Balancing the Centre of Body Mass During Standing Up

When individuals stand up from the sitting position, the body mass must be controlled within the limits imposed by the need to balance the massive upper body segment while the total body rotates and pivots over a fixed foot. Since postural adjustments occur in any movement of the body over a fixed base of support, standing up provides an interesting model for studying balance. Standing up requires an initial impulse in the horizontal direction, changing to the vertical direction when the thighs are lifted off the supporting surface. The horizontal distance moved by the centre of body mass (CBM) and the timing of lower limb extensor force in relation to the position of the CBM appear to be critical to ensure both a change in direction and the preservation of equilibrium. The findings from a biomechanical study of standing up, in which the extent of arm movement was varied, indicated that restricting arm movement affected the momentum and position of the CBM in relation to the base of support at thighs-off.