Several treatment modalities to manage or prevent lower-limb musculoskeletal pathologies can be listed. Adding foot orthoses (FOs) to the patient's shoes is one of these treatments. In the scientific literature, FOs are described as efficient to treat several pathologies such as low back pain, posterior tibial tendon dysfunction, plantar fasciitis, and patellofemoral pain syndrome.1–4 However, to this day, there is still no consensus on the FOs' mechanism of action among the available data. Previously, it was believed that FOs realign the lower-limb skeleton, but recent literature has shown that they have small and inconsistent effects on lower-limb kinematics.5–9 Their effects on muscle activity have been proposed as an alternative mechanism of action. In fact, according to the preferred movement pathway theory,10 if FOs support the natural joint movement, muscle activity will be decreased. Inversely, if FOs counteract the natural joint movement, muscle activity will be increased. Although more studies are needed to validate this theory, it has been shown by a recent systematic review that FOs affect lower-limb muscle activity during gait.11 In this review, it was found that FOs increase the activation of the tibialis anterior and the fibularis longus muscles and that they may alter lower-back muscle activation. Better understanding of their effects on muscle activity could help to optimize FO treatment for some musculoskeletal pathologies.
Foot orthoses modifications were previously defined as material that is added or removed to the original shell of the orthoses to increase or decrease their effects.12 Among these modifications, deep heel cup,13 medial and lateral flanges,13 lateral bar,12 top covers,14 or extrinsic rearfoot and forefoot posts15 can be listed. Of these modifications, extrinsic rearfoot and forefoot posts are regularly used clinically and in research. Extrinsic rearfoot and forefoot posts can be used whether alone or in combination. These posts can be inclined in the frontal and transverse planes. In the frontal plane, the FOs can be inclined in varus or in valgus. In the transverse plane, the medial or lateral side of the extrinsic rearfoot post can be lengthened. Their utilization is based on the subtalar joint axis location and rotational equilibrium theory of foot function (SALRE).16 According to this theory, any force created by the FOs acting medially to the subtalar and ankle joints axes will produce a supinatory moment of force around these joints and therefore will decrease the lower-limb supinator muscles activity. The opposite applies for a force acting laterally to these joints axes. According to this theory, by adding a varus inclination to the extrinsic rearfoot and forefoot posts, the supinatory moment of force around the ankle and subtalar joints will be increased, which will decrease the activity of the lower-limb supinator muscles and increase the activity of the lower-limb pronator muscles. By increasing the medial length of the extrinsic rearfoot post in the transverse plane, the supinatory moment of force around these joints will also be increased. Inversely, increasing the lateral length of the extrinsic rearfoot post in the transverse plane will decrease the supinatory moment of force and therefore decrease the activity of the lower-limb pronator muscles and increase the activity of the lower-limb supinator muscles.
Although some studies provided evidences that certain types of extrinsic rearfoot and forefoot posts can affect the lower-limb biomechanics during locomotion,9,15,17 only one study quantified the effects of transverse plane inclination of the extrinsic rearfoot posts.17 However, few studies quantified the effects of different types of extrinsic rearfoot and forefoot posts on muscle activity during a walking task. Their effects on muscle activity are still unclear in the scientific literature.
The objective of this case study was to generate hypotheses with the goal to develop a more complex experimental design to better understand the effects of extrinsic rearfoot and forefoot posts on muscle activity in further studies.
MATERIALS AND METHODS
A single subject was used in this case study. The participant was a 26-year-old man (weight, 182 lbs; height, 5'9"; body mass index, 26.9) with no history of macrovascular symptoms, neuromuscular diseases, or traumatic injuries affecting his ability to walk 6 months before the study. A biomechanical assessment was performed on the subject and revealed a bilateral fully compensated rearfoot varus, bilateral forefoot supinatus, and a relaxed calcaneal stance position of 9° everted left and 8° everted right. No limb-length discrepancy was found by measuring from the medial malleolus to the anterior superior iliac spine of both legs. The subject had bilateral diminished internal rotation of the hip, both in extension and in flexion. Range of motion of the subtalar joint was increased, and Foot Posture Index score was +11 bilaterally. No limitation of the first metatarsophalangeal joint was found during non–weight-bearing assessment. When weight bearing in relaxed calcaneal stance position, both subtalar joints were maximally pronated. Using Kirby's method16 to determine the position of the subtalar joint axis, a medial deviation was found bilaterally. Before his participation, the subject gave his informed written consent according to a protocol approved by the Université du Québec à Trois-Rivières (Canada) Ethics Committee.
