric twitch mechanomyography (ETM). Methods: Surface electrodes and a tri-axial accelerometer were placed over the left trapezius muscle. Muscles were stimulated for 3 min each at 2 Hz, 4 Hz and 6 Hz. Maintenance of acceleration during muscle twitches was used to calculate an endurance index (EI). Subjects (n = 9) were tested on two separate days to assess reproducibility of the trapezius EI measurements. The endurance measurements were made on the wrist flexor and vastus lateralis muscles for comparison. Near infrared spectroscopy was used to measure muscle oxygenation (HbO2) during the stimulation protocol (n = 8). Results: Mean (SD) EI was 84.9% (8.7%), 63.3% (19.1%) and 41.7% (20.0%) for 2 Hz, 4 Hz and 6 Hz, respectively. The coefficients of variation were 7.4%, 11.3% and 24.0% for 2 Hz, 4 Hz and 6 Hz, respectively. EI values were significantly lower in the trapezius compared to arm and leg muscles (p < 0.05). HbO2 values were unchanged from resting values with electrical stimulation. Conclusions: The EI as measured by ETM may provide a reproducible method of evaluating function in trapezius muscles that is not influenced by oxygen saturation.