Soleus
| |
| Soleus | |
|---|---|
| Muscle Type | |
| Origin | Posterior aspect of head of fibula; proximal fibular shaft; soleal line of tibia; middle third of medial tibial border |
| Insertion | Posterior calcaneus via calcaneal tendon |
| Action | Plantarflexes ankle (independent of knee position); stabilizes leg over foot (postural) |
| Synergists | |
| Antagonists | Tibialis Anterior, Extensor Digitorum Longus |
| Spinal innervation | |
| Peripheral Innervation | Tibial nerve (S1–S2) |
| Vasculature | Posterior tibial artery; peroneal artery |
The soleus is a broad, flat muscle lying deep to the gastrocnemius in the calf. Together with gastrocnemius, it forms the triceps surae and shares the Achilles tendon insertion. Soleus is a powerful plantarflexor of the ankle and is primarily active in postural maintenance and endurance activities (standing, walking slowly). Unlike gastrocnemius, it does not cross the knee, so its plantarflexion power is not influenced by knee position. It is sometimes called the “workhorse” of plantarflexion for walking and standing, as it is constantly active to prevent the body from falling forward.
Structure
Soleus is a flat, broad muscle that lies in the superficial posterior compartment (deep to gastrocnemius). It has a dense, slow-twitch fiber composition making it highly fatigue-resistant for continuous activity. Its shape is like a sole fish (hence the name), and it forms the bulk of the calf beneath the gastroc heads.
Origin
It originates from the posterior head and upper shaft of the fibula, the soleal line on the posterior tibia, and the middle third of the medial border of the tibia. There is also a tendinous arch between tibia and fibula origins (the fibrous band under which the popliteal vessels pass). Essentially, it arises from the upper tibia and fibula, making a U-shape origin around the back of the leg.
Insertion
The soleus inserts with the gastrocnemius via the Achilles (calcaneal) tendon onto the posterior surface of the calcaneus. It contributes a large portion of the Achilles tendon’s substance. Some deep fibers of soleus may directly attach to the tibia via fascia, but functionally its insertion is the heel bone.
Action
Soleus plantarflexes the foot at the ankle strongly. Because it doesn’t cross the knee, it generates plantarflexion regardless of knee position (unlike gastroc which is less effective when knee is flexed). Soleus is heavily used in standing to maintain the center of gravity – it contracts to prevent the body from tipping forward (countering dorsiflexion moment at the ankle). During walking, it is active in the stance phase (eccentrically controlling tibial advancement, then concentrically in heel rise). It’s also active in low-intensity activities like posture maintenance (why it’s predominantly slow-twitch). It does not contribute to knee flexion. Additionally, by virtue of connecting tibia to heel, it can help stabilize the tibia on the foot (thus important in balance).
Synergists
Gastrocnemius and plantaris are synergists for plantarflexion. Gastrocnemius provides power for fast/explosive movements, while soleus provides endurance and static hold. They work together for tasks like rising on tiptoes (gastrocnemius initiates with force, soleus sustains).
Tibialis posterior, flexor hallucis longus, flexor digitorum longus (deep posterior compartment) also assist in plantarflexion (and inversion). Soleus works with tibialis posterior especially in stance to control dorsiflexion (tib post mainly inverts and supports arch, but also plantarflexes).
In controlling sway, gastrocnemius and soleus co-contract with tibialis anterior in subtle ways: when one is too relaxed, the other kicks in to reposition.
With knee flexed (like doing seated calf raises), soleus is the primary plantarflexor as gastroc is less taut; so for tasks like slow walking or standing, soleus is more consistently active whereas gastroc might be silent (if no knee extension or heavy push-off needed).
Antagonists
The dorsiflexors (tibialis anterior, extensor digitorum longus, extensor hallucis longus) are antagonists. When soleus plantarflexes, these must relax or lengthen. Conversely, when tibialis anterior holds foot up (as in swing phase), soleus must be off or working eccentrically as needed.
Soleus particularly opposes tibialis anterior's dorsiflexion. For instance, during stance, the tibia tends to roll forward (dorsiflex) – soleus eccentrically resists that (antagonizing tib ant which would pull leg further forward).
At the knee, soleus has no direct effect, but indirectly, if foot is fixed, contraction of soleus tends to pull tibia backward (contributing to knee extension) – which can antagonize knee flexors in some sense. But practically, we consider antagonism at the ankle.
