The primary action of the soleus is plantarflexion, the movement of pointing your foot downward. But this muscle does far more than move your ankle. It keeps you upright when you stand, pumps blood back toward your heart, and plays a surprisingly large role in how your body processes blood sugar. Because it only crosses the ankle joint (not the knee), the soleus is uniquely built for sustained, low-level work that other calf muscles can’t match.
Plantarflexion: The Primary Action
Every time you press your foot down, whether pushing off the ground while walking, stepping on a gas pedal, or rising onto your toes, your soleus is generating plantarflexion force. It shares this job with the gastrocnemius, the more visible calf muscle that sits on top of it. Together, these two muscles merge into the Achilles tendon and attach to the back of the heel bone.
The critical difference is that the soleus only crosses the ankle joint, while the gastrocnemius crosses both the ankle and the knee. This means the soleus can produce plantarflexion force regardless of knee position. The gastrocnemius, by contrast, loses mechanical advantage when the knee is bent because its fibers shorten and can’t generate as much tension. Research in Biology Open found that when people performed plantarflexion with a bent knee, gastrocnemius activation dropped significantly while soleus activation increased by 14 to 19 percent at moderate speeds. This is why a seated calf raise isolates the soleus: bending the knee to 90 degrees forces the soleus to do the heavy lifting.
Postural Stability During Standing
When you stand upright, your center of mass sits slightly in front of your ankle joints. Without something actively preventing it, you’d tip forward. The soleus provides that something: a continuous, low-grade plantarflexion force that counteracts forward sway and keeps you balanced.
This is where the soleus’s fiber composition matters enormously. Roughly 80 to 88 percent of the soleus is made up of slow-twitch (Type I) muscle fibers, a higher proportion than in 36 other human muscles that have been studied. Slow-twitch fibers are built for endurance. They resist fatigue, generate force for long periods, and rely on aerobic metabolism. The gastrocnemius, with a more mixed fiber profile, is better suited to explosive movements like jumping or recovering from a stumble. The soleus handles the quiet, constant work of keeping you from falling over in the first place.
The soleus also contributes passively to stability. Its short, slow-contracting fibers attach to a long, compliant Achilles tendon that acts as a shock absorber, buffering small perturbations without requiring active muscle engagement. When those passive forces aren’t enough, reflex pathways through the spinal cord, skin receptors, and the inner ear trigger rapid adjustments in soleus activation. This combination of passive stiffness and fast reflexive correction makes the soleus the body’s primary anti-gravity muscle at the ankle.
Venous Pump: The “Second Heart”
The soleus contains a network of veins, with larger ones on the lateral side draining into the fibular veins and smaller medial ones feeding into the posterior tibial veins. When the soleus contracts during walking, it squeezes these veins and pushes blood upward against gravity toward the heart. One-way valves in the veins prevent the blood from flowing back down between contractions.
This mechanism, sometimes called the calf muscle pump or “peripheral heart,” is essential for circulation in the lower legs. During prolonged sitting or standing without movement, the pump doesn’t activate, which allows blood to pool in the lower extremities. This is one reason extended immobility raises the risk of blood clots in the deep veins of the calf.
Metabolic Effects on Blood Sugar and Fat
A 2022 study published in iScience found that sustained, isolated soleus contractions (called “soleus pushups,” a small movement of raising the heel while seated) produced striking metabolic effects. The soleus, despite making up only about 1 percent of body weight, increased its rate of glucose oxidation by 113 milligrams per minute during these contractions. Blood levels of a type of fat carried by VLDL particles also dropped significantly.
What makes this possible is the soleus’s heavy reliance on aerobic metabolism and blood-borne fuel rather than stored glycogen. During nearly 4.5 hours of continuous soleus contractions, glycogen stores in the muscle dropped by only about 4 percent of total energy expenditure. Because the muscle wasn’t burning through its glycogen reserves, it drew glucose and fat directly from the bloodstream. Participants reported no sensation of local fatigue, consistent with a muscle that evolved to work for hours without rest.
Anatomy and Nerve Supply
The soleus originates from the back of the fibula head and upper third of the fibula, the middle third of the inner border of the tibia, and a tendinous arch that bridges between these two bones. It sits deep to the gastrocnemius and is wider and flatter. Its fibers converge into the Achilles tendon, which inserts on the middle third of the back surface of the heel bone. The tibial nerve, carrying signals from the S1 and S2 spinal nerve roots, controls the muscle.
How Soleus Strains Differ From Gastrocnemius Strains
Calf strains most commonly occur in the medial head of the gastrocnemius, often during explosive movements with the knee extended. Soleus strains are less common and tend to occur during sustained or repetitive activity. On physical exam, the key distinguishing feature is location: gastrocnemius strains produce tenderness in the inner belly of the calf or at the junction where muscle meets tendon, while soleus strains typically cause pain more toward the outer (lateral) side and deeper in the calf. A palpable gap in the muscle suggests a more severe tear regardless of which muscle is involved.
Because the soleus works independently of knee position, clinicians can help isolate it during testing. Plantarflexion against resistance with the knee bent loads the soleus more than the gastrocnemius. If that movement reproduces pain while straight-knee plantarflexion does not, the soleus is the more likely source.
Targeting the Soleus in Exercise
The most effective way to isolate the soleus is to perform calf raises with the knee bent to about 90 degrees. The seated calf raise, either on a machine or with a dumbbell across the thighs, is the standard exercise. The further the knee bends, the more the soleus becomes the dominant force producer in plantarflexion, since the gastrocnemius is progressively shortened and weakened at that length.
Standing calf raises still work the soleus, but the gastrocnemius takes a larger share of the load when the knee is straight. For runners, endurance athletes, or anyone focused on ankle stability and fatigue resistance, dedicated bent-knee calf work ensures the soleus gets the training stimulus it needs. Given its role in posture, circulation, and metabolic health, the soleus is one of the most functionally important muscles to keep strong.