Soleus Muscle

The soleus muscle is a powerful and endurance-oriented muscle located in the posterior compartment of the leg. It plays a vital role in plantar flexion of the ankle and is fundamental for maintaining upright posture and locomotion. Together with the gastrocnemius, it forms the calf muscle group and contributes significantly to activities such as walking, running, and jumping.

Anatomy of the Soleus Muscle

Location and General Description

The soleus muscle is a broad, flat muscle situated deep to the gastrocnemius in the superficial posterior compartment of the leg. It lies posterior to the tibia and fibula and extends from the upper part of these bones to the heel. Because of its deep position and flat shape, it is sometimes referred to as the “second heart” of the leg due to its role in venous return during standing and walking.

Origin and Insertion

The origin of the soleus muscle arises from the posterior aspect of the fibular head, the upper one-third of the posterior fibular surface, and the soleal line on the posterior surface of the tibia. It also has a fibrous arch between the tibia and fibula through which the popliteal artery and tibial nerve pass. The fibers descend vertically and converge into a thick aponeurosis that fuses with that of the gastrocnemius to form the Achilles tendon. This tendon inserts into the posterior surface of the calcaneus.

Shape and Structural Features

The soleus is a broad and pennate muscle, characterized by short, obliquely arranged muscle fibers that allow for sustained contractions and high endurance. Its flattened belly makes it distinct from the more bulky gastrocnemius. The muscle is composed predominantly of slow-twitch (type I) fibers, which are adapted for continuous postural support and resistance to fatigue.

Relations with Surrounding Structures

Superficially, the soleus is covered by the gastrocnemius and the deep fascia of the leg. Deep to the muscle lie the tibial nerve, posterior tibial artery, and fibular artery. The plantaris tendon often courses between the gastrocnemius and soleus, joining the Achilles tendon inferiorly. Medially, it relates to the posterior tibial vein, while laterally, it is adjacent to the fibular artery and vein.

Innervation and Blood Supply

Nerve Supply

The soleus muscle is innervated by the tibial nerve, a branch of the sciatic nerve, with root values typically derived from L5 to S2. The nerve enters the deep surface of the muscle and provides motor branches that supply its entire extent. This innervation enables both voluntary contraction and reflex control during balance and locomotion.

Vascular Supply

The arterial supply to the soleus is provided mainly by the posterior tibial and fibular arteries. These branches ensure a rich capillary network suitable for the muscle’s continuous activity. Venous drainage corresponds with the arterial pattern, emptying into the posterior tibial and fibular veins, which assist in the venous return from the lower limb during muscular contraction.

Lymphatic Drainage

Lymph from the soleus drains primarily into the deep lymphatic vessels accompanying the posterior tibial veins. These vessels eventually lead to the popliteal lymph nodes located in the popliteal fossa, contributing to immune surveillance and fluid balance within the leg.

Function of the Soleus Muscle

Role in Plantar Flexion

The primary function of the soleus muscle is plantar flexion of the foot at the ankle joint. It acts to pull the heel upward while the forefoot is fixed, enabling powerful movements such as pushing off the ground during walking, running, or jumping. Because it acts independently of the knee joint, unlike the gastrocnemius, the soleus can maintain its action even when the knee is bent.

Contribution to Postural Stability

The soleus is often referred to as a postural or anti-gravity muscle due to its role in maintaining balance while standing. Its continuous low-level contraction prevents the body from swaying forward at the ankle joint. This tonic activity provides stability during quiet standing and allows efficient control of the center of gravity over the base of support.

Action During Gait Cycle

During the gait cycle, the soleus becomes particularly active in the stance phase. It controls the forward movement of the tibia over the foot and provides a propulsive force at toe-off. Its coordinated function with the gastrocnemius and other lower limb muscles ensures smooth transition between stance and swing phases, aiding in efficient locomotion.

Synergistic and Antagonistic Muscles

The soleus works synergistically with the gastrocnemius and plantaris muscles to form the triceps surae complex, which collectively contributes to plantar flexion. Antagonistically, it opposes the action of the tibialis anterior and extensor digitorum longus, which produce dorsiflexion. This balanced relationship maintains smooth ankle motion and prevents instability during movement.

