Effects of two types of foot orthoses on lower limb muscle activity before and after a one-month period of wear

Highlights

• Foot orthoses with and without lateral bar altered muscle activity during walking.

• Foot orthoses decreased activity of the tibialis anterior muscle.

• Foot orthoses with lateral bar decreased activity of the peroneus longus muscle.

• This study was the first to quantify the effects of the lateral bar.

• Unacceptable level of variability was found for the between-session EMG reliability.

Abstract

The purpose of this study was to quantify the effects of two types of foot orthoses (FOs) on muscle activity during walking. Twenty-one healthy participants were recruited to walk on a five-meter walkway with a control condition (no FOs) and two experimental conditions (FOs and FOs with lateral bar). The experimental protocol was performed before and after a one-month period of wear for each experimental condition. Electromyographic signals were recorded for six muscles (gluteus medius, vastus lateralis, medial gastrocnemius, lateral gastrocnemius, peroneus longus and tibialis anterior). Mean muscle activity was analyzed during the contact, the combined midstance/terminal stance and the pre-swing phases of gait. Peak amplitude and time to peak amplitude were quantified during the stance phase. Unacceptable level of variability was observed between the testing sessions. Therefore, no comparisons were performed to compare the effects of the experimental conditions between testing sessions. After a one-month period of wear, FOs with lateral bar decreased peak amplitude and mean activity of the peroneus longus muscle during the combined midstance/terminal stance phase and FOs decreased peak amplitude and mean activity of the tibialis anterior muscle during the contact phase compared to a control condition. In conclusion, repeated-test design should be used with caution when assessing the muscular adaptation to the wear of FOs for a certain period of time. More studies are needed to determine if the decreased activity of the peroneus longus muscle could be of benefit to treat pathologies such as peroneal tendinopathy or lateral ankle instability.

Introduction

Foot orthoses (FOs) are regularly prescribed to treat numerous lower limb pathologies [1], [2], [3], [4]. A variety of FOs has been described in the literature, but most custom-made FOs are fabricated from a negative cast impression based on subtalar joint neutral position theory [5]. According to this theory, the position in the cast should enable the foot to be in subtalar joint neutral position immediately after heel strike and at the end of the midstance phase of gait when wearing FOs. They were believed to realign the lower-extremity skeleton and therefore affect lower extremity kinematics. However, the latest literature tends to prove otherwise. Numerous studies have quantified the effects of FOs on lower limb kinematics during gait with mixed results. Most studies found small and inconsistent differences in kinematic variables when comparing FOs to a control condition [6], [7], [8], [9], [10]. Considering the positive outcomes when using FOs as treatment [1], [2], [3], [4] and their relatively small effect on lower limb kinematics, other mechanisms of action have been considered.

One of these is based on the FOs’ effects on lower limb muscle activity. Even though some authors have quantified their effects on muscle activity during walking, there is still no consensus among the available literature. A systematic review provided some evidence that FOs affect muscle activity during gait [11], but there are contradictory results in the scientific literature. It is mentioned, for example, that FOs can increase [12] or have no effect [13] on peroneus longus peak amplitude during walking. The contradiction in the scientific literature could perhaps be explained by the variability between the types of FOs used in the studies.

In general, FOs are clinically used for a long-term period and it can take a certain period of time for the patients to be comfortable wearing them. Thus, one can say that the human body needs to adapt to the wear of FOs. However, most studies quantified their instantaneous effects on muscular activity or their effects after a short-term period of wear (2 weeks or less), and have not quantified if these effects change with time [12], [14], [15], [16]. It is therefore unknown if wearing FOs for a certain period of time induces muscular adaptations. However, it is still unclear in the scientific literature if repeated-test designs are reliable to assess muscle activity during walking. Some authors [17], [18], [19], [20] showed that surface electromyography have a good relative reliability when studying muscle activity between different testing sessions for healthy and pathologic patients during walking. However, according to Atkinson and Nevill [21], absolute measures of reliability (e.g. limits of agreement (LoA)) should be used. Using these absolute measures of reliability, Murley et al. [22] observed great between-session variability when comparing timing and amplitude of EMG parameters of muscles of the leg. They recommend reporting the absolute reliability when using a repeated-test experimental design to demonstrate that the magnitude of test-retest error is smaller than the significant effects found.

Moreover, few authors have studied if FOs with and without modification have different effects on the lower limb biomechanics during the gait cycle. In clinical practice, FOs modifications are added to increase the specificity of the intervention. One of these modifications is a lateral bar. The lateral bar consists of a one-centimeter wide ethylene vinyl acetate (EVA) bar glued under the lateral part of the FOs lying from the rearfoot post to the distal end of the shell. No study to date has quantified the effects of this modification. However, this modification is added to FOs in clinical practice to limit foot inversion during locomotion. It is mostly used to treat patients with excessive foot inversion during the gait cycle, or to treat pathologies such as functional ankle instability and peroneal tendinopathy with generally good results. Its utilization is based on the subtalar joint axis location and rotational equilibrium theory of foot function (SALRE) [23]. According to this theory, any force acting laterally to the subtalar axis of rotation creates a pronatory moment of force on this joint. The lateral bar should create a pronatory moment of force around the subtalar joint axis and therefore increase supinator muscles activity and decrease pronator muscles activity during gait.

The principal objective of this study was to evaluate the effects of two types of FOs on the muscle activity of the lower extremity during walking. A secondary objective was to determine if between-session muscle activity assessment is reliable when using absolute measures of reliability. It was hypothesized that (a) muscular activity would be increased for supinator muscles with FOs with lateral bar and muscular activity of the pronator muscles would be decreased with FOs and that (b) between-session variability would be too high to compare different testing sessions.