PHYS THER
Vol. 87, No. 5, May 2007, pp. 570-571
DOI: 10.2522/ptj.20060262.ar
Author Response
Kelly P Westlake and
Elsie G Culham
We would first like to thank Madhavan and Shields for their insightful comments regarding our research, and we appreciate having the opportunity to respond. They describe the importance of including a measure of dynamic position sense to enhance the functional and scientific meaning of our results.
We concur with the importance of targeting the peripheral and central aspects of proprioception during measurement. Madhavan and Shields postulated that voluntary motor responses to fast testing velocities reflect the ability to extract early velocity information needed to recover from a fall. Indeed, a measure of dynamic position sense could enhance our results regarding the sense of position, velocity, and central processing time. However, the aim of the current study was to identify training effects on the conscious perception of position and movement regardless of central processing time. Our interest in the central effects of training is apparent in part II of the current study in which we focus on the central integration of proprioceptive inputs during a postural task.
Despite the high velocity requirements during fall recovery, our decision to select slow testing velocities, particularly for velocity discrimination, was twofold. First, the velocity-dependent nature of movement detection saturates at speeds above 5°/s in both younger and older adults.1,2 Second, the chosen velocities were deemed low enough to reduce age-related differences in central processing time of proprioceptive signals.3 Therefore, for the purposes of our study, we did not consider high testing velocities advantageous for our desired outcomes.
Madhavan and Shields also make a valid point in that electromyographic recordings may have given us insight as to whether improved velocity discrimination values were due to pre-exercise and post-exercise differences in the ability to relax the muscles. However, assuming that older adults are consistent in their attempt to increase muscle spindle gain through co-contraction of the muscles, this effect would have been seen across all 3 of our tests. With only one of our measures demonstrating improvements, we may assume that the post-exercise gain of the muscle spindle did not contribute to our outcomes.
At this point, we should mention that our improvements in velocity discrimination were noted under passive conditions. Thus, characterization of influence of cortical drive was not relevant to this protocol. We agree, however, that cortical influence to position sense represents an important component of proprioception, and we hope that future research may further probe this potential avenue for improvement.
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References
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- Thelen DG, Brockmiller C, Ashton-Miller JA, et al. Thresholds for sensing foot dorsi- and plantarflexion during upright stance: effects of age and velocity.
J Gerontol A Biol Sci Med Sci. 1998;53:M33–M38.[Abstract]
- Chaput S, Proteau L. Modifications with aging in the role played by vision and proprioception for movement control.
Exp Aging Res. 1996;22:1–21.[Medline]
Copyright © 2007 by the American Physical Therapy Association.
