The role of anticipatory postural adjustments in compensatory control of posture: 2. Biomechanical analysis
Introduction
The equilibrium of vertical posture is achieved when the center of mass (COM) of the body is positioned over the base of support (BOS) and is aligned with the center of pressure (COP). Any body perturbation, either external such as a sudden translation of the support surface or internal such as fast arm or leg movement, shifts the projection of the COM closer to the borders of the BOS and the alignment between the COM and COP is disrupted: this may result in the loss of body equilibrium. To minimize the danger of losing equilibrium, the central nervous system (CNS) utilizes anticipatory postural adjustments (APAs) by activating the trunk and leg muscles prior to the forthcoming body perturbation. (Belenkiy et al., 1967, Massion, 1992, Aruin and Latash, 1995, Li and Aruin, 2007). Theoretically, minor equilibrium disturbances could be counteracted with involvement of APAs only. In the real world, however, this happens rarely since the body perturbations are too large to be counteracted using just APAs. Thus, the CNS uses compensatory postural adjustments (CPAs) that are initiated by the sensory feedback signals (Park et al., 2004, Alexandrov et al., 2005). As such, CPAs serve as a mechanism of restoration of the position of the COM after a perturbation has already occurred.
While important information about the individual role of APAs and CPAs in control of posture is available in the literature, to the best of our knowledge, there are no studies investigating systematically the role of APAs in subsequent control of posture after a perturbation has occurred, i.e., during the CPA phase. Understanding the role of APAs in compensatory control of posture is important because activities such as throwing or catching a ball (that induce expected perturbations) or sudden trunk disturbances such as pulling or pushing (that might be considered as unexpected perturbations) are commonly used by clinicians to treat individuals with orthopedic and neurologic impairments (Kisner and Colby, 2007). Such perturbations are also used for balance control training or physical fitness in the elderly. However, little is known about the role of anticipatory postural adjustments, specifically, its relationship with CPAs in controlling body balance.
In a recent study, we investigated a relationship between the anticipatory and compensatory EMG activity recorded in the trunk and leg muscles prior to and after external perturbations (Santos et al., 2009). It was shown that if present, anticipatory EMG activity in the trunk and leg muscles scales down the magnitudes of compensatory EMGs. In contrast, when no APAs were generated, greater activity and opposite sequence of muscle activation were seen during the CPA phases.
The principal purpose of the present study was to investigate the relationship between APAs and CPAs from a kinetic and kinematic perspective. This study tested the hypothesis that the predictability of a perturbation influences the relationship between COM and COP displacements. Our second hypothesis was that the joint angular displacements after the perturbation would be larger for the unpredictable as compared to the predictable conditions.
To test these hypotheses we utilized an experimental paradigm that induces external perturbations at the shoulder level; this type of perturbation, for example, catching a ball, is commonly used in clinical settings to enhance/restore balance control, (Kisner and Colby, 2007). The perturbations were applied with the subjects in the standing position, with their eyes open and eyes closed: this introduced predictable and unpredictable body perturbations of identical magnitudes.
Section snippets
Subjects
Eight subjects (four males and four females) free from any neurological or musculoskeletal disorders participated in the experiments. The mean age of the subjects was 25 ± 2 years, mean body mass 59.1 ± 6.5 kg, and mean height 1.67 ± 0.08 m. They all signed a written informed consent approved by the Institutional Review Board of the University of Illinois at Chicago.
Procedure
The subjects were instructed to maintain upright stance while standing barefoot on the force platform with their feet shoulder width apart.
Angular displacements
Changes in the angular position of the ankle, knee, hip, spine, thorax, and head are shown in Fig. 1. Note that the angular position was calculated during the interval from −200 ms to +400 ms in relation to T0, each point represents the average of 50 ms time window and their respective standard errors, and the four 150 ms epochs are shown. There were no anticipatory displacements in the ankle, knee, and hip joints while small anticipatory displacements could be seen in the spine, thorax, and head
Discussion
In this study, we analyzed the anticipatory and compensatory postural adjustments triggered by external predictable and unpredictable perturbations of the same magnitude applied at the shoulder level of standing subjects. There were several principal findings. First, the patterns and magnitudes of joint angular displacements differed between the predictable and unpredictable conditions. Thus, small changes in the lower extremities joint angular positions were seen after the predictable pendulum
Conclusion
Unpredictable perturbations, that were not associated with any anticipatory corrections, induced large compensatory changes in the angular positions of the ankle, knee, and hip joints, and larger displacements of the COM followed by displacements of the COP. In contrast, APAs seen in conditions with predictable perturbations initiated COP displacements, resulting in better arrangement of the body position prior to the impact; this led to smaller compensatory COP-COM excursions and smaller
Acknowledgement
This work was supported in part by NIH grants HD-50457 and HD-51628 and NIDRR grant H133P060003.
