The soleus is also known as the ‘runner’s muscle’, as it is the major force producer of the lower limb and contributes enormously to locomotion during running. It has been shown to not only contribute to lower limb acceleration during mid stance, but also to the acceleration of the trunk, owing to the way in which energy is transferred through the body.

In this article, we are going to look into the architecture of this muscle, implications for loading in a well designed strength training programme, and how it contributes to running performance.

Soleus

The soleus is the smaller of two muscles that make up the triceps surae muscle group, the other being the two headed calf (gastrocnemius muscle). Both have responsibility for plantar flexion at the ankle, although the soleus is the prime mover in positions of knee flexion. Both muscles attach to the Achilles tendon, which then inserts into the calcaneus of the foot.

The soleus is known to be able to produce up to 8 times bodyweight of force during high speed running. That is the equivalent of a 70kg runner producing 560kg of force during foot contact!

The reason for it’s huge force potential is down to two factors primarily: it’s cross sectional area and it’s pennation angle. Let’s take a look into those two terms to understand the implications for designing a training programme to strengthen this muscle.

Cross sectional area relates to the size of a muscle. Generally speaking, the larger the cross sectional area of a muscle, the more force that it is capable of exerting, as it has a greater number of fibres which can produce force.

Pennation angle is the orientation of a muscle’s fibres in relation to its longitudinal axis (the centre of the muscle).

As well as high force, the soleus also responds to high volume training, as it is predominantly composed of slow twitch muscle fibres. This means that it is relatively resistant to fatigue, which makes sense due to its role in everyday life such as walking.

It is a uniarticular muscle meaning that it only crosses one joint (the ankle), whereas the calf muscle is biarticular, crossing more than one joint (the ankle and knee). This means that the primary role of the soleus is plantarflexion of the ankle joint, exerting mechanical power on the achilles tendon, and subsequently on the calcaneus. In order to train it optimally therefore, we should train it in positions of knee flexion, where the calf is not required to both plantarflex the ankle and contribute to knee extension. The calf in this instance experiences what is known as ‘active insufficiency’ meaning that it cannot perform two joint actions at once.

Programme design

When considering programme design for any given muscle group we can use the following flow of questions to get to our specifics.

1. Frequency - how often can I train the muscle group based on its characteristics?

2. Intensity - how intense does the training stimulus need to be to elicit the training adaptation I want?

3. Volume - how much training is going to be required to elicit the adaptation?

4. Exercises - which specific movements are needed to deliver the stimulus to the muscle?

With these elements in mind, we can put together a programme that allows us to develop the strength of the soleus, meeting its needs from a loading perspective. Firstly, we know that based on it’s architecture, it requires high volume and frequency owing to its high proportion of slow twitch muscle fibres. Secondly, we know that is has a very high force potential, and therefore requires high load strength training activities as well. And lastly, it is best trained in knee flexion, so seated calf raises are the best option for developing strength of the muscle.

Putting it all together, we could load it in a range of ways across the training week:

• Seated and bent knee calf raises (single and double leg)

• Loaded 2-4 x per week

• 3-4 sets x 15-20 repetitions

• Loading options: manually, barbell, machine

Performance impact

Increased force expression at the soleus contributes to increased running speed. For example, a study in correlates of marathon running performance found that morphological characteristics of the lower leg muscle–tendon unit including soleus cross-sectional area and tendon thickness correlate with running performance. Interestingly, cross sectional area of the calf did not correlate with running performance.

Summary

The soleus is the key force producer of the lower limb, and is hugely important in running performance. It can often be neglected which can lead to performance deficits and injury risk factors including tendinopathy. To train it effectively, it requires volume owing to its natural characteristics. The impact of increasing its strength potential are significant on running speed and injury risk.

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References

Bohm, Sebastian & Mersmann, Falk & Santuz, Alessandro & Arampatzis, Adamantios. (2019). The force-length-velocity potential of the human soleus muscle is related to the energetic cost of running. Proceedings of the Royal Society B: Biological Sciences. 286.

Kovács, B., Kóbor, I., Gyimes, Z. et al. (2020) Lower leg muscle–tendon unit characteristics are related to marathon running performance. Sci Rep 10, 17870.

O’Neill S, Watson P, Barry S (2014) 75 Plantarflexor Muscle Power Deficits In Runners With Achilles Tendinopathy. British Journal of Sports Medicine. 48:A49.