Original articleEffect of plantar intrinsic muscle training on medial longitudinal arch morphology and dynamic function
Introduction
The role of the plantar fascia, calcaneonavicular (spring) ligament, posterior tibialis, and the closed pack nature of the subtalar joint (STJ) bony architecture in supporting the medial longitudinal arch (MLA) is well established (Saltzman et al., 1995; Donatelli, 1996; Carlson et al., 2000; Fuller, 2000) Less is known regarding the role of the intrinsic foot musculature in contributing to the stability, shock absorption, and force attenuation value of the smaller plantar muscles on the sole of the foot. Excessive pronation, represented by a loss of MLA height, is a common finding in many pathological conditions involving the lower extremity including, plantar fasciitis, patellofemoral pain syndrome, posterior tibialis tendinopathy, Achilles tendinopathy, and posterior and anterior tibialis overuse syndrome (Jung et al., 2011).
There are both passive and active systems that support the MLA. The passive support system includes the inherent shape of the MLA and its ligamentous support. The bony arch consists of the calcaneus, talus, navicular, medial cuneiform and the first metatarsal. The ligamentous support system includes the plantar aponeurosis, long and short plantar ligaments, and the spring ligament. The navicular bone serves as the keystone for the MLA and plays a pivotal role in STJ pronation and supination. Previous research has demonstrated that a modified low-Dye taping procedure can passively support the MLA and reduce the amount of navicular drop (ND) during a functional task (Holmes et al., 2002).
The active support system for the MLA includes the anterior and posterior tibialis, fibularis longus and the plantar foot intrinsic muscles. If any of these contractile structures fails, prolonged or excessive pronation that leads to an injury may occur. Many therapeutic activities focus on the extrinsic muscles to support the MLA. Conversely, short foot exercises target recruitment of the plantar intrinsic foot musculature yet few studies have been conducted on the effect of plantar foot intrinsic motor training to control the MLA.
Previously conducted electromyographic (EMG) studies have demonstrated that alteration or impairment of plantar foot intrinsic muscles influence the height of the navicular and shape of the MLA. A study by Fiolkowski et al. (2008), found that a tibial nerve block to ablate intrinsic muscle activity caused a large decrease in the EMG activity of the abductor hallucis which corresponded to a significant increase in ND. In another study, exercise-induced fatigue of the plantar intrinsics musculature correlated with a significant increase in ND (Headlee et al., 2008).
Short foot exercise (SFE) training is a specific exercise activity that has been shown to be an effective means to recruit the abductor hallucis and prevent excessive lowering of the MLA (Janda et al., 2007; Campbell et al., 2008; Sauer et al., 2011). The exercise aims to activate weakened or inhibited intrinsic plantar foot muscles by intensifying and optimizing the sole's contact with the floor. Plantar intrinsic muscles originate and insert within the foot itself and function to improve dynamic alignment, control the arch position, and stimulate proprioceptors on the sole of the foot to improve balance (Newsham, 2010).
Clinically, the SFE emphasizes metatarsophalangeal and proximal interphalangeal joint flexion during balancing activities while minimizing distal interphalangeal flexion. This activity has been recommended as a means to improve neuromuscular control and intrinsic foot strength (Greenman, 2003; Rothermel et al., 2004, Jam, 2006; Prentice, 2009). The exercise is performed by the subject utilizing the intrinsic muscles of the foot to draw the metatarsal heads back toward the heel. The intent of this exercise is to “shorten the foot” without curling the toes. This motion should cause slight arch elevation without engaging the long toe flexors. The intensity of the exercise is submaximal in nature and emphasizes proper technique and the acquisition of new physical skill. This type of training was shown to challenge the abductor hallucis at 45% of a maximal voluntary isometric contraction in a sitting position that increased to 73% when the muscle was activated in a standing position (Jung et al., 2011).
The results of these studies seem to indicate that intrinsic plantar musculature may have an important role in supporting the MLA; however, to date, no studies have evaluated the ability of a specific intrinsic foot musculature (IFM) training program to decrease the ND, improve the arch height index (AHI), effect muscular balancing strategies in static stance, or impact function during a dynamic balancing activity. The purpose of this investigation was to evaluate the impact of a 4-week training program on the active support of the MLA and its ability to translate into dynamic control during functional tasks.
Section snippets
Methods
A pre-test-post-test design was used for this pilot study. The independent variable was a 4-week IFM training program. The dependent variables were ND, change in AHI, intrinsic foot musculature performance, and performance on the star excursion balance test (SEBT) as measured at baseline, 4 weeks, and 8 weeks.
Data analysis
The amount of ND and the distance reached on the SEBT were both analyzed by repeated measures analysis of variance (ANOVA) to determine if statistically significant differences existed between measurements taken at baseline, 4 weeks post-intervention, and at 8 weeks post-intervention. To protect against Type I error, Tukey tests were used to ensure the alpha levels (p < 0.05) identified the probability that one or more of the pairwise comparisons would be falsely declared significant. The
Results
The subjects reported a daily exercise compliance of 85% or an average of 5.9 training sessions per week. At baseline the subjects had a mean ND of 12.7 mm ± 6.0 mm which had a significant decrease to 10.9 ± 5.5 mm drop (p = 0.04) at 4 weeks and 10.5 ± 5.7 at 8 weeks (p = 0.01) (Fig. 3). There was not a significant change between the ND findings at 4 and 8 weeks after the exercise training ceased (p > 0.05). AHI and IFMT also had a significant change from baseline to 4 and 8 weeks
Discussion
Our initial findings support that a simple training program has an impact on ND, arch height morphology, foot intrinsic function in static stance, and dynamic balance and reach abilities in an asymptomatic population. These findings are similar to others that have found that alterations in IFM activity impact navicular positioning (Fiolkowski et al., 2008; Campbell et al., 2008; Headlee et al., 2008). These results correlate with previous EMG studies that suggest IFM function has a supportive
Conclusion
A four week training program emphasizing recruitment of the plantar intrinsic muscles may have value in dynamically supporting the MLA. IFM training may have an adjunctive role in preventing excessive ND, influencing dynamic control during functional tasks, and have potential value as part of a comprehensive approach to addressing abnormal hyperpronation mechanics in a selected subset of subjects.
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