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THE EFFECTS OF PLANTAR DESENSITIZATION ON THE VARIABILITY AND LOCAL STABILITY OF TREADMILL WALKING IN HEALTHY ADULTS
Presentation SM13, Gait and Balance 3 – Quantification of Gait Stability. 8:30, Room 101CD
S208
THE EFFECTS OF PLANTAR DESENSITIZATION ON THE VARIABILITY AND LOCAL STABILITY OF TREADMILL WALKING IN HEALTHY ADULTS 1 B. Manor, 1O. Smith, 2A. Guevara, 2P. Wolenski, 1L. Li 1 Department of Kinesiology, 2Department of Mathematics, Louisiana State University, USA Table 1). Further, after desensitization the correlation between stride duration VAR to both short- and long-term local stability was not significant, whereas the degree of sensory loss was positively correlated with ȜLT (R2 = 0.40). 160
140
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ln(divergence)
INTRODUCTION The soles of the feet are the exclusive points of contact with the environment when walking. Feedback (FB) from plantar cutaneous mechanoreceptors therefore likely contributes to the integrity of walking by providing the central nervous system (CNS) with detailed pressure distribution information that may help control stride-to-stride temporospatial fluctuations [1]. Gait variability (VAR) quantifies the average magnitude of these fluctuations, whereas local stability quantifies the sensitivity of the system to small perturbations that arise as a consequence of VAR [2]. Simultaneously assessing VAR and local stability with desensitized foot soles (thereby reducing pressure-related FB) provides a unique opportunity to examine the role of plantar cutaneous FB in the control of human walking, and the relationship between variability and stability [3]. The purpose of this study was to investigate the effects of foot sole desensitization through ice immersion on the VAR and local stability of walking.
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METHODS Healthy college-aged participants (six men, seven women) completed two minute bouts of preferred speed treadmill walking under both normal and desensitized conditions.
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Desensitization was induced by exposure of the foot soles to shaved ice, and sensory loss was assessed by determining pressure detection threshold at five sites on the plantar aspect of the foot using a 5.07 gauge monofilament.
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Sagittal-plane joint position data were acquired using one high-speed camera (60 Hz) and reflective markers placed at the hip, knee, and ankle joints. Continuous knee joint angles were computed over 45 consecutive strides. VAR was calculated by determining the standard deviation away from the mean stride duration. Local stability was determined by computing both short-term (ȜST) and long-term (ȜLT) finite-time Lyapunov exponents. Specifically, the average exponential rate of divergence between neighboring trajectories in reconstructed phase-space was approximated by calculating the slope of the linear best-fit lines from 0-1 strides and from 4-10 strides, respectively [2,4] (Figure 1). RESULTS AND DISCUSSION Plantar sole desensitization significantly reduced plantar cutaneous pressure sensitivity from 4.8 ± 0.3 to 0.60 ± .70 sites (p < 0.001). While this procedure had no effect on stride duration VAR or ȜST, it led to significantly increased ȜLT values (p = 0.001), indicating an increased sensitivity to small-scale perturbations during this time-frame (Figure 1,
Normal Desensitized ȜST, Normal & Desensitized ȜLT, Desensitized ȜLT, Normal
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Number of Strides Figure 1: Representative logarithmic divergence curves, and linear best-fit lines, generated from continuous sagittal-plane knee joint angles of an exemplar participant while walking under both normal and desensitized conditions. CONCLUSIONS Peripheral sensory FB related to plantar cutaneous pressure distribution does not appear to play a major role in the control of stride duration VAR. It does, however, contribute considerably to long-term local stability, likely by providing the CNS with timely information regarding the characteristics of small-scale perturbations. Furthermore, the results of the present study provide direct evidence that stride duration VAR should not necessarily be equated with stability when assessing human walking. REFERENCES 1. Kavounoudias A, et al. J Physiol 532, 869-878, 2001. 2. Dingwell JB, et al. Chaos 10, 848-863, 2000. 3. Li L et al. Human Movement Science 24, 257-267, 2006 4. Rosenstein MT, et al. Phsical D 65, 117-134, 1993.