During the experiment, the participant wore neutral cushioned athletic shoes with no intrinsic correction and with a Velcro closure (Athletic Works, Model: Rupert). The prone casting technique described by McPoil et al.18 was used to produce the negative impression of the foot. This technique consisted of palpating the subtalar joint with one hand to place the joint in neutral position. With the other hand, a force was applied on the fourth and fifth metatarsal heads to dorsiflex and abduct the forefoot. The forefoot supinatus of the participant was also intrinsically reduced in the cast impression. A trained podiatrist (3 years of experience) took the plaster casts and a certified orthotic technician (15 years of experience) manufactured the seven pairs of FOs. No modifications of the positive cast were performed, except the removal of gross abnormalities. The FOs used in this case study were made of a 3.2-mm polypropylene shell. The shell ended proximally to the metatarsal heads to not limit their range of motion during walking. The extrinsic posts added to the FOs were as follows: 1) no post, 2) external oblique rearfoot post (EORP), 3) internal oblique rearfoot post (IORP), 4) straight rearfoot post (SRP), 5) rearfoot and forefoot posts (RFP), 6) rearfoot and forefoot posts at 2° varus (RFP2), and 7) rearfoot and forefoot posts at 5° varus (RFP5) (Figure 1). All extrinsic rearfoot and forefoot posts were made of ethylene vinyl acetate. The IORP was manufactured with the medial side at 100% of the length of the heel cup and the lateral side at 0%. The EORP was manufactured with the medial side at 50% of the length of the heel cup and the lateral side at 100%. The SRP was cut perpendicularly to the shell at 50% of the length of the heel cup.
To familiarize himself with the experimental protocol, the participant was instructed to walk five times on a 5-meter walkway at comfortable speed. Then, stance duration, which was determined by the time between heel strikes of ipsilateral and contralateral limbs, was recorded and averaged for 10 trials using two force platforms separated by a step length on the participant's path. During the data collection, all trials with stance duration varying from ±10% of the mean stance duration were rejected and retaken. The participant initiated the walk two steps before the second force platform, according to a two-step protocol.19
For the data collection, the first three trials were performed in the control condition. Then three trials were performed for all experimental conditions, which were randomly selected. After all trials with experimental conditions were completed, three more trials with the control condition were performed. The participant was then asked to perform three trials for all experimental conditions in the opposite order and finished with three trials in the control condition. The participant was given a 2-minute break between each experimental condition to rest and for the research team to change the experimental condition in the participant's shoes.
The application of the bipolar disposable Ag-AgCl electromyography (EMG) electrodes (Bortec Biomedical, Alberta, Canada) was based on the recommendations of SENIAM.20 They were applied over the gluteus medius, vastus medialis and lateralis, biceps femoris, fibularis longus, tibialis anterior, and lateral and medial gastrocnemius. The muscles chosen are commonly assessed in gait analysis with and without FOs11 and represent the different muscle groups of the lower limb that are the most solicited during walking.
EMG signals were differentially amplified (AMT-8, common mode rejection ratio of 115 dB at 60 Hz, input impedance of 10 GW; 12-bit A/D converter) and sampled at 1000 Hz. The EMG data were digitally filtered with a zero phase lag, bidirectional, 10- to 450-Hz bandpass fourth-order Butterworth filter. Analyses were performed on the root mean square (RMS) of these data, calculated with a moving window of 50-millisecond width with an overlap of 25 milliseconds. The RMS data of each muscle were normalized with the mean peak RMS amplitude of the three trials of the control condition. The parameters analyzed were the mean activity during the contact, midstance, and propulsion phases. Gait cycle phases were determined by sequencing the ground reaction force signal collected with the second force platform (AMTI) on the participant's path, whereas the first force plate was only used to determine the participant's speed. All measurements were taken on the dominant leg, which was determined by asking the participant to kick a ball, write his name on the floor with one foot, and stamp out a simulated fire. The leg chosen by the participant to perform the majority of the tasks was considered dominant.21
The differences in muscle activity were considered significant when the 95% confidence intervals did not overlap between two conditions. Mean differences (MDs) in percentage were calculated for the significant differences.
MUSCLE ACTIVITY DURING THE CONTACT PHASE
For the lateral gastrocnemius, mean activity during the contact phase was increased for RFP compared with the control condition (31.52% [confidence interval (CI), 27.46–35.57] vs 21.13% [CI, 15.70–26.57]; MD, +49%) (Figures 2A, B) (Table 1).
MUSCLE ACTIVITY DURING THE MIDSTANCE PHASE
For the gluteus medius muscle, mean activity was decreased for RFP2 compared with RFP (30.32% [CI, 24.33–36.31] vs 43.57% [CI, 37.63–49.51]; MD, −30%). For the biceps femoris, mean activity was increased for RFP5 (22.44% [CI, 18.57–26.31] vs 12.33% [CI, 8.90–15.75]; MD, +82%), SRP (24.16% [CI, 16.57–31.76] vs 12.33% [CI, 8.90–15.75]; MD, +96%), and RFP (25.78% [CI, 17.66–33.89] vs 12.33% [CI, 8.90–15.75]; MD, +109%) compared with the control condition. For the medial gastrocnemius muscle, mean activity was increased for RFP2 compared with RFP (30.60% [CI, 27.43–33.76] vs 18.81% [CI, 14.96–22.66]; MD, +63%). For the fibularis longus muscle, mean activity was decreased for RFP2 compared with RFP5 (23.51% [CI, 21.46–25.56] vs 48.79% [CI, 27.00–70.58]; MD, −52%), SRP (23.51% [CI, 21.46–25.56] vs 50.71% [CI, 32.11–69.31]; MD, −54%), and RFP (23.51% [CI, 21.46–25.56] vs 50.19% [CI, 31.59–68.78]; MD, −53%) (Figures 3A, B) (Table 1).
