Fatigue of the plantar intrinsic foot muscles increases navicular drop

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Abstract

Our purpose was to assess the effect of foot intrinsic muscle fatigue on pronation, as assessed with navicular drop, during static stance. Twenty-one healthy young adults participated. Navicular drop was measured before and after fatiguing exercise of the plantar foot intrinsic muscles. Surface electromyography of the abductor hallucis muscle was recorded during maximum voluntary isometric contractions (MVIC) in order to find the baseline median frequency (MedF). Subjects then performed sets of 75 repetitions of isotonic flexion contractions of the intrinsic foot muscles against a 4.55 kg weight on a custom pulley system. After each set an MVIC was performed to track shifts in MedF. After a MedF shift of at least 10%, navicular drop measurements were repeated. Subjects exhibited 10.0 ± 3.8 mm of navicular drop at baseline and 11.8 ± 3.8 mm after fatigue (p < 0.0005). The change in navicular drop was significantly correlated with change in MedF (r = .47, p = .03). The intrinsic foot muscles play a role in support of the medial longitudinal arch in static stance. Disrupting the function of these muscles through fatigue resulted in an increase in pronation as assessed by navicular drop.

Introduction

Several lower extremity pathologies such as tarsal tunnel syndrome (Sherman, 1999), metatarsalgia (Sherman, 1999), plantar fasciitis (Jam, 2006, Sherman, 1999), and medial tibial stress syndrome (Jam, 2006) have been associated with hyperpronation. Pronation is a complicated triplanar motion of the foot. The bony structure, ligamentous support, and extrinsic and intrinsic foot muscles all contribute to maintenance of the medial longitudinal arch (MLA) and play a role in controlling pronation during gait. If one of these structures contributing to the MLA fails, prolonged or excessive pronation can occur, and overuse injury may result.

The exact contribution of each component of the MLA in preventing excessive pronation has not been established. The bony structure of the arch, specifically the stability of the navicular, serves as a primary support structure for the MLA (Franco, 1987). Various ligaments have been shown to maintain the structural integrity, including the spring ligament (Brody, 1982, Davis et al., 1996, Franco, 1987), the plantar fascia (Cheung et al., 2004, Huang et al., 1993, Sherman, 1999), and the deltoid ligament (Kitaoka et al., 1998). The role of muscles in arch support is not fully understood. Extrinsic muscles, including the posterior tibialis (Kitaoka et al., 1997, Thordarson et al., 1995), anterior tibialis (O’Connor and Hamill, 2004, Sherman, 1999), peroneous longus (O’Connor and Hamill, 2004, Sherman, 1999, Thordarson et al., 1995), and peroneous tertius (O’Connor and Hamill, 2004) muscles have all been shown to provide dynamic support of the MLA. Studies of the plantar intrinsic foot muscles such as the abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, abductor digiti minimi, and dorsal interossei muscles have shown EMG activity during gait but not in static stance (Mann and Inman, 1964). Dysfunction of any of these intrinsic muscles supporting the MLA may predispose individuals to hyperpronation-related overuse injuries.

Navicular drop provides a composite measure of pronation with greater inferior displacement of the navicular during weight bearing representing more pronation (Brody, 1982, Kitaoka et al., 1998, Menz, 1998, Picciano et al., 1993). Recent research has shown that blocking tibial nerve transmission to the intrinsic foot muscles produces a significant increase in pronation in static stance as assessed by navicular drop (Fiolkowski et al., 2003). Adequate neuromuscular function of the intrinsic foot muscles may be necessary to support the MLA (Jam, 2006). While complete loss of nerve transmission rarely occurs clinically, this research provides evidence of a link between foot intrinsic muscular function and pronation during static stance.

A more functional way to assess the contribution of the intrinsic foot muscles in maintaining the MLA may be through fatiguing exercise of these muscles. Muscle fatigue causes a decrease in power output resulting in a reduced work capacity (Fitts, 1996). A loss of foot intrinsic muscular function due to fatigue could result in a loss of structural support and motion control for MLA (Jam, 2006). By better understanding the role of intrinsic muscle function in maintaining the integrity of the MLA, we may be able to more effectively treat injuries that are linked to hyperpronation. The purpose of this study was to compare pronation during static stance, as assessed by navicular drop, in feet before and immediately after fatiguing the plantar intrinsic foot muscles. We hypothesize that fatigue of the plantar intrinsics will lead to increased navicular drop during static stance.

