We spontaneously outstretch our arms when standing upon challenging surfaces, yet the effect of stretching the arms upon postural stability is unknown. We investigated whether stretching out the arms laterally improves postural control during tandem stance on a narrow beam. Twelve healthy participants stood upon a beam, right foot in front of the left foot, for 30 s with arms outstretched or down to the side, with eyes open and closed. Mediolateral head movement was characterised by Root Mean Square amplitude (RMS), sway path, velocity during the largest excursion and power spectrum. Spectra for lateral forces from a force platform beneath the beam were also recorded. Outstretching the arms significantly reduced RMS, sway path and velocity of maximum displacement of head movement with eyes closed but not with eyes open. A similar trend was present in the power spectra of head motion and sway platform lateral forces. In conclusion, outstretching the arms helps postural stability in challenging situations such as tandem stance on a narrow beam with eyes closed. Although the exact mechanisms require further investigation, the effects are most likely mediated by changes in segmental inertia and the ability to make corrective arm movements.