Human Postural Sway - Essay Example

Postural Control Advanced Lab Write-up Postural sway during quiet standing is the resultant action of various destabilizing forces acting on the body and actions by the postural control system to prevent a loss of balance. Human postural sway is a stochastic process and can be represented in terms of the fluctuations of center of pressure (COP) under the feet of a quietly standing subject (Chiari et al., 2000; Chow and Collins, 1995; Collins and De Luca, 1993, 1994, 1995; Ohira and Milton, 1995; Peterka, 2000; Sabatini, 2000; Yao et al., 2001). According to the fluctuation-dissipation theorem (FDT), the fluctuations of a quasi-static, stochastic system and same system's relaxation to equilibrium following a perturbation can be linearly correlated [Honerkamp, 1998; Kubo, 1966; Pathria, 1972]. This paper explores whether anterior-posterior (AP) sway of center of pressure (COP) can be linearly applied to predict the AP response of a weak posterior directed mechanical perturbation (tug or pull at the waist) in a healthy subject and effects of muscle fatigue on postural control system. In this paper we present the observation where 10 healthy subjects of mean age 25 are examined to find out a relationship between postural sway during quite-stance and perturbation with and without muscle fatigue. We also observe the effect of vision on postural sway on both normal and fatigued conditions. The results suggest that it is possible to predict an individual's dynamic response to a mild perturbation using quiet-stance data, regardless of age and health. Introduction Human control of upright body posture involves inputs from several senses (visual, vestibular, proprioceptive, somatosensory) and their central interactions. Multiple sensory systems of human body are involved in tandem for controlling quite standing. Studies show that there is an indirect and presumably cognitive relationship between visual effects on posture control and their intersensory interactions (BLMLEA. et al, 2006). Human postural control during upright stance is typically assessed in either of two modes: 1. Quiet stance: Stance with no intended movement (a ''quasi-static'' condition) 2. Perturbed stance: Quiet stance that is disturbed by an external perturbation, such as a push at the sternum or a sudden shift of the support surface (a ''dynamic'' condition) During quite stance position, center of mass (COM) is stabilized over base of support by using low level muscular movements and body sways around the point of support like an inverted pendulum (Johansson R, Magnusson M, Akesson M. 1988). This led to the hypothesis of inverted pendulum. Any defect, alteration or malfunctioning of the sensory or motor components increases body sway and hence increases the muscle activity to maintain postural equilibrium (Dietz V. 1992). Minor perturbations occurring during normal stance can be counteracted by the regulation of ankle muscles (Schieppati M et al, 1994 and McClenaghan BA et al 1996). Muscle fatigue is a complex phenomenon that has been defined as a reduction in the force-generating capacity of muscles, regardless of the task performed (Bigland-Ritchie B, Woods JJ.1984). Though, how fatigues affect the postural control system is not clear there are several fatigue related mechanisms involved at different levels of the nervous system that could affect the regulation of these small forces. Muscle fatigue causes failure of transmission of neural signals and disables the muscles to respond to the neural currents (Bigland-Ritchie B, Woods JJ.1984). Muscle fatigue also alters the basic functioning of complete nervous system and causes failure of motoneurons excitement. Effects of a muscle fatigue on human postural sway can be studied by inducing momentary fatigue by physical exertion. Studies show a mild difference in effects of a fatigue on sway with and without vision (Lepers R. and Nardone A. et al). To examine the effects of a fatigue on human postural control, in this experiment muscle fatigue is induced in ankle plantar-flexors with some artificial setup. Postural stability was measured over 60-s periods and sway areas, diameters and velocities of COP was examined. The data gathered was fed on Labview for time/ frequency domain and stabilogram-diffusion analyses. Time and frequency domain analysis gives the idea of dimensional aspects of sway whereas stabilogram-diffusion analyses can be used to identify underlying motor control processes (Collins JJ et al). Stabilography technique is also known as posturography or force-plate technique. It is used for the analysis of upright postural control forces acting on body and evaluation of functional state of human body. In stabilography technique, small anterior-posterior and m-lateral postural sways are measured simultaneously. In this experiment, postural sway in subjects was examined in four conditions: Sway in quite stance condition with and without vision Sway in perturbed condition with and without sway Effect of vision on postural control was analyzed as absence of vision may exacerbate the sway in normal as well as fatigued state. Human body need to rely more on ankle proprioceptive inputs for postural control in the absence of vision. Lauk et al. (1998) applied a linear relationship based on the FDT to the human postural control system. By conducting quiet- and perturbed-stance experiments, they were able to show in young adult subjects that anterior-posterior (AP) fluctuations of the COP during quiet stance can be used to predict the AP response of the dynamic postural control system to a weak posteriorly directed perturbation through a linear relationship. They also concluded that, since this FDT-based linear relationship held for these conditions, the postural control system uses the same neuromuscular control mechanisms during