The Soleus: An Unsung Hero in Runners

There are over 600 muscles found in a human body. This number makes it quite difficult to give every muscle the attention that it deserves. Based on my clinical experience, there are certain muscles that I have found to be the most overlooked, underappreciated, and neglected, leading to muscle imbalances throughout the body with subsequent injury. This blog series will be dedicated to educating our readers about specific undervalued muscles that are extremely pertinent to optimal performance and injury prevention in endurance athletes. The first muscle that I want to feature in this series of unfortunately underrated muscles is the soleus muscle.

First, let’s start with the anatomy of the soleus muscle. The soleus muscle is one of the main muscles that make up the calf complex. Most people are familiar with their “calf muscle”, however, many people do not realize that it is actually composed of several different muscles. The two main muscles that make up our calf complex include the gastrocnemius and the soleus. These muscles are stacked on top of one another. The gastrocnemius is the most superficial and the large, broad, and flat soleus muscle is located just underneath. The gastrocnemius muscle crosses the back of the knee joint, and the soleus does not. They both come together to form the Achilles tendon which attaches at the heel, but is continuous with the fascia throughout the bottom of the foot, our plantar fascia.

What does the soleus do?

The gastrocnemius and soleus work together to perform plantar flexion (aka pushing the foot downward toward the ground in a heel lifting motion), however, these muscles also work heavily in isolation. The gastrocnemius is responsible for plantar flexion with the knee straight, as well as assisting with knee flexion or bending the knee. The soleus is responsible for plantar flexion of the foot with the knee bent, leading to propulsion and forward acceleration of our trunk during activities such as walking and running. The soleus also helps stabilize and decelerate the tibia, or lower leg, through the stance phase of gait [6]. The soleus muscle plays a major role in producing propulsive force during endurance running.

What makes the soleus so important for runners?

The soleus is the largest muscle in weight and muscle fiber size of the entire lower leg [4]. It is made up of almost 100% Type 1 (slow twitch) muscle fibers, which is what makes this muscle a top priority to train for endurance activities such as running [4]. To further emphasize the importance of this muscle for runners, research has also shown that the soleus muscle provides the greatest upward mass center acceleration at all running speeds and the greatest contribution to forward mass center acceleration [1]. It takes a large amount of the load during running, much more than the gastrocnemius. In one study looking at individual leg muscles during running, they found that the soleus muscle was able to produce vertical forces EIGHT TIMES our body weight, in comparison to the gastrocnemius, which was only able to produce vertical forces a mere three times our body weight [3]. Another study was performed in 2020 analyzing the role of the soleus in long distance running. They found that a larger and better trained soleus muscle was correlated with better marathon performance [5]. If the evidence of this muscle's huge contribution to running performance isn't enough to convince you to include it in your training plan, then maybe learning about its role in injury prevention will.

The Soleus and Injury Prevention

Not only is strength in this muscle valuable for activities including walking, running, cycling, hiking, and more, it is also extremely important for treatment and prevention of many injuries. To avoid potential injury, the soleus must be strong enough to tolerate the load of these activities for long durations. A well trained soleus muscle can help prevent many injuries including hip, knee, ankle, and/or foot pain, calf strains, Achilles tendinopathy, medial tibial stress syndrome or "shin splints", and plantar fasciitis, to name a few. This muscle has even been linked to prevention of ACL injuries due to the knee-stabilizing function of the soleus [7]. Weakness and under training of this muscle can lead to overuse or excess strain of the muscle itself, or in other areas of the body resulting in the many of the conditions listed above. For example, in one study performed by O'Neill, et al., it was demonstrated that Achilles tendinopathy in runners is associated with decreased force generating capacity of the soleus muscle [2]. This evidence demonstrates that if you do not address the weakness and deconditioning of this muscle, these conditions are likely to occur again and again.

Strengthen Your Soleus

The best way to activate and strengthen your soleus is to plantarflex (lift the heels) while our knees are bent. High repetitions (15-25) are best due to the type of muscle fibers that make up the soleus.

A few of my favorite ways to strengthen the soleus include:

• Seated calf raises with weight

• Standing calf raises with the knees bent

• Wall squats holds with elevated heels

• Bridges with the heels lifted

In summary, the soleus is fairly underappreciated in an endurance athlete's world, and we would be amiss as runners and endurance athletes to neglect this muscle. The soleus muscle is responsible for a powerful push off and stabilizing our leg during stance when running. It plays a huge role in injury prevention for runners, as well. Including this muscle in your strength work is key to happy, healthy running! Now get out there and stabilize your stance by showing your soleus some love!

Thanks for reading!

References:

1. Hamner SR, Delp SL. Muscle contributions to fore-aft and vertical body mass center accelerations over a range of running speeds. J Biomech. 2013 Feb 22;46(4):780-7. doi: 10.1016/j.jbiomech.2012.11.024. Epub 2012 Dec 11. PMID: 23246045; PMCID: PMC3979434.

2. O'Neill S, Barry S, Watson P. Plantarflexor strength and endurance deficits associated with mid-portion Achilles tendinopathy: The role of soleus. Phys Ther Sport. 2019 May;37:69-76. doi: 10.1016/j.ptsp.2019.03.002. Epub 2019 Mar 9. PMID: 30884279.

3. Dorn TW, Schache AG, Pandy MG. Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance. J Exp Biol. 2012 Jun 1;215(Pt 11):1944-56. doi: 10.1242/jeb.064527. Erratum in: J Exp Biol. 2012 Jul 1;215(Pt 13):2347. PMID: 22573774.

4. Ito J, Moriyama H, Seiichiro S, et al. Human lower limb muscles: an evaluation of weight and fiber size. Okajimas Folia Anat Jpn, August 2003;80(2-3):47-56.

5. Kovács B, Kóbor I, Gyimes Z, Sebestyén Ö, Tihanyi J. Lower leg muscle-tendon unit characteristics are related to marathon running performance. Sci Rep. 2020 Oct 21;10(1):17870. doi: 10.1038/s41598-020-73742-5. PMID: 33087749; PMCID: PMC7578824.

6. Francis CA, Lenz AL, Lenhart RL, Thelen DG. The modulation of forward propulsion, vertical support, and center of pressure by the plantarflexors during human walking. Gait Posture. 2013 Sep;38(4):993-7. doi: 10.1016/j.gaitpost.2013.05.009. Epub 2013 Jun 17. PMID: 23787149; PMCID: PMC3795949.

7. Maniar N, Schache A, Pizzolato C, et al. Muscle contributions to tibiofemoral shear forces and valgus rotational joint movements during single leg drop landing. Scand J Med Sci Sports, 2020 Sep;30(9):1664-1674.