Biomechanics and Physiology

Muscle Fiber Composition (Slow vs Fast Twitch)

The soleus muscle contains a predominance of slow-twitch (type I) muscle fibers, which are highly oxidative and fatigue-resistant. These fibers allow for sustained contractions that are essential for maintaining posture and supporting prolonged standing. The minimal presence of fast-twitch (type II) fibers reflects its specialization for endurance rather than speed or explosive strength.

Force Generation and Endurance

The architecture of the soleus, with its short pennate fibers and large physiological cross-sectional area, enables it to generate substantial force at low contraction speeds. This mechanical design, combined with its high mitochondrial density and myoglobin content, makes the muscle capable of continuous low-intensity work for extended durations without fatigue.

Electromyographic Activity Patterns

Electromyographic (EMG) studies have shown that the soleus exhibits sustained low-frequency activation during standing and slow walking. Its activity increases during running and jumping, reflecting its contribution to propulsion and stabilization. Unlike the gastrocnemius, which shows phasic bursts of activity, the soleus demonstrates tonic activation, underscoring its postural role in the lower limb.

Development and Variations

Embryological Development

The soleus muscle develops from the mesodermal myogenic precursor cells of the posterior compartment of the lower limb bud during embryogenesis. By the seventh week of gestation, myoblasts differentiate into myotubes that form the primitive muscle mass destined to become the triceps surae group. The innervation by the tibial nerve is established early in fetal life, ensuring proper coordination with other posterior compartment muscles as the lower limb matures.

Anatomical Variations

Although the soleus muscle is generally consistent in structure, certain anatomical variations may occur. In some individuals, the extent of the tibial or fibular origin may differ, leading to slight differences in muscle width or attachment levels. Occasionally, the muscle fibers may extend further inferiorly than usual, influencing the length of the Achilles tendon or the shape of the calf.

Accessory or Absent Soleus Muscle

Rarely, accessory slips of the soleus muscle may be present, termed the “accessory soleus.” This variant typically originates from the tibia or fibula and inserts independently onto the calcaneus or adjacent tendons. It may form a visible mass in the posteromedial aspect of the leg or ankle and can sometimes be mistaken for a soft tissue tumor on imaging. Complete absence of the soleus muscle is extremely uncommon and may be associated with other developmental anomalies of the lower limb.

Clinical Relevance

Common Injuries and Conditions

• Soleus Strain: Overstretching or excessive contraction of the muscle can result in microscopic tearing of fibers, leading to pain, tenderness, and stiffness in the calf region. Such strains often occur during activities involving sudden acceleration or changes in direction.
• Chronic Exertional Compartment Syndrome: Due to its deep position within the posterior compartment, swelling of the soleus may increase intracompartmental pressure, compressing vessels and nerves, and causing pain during prolonged exercise.
• Deep Vein Thrombosis Mimicry (“Tennis Leg”): Partial rupture of the medial soleus or plantaris muscle can present as acute calf pain and swelling, symptoms that may mimic venous thrombosis. Proper imaging is required for accurate differentiation.

Overuse and Fatigue Syndromes

Repetitive loading during endurance sports such as running, cycling, or hiking can lead to overuse injury of the soleus. The slow-twitch fibers, though resistant to fatigue, can develop microtrauma when subjected to prolonged strain without adequate recovery. Symptoms may include deep calf soreness, reduced flexibility, and performance decline, often misdiagnosed as Achilles tendinopathy or deep calf strain.

Trigger Points and Myofascial Pain

The soleus muscle is a common site for myofascial trigger points that may refer pain to the heel or posterior ankle region. These hyperirritable nodules can cause persistent discomfort, particularly during dorsiflexion or prolonged standing. Manual therapy, stretching, and dry needling are often used to relieve these localized areas of muscle tension.

Diagnostic Evaluation

Clinical Examination

Evaluation of the soleus muscle begins with a thorough history and physical examination. Patients typically report deep, aching pain in the lower calf, aggravated by walking or pushing off the foot. Palpation of the deep posterior compartment may reproduce discomfort. The practitioner may also perform resisted plantar flexion with the knee flexed at 90 degrees, which isolates the soleus from the gastrocnemius and helps in localizing injury.

