Willemse et al. (2023)

Exercises for activation of the intrinsic foot muscles

Functional exercises provoked comparable or even more activation of the intrinsic foot muscles than isolated foot exercises

By integrating some functional exercises into your daily routine, you can train the intrinsic muscles of the plantar foot in a time-efficient manner

Isometric exercises produced more muscle activation of the intrinsic foot muscles than concentric exercises, except for the FDB muscle

Introduction

The foot is our base of support for everyday and athletic activities. A good base of support is therefore important to prevent and overcome injuries of the foot and ankle and even higher up the kinetic chain. In this light, the intrinsic foot muscles are especially important in controlling the medial longitudinal arch of the foot and acting as dynamic stabilizers. We previously reviewed a study on this topic, which you can read here. To strengthen the intrinsic foot muscles, the use of isolated foot exercises has widely been used. A disadvantage of this type of exercise is that for many, contracting these foot muscles is difficult or impossible. Despite your encouragement and demonstration, a lot of individuals won’t be able to contract these muscles. It is known that, as functional exercises create unstable positions, the foot muscles will react and try to provide a stable base of support. In this light, this study aimed to compare the muscle activation of the plantar intrinsic muscles of the foot during the execution of functional exercises compared to the more difficult isolated foot exercises.

 

Methods

To compare the muscle activation of the plantar intrinsic foot muscles, surface EMG was used to measure the activity of the following muscles:

  • M. abductor hallucis (AbH)
  • M. flexor digitorum brevis (FDB)
  • M. flexor hallucis brevis (FHB)

The M. flexor hallucis longus (FHL) was selected to represent the extrinsic toe flexor muscle.

 

Activation of the intrinsic foot muscles
From: Willemse et al., Physiother Theory Pract. (2023)

 

One reference exercise was done at the start of the experiment to normalize the EMG amplitude. Following this, 5 static foot exercises were performed:

  1. Hallux grip
  2. Lesser toe grip
  3. Toe spread-out
  4. Short foot
  5. Toe curl

 

Activation of the intrinsic foot muscles 2
From: Willemse et al., Physiother Theory Pract. (2023)

 

These exercises were compared to five functional exercises for their activation of the intrinsic foot muscles:

  1. Toe stance
  2. Toe stance with forward lean
  3. Toe stance on a compliant surface
  4. Toe walking
  5. Hopping

 

Activation of the intrinsic foot muscles 3
From: Willemse et al., Physiother Theory Pract. (2023)

 

To make the comparison with the functional exercise, one muscle-specific isolated foot exercise was selected. This was the exercise that provoked the largest mean EMG amplitude for that specific intrinsic foot muscle. The muscle-specific isolated foot exercise was the hallux grip for the FHB, toe curl for the FDB, and toe spread-out for the AbH.

Comparisons were made between concentric and isometric exercises. The concentric toe curl exercise was compared to the isometric toe grip, hallux grip and lesser toe grip. The influence of body weight on the activation of the intrinsic foot muscles was studied by comparing a single-leg toe stance to a single-leg toe stance with forward lean.

Moreover, the EMG amplitude integrated over time was measured. That is, the muscle activity is measured with the contraction time in mind to determine the total activity in the muscle. For example, a hop is a very brief moment but this may produce a lot of muscle activity in that short time. With measuring the iEMG, the duration of the movement is taken into account.

 

Results

Twenty-nine healthy and asymptomatic participants were included in the trial. They were on average 23 years old.

Activation of the intrinsic foot muscles 4
From: Willemse et al., Physiother Theory Pract. (2023)

 

The difference in muscle activation between the muscle-specific isolated foot exercises and the functional exercises is shown in the picture hereunder.

  • Toe stance on a compliant surface and hopping produced significantly more activity in the FHB, compared to the Hallux Grip exercise.
  • Compared to the isolated foot exercise “toe spread out”, hopping produced more activity in the AbH

 

Activation of the intrinsic foot muscles 5
From: Willemse et al., Physiother Theory Pract. (2023)

 

  • During toe walking and hopping, the iEMG of the FHB muscle was smaller than during the muscle-specific isolated foot exercise Hallux Grip isolated foot exercise. When toe walking, this difference was 2.8 times smaller, and when hopping, this was 9.1 times smaller. This means that you need on average 3 steps of toe walking and 9 hops to create the same amount of muscle activity of the 3-second hold Hallux grip.
  • The iEMG of the toe curl for the FDB was significantly larger than that of toe walking and hopping. Toe walking had a 2.9 times smaller iEMG and hopping a 7.7 times smaller iEMG. This means that around 3 steps of toe walking and around 8 hops are needed to approximate the muscle activation of the 3-second intrinsic foot toe curl exercise.
  • The iEMG for hopping was 2.4 times smaller for AbH when compared to the 3-second toe spread-out exercise, meaning you’d need to do approximately 3 hops to create the same amount of muscle activity in the AbH muscle compared to doing 3 seconds of the toe spread-out exercise.