Section snippets

Methods

A pre-test–post-test design was used for this study. The independent variable was muscle condition at two levels, pre- and post-fatigue, and the primary dependent variable was navicular drop.

Procedures

Subjects assigned odd numbers had their left foot tested while those assigned even numbers had their right foot tested. Brody’s method was used to assess navicular drop (Brody, 1982). The subject sat with hips, knees, and ankles bent to 90° in an adjustable-height chair with the tibia perpendicular to the floor and the foot resting on the floor. In order to measure navicular drop, the tester palpated the talar dome of the subject’s foot by placing the thumb and index finger on the medial and

Statistical analysis

Pre-fatigue and post-fatigue navicular drop measurements were compared with a dependent t-test. The correlations between: (1) the pre–post change in navicular drop and the pre–post change in MedF, and (2) baseline navicular drop measures and pre–post change in navicular drop were determined with Pearson product moment calculations. The level of significance was set at p < .05 for all analyses. SPSS 12.0 software (SPSS Inc., Chicago, IL) was used for all statistical analysis.

Results

Navicular drop significantly increased after fatiguing exercise of the foot intrinsic muscles, as determined by shift in the MedF of the EMG signal from the abductor hallucis muscle (p < 0.001). Subjects exhibited 10.0 ± 3.8 mm of navicular drop at baseline and 11.8 ± 3.8 mm after fatigue. Pre–post fatigue changes in navicular drop ranged from 0 to 5 mm with a mean change of 1.8 ± 1.3 mm, while changes in MedF ranged from 10.4% to 22.2% with a mean change of 16.6% ± 3.4%. The change in navicular drop was

Discussion

Our primary finding was that navicular drop, a measure of pronation in static stance, increased significantly after fatiguing contractions of the intrinsic foot muscles. These results reinforce recent findings that the intrinsic foot muscles provide substantial support to the MLA in static stance (Fiolkowski et al., 2003). Increased pronation was seen in subjects after fatigue regardless of baseline arch height.

In our study we aimed to minimize contraction of the extrinsic muscles that

Donella Headlee, MEd, ATC holds a B.S. in athletic training from Dakota Wesleyan University and an M.Ed. in athletic training from the University of Virginia. She is currently an athletic trainer with ACAC Physical Therapy in Charlottesville, Virginia.

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    Donella Headlee, MEd, ATC holds a B.S. in athletic training from Dakota Wesleyan University and an M.Ed. in athletic training from the University of Virginia. She is currently an athletic trainer with ACAC Physical Therapy in Charlottesville, Virginia.

    Jamie Leonard-Frye, PhD, ATC earned her PhD in sports medicine from the University of Virginia. She is currently an assistant professor in the Athletic Training Program at the University of Toledo. Her current research focuses on understanding biomechanical and muscle adaptations to chronic knee injuries.

    Joseph M. Hart, PhD, ATC earned his PhD from the University of Virginia in sports medicine. He is currently an assistant professor of orthopaedic research in the Department of Orthopaedic Surgery at the University of Virginia. His current research interests include fatigue-related lower extremity neuromuscular compensations in patients with recurrent low back pain.

    Christopher D. Ingersoll, PhD, ATC is the Joe H. Gieck Professor of Sports Medicine, director of the graduate programs in sports medicine/athletic training, and adjunct professor in the Department of Physical Medicine and Rehabilitation at the University of Virginia. He earned his PhD in biomechanics at the University of Toledo. He currently serves as Editor-in-Chief of the Journal of Athletic Training and is a fellow of the American College of Sports Medicine. He has authored or coauthored over 90 peer reviewed publications, 210 presentations at scientific meetings, and 50 invited presentations or symposia in the areas of sports medicine, athletic training, and neuroscience.

    Jay Hertel, PhD, ATC is an assistant professor of Kinesiology and Physical Medicine and Rehabilitation at the University of Virginia. He holds a PhD in kinesiology from the Pennsylvania State University. He serves as a section editor for the Journal of Athletic Training and is a fellow of the American College of Sports Medicine. His research interests include neuromuscular control of the lower extremity with particular interest in the prevention, assessment, and rehabilitation of lower extremity injuries in athletes.

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