Base of Support
Original Editor - The Open Physio project
The base of support (BOS) refers to the area beneath an object or person that includes every point of contact that the object or person makes with the supporting surface. These points of contact may be body parts e.g. feet or hands, or they may include things like crutches or the chair a person is sitting in.
The BOS is an important concept to understand to understand an individual's ability to Balance, as balance is defined as the ability to maintain the line of gravity (passing through the Centre of Gravity) within the BOS.
In gait, the base of support has been defined as the horizontal stride width during the double-support phase when both feet are in contact with the ground and the whole-body center of gravity (CG) remains within the BOS. A wide base of support (BOS) has long been believed to be a hallmark of unsteady gait. 
- Wellar et al found that decreased stride width was closely correlated with fall frequency and dynamic stability. 
- The majority of the gait cycle is spent in single leg support during which the BOS is minimized to the width of the supporting foot. 
- Thus when a patient is tried for gait, the wider base of support by means of walking aids such as walker, crutches,cane,etc is provided to acquire more stability. As the condition of the patient improves, the BOS can be reduced.
- Krebs DE,Goldvasser D,Lockert JD,Portney LG,Gill-Body KM ;Is base of support greater in unsteady gait?:Phys Ther. 2002 Feb;82(2):138-47
- Nutt JG, Marsden CD, Thompson PD. Human walking and higher-level gait disorders, particularly in the elderly. Neurology.1993 ;43:268–279.