Flexor Hallucis Brevis Morphology is Associated with Visual Reliance While Balancing in Previously Injured Runners

Abstract

Running related injuries to the foot and lower leg are pervasive and thought to be due to poor plantar intrinsic muscle (foot core) function. Previously injured runners have decreased cross sectional area (CSA) and thickness of the flexor hallucis brevis (FHB) and increased reliance on visual information while balancing relative to uninjured runners. However, it remains unknown if FHB morphology is associated with an increased reliance on visual information. PURPOSE: To determine if visual reliance while balancing is associated with FHB morphology in those with a history of running related injuries. METHODS: Twenty-four runners with foot and/or leg running injuries within the past three years but were currently asymptomatic (age: 21.66±2.44 years, mass: 66.84±10.03kg, height: 169.21±19.34cm, runs per week: 4.37±1.30, miles per week: 14.16±8.88) volunteered. Three, 10-second eyes open and eyes closed single leg stance trials on the injured limb were recorded. Center of pressure velocity (cm/s) for each condition was used to calculate %-modulation (%-modulation=[(eyes open-eyes closed)/eyes open]. Larger negative scores indicates greater reliance on visual information. While standing in a weight-bearing, subtalar neutral position, diagnostic ultrasound was used to image the FHB in a relaxed state and contracted state (while holding a short foot contraction). Images were processed to calculate CSA and FHB thickness. CSA was defined as the area (cm2) within the fascial borders of the muscle. Thickness was the distance (cm) between superior to inferior fascia perpendicular to the muscle fibers. Pearson product moment correlations determined the strength of associations and an a priori alpha level of 0.05 was used for all analyses. RESULTS: Larger negative %-modulation was associated with the less contracted FHB thickness (r=0.451, p=0.027) and trended towards being associated with less relaxed FHB thickness (r=0.368, p=0.077). No association was noted with the FHB CSA (r=0.273, p=0.198). CONCLUSIONS: In previously injured runners decreased FHB thickness is associated with an increased reliance on visual information while balancing. Short foot exercises, aimed at increased FHB strength may decrease reliance on visual information but future research is needed to confirm this hypothesis.