Marcio J. Santos graduated in Physical Therapy (PT) from the Universidade Estadual de Londrina (UEL), Brazil. He obtained his Master Degree (M.S.) in Human Physiology from the Universidade Estadual de Campinas (UNICAMP), Brazil, and his Ph.D. in Rehabilitation Science from the University of Kansas Medical Center (KUMC), USA. He worked as a postdoctoral research associate in the Knecht Movement Science Laboratory at the Department of Physical Therapy at the University of Illinois at Chicago
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Marcio J. Santos graduated in Physical Therapy (PT) from the Universidade Estadual de Londrina (UEL), Brazil. He obtained his Master Degree (M.S.) in Human Physiology from the Universidade Estadual de Campinas (UNICAMP), Brazil, and his Ph.D. in Rehabilitation Science from the University of Kansas Medical Center (KUMC), USA. He worked as a postdoctoral research associate in the Knecht Movement Science Laboratory at the Department of Physical Therapy at the University of Illinois at Chicago (UIC), USA. He is currently assistant professor at the Department of Physical Therapy at the Universidade Estadual de Santa Catarina (UDESC), Brazil. He has co-authored various scientific papers in the motor control, biomechanics and rehabilitation fields, testing healthy individuals and those with disabilities. His present research projects focused on: (1) feedforward and feedback postural control and (2) grip force control in healthy individuals and orthopedic and neurologic patients.
Neeta Kanekar received her Bachelor’s degree in Physiotherapy from Maharashtra University of Health Sciences, Seth G.S. Medical College and K.E.M Hospital, India and her Master’s degree in Physical Therapy from University of Illinois at Chicago (UIC), USA. After practicing as a PT for a few years, she is now pursuing her Ph.D. degree in the program of Kinesiology, Nutrition, and Rehabilitation with a major in Rehabilitation at UIC. She is currently working in the Knecht Movement Science Laboratory in the Department of Physical Therapy at UIC. Her primary area of research is related to rehabilitation, biomechanics, and motor control. Presently her research projects are focused on: (1) feedforward and feedback mechanisms of balance control, (2) effect of fatigue on postural control, (3) effect of balance training in healthy individuals, the elderly, and individuals with musculoskeletal and neurological disorders. She is a member of the Indian Association of Physiotherapists and the National Honor Society of Phi Kappa Phi, USA.
Alexander S. Aruin received his Ph.D. in Medical Cybernetics from the Moscow Institute of Artificial Organs and Transplantation in 1978 and a D.Sc. (Ph.D.) in Biomechanics from the Latvian Institute of Traumatic Injuries and Orthopedics in 1990. He was affiliated with the Moscow Institute of Electronic Engineering as Full Professor and Director of the Laboratory of Ergonomics and Biomechanics. After moving to the United States in 1992, he became a faculty at Rush-Presbyterian St. Luke’s Medical Center in Chicago and later a faculty member at Pennsylvania State University. He is currently a Professor of Physical Therapy, Bioengineering, and Kinesiology and the Director of the Knecht Movement Science Laboratory at the University of Illinois at Chicago, a Professor of Physical Medicine and Rehabilitation at Rush University, Chicago, and a Senior Clinical Researcher at Marianjoy Rehabilitation Hospital in Wheaton, Illinois. He has co-authored more than 100 referred papers and two monographs in the fields of biomechanics, kinesiology, motor control and neuromuscular disorders. His primary research interests have been focused on motor disorders and rehabilitation, biomechanics, and motor control. Dr. Aruin develops new technologies for training healthy people and for providing physical therapy and rehabilitation to injured and disabled individuals. He is a member of the American Society for Biomechanics, Society for Neuroscience, and International Society of Motor Control.