Unified theory regarding A/P and M/L balance in quiet stance

Abstract

1. Control of posture in quiet stance has been quantified by center of pressure (COP) changes in the anterior-posterior (A/P) and medial-lateral (M/L) directions from a single force platform. Recording from a single force platform, researchers are unable to recognize two separate mechanisms that become evident when two force platforms are used. Depending on the stance position taken, many combinations of an ankle mechanism and a hip (load/unload) mechanism are evident. In side-by-side stance, A/P balance is totally under ankle (plantar/dorsiflexor) control, whereas M/L balance is under hip (abductor/adductor) control. In tandem stance, the A/P balance is dominated by the hip mechanism, with mixed and small or sometimes negligible contributions by the ankle plantar/dorsiflexors: for M/L balance, the reverse is evident; ankle invertors/evertors dominate, with mixed and small contribution from the hip load/unload mechanism. In an intermediate 45 degrees stance position, both ankle and hip mechanisms contribute to the net balance control in totally different ways. In the M/L direction the two strategies reinforce, whereas in the A/P direction the ankle mechanism must overcome and cancel most of the inappropriate contribution by the hip load/unload mechanism. A spatial plot of the separate mechanisms reveals the fact that the random-looking COP scatter plot is nothing more than a spatial and temporal summation of two separate spatial plots. A straight line joining the individual COPs under each foot is the load/unload line controlled by the hip mechanism. At right angles to this load/unload line in the side-by-side and tandem positions is the independent control line by the ankle muscles. In an intermediate standing position, the separate control lines exist, but now the ankle control is not orthogonal to the load/unload line; rather, it acts at an angle of approximately 60 degrees. The direction of these ankle control and load/unload lines also allows us to pinpoint the muscle groups responsible at the ankle and hip in any of the stance positions.

Relationship between body balance, lung function, nutritional status and functional capacity in adults with cystic fibrosis

“…Similarly to the current study, some authors demonstrated greater displacement in the anterior-posterior direction in COPD patients 3 ; however, Rocco et al 22 observed increased lateral center of pressure displacement (but no increased anterior-posterior center of pressure displacement) compared to healthy controls. Interestingly, balance in the lateral direction is primarily maintained by torques at the trunk muscles and movement, whereas anterior-posterior direction is primarily maintained by torques at the ankle 23 . Despite not having a control group, our study suggests that lateral imbalance in adults with CF may also be due to a marked impairment in respiratory muscle strength.…”

The influence of artificially increased hip and trunk stiffness on balance control in man

“…The ankle strategy assumes that the body's COM is repositioned by moving the whole body as a single-segment, inverted pendulum, about the ankle joints, either by controlling ankle muscle stiffness or by increasing ankle muscle activity and changing the "muscle-spring" offset position (Nashner and McCollum 1985;Horak et al 1990;Winter et al 1996Winter et al , 1998Morasso and Sanguinetti 2002). The hip strategy involves antiphasic motion about both the ankle and hip joints (Nashner and McCollum 1985;Horak et al 1990;Winter et al 1996Winter et al , 1998Runge et al 1999). Extensions of these strategies include a mixed hip-ankle strategy (Kuo and Zajak 1993;Kuo 1995).…”

“…Recently, however, this concept has been challenged for motion occurring during quiet stance (Aramaki et al 2001;Loram and Lakie 2002). Indeed, as body motion increases in amplitude and direction from that of quiet standing (Fitzpatrick et al 1992(Fitzpatrick et al , 1994Winter et al 1996Winter et al , 1998Gatev et al 1999;Accorneo et al 1997), to that induced by a support surface perturbation in the pitch plane alone (Cordo and Nashner 1982;Allum et al 1993;Horak et al 1997), and finally to that induced by combined roll and pitch plane perturbations (Moore et al 1988;Maki et al 1994aMaki et al , 1994bHenry et al 1998b;Carpenter et al 1999;Allum et al 2002), the multi-link nature of human postural corrections becomes increasingly prominent. This multi-link strategy involves hinging at the knees, hips and lower vertebral column, in addition to ankle joint motion .…”

Balance recovery is compromised and trunk muscle activity is increased in chronic obstructive pulmonary disease

“…The mechanisms underlying impaired balance in COPD are poorly understood; however, mediolateral (ML) balance control appears more impaired than anteroposterior (AP) control [4]. In quiet stance, ML balance depends on hip and trunk moments/movements, whereas AP control is controlled by ankle moments/movements [5,6].…”