quiet-standing and mild-perturbation conditions. Methods Subjects Ten healthy male subjects of mean age 25, height 177 cm and body weight 76 kg with no gait, postural, neuromuscular, or musculo-skeletal abnormalities were observed. All the subjects were aware of study and a formal informed consent has been obtained from each subject. Experiment procedures: Twenty randomized trials were conducted on each subject: 10 quiet-standing trials and 10 perturbed state trials, all 30 s in duration. For all 20 trials, the subject was instructed to maintain a quiet, upright posture throughout the recording (Fig. 1). To initiate a perturbation, which was a weak impulse disturbance (a backward tug), a mechanical trigger was activated that released a 2.3 kg mass. After the weight fell, the perturbation mechanism caused the tether to quickly slacken, thus allowing the subject to adjust to an upright posture. The tug necessitated only a postural sway response, i.e., the subject did not need to take a step to maintain balance. The trigger was activated after a random delay of 10- 20 s from the start of the trial. The subject received no cues as to if or when the perturbation would occur. For all trials, the COP time series data in the AP (y) direction was computed from force-plate measurements, which were sampled at 100 Hz. A series of ten trials was performed with eyes open. Another series of ten trials was conducted with eyes closed. The order of testing (eyes open versus eyes closed) was randomized between subjects. Rest periods of 30 seconds were provided between each trial and five minutes between conditions. Two identical series of ten trials with fatigue were then performed. Trials without fatigue were always performed before trials with fatigue. Results After experiment, sway trajectory, mean velocity, standard deviation and maximal instantaneous velocity of the COP along A-P and M-L axes were calculated for all conditions. Standard deviation of gathered data is calculated to measure amplitude variability of the COP around the mean position. The mean velocity represents the total distance covered by the COP or total sway path) divided by the duration of the sampled period and constitutes a good index of the amount of activity required to maintain stability. The maximal instantaneous velocity represents the worst case scenario of the anterior COP velocity occurring during a trial. Analysis results show that there is an increases postural sway in subjects with muscular fatigue. They exhibited an increased center of pressure (COP) velocity, greater COP mean and median frequency] and a decreased long term scaling exponent compared to the control conditions. It was also observed that though fatigue enhances postural sway, it did not modify the range of oscillations and the variability of the postural oscillations around the mean position of the COP. The effect of muscular fatigue on postural sway was found to be similar in both with and without vision condition. These results suggest that fatigue did induce some changes in the control mode of postural stability but that the detection/action capabilities (due to vision) of the sensorimotor system remained partly efficient when the ankle plantar-flexors were fatigued. Fig: A normal stabilogram Velocity of the COP Fig: Graphs showing mean COP velocity in normal and fatigued state with and without vision. Fig: Spectral enveloppes calculated by averaging the spectral signatures of each subject for the M-L and the A-P axes Frequency of COP Fig: Graphs showing median COP frequency in normal and fatigued state with and without vision. Fig: Graphs showing mean COP frequency in normal and fatigued state with and without vision. Discussion Our results suggest that, by applying a linear relationship derived from the fluctuation-dissipation theorem (FDT), it is possible to predict the dynamic response of the postural control system after a mild perturbation using only quiet-standing COP data. From above graphs it can be observed that the effects of fatigue are similar with and without vision which means postural sway increases if there is muscle fatigue. Studies by various researchers shows that vision did not interact with fatigue with the possible exception of a one-leg stance experiment (Vuillerme N et al, 2001) where reinserting vision interacted with localized fatigue of the ankle muscles for a few seconds only. Altogether, these observations suggest that the effect of fatigue may affect more the motor output than the sensory system. Alternatively, it is possible that the sensory system is affected mainly at the peripheral level through a change in the spindle threshold of the fatigued muscles but other receptors (e.g. tactile and pressure sensors) could compensate for this increased threshold. This could explain the absence of a differential effect when vision was withdrawn. Findings of this experiment also suggest that the neuromotor system uses the same postural control mechanisms for quiet stance and perturbed conditions. This stems from the FDT-based conclusion that when the postural control system is not in equilibrium, it cannot distinguish whether it was brought into that state by an intrinsic, random fluctuation or a mild, external perturbation. These findings may have promising clinical implications. They suggest that it may not be necessary to perform tests that involve mild perturbations to assess postural control capacity in healthy adults. Further studies are necessary to determine whether this technique is applicable to individuals with impairments in balance that may place them at a greater risk for falls, and to tests using larger perturbations. If it can be shown that this FDT-based technique is applicable to such conditions and individuals, then perturbation tests may be deemed unnecessary. 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