Effect of base of support size on ability to recover balance

Humans' ability to recover balance should depend on the size of the base-ofsupport (BOS) between the feet and ground. To test this hypothesis, I conducted experiments where subjects (n=15) were released suddenly from an inclined position by means of a tether and electromagnet, and recovered upright stance using the feet-in-place ankle strategy or mixed (hiplankle) strategy. I varied the size of the available BOS by adjusting the length of a block that the subject stood upon. I found that the maximum angle where subjects were able to recover balance (THETA - MAX) declined from 8.8 to 7.3 deg when BOS decreased from 100% to 75%, and from 7.3 to 5.0 deg when BOS decreased from 75% to 50%. THETA - MAX was 19% larger for the mixed than ankle strategy. However, recovery strategy did not influence the effect of BOS on THETA - MAX. These results confirm that BOS size strongly influences ability to recover balance.