PHYS THER
Vol. 87, No. 5, May 2007, pp. 569-570
DOI: 10.2522/ptj.20060262.ic
Invited Commentary
Sangeetha Madhavan and
Richard K Shields
S Madhavan, PT, MA, is a doctoral student, Graduate Program in Physical Therapy and Rehabilitation Science, Carver College of Medicine, The University of Iowa, Iowa City, Iowa
RK Shields, PT, PhD, FAPTA, is Professor and Director, Graduate Program in Physical Therapy and Rehabilitation Science, Carver College of Medicine, The University of Iowa, 1–252 Medical Education Building, Iowa City, IA 52242
Address all correspondence to Dr Shields at: richard-shields{at}uiowa.edu
We commend Westlake and colleagues for an important study examining the benefits of a specialized balance training program on ankle proprioception of older adults. Few studies have examined the effect of a training program on the position and movement sense of the ankle in the aging population. The authors of this study provide evidence that a prescribed set of specific balance exercises can improve the ability of older adults to detect certain movement velocities at the ankle. This study is an important first step to understanding whether training programs that improve impairment (proprioception) translate into reduced falls in elderly people.
Proprioception, as routinely defined, is the ability to sense (detect) the position and motion of a joint in space. The authors of this study measured proprioception by having the individual reproduce a target joint angle passively, by having the individual detect the threshold to the start of a passive motion, and by having the individual detect the faster of 3 passive movements. Assessing joint position sense statically or at very slow velocities is important, but does not include the dynamic range of many movements (perturbations) associated with triggering falls. Therefore, we would like to expound on the importance of testing dynamic joint motions, including higher velocities; and emphasize that the central nervous system processing of proprioceptive information may be more important than merely detecting when the limb is moving. Perhaps there is a large "safety margin" for an altered ability to detect limb motion, but a more "stringent margin" for an altered ability to interpret what the incoming sensory information means. We believe that dynamic position testing methods probe the peripheral and central sources that may be impaired with aging.
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Dynamic Position Sense
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Dynamic position sense is defined as the knowledge of the angular position and velocity of a moving joint obtained through various sensory receptors and includes central processing of this information.1–3 Dynamic position sense may play an important role in postural stability by triggering another coordinated sequential movement in response to a joint position change. For example, postural perturbations causing movement at the ankle may trigger an upper-extremity movement that prevents a fall. During slow-velocity movements, the nervous system has the time to rely on information about the position of the joint (muscle spindle group II, III, IV afferents) before responding. Conversely, during higher-velocity movements, the nervous system must extract the early velocity information (muscle spindle group 1A afferents) in order to predict where the joint will be in order to "trigger" an appropriate response. The extent to which the central nervous system can "process" early velocity information in order to prepare a purposeful response (such as a sequential movement to prevent a fall) also is important when assessing the effect of therapeutic interventions on proprioception. The processing time during a perturbation at the ankle (or any joint) represents the minimum time needed by the central nervous system to extract the kinesthetic information, process the information, and trigger a purposeful response (ideally, in this case, to prevent a fall).1 Exercise interventions that affect the ability to process of proprioceptive information may have better scientific grounding for reducing falls in elderly people.
We previously reported that the ability to trigger an upper-extremity movement sequence using dynamic position sense testing of the ankle was decreased in elderly people.4 Elderly individuals did not have sufficient time to process sensory information obtained from the ankle, especially at the faster velocities of ankle rotation, causing them to have greater errors in triggering another movement response.4 The balance training program used by Westlake and colleagues was successful in improving slow-velocity discrimination sense in the older individuals. However, we wonder whether this intervention also would improve dynamic position sense, including central processing, as tested by others at the ankle of elderly clients.4,5
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Cortical Influence on Proprioception
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Westlake and colleagues used passive and active joint position sense to ensure that the training effects were not due to an improved motor response. By monitoring muscle electromyographic (EMG) activity, the authors could have verified this assertion. It appears conceivable, in the absence of EMG quantification, that the training program increased the gain to the spindle (alpha-gamma coactivation), which may have contributed to the improved detection to slow velocities. We previously observed that elderly people struggle to relax during many blinded passive movements and that this may be in an effort to increase the gain of the sensory system.4 This proposed increased gain does not detract from the findings of the study by Westlake and colleagues, but merely provides a mechanism to understand why the exercise intervention may have been effective.
Central cortical training also may influence proprioception. Cortical motor commands, through corollary discharge, are an important component contributing to the perception of joint angles during active movements.6 Westlake and colleagues demonstrated quite convincingly that proprioception, as classically tested, improved in the trained group and not in the control group. However, there is less certainty about the central adaptations, which may be most important in ultimately improving balance, decreasing falls, and improving perception of health quality in the elderly population. Impaired dynamic position sense, which includes central processing, has been shown to be associated with decreased single-limb stance time in elderly people who are healthy.4
Balance control involves a complex coordination of muscular responses, dependent on somatosensory, vestibular, and visual information. Therefore, any link between improved tests of somatosensory impairment and balance control would enhance the importance of the training program used by Westlake and colleagues. For example, if these same subjects trained with the proprioceptive testing method used in this study, we also would expect to see an improvement in proprioception. Indeed, would we also then expect an improvement in balance control? Accordingly, there is a need to probe into relationships among proprioception, balance, falls, and health-related quality of life to understand the specificity and optimal dose of therapeutic interventions.
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Summary
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Westlake and colleagues provide important groundwork for eventually understanding the relationship between certain methods of training and their effect on proprioception. Indeed, extending this work to dynamic position testing methods,1–5 characterizing the influence of cortical drive,6 and testing the extent to which changes in impairment (proprioception) will affect falls and health quality in the elderly population are all important considerations for future studies. We thank the authors for reporting this thought-provoking study, and we appreciate the opportunity to comment on this work.
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References
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- Cordo P, Carlton L, Bevan L, et al. Proprioceptive coordination of movement sequences: role of velocity and position information.
J Neurophysiol. 1994;71:1848–1861.[Abstract/Free Full Text]
- Shields RK, Madhavan S, Cole K. Sustained muscle activity minimally influences dynamic position sense of the ankle.
J Orthop Sports Phys Ther. 2005;35:443–451.[ISI][Medline]
- Shields RK, Madhavan S, Cole KR, et al. Proprioceptive coordination of movement sequences in humans.
Clin Neurophysiol. 2005;116:87–92.[CrossRef][ISI][Medline]
- Madhavan S, Shields RK. Influence of age on dynamic position sense: evidence using a sequential movement task.
Exp Brain Res. 2005;164:18–28.[CrossRef][ISI][Medline]
- Verschueren SM, Brumagne S, Swinnen SP, Cordo PJ. The effect of aging on dynamic position sense at the ankle.
Behav Brain Res. 2002;136:593–603.[CrossRef][ISI][Medline]
- Gandevia SC, Smith JL, Crawford M, et al. Motor commands contribute to human position sense.
J Physiol. 2006;571:703–710.[Abstract/Free Full Text]
Copyright © 2007 by the American Physical Therapy Association.
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