 

Activation of the intrinsic foot muscles 6
From: Willemse et al., Physiother Theory Pract. (2023)

 

The isometric exercises produced significantly larger mean muscle activation than the concentric exercises for the FHB and the AbH. On the other hand, the concentric exercises produced more muscle activity in the FDB and FHL muscles.

Increasing body weight on the foot during toe stance with forward lean did not influence muscle activation, compared to the normal stance.

 

Questions and thoughts

Where did this idea of isolating the intrinsic foot muscles come from in the first place? This was probably to avoid compensation from extrinsic foot muscles. Probably, this was a way to study the actions of the intrinsic foot muscles, but apart from studying this, I don’t see the benefit of it for rehab. You will need more than to just activate the plantar muscles of the foot in daily life, and you’ll probably compensate with other muscles higher up the chain, so why would we want to avoid that? You don’t target the vastus medialis obliquus muscle alone for increasing stability around the knee joint, do you?

This study may have a promising impact on the rehabilitation of foot and ankle problems in children. Potentially, it may be helpful in increasing a good foot posture for efficient load transmission and shock absorption. As the isolated foot exercises may be difficult to explain, and even more difficult to teach young children and adolescents, this study gives us a wide array of other potentially useful movements. It can even be more playful to give hopping exercises and tip-toeing than to just let them spread out their toes.

 

Talk nerdy to me

  • This study made use of surface EMG to measure the muscle activity in the intrinsic foot muscles (and in the extrinsic FHL muscle). Although it is a non-invasive way to measure muscle signals, it cannot exclude the possibility of cross-talk from other muscles.
  • Data from this study cannot be used to predict training effectiveness of these exercises. Nonetheless, it can be a starting point for trials comparing both types of training on muscle strength and adaptation.
  • Furthermore, this population was asymptomatic and therefore, we cannot draw conclusions about the muscle activity in people experiencing pain or foot and ankle (muscle) injuries.
  • The p-values were corrected for multiple testing using the Bonferroni correction.
  • Only the data that produced a good motor performance (as rated by the examiner) were included in the trial so that the data contained less “waste”. As such, it reflected the muscle activities during the “ideal” performance.
  • The exercises were performed in a standard order (first static foot, then static functional, then dynamic functional) to avoid detaching the sensors. This may have caused fatigue to arise and may have impacted the performance of the last performed exercises.

 

Take home messages

This study compared the activation of the intrinsic foot muscles between isolated foot exercises and functional exercises. The results showed that it is not necessary to perform isolated foot exercises to get the intrinsic foot muscles working. Instead, by standing on your toes, walking on your toes, or hopping, you can generate the same amount or even more muscle activity in the plantar muscles. For many, these exercises are potentially easier to perform and can easily be integrated into daily routines, making the training more time-efficient.

 

Reference

Willemse L, Wouters EJM, Pisters MF, Vanwanseele B. Plantar intrinsic foot muscle activation during functional exercises compared to isolated foot exercises in younger adults. Physiother Theory Pract. 2023 Apr 26:1-13. doi: 10.1080/09593985.2023.2204947. Epub ahead of print. PMID: 37126537.

Additional reference

Kurihara T, Yamauchi J, Otsuka M, Tottori N, Hashimoto T, Isaka T. Maximum toe flexor muscle strength and quantitative analysis of human plantar intrinsic and extrinsic muscles by a magnetic resonance imaging technique. J Foot Ankle Res. 2014 May 5;7:26. doi: 10.1186/1757-1146-7-26. PMID: 24955128; PMCID: PMC4049512. 

FREE WEBINAR FOR ATHLETE REHAB

WHAT TO LOOK FOR TO PREVENT HAMSTRING, CALF & QUADRICEPS INJURIES

Whether you’re working with high-level or amateur athletes you don’t want to miss these risk factors which could expose them to higher risk of injury. This webinar will enable you to spot those risk factors to work on them during rehab!

 

Lower limb muscle injuries webinar cta
Download our FREE app