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Effect of functional fatigue on vertical ground reaction force among individuals with flat feet

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Flat foot as one of the lower extremity deformities might change some kinetic variables of gait. Fatigue can deteriorate the muscle ability in supporting joints and can alter the vertical ground reaction force (GRF) [1, 2]. This study examined the fatigue effect on vertical GRF in individuals with flat feet compared with a normal group during barefoot walking.

Materials and methods

Seventeen subjects with flat feet and 17 normal subjects completed the test. Three vertical GRF measures (F1; the first peak force, F2; minimum force; and F3; the second peak force) were extracted before and after a functional fatigue protocol. To check the homogeneity of the velocity among conditions, the average velocity of the anteroposterior center of pressure (COPy) excursion was calculated. A repeated measure ANOVA was conducted to analyze data.


For the average COPy velocity, no significant fatigue, group and interaction effects were seen. F2 was higher in the flat feet group compared with the normal group (p < 0.05). The fatigue protocol resulted in higher F2 and lower F3 in both groups (p < 0.05). See Figure 1. as the sample vertical GRF curves for a subject with flat feet and a subject with normal feet after fatigue.

Figure 1

Vertical GRF curves for a subject with flat feet and a subject with normal feet after fatigue.


The higher F2 in the flat feet group, which results in a decrease drop in vertical GRF, might be due to more flexible foot joints. Foot muscles lose their appropriate ability to control the foot joints and MLA due to fatigue [24] which results in higher F2 for both groups. Furthermore the muscles could not make a proper lever arm for the propulsion gait phase after fatigue [2] resulting in lower F3 for both groups.


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    Christina K, White S, Gilchrist L: Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running. Hum Mov Sci. 2001, 20: 257-276. 10.1016/S0167-9457(01)00048-3.

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    Headlee D, Leonard J, Hart J, Ingersoll C, Hertel J: Fatigue of the plantar intrinsic foot muscles increases navicular drop. J Electromyogr Kinesiol. 2008, 18: 420-425. 10.1016/j.jelekin.2006.11.004.

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This study was funded and supported by Tehran University of Medical Sciences.

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Correspondence to Ali Ashraf Jamshidi.

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  • Normal Group
  • Peak Force
  • Ground Reaction Force
  • Fatigue Effect
  • Average COPy