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Nicolas Termoz, Luc Martin and François Prince

The aim of this study was to assess postural response efficiency to a self-initiated perturbation using an original method based on the inverted pendulum model. Eight young subjects were asked to perform bilateral arm raising and lowering at 3 different speeds while standing on a force plate. The time necessary to recover a steady state following the movement was computed by analyzing the time evolution of the coefficient of determination between the center of pressure and center of mass difference variable (COP-COM) and the horizontal acceleration of the COM. Results show a spatial reorganization (hip strategy) of the segments following the perturbation and a strong influence of the linear relationship to the arm velocity. However, the conditions of arm velocity did not have any effect on the time response of the postural control, suggesting that this parameter would be an invariant characteristic of the movement. These results support the existence of an internal representation of the inertial constraints related to the movement execution.

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Johanne Desrosiers, François Prince, Annie Rochette and Michel Raîche

The objectives of this study were to standardize measurement procedures and study the test-retest and interrater reliability of the belt-resisted method for measuring the lower extremity isometric strength of three muscle groups. The strength of 33 healthy, elderly, community-dwelling subjects was evaluated with a hand-held dynamometer using the belt-resisted method. Isometric strength testing of three muscle groups (hip flexors, knee extensors, and ankle dorsiflexors) was performed on two separate occasions, I week apart, by the same tester to determine test-retest reliability. The test results of two different examiners testing on different days were used to determine interrater reliability. Test-retest reliability was higher than interrater reliability. Test-retest reliability coefficients of the three muscle groups were high (J9-.95). For interrater reliability, intraclass correlation coefficients varied from .64 to .92. depending on the muscle group and side. For the two kinds of reliability, intraclass correlation coefficients increased from proximal to distal. The method for the hip muscle group should be modified to increase reliability of the measure.

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Heydar Sadeghi, Francois Prince, Karl F. Zabjek and Paul Allard

In this study, tasks of the hip in elderly and young gait were identified using principal-component analysis (PCA). Discrepancies between older and younger participants for hip-flexor and -extensor action during stance were also investigated. PCA was applied to the sagittal-hip-muscle-power curves of participants. Three principal components (PCs) were retained for further analysis. A t test revealed that all measures of gait spatiotemporal parameters were significantly lower in the older participants (p < .05). The first PC for both groups extracted the largest variation and described hip-power action during midistance. The second and third PCs in the older participants highlighted the role of the hip extensors and flexors during weight acceptance, late stance, and pull-off. The corresponding PCs for the younger participants were mainly associated with hip-extensor/flexor action during pull-off and weight acceptance. The results indicate that the hip-extensor/flexor muscles are mainly responsible for balance control in elderly gait but contribute to both balance control and propulsion in the gait of younger individuals.