The habitual activity patterns of trapezius and postural back muscles (multifidus, iliocostalis, longissimus) of 23 female subjects with moderate physical activity were studied. Bilateral surface electromyographic (SEMG) recordings from start of work until bedtime were analyzed. The activity level was calibrated as percentage of root-mean-square detected muscle activity at maximal voluntary contraction (%EMG_max). Sixty-six previous trapezius recordings of females with moderate physical activity were included in some analyses to pursue the full range of variation in trapezius activity. Twenty-six of these were recorded twice, separated by 16-28 months. Median activity level and duration of periods with SEMG activity <0.5% EMG_max ("rest time"; only trapezius) and exceeding 2% ("burst time"), 10%, 30% and 50% EMG_max was determined. The trapezius median activity level ranged from 0.6 to 8.8% EMG_max, burst time from 9 to 84% and rest time from 2 to 84%. The activity patterns of the back muscles showed similar large inter-individual variation. Repeated trapezius recordings of the same subject showed high consistency, intra-class correlation coefficients ranged from 0.62 to 0.79 for different SEMG variables. Periods with high SEMG amplitude were of short duration; 7% of the trapezius recordings did not present time intervals (0.2 s duration) above 50% EMG_max. The activity patterns of the postural muscles, despite large inter-individual variability, were distinctly different from activity patterns of upper and lower limb muscles, reported by others (e.g., mean burst time 40-50% vs. 10-20%). We conclude that postural trunk muscles show idiosyncratic activity patterns with large inter-individual variation. High-threshold motor units are activated to a very minor extent.