Background Ergogenic evidence for caffeine is robust in sports and isolated limb tasks. Our objective was to evaluate a possible ergogenic effect on postural muscles, still unknown, through the Biering-Sorensens lumbar extension test. Methods A double-blind, controlled placebo, crossover study. Fifty-one healthy, physically inactive male subjects (18-25 years) with average body mass (BMI 18.5-24.9 kg/m2) were recruited. The subjects received oral caffeine (6 mg/kg) and saline (0.3%) in two cross-evaluations separated by one week. The primary outcome was the time in the Biering-Sorensen test after 1 hour of treatment. The secondary outcomes were peak lumbar extension force, rating of perceived exertion, EMGs median frequency and muscle recruitment of multifidus spinae and transversalis/interne oblique muscles, and cardiovascular variables (heart rate and blood pressure). Results 27 subjects were blindly treated with caffeine and saline. Caffeine was ergogenic during the Biering-Sorensen test. It increased lumbar extension time (d=0.34, P<0.05), but not peak force. The perception of effort decreased with caffeine (d=0.37, P<0.05). Caffeine increased muscle stimulation frequency (P<0.05) and recruitment (η²=0.49, P<0.05) of multifidus spinae. In the transversalis/interne oblique muscles, caffeine increased the median frequency (η² = 0.13, P<0.05) and the distribution of higher frequencies (P<0.05). Caffeine also increased muscle recruitment in the transversalis/interne oblique muscles (η²=0.94, P<0.05). Tachycardia and increased blood pressure at the lumbar test were higher in the caffeine condition (P<0.05). Conclusions Our results show that caffeine is ergogenic for postural muscles. Decreased RPE and improved muscle activity suggest central mechanisms of caffeine. Trial registration UTN U1111-1234-2079.