Instant Equilibrium Point and Its Migration in Standing Tasks: Rambling and Trembling Components of the Stabilogram

Abstract

A method of decomposing stabilograms into two components, termed rambling and trembling, was developed. The rambling component reveals the motion of a moving reference point with respect to which the body's equilibrium is instantantly maintained. The trembling component reflects body oscillation around the reference point trajectory. The concepts of instant equilibrium point (IEP) and discrete IEP trajectory are introduced. The rambling trajectory was computed by interpolating the discrete IEP trajectory with cubic spline functions. The trembling trajectory is found as a difference between the approximated rambling trajectory and the COP trajectory. Instant values of the trembling trajectory are negatively correlated with the values of the horizontal ground reaction force at a zero time lag. It suggests that trembling is strongly influenced by a restoring force proportional to the magnitude of COP deviation from the rambling trajectory and acts without a time delay. An increment in relative COP position per unit of the restoring force, in mm/N, was on average 1.4 ± 0.4. The contribution of rambling and trembling components in the stabilogram was ascertained. The rambling variability is approximately three times larger than the trembling variability.

Influence of prolonged wearing of unstable shoes on upright standing postural control

“…The RM and TR displacement components were obtained according to the method proposed in (Zatsiorsky & Duarte, 1999). In brief, the RM component expresses the movement of a moving reference point (an attractor point), with respect to which the balance of the body is maintained instantly.…”

“…The reduction of the P-P of RM trajectory in the anteroposterior direction reflects a higher efficiency of the postural control system (Bennell & Goldie, 1994;Kinzey, et al, 1997;Norris, et al, 2005;Prieto, et al, 1996) related to supraspinal processes that define an instantaneous point about which the body is stabilised (Zatsiorsky & Duarte, 1999. The increased MV of the RM component could be related to a reweighted combination of reciprocal activation and co-activation commands (Drew & Rossignol, 1987;Feldman, 1980aFeldman, , 1980bFeldman & Levin, 1995;Lacquaniti, 1992;Lacquaniti et al, 1991;Levin et al, 1992).…”

“…This association is corroborated not only by the decrease of the most representative CoP displacement parameters and RM P-P, but also by decreased anterior-posterior TR RMS. Changes in the TR RMS indicate an increased effectiveness provided by an adaptation of spinal reflexes and changes in the intrinsic mechanical properties of muscles and joints (Zatsiorsky & Duarte, 1999).…”

“…The importance of CoM measurements in association with CoP measurements is because the difference between the two variables is proportional to the horizontal acceleration of the CoM representing the "error" signal in the balance control system (Winter, 1995b). According to Zatsiorsky and Duarte, 1999, the nature of postural sway is the result of a moving reference point (rambling, RM). This moving point is related to the supraspinal process and constitutes a reference about which the body oscillates (trembling, TR) through the action of spinal reflexes and changes in the intrinsic mechanical properties of muscles and joints (Zatsiorsky & Duarte, 1999).…”

“…According to Zatsiorsky and Duarte, 1999, the nature of postural sway is the result of a moving reference point (rambling, RM). This moving point is related to the supraspinal process and constitutes a reference about which the body oscillates (trembling, TR) through the action of spinal reflexes and changes in the intrinsic mechanical properties of muscles and joints (Zatsiorsky & Duarte, 1999). The decomposition technique of CoP time series proposed by authors to assess RM and TR has been demonstrated to provide a very good estimate for both components (Lafond et al, 2004).…”