Do horizontal propulsive forces influence the nonlinear structure of locomotion?
BACKGROUND
Several investigations have suggested that changes in the nonlinear gait dynamics are related to the neural control of locomotion. However, no investigations have provided insight on how neural control of the locomotive pattern may be directly reflected in changes in the nonlinear gait dynamics. Our simulations with a passive dynamic walking model predicted that toe-off impulses that assist the forward motion of the center of mass influence the nonlinear gait dynamics. Here we tested this prediction in humans as they walked on the treadmill while the forward progression of the center of mass was assisted by a custom built mechanical horizontal actuator.


METHODS
Nineteen participants walked for two minutes on a motorized treadmill as a horizontal actuator assisted the forward translation of the center of mass during the stance phase. All subjects walked at a self-select speed that had a medium-high velocity. The actuator provided assistive forces equal to 0, 3, 6 and 9 percent of the participant's body weight. The largest Lyapunov exponent, which measures the nonlinear structure, was calculated for the hip, knee and ankle joint time series. A repeated measures one-way analysis of variance with a t-test post hoc was used to determine significant differences in the nonlinear gait dynamics.


RESULTS
The magnitude of the largest Lyapunov exponent systematically increased as the percent assistance provided by the mechanical actuator was increased.


CONCLUSIONS
These results support our model's prediction that control of the forward progression of the center of mass influences the nonlinear gait dynamics. The inability to control the forward progression of the center of mass during the stance phase may be the reason the nonlinear gait dynamics are altered in pathological populations. However, these conclusions need to be further explored at a range of walking speeds.
Kurz
Max J
MJ
Laboratory of Integrated Physiology, University of Houston, Department of Health and Human Performance, Houston, Texas, USA. mkurz@uh.edu
Stergiou
Nicholas
N
D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D003198:Computer Simulation; D005260:Female; D005684:Gait / Q000502:physiology*; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D017711:Nonlinear Dynamics; D005082:Physical Exertion / Q000502:physiology*; D013314:Stress, Mechanical; D016138:Walking / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology
Exerc Sport Sci Rev. 2005 Apr;33(2):88-97
15821430
Nature. 2001 Mar 8;410(6825):277-84
11258383
Am J Physiol. 1977 Nov;233(5):R243-61
411381
Child Dev. 1993 Aug;64(4):1143-57
8404261
J Biomech. 2006;39(15):2873-6
16325825
Clin Orthop Relat Res. 1983 May;(175):147-54
6839580
J Appl Physiol (1985). 1995 Jan;78(1):349-58
7713836
J Appl Physiol (1985). 1997 Jan;82(1):262-9
9029225
J Biomech Eng. 2002 Feb;124(1):113-20
11871597
J Exp Biol. 1999 Dec;202(Pt 23):3325-32
10562515
J Biomech Eng. 2007 Apr;129(2):216-22
17408326
Clin Biomech (Bristol, Avon). 2003 Jun;18(5):435-43
12763440
J Mot Behav. 2002 Jun;34(2):99-125
12057885
J Biomech. 2001 May;34(5):679-85
11311709
Biol Cybern. 2005 Sep;93(3):213-21
16059784
J Appl Physiol (1985). 1999 May;86(5):1657-62
10233132
Chaos. 2000 Dec;10(4):848-863
12779434
J Appl Physiol (1985). 2005 Jul;99(1):23-30
16036902
Integr Physiol Behav Sci. 1994 Jul-Sep;29(3):311-27
7811651
Clin Biomech (Bristol, Avon). 2004 Nov;19(9):957-63
15475129
J Biomech Eng. 1998 Apr;120(2):281-8
10412391