MUSCLE ACTIVITY DURING THE PROPULSION PHASE
For the vastus medialis muscle, mean activity was decreased for OBE compared with RFP2 (5.97% [CI, 3.75–8.20] vs 10.63% [CI, 8.74–12.51]; MD, −44%). For the fibularis longus muscle, mean activity was increased for RFP5 compared with RFP2 (53.05% [CI, 44.73–61.37] vs 36.88% [CI, 31.10–42.66]; MD, +44%) (Figures 4A, B) (Table 1).
The principal objective of this case study was to compare the effects of FOs with different extrinsic rearfoot and forefoot posts on muscle activity of a participant with hyperpronated feet, during a walking task, to generate hypotheses with the goal to develop a more complex experimental design for further studies. A single-subject design was used in this study to preserve the participant-specific information as previously described in other studies.17,22,23 As FOs therapy is a patient-specific treatment, it is possible that a case study is more appropriate to study FOs' effects.17
Differences in muscle activity were observed when comparing the experimental conditions with each other and with the control condition. RFP2 and RPF5 seem to be the conditions with the greatest impact on muscle activity. They affected the activity of the gluteus medius, the biceps femoris, the medial gastrocnemius, the fibularis longus, and the vastus medialis muscles. In general, adding an extrinsic varus post to the FOs seems to increase the activity of these muscles. These results are in agreement with a previous study that showed that posting FOs changes muscle activity of the lower limb.15 However, for this participant, FOs with no post had a minimal impact on the lower-limb muscle activity. It raises the hypothesis that FOs may need to be patient-specific to have an impact on muscle activity. However, it is still unknown in the scientific literature on how much increase or decrease muscle activity is needed to produce a clinical impact. The results of this case study also suggest that larger-scale studies are needed and are of interest to generalize the effects of these FO modifications on muscle activity for patients with hyperpronated feet. It would also be interesting to quantify in further studies whether muscle activity can be modulated by adding extrinsic posts to prefabricated FOs.
On a patient-specific approach, mean activity of the fibularis longus muscle was decreased for RFP2 compared with RFP and RFP5 during the midstance and the propulsion phases. The role of the fibularis longus during these two phases of gait is to assist the stabilization of the midfoot joint and the plantarflexion of the first metatarso-cuneiform joint. It can be hypothesized that, for this participant, a small extrinsic varus post on FOs (RFP2) could help stabilize the midfoot joints and assist the plantarflexion of the first metatarso-cuneiform joint. Consequently, the activity of the fibularis longus muscle is decreased according to the preferred motion pathway theory.10 Increasing the extrinsic varus post (RFP5) may destabilize the ankle complex by creating an excessive inversion moment of force around the ankle and subtalar joints, according to the SALRE theory.16 Therefore, the fibularis longus muscle must compensate to stabilize the subtalar and ankle joints by increasing the pronatory moment of force around them, thus explaining the increased mean activity.
Some limitations should be taken into account. The single-case design used in this case study enables the preservation of more participant-specific information but decreases the external validity of the results. However, this design was used to generate hypotheses and research questions and develop a more complete experimental design for further studies to verify if these results would translate to a larger population. Moreover, this study only quantified the instantaneous effects of FOs even if they are generally worn for a long-term period. It would be interesting to quantify the biomechanical differences during walking with FOs before and after a period of adaptation. However, it has been shown that the between-session EMG assessment reliability may be too poor to quantify these comparisons.12,24 It is possible that the results of this case study are not representative of the long-term effects of these FOs. The results of this case study are also limited to a walking task. Further studies assessing muscle activity during dynamic and multidirectional tasks are needed to better understand the impact of wearing FOs in a clinical situation. Furthermore, the participant recruited in this case study was asymptomatic. It is still unclear in the scientific literature if the effects of FOs on muscle activity change whether the participants are symptomatic or not. However, it is possible that pain or other symptoms affect muscle activity during walking. The results of this case study may be different with a symptomatic participant.
The results of this case study suggest that the addition of rearfoot and forefoot extrinsic posts to the FOs can influence muscle function during walking. In general, more muscle activity was observed by increasing the frontal plane inclination of the extrinsic rearfoot and forefoot posts. However, further studies are needed to develop a clinical guideline and to quantify if these extrinsic posts can be of benefit for the treatment of musculoskeletal pathologies.
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