Spinal Innervation
S1–S2 (via tibial nerve).
Peripheral Innervation
Tibial nerve innervates the soleus (branches given off in the popliteal fossa specifically to soleus, often a separate branch from gastroc's). In a tibial nerve injury, both gastroc and soleus are out, causing inability to plantarflex or stand on toes. In an S1 radiculopathy (due to disc herniation), weakness of gastrocnemius/soleus (especially soleus) leads to difficulty with toe raises and an absent Achilles reflex (which tests primarily S1 via tibial nerve). Achilles reflex primarily tests the soleus (with some gastroc) because tapping Achilles causes slight ankle plantarflexion (soleus is main posture muscle engaged reflexively).
Vasculature
The posterior tibial artery (and its sural branches) and the peroneal (fibular) artery supply the soleus via muscular branches. Soleus has a rich blood supply (numerous small vessels). It also has a venous plexus (soleal veins) that are important in the calf muscle pump for venous return; stasis in these veins can predispose to DVT if not mobilizing (why calf pumps help in flights or prolonged immobility). These veins drain into posterior tibial and peroneal veins.
Clinical Relevance
Achilles Reflex: As mentioned, the Achilles tendon reflex (S1) is primarily assessing soleus and gastroc function. Loss of reflex indicates potential tibial nerve or S1 root problem.
DVT risk: Because soleus has large venous sinuses that empty with muscle contraction, inactivity (bed rest, long travel) can cause blood pooling in soleal sinuses. This is a common site for deep venous thrombosis origin. Activation of soleus by walking or exercises (ankle pumps) is key in prophylaxis. Many DVTs in calf start in soleal veins.
Isolated Soleus weakness: Rarely, selective soleus issues might occur (e.g., intramuscular tear). It's hard to detect clinically because gastroc compensates unless knee is flexed (in which case solely soleus should plantarflex strongly – a test done by doing a calf raise with knee bent isolates soleus). If someone can do a heel raise with knee straight but not with knee bent, that indicates soleus weakness or Achilles issue.
Soleus Strain: Typically slower to injure than gastroc due to posture role, but long-distance runners can get soleus strains (especially medial soleus). Presents as deep calf ache, worse with push-off but not as sudden as gastroc tear. Treated with RICE and load management.
Postural sway: In quiet standing, soleus is continuously active to counteract forward sway. If someone has compromised soleus (e.g., neuropathy or muscle fatigue), they may have more postural oscillation. They might also hyperextend knees to rely on bone-ligament support and less on muscle, which can then stress knees.
Exertional Compartment Syndrome: Soleus is in the deep posterior compartment (some classify it superficial, but functionally often considered with deep due to fascia). Chronic compartment syndrome usually affects anterior or lateral compartments; deep posterior is less common but if occurs, pain with toe flexion (FHL, FDL) and plantarflexion (TP, soleus partly) can happen. Difficult to diagnose, often requiring intracompartmental pressure measurements.
Calf Strengthening: To focus on soleus, exercises are done with knees bent (like seated calf raises, or calf raises performed with a bent knee). This is often included in rehab for Achilles injuries or general calf strengthening to ensure both gastrocnemius and soleus are targeted.
Achilles Tendonitis: Soleus tightness often contributes to Achilles tension. Many Achilles stretches involve bending knee slightly to specifically stretch soleus (since gastroc slackens with bent knee). People with chronic Achilles issues often have a tight soleus which must be stretched (the runner's stretch with knee bent for soleus vs knee straight for gastroc).
Walking vs Sprinting: Soleus is more active during slow walking and stance support (low-intensity, long-duration). In sprinting or jumping, gastroc contributes more to the power. Thus, endurance athletes focus on soleus conditioning, while sprinters ensure both gastroc and soleus (but often gastroc for the explosive part).
Soleus Pseudohypertrophy: In some neuromuscular conditions like DMD, calf muscles enlarge (pseudohypertrophy) due to fat infiltration, giving the appearance of strong soleus/gastroc even though they're weak.
Selective Activation Issues: Sometimes after Achilles rupture repair, patients regain gastroc strength but soleus may lag (or vice versa) due to differential atrophy. Emphasis on closed-chain exercises helps engage both. EMG studies show soleus can be trained with high reps/low weight (endurance) whereas gastroc might need more explosive training for full rehabilitation.
