Plantar intrinsic foot muscle activation during functional exercises compared to isolated foot exercises in younger adults

Key Points

1. This study used surface electromyography (EMG) to compare muscle activation of three plantar intrinsic foot muscles during isolated foot exercises and functional exercises.
2. Functional exercises showed comparable or higher mean EMG amplitude of the plantar intrinsic muscles as isolated foot exercises, but the assumption that exercises that elicit the greatest amplitude confers the most favourable/effective exercise warrants further investigation.
3. In the clinical setting there are many factors to consider in therapeutic exercise selection (and potential effectiveness) including the targeted impairment, condition specific load limitations, and physical/psychosocial characteristics.

BACKGROUND & OBJECTIVE

In both clinical and research settings there has been a rapid increase in the use of intrinsic foot muscle training. This is not surprising considering biomechanical evidence that the intrinsic foot muscles make important contributions to foot function, as well as reports that altered intrinsic foot muscle size and strength has been associated with aging, lower extremity musculoskeletal conditions and diabetic peripheral neuropathy.

With the goal to facilitate better design of exercise interventions that target the intrinsic foot muscles, the aim of this study was to compare the activation of plantar intrinsic foot muscles between functional exercises and isolated foot exercises.

The intrinsic foot muscles make important contributions to foot function.

Functional exercises are perceived as easier to perform, more readily integrated into daily routines, easily transferrable to functional tasks and activate multiple muscles.

METHODS

• 29 asymptomatic adults (21 women; aged 18-40 years) participated in the study.

• Surface electromyography (EMG) of four muscles during various exercises was performed: three plantar intrinsic muscles (abductor hallucis, flexor digitorum brevis, flexor hallucis brevis) and one extrinsic muscle (flexor hallucis longus).

• Ultrasound guidance was used for electrode placement.

• Table 1 describes the five isolated foot exercises and five functional exercises that were evaluated. For static exercises, three trials with a 5-second contraction were performed with a 20-second rest between trials. For dynamic exercises three repetitions were performed within one recording.

• Verbal instructions for each exercise were provided and the participant was allowed to practice the exercise once. The researcher rated the motor performance of each exercise on a 3-point scale (0/1/2) and trials were excluded if the exercise was not completed with a standard pattern (<2).

• Mean EMG amplitude (% - normalised to reference exercise) and EMG amplitude integrated over time (%.s) were calculated.

RESULTS

• Functional exercises showed similar or greater mean EMG amplitude when compared to isolated muscle specific exercises (see Figure 1)

• Magnitude of integrated EMG was greater for muscle-specific isolated foot exercise than functional exercises for most comparisons.

• Compared to concentric exercise (toe curl), isometric exercise (toe grip) showed greater mean EMG amplitude for flexor hallucis brevis and abductor hallucis but lower mean EMG amplitude for flexor digitorum brevis and flexor hallucis longus.

• Mean EMG amplitude was not different during toe stance with forward lean compared to normal toe stance.

LIMITATIONS

• Surface electrodes are prone to crosstalk from adjacent and underlying muscles (1). This is relevant given the small size and close anatomical arrangement of the intrinsic foot muscles, but also for flexor hallucis longus which lies in proximity with other extrinsic muscles in the leg. Previous research has demonstrated that electrode selection (surface versus intramuscular) affects interpretation of EMG data from flexor hallucis longus during walking (2).

• Many factors influence the myoelectric signal including muscle length, contraction type and contraction speed, and comparing surface EMG signals between different exercises that do not control for these factors may not be appropriate (3).

CLINICAL IMPLICATIONS

The authors concluded functional exercises provoked comparable or higher activation of the plantar intrinsic foot muscles when compared to isolated foot exercises. Additionally, isometric exercises seem more appropriate to train the plantar intrinsic foot muscles because higher muscle activation was demonstrated during these exercises (hallux/lesser toe grip) compared to concentric exercises (toe curl). The assumption that exercises that elicit the greatest EMG amplitude confers the most effective exercise warrants further investigation.

In the clinical setting there are many factors that may influence the selection of an exercise and/or its effectiveness, including the targeted impairment, condition specific load limitations, physical/psychosocial characteristics and adherence.

In that regard, the authors propose some advantages of functional exercises such as being perceived as easier to perform, more readily integrated into daily routines (which may improve adherence), easily transferable to functional tasks (like balance, walking, running), and activate multiple muscles (which may reduce time burden).

I would also add the potential for progressive loading including beyond body weight which may be relevant for populations including running and jumping athletes. One could also propose advantages of isolated foot exercises, for example when weight bearing is pain limited and/or restricted. It is therefore unlikely that the “best” exercise for intrinsic foot muscle training exists, but rather the most appropriate exercise for a given individual at a given point in time.

+STUDY REFERENCE

Willemse L, Wouters, E, Pisters M, Vanwanseele, B (2023) Plantar intrinsic foot muscle activation during functional exercises compared to isolated foot exercises in younger adults. Physiotherapy Theory and Practice, 1–13.

SUPPORTING REFERENCE

1. Besomi, M., Hodges, P. W., Van Dieën, J., Carson, R. G., Clancy, E. A., Disselhorst-Klug, C., Holobar, A., Hug, F., Kiernan, M. C., Lowery, M., McGill, K., Merletti, R., Perreault, E., Søgaard, K., Tucker, K., Besier, T., Enoka, R., Falla, D., Farina, D… Wrigley, T. (2019). Consensus for experimental design in electromyography (CEDE) project: Electrode selection matrix. J Electromyogr Kinesiol, 48, 128-144.
2. Knox, J., Gupta, A., Banwell, H. A., Matricciani, L., & Turner, D. (2021). Comparison of EMG signal of the flexor hallucis longus recorded using surface and intramuscular electrodes during walking. J Electromyogr Kinesiol, 60, 102574.
3. Vigotsky, A. D., Halperin, I., Lehman, G. J., Trajano, G. S., & Vieira, T. M. (2018). Interpreting Signal Amplitudes in Surface Electromyography Studies in Sport and Rehabilitation Sciences [Review]. Frontiers in Physiology, 8.