If you strength train major muscles like your quads, hamstrings, and abs, you're off to a great start supporting your running routine. But stop there and you might not be performing as well or going as long as you could be.
Enter: soleus exercises. This wide, flat muscle in your lower leg tends to get less attention than the more prominent muscle of your calves -- a.k.a., your gastrocnemius (or gastroc), which the soleus lies beneath.
"The soleus muscle is a deep calf muscle that, along with the gastrocnemius, attaches to the Achilles tendon," says Anh Bui, D.P.T., C.S.C.S., a former collegiate runner, physical therapist, and biomechanics specialist in Oakland, California. "Its postural function is to hold you upright to resist a forward lean when you are standing, and its dynamic function is to plantarflex the foot and propel the body forward during running."
Unfortunately, it's an easy muscle to ignore -- and you might run for quite a while on a weak soleus before it starts causing you trouble. Here's how to give it the love it deserves so you can both run with fewer aches and have a more propulsive gait.
There are a few ways this muscle can help give your running a boost.
The soleus might not seem as essential to a powerful stride as some bigger muscles, but it truly fights above its weight class, so to speak. "The soleus is responsible for some of the greatest force production in the human body during running," says Bui.
In fact, even though it seems secondary to the gastrocnemius, it's much more forceful: The soleus produced vertical forces of about eight times runners' bodyweight, while the gastroc produced only about three times their bodyweight in a 2012 Australian study. What's more: Across running speeds, the soleus -- along with the gastroc and three of the four main muscles of the quads -- together provide about 75 percent of the total vertical support impulse needed to move the body upward on the run, with the soleus itself contributing up to 50 percent.
"The muscle fibers of the soleus are predominantly type 1 slow-twitch muscle fibers, whereas the gastrocnemius is most type 2 fast-twitch muscle fibers," explains Brian Gurney, D.P.T., C.S.C.S., a trainer, board-certified sports clinical specialist, and physical therapist at BeFit Therapy in New York City. "Fast-twitch muscle fibers are responsible for quick bursts of energy while slow-twitch fibers are responsible for endurance or long-duration activity."
That doesn't mean, however, that if you're a marathoner you can get away with ignoring your gastroc or that sprinters can forget about the soleus. "Both are important for training for any distance of running," he adds.
Pain in your lower leg or foot could stem from a weak or overused soleus. "The risk of running without strengthening this muscle will increase your chances of Achilles tendonitis in particular because of the repetitive stress on your ankle from the lack of stability and control of your lower leg each time you land," says Gurney. That's because as you land, your soleus lengthens and it stabilizes your tibia (shin bone) so it doesn't move too far forward, he explains.
Pain along the back of your ankle is the main indicator that there's a problem, and you'll likely need a combo of "strengthening and mobility exercises to correct the mechanics of your movement patterns," he says.
If you feel sore in your calves and/or shins but nowhere else after a big workout, that's a big sign that your soleus could be weak or overworked, says Bui. So you probably need to turn to some exercises that strengthen this muscle.
Tightness in your calves, or even just feeling like your ankles can't move fully in a good range of motion (especially flexing your foot upward), can point out that the soleus, gastroc, and stabilizing muscles that all need stretching.
A physical therapist can take precise measurements of your range of motion, but to get an idea at home, Bui recommends a knee to wall test: Facing a wall, stand with your foot 4 to 6 inches back. Bend your knee and try to tap it against wall without letting your heel come up. "If you can't get your knee over your toe, you're lacking ankle dorsiflexion," she says.
Even if you don't notice the above signs that your soleus needs improvement, any runner can benefit from giving this muscle a little extra TLC. Bui and Gurney recommend these exercises to strengthen and stretch the soleus.