BACKGROUND This study aimed to determine whether trunk stability muscles co-contract with body position as a factor of pelvic floor muscle (PFM) activity. MATERIAL AND METHODS Sixty-one healthy adults without pelvic floor dysfunction were examined for pelvic floor and trunk stability muscle activity in 4 body positions (ankle dorsiflexion and plantar flexion in standing position, and ankle dorsiflexion and plantar flexion in long sitting position). The activities of the PFMs via anal/vaginal probes, internal oblique (IO), multifidus (MF), tibialis anterior, and gastrocnemius muscles were measured by surface electromyography. Three-dimensional motion analysis measured the movement of the pelvis in real time according to the change in body position. RESULTS There was a significant increase in PFM activity from the ankle neutral position while standing for both ankle dorsiflexion and plantar flexion in standing position (p<0.05). In maximal contraction of PFM in the standing position, IO and MF were found to co-activate (p<0.05). CONCLUSIONS In standing position, the ankle dorsiflexion and plantar flexion positions activated PFMs, which was found to co-activate with trunk stability muscles. Pelvic floor training programs based on the results of this study may be helpful in patients with incontinence.