Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles
Introduction
Shoes are traditionally designed to improve stability and support the foot during locomotion, often with the goal of controlling such movements as pronation. It has been suggested in coaching circles and in the biomechanical community that (i) these shoe design features may lead to an overprotection and/or under-utilization of the smaller extrinsic foot muscles and (ii) training barefoot can be an effective method for strengthening foot and ankle muscles [1]. The under-utilization of muscles over time can result in reduced strength or weakness [2] and combined with muscle imbalances, can lead to increased injury susceptibility of the lower limb and back [3], [4], [5], [6], [7]. Unstable training devices such as wobble boards have proven effective in reducing injury in younger and older populations and this training enhances proprioception, improves muscle coordination and may even strengthen select muscles [8], [9], [10]. Recently, unstable shoes have been developed to simulate an effect similar to the wobble board, with the primary purpose of activating and strengthening muscles that may be relatively inactive and under-utilized while wearing a more stable shoe.
Masai Barefoot Technology (MBT) is an unstable shoe with a rounded sole that provides instability in the anterior–posterior direction and a cushioned heel sensor that provides instability in the medial–lateral direction. This design attempts to simulate an unstable surface, thereby requiring continual activation of important stabilizing muscles of the lower limb to maintain proper balance. Evidence exists that wearing the unstable MBT can contribute to a significant reduction in knee and low back pain [11], [12].
The biomechanical and neuromuscular changes introduced by wearing unstable shoes have also been investigated [13], [14], [15]. Walking and standing in an unstable MBT shoe provides training to some of the larger extrinsic foot muscles crossing the ankle joint complex, specifically increasing activity of the gastrocnemius and tibialis anterior [14], [15]. No study, however, has quantified muscle activity for some of the smaller extrinsic foot muscles important for stabilizing and controlling the foot, such as the peroneus longus and flexor digitorum longus.
Most small extrinsic foot muscles (e.g. peroneus longus and brevis, flexor hallucis longus and tibialis posterior) have short moment arms about the talocrural joint (plantar- and dorsi-flexion) but more effective moment arms about the subtalar joint (inversion–eversion) [16]. A continuous engagement of these smaller muscles could lead to more effective control of movements and moments about the subtalar joint and potentially contribute to a reduction in joint loading and joint pain [1], [17].
Quantification of activity of some smaller extrinsic foot muscles can be done invasively with indwelling electrodes [18] or non-invasively with a custom designed EMG circumferential linear array of several bipolar EMG electrode pairs adjacent to each. This array is capable of measuring individual activity of selected small extrinsic foot muscles and is suitable for studying the interplay between multiple muscles during tasks such as walking and standing [19].
The purpose of this study was to (i) quantify muscle activity of select small extrinsic foot muscles using a circumferential linear EMG array proximal to the ankle joint and (ii) quantify postural sway while standing barefoot, in an unstable MBT shoe and in a stable control shoe. Measurements were also compared before and after a 6-week accommodation period of wearing the unstable MBT shoe to determine if improvements in postural control occurred as a possible result of increased muscle training and strengthening.
The following hypotheses were tested: H1 For selected smaller muscles including the peroneus brevis/longus and flexor digitorum longus, the initial muscle activity while standing in an unstable MBT shoe will be greater than standing barefoot or in a stable control shoe. H2 After using the unstable MBT shoe for 6 weeks, the muscle activity for the same smaller muscles while standing in an unstable MBT shoe will remain greater than standing barefoot or in a stable control shoe. H3 Initial postural sway while standing in an unstable MBT shoe will be greater than standing barefoot or in a stable control shoe. H4 After using the unstable MBT shoe for 6 weeks, postural sway while standing in an unstable MBT shoe will be greater than standing barefoot or in a stable control shoe.
Section snippets
Subjects
Twenty-eight subjects (19 females, 9 males) participated in this study and meet the criteria in Table 1. The study was approved by the University of Calgary's Office of Medical Bioethics and all subjects signed a written consent form prior to testing. A priori power analysis (β = 0.2, p = 0.05) using previous Center of Pressure (CoP) excursion and EMG data [14] demonstrated adequate power would be achieved with 28 subjects.
Testing protocol overview
The study was completed in the Human Performance Laboratory (HPL) and
EMG-activity intensities
Comparing activity wavelet intensities, neither visit (pre- and post-accommodation testing) nor interaction effects were identified for all muscle groups (Table 2 and Fig. 2). A shoe effect was evident for the four muscle groups and based on the small p-values, Bonferroni pairwise comparisons were performed (Table 3). For both testing visits, the unstable MBT shoe produced larger intensities compared to the other two footwear conditions for the FDL and AC muscle group. No differences were
Discussion
Several shoe companies have recently developed so called “barefoot” shoes [1] with the purpose of providing some of the suggested benefits of barefoot locomotion. Being conceptually similar to wobble board training during injury rehabilitation, the unstable MBT shoe is one of these “barefoot” shoes designed to train or activate some of the smaller extrinsic foot muscles while standing or walking. Evidence for the effectiveness of wobble board training does exists [9], [10], [21], however, such
Conflict of interest statement
While Masai Barefoot Technology (MBT) provided the unstable shoes and financial support, they had no role in (i) the study design, (ii) the collection, analysis and interpretation of the data, (iii) the writing of the manuscript or (iv) the decision to submit the manuscript for publication.
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