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In simplest terms, humans prevent themselves from falling by using two different types of balance recovery strategies to bring the center of mass (COM) back over the base of support (BOS) and maintain an upright posture – fixed-support strategies and change-in-support strategies [1].

Typically, our BOS is defined by the boundaries of our feet, both forward/backward and side to side. If one is using an ambulatory aid such as a cane or walker, this will increase the size of the BOS. In response to a smaller disturbance to our balance, humans commonly will use a fixed-support strategy. Fixed support strategies involve the generation of torques (rotation) about the hips, knees and/or ankles to help control the movement of the COM without changing the size of the BOS [2]. The arms may also be used to help create the necessary rotation to effectively keep the COM within the BOS. Think of riding public transit when there are no seats left on the bus - when you are standing and the bus comes to controlled stop, we often rotate about the hips, knees and/or ankles to help keep our COM within our BOS, without moving our feet. 

In response to a larger disturbance to our balance, one where we cannot use a fixed-support strategy, humans will typically use a change-in-support strategy. These responses involve actions that increase the size of the BOS. For example, taking a forward or backward step, or reaching out and grasping an object such as a handrail or similar [2]. Although these change-in-support strategies commonly require larger movements than the fixed-support strategies, they still occur incredibly fast, on the order of milliseconds. Of course, external factors can influence the effectiveness of these responses, for example an icy surface may make a reactive step much less effective due to the reduced coefficient of friction between the foot and the ground causing further slipping. Nonetheless, because a change-in-support strategy causes a large increase in the size of the BOS, a much larger range of COM motion can be accommodated before the COM is brought to a rest and upright posture restored/maintained [2].

If you would like to learn more about this subject, contact us today and we can connect you with one of our LISKE Team Experts who have studied this topic extensively.

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[1] Maki BE, McIlroy WE. The role of limb movements in maintaining upright stance: the "change-in-support" strategy. Phys Ther. 1997 May;77(5):488-507. doi: 10.1093/ptj/77.5.488. PMID: 9149760.

[2] Maki BE, McIlroy WE. Control of rapid limb movements for balance recovery: age-related changes and implications for fall prevention. Age Ageing. 2006 Sep;35 Suppl 2:ii12-ii18. doi: 10.1093/ageing/afl078. PMID: 16926197.