Table 1: Effects of Ice Immersion on the VAR and local stability of treadmill walking Normal Desensitized
Journal of Biomechanics 40(S2)
VAR (mean ± SD) 1.2 ± 0.2 1.4 ± 0.3
ȜST (mean ± SD)*100 17.3 ± 4.1 18.1 ± 4.7
ȜLT (mean ± SD)*100 0.46 ± 0.29 0.71 ± 0.32
XXI ISB Congress, Podium Sessions, Tuesday 3 July 2007
S208
THE EFFECTS OF PLANTAR DESENSITIZATION ON THE VARIABILITY AND LOCAL STABILITY OF TREADMILL WALKING IN HEALTHY ADULTS 1 B. Manor, 1O. Smith, 2A. Guevara, 2P. Wolenski, 1L. Li 1 Department of Kinesiology, 2Department of Mathematics, Louisiana State University, USA Table 1). Further, after desensitization the correlation between stride duration VAR to both short- and long-term local stability was not significant, whereas the degree of sensory loss was positively correlated with ȜLT (R2 = 0.40). 160
140
120
ln(divergence)
INTRODUCTION The soles of the feet are the exclusive points of contact with the environment when walking. Feedback (FB) from plantar cutaneous mechanoreceptors therefore likely contributes to the integrity of walking by providing the central nervous system (CNS) with detailed pressure distribution information that may help control stride-to-stride temporospatial fluctuations [1]. Gait variability (VAR) quantifies the average magnitude of these fluctuations, whereas local stability quantifies the sensitivity of the system to small perturbations that arise as a consequence of VAR [2]. Simultaneously assessing VAR and local stability with desensitized foot soles (thereby reducing pressure-related FB) provides a unique opportunity to examine the role of plantar cutaneous FB in the control of human walking, and the relationship between variability and stability [3]. The purpose of this study was to investigate the effects of foot sole desensitization through ice immersion on the VAR and local stability of walking.
100
80
60
METHODS Healthy college-aged participants (six men, seven women) completed two minute bouts of preferred speed treadmill walking under both normal and desensitized conditions.
40
Desensitization was induced by exposure of the foot soles to shaved ice, and sensory loss was assessed by determining pressure detection threshold at five sites on the plantar aspect of the foot using a 5.07 gauge monofilament.
0
Sagittal-plane joint position data were acquired using one high-speed camera (60 Hz) and reflective markers placed at the hip, knee, and ankle joints. Continuous knee joint angles were computed over 45 consecutive strides. VAR was calculated by determining the standard deviation away from the mean stride duration. Local stability was determined by computing both short-term (ȜST) and long-term (ȜLT) finite-time Lyapunov exponents. Specifically, the average exponential rate of divergence between neighboring trajectories in reconstructed phase-space was approximated by calculating the slope of the linear best-fit lines from 0-1 strides and from 4-10 strides, respectively [2,4] (Figure 1). RESULTS AND DISCUSSION Plantar sole desensitization significantly reduced plantar cutaneous pressure sensitivity from 4.8 ± 0.3 to 0.60 ± .70 sites (p < 0.001). While this procedure had no effect on stride duration VAR or ȜST, it led to significantly increased ȜLT values (p = 0.001), indicating an increased sensitivity to small-scale perturbations during this time-frame (Figure 1,
Normal Desensitized ȜST, Normal & Desensitized ȜLT, Desensitized ȜLT, Normal
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0
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2
3
4
5
6
7
8
9
10
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Number of Strides Figure 1: Representative logarithmic divergence curves, and linear best-fit lines, generated from continuous sagittal-plane knee joint angles of an exemplar participant while walking under both normal and desensitized conditions. CONCLUSIONS Peripheral sensory FB related to plantar cutaneous pressure distribution does not appear to play a major role in the control of stride duration VAR. It does, however, contribute considerably to long-term local stability, likely by providing the CNS with timely information regarding the characteristics of small-scale perturbations. Furthermore, the results of the present study provide direct evidence that stride duration VAR should not necessarily be equated with stability when assessing human walking. REFERENCES 1. Kavounoudias A, et al. J Physiol 532, 869-878, 2001. 2. Dingwell JB, et al. Chaos 10, 848-863, 2000. 3. Li L et al. Human Movement Science 24, 257-267, 2006 4. Rosenstein MT, et al. Phsical D 65, 117-134, 1993.
Table 1: Effects of Ice Immersion on the VAR and local stability of treadmill walking Normal Desensitized
Journal of Biomechanics 40(S2)
VAR (mean ± SD) 1.2 ± 0.2 1.4 ± 0.3
ȜST (mean ± SD)*100 17.3 ± 4.1 18.1 ± 4.7
ȜLT (mean ± SD)*100 0.46 ± 0.29 0.71 ± 0.32
XXI ISB Congress, Podium Sessions, Tuesday 3 July 2007