Imaging Techniques

• Ultrasound: High-resolution musculoskeletal ultrasound allows visualization of muscle architecture, detecting tears, edema, or hematoma within the soleus. It is particularly useful for dynamic assessment during movement.
• MRI: Magnetic Resonance Imaging is the gold standard for assessing soft tissue injuries. It can clearly delineate partial or complete tears, inflammation, and fascial compartment involvement. T2-weighted images often reveal hyperintensity in acute muscle strain.
• CT Scans: Computed Tomography is less frequently used but may assist in identifying calcifications or ossification within chronic lesions or in cases where bone involvement is suspected.

Functional Assessment Tests

Functional testing evaluates the strength and endurance of the soleus muscle. The single-leg heel raise test with the knee flexed primarily activates the soleus and is used to assess recovery following injury or surgery. Electromyography (EMG) may also be employed to measure muscle activity and detect neuromuscular dysfunction or denervation.

Rehabilitation and Treatment

Acute Injury Management

Initial treatment of soleus injuries follows the RICE protocol: rest, ice, compression, and elevation. In mild strains, short-term immobilization may be applied to limit further damage. Nonsteroidal anti-inflammatory drugs (NSAIDs) can help reduce inflammation and pain. Gradual reintroduction of movement is recommended as symptoms improve.

Physical Therapy and Strengthening Exercises

Physical therapy focuses on restoring flexibility, strength, and endurance. Progressive loading through calf raises, eccentric contractions, and proprioceptive exercises helps regain functional capacity. Therapists often emphasize closed-chain movements to retrain the soleus in its postural role and improve ankle stability during dynamic activities.

Stretching and Mobility Techniques

Gentle stretching of the calf muscles, performed with the knee flexed, specifically targets the soleus. Regular mobility drills enhance muscle elasticity and circulation, reducing the risk of reinjury. Foam rolling and myofascial release techniques are also used to alleviate tension and improve muscle recovery after strenuous exercise.

Post-Surgical Rehabilitation

In rare cases where the Achilles tendon or associated structures require surgical repair, the soleus plays a critical role in rehabilitation. Early controlled motion and gradual strengthening are key to preventing stiffness and muscle atrophy. Physiotherapists monitor gait retraining and progressive resistance exercises to ensure full functional restoration before returning to sports or heavy activity.

Applied Anatomy in Clinical and Sports Contexts

Role in Running, Jumping, and Balance

The soleus muscle serves as a vital stabilizer and power generator in dynamic lower limb activities. During running, it acts eccentrically to control ankle dorsiflexion and concentrically to propel the body forward during toe-off. In jumping, the soleus contributes to explosive plantar flexion, providing a steady base for the gastrocnemius and other leg muscles to generate vertical lift. It also plays a crucial role in maintaining balance during one-legged stances and uneven terrain by adjusting ankle tension to counteract gravitational forces.

Importance in Orthopedic and Physiotherapy Practice

From a clinical standpoint, the soleus is of great interest to orthopedic surgeons and physiotherapists due to its deep anatomical position and postural function. Injuries or dysfunction of the soleus can affect gait mechanics, ankle stability, and lower limb circulation. In physiotherapy, strengthening the soleus is essential for rehabilitation following Achilles tendon injuries, ankle sprains, or knee surgeries. Targeted soleus activation exercises, such as seated calf raises, are used to enhance muscular endurance and reduce the risk of re-injury.

Relevance in Lower Limb Biomechanics

Biomechanically, the soleus acts as a key decelerator of forward motion of the tibia over the foot. It counterbalances the anterior pull of the body’s center of gravity during walking or standing, helping maintain joint alignment and efficient force transmission through the lower limb. A weak or fatigued soleus can lead to compensatory overuse of other muscles, predisposing individuals to conditions such as Achilles tendinopathy, shin splints, or plantar fasciitis.

Comparison with the Gastrocnemius Muscle

Structural and Functional Differences

Although the soleus and gastrocnemius together form the triceps surae complex, they differ in both structure and function. The gastrocnemius is a biarticular muscle that crosses both the knee and ankle joints, whereas the soleus is monoarticular, acting only at the ankle. This distinction gives the soleus a specialized role in maintaining posture and performing slow, sustained contractions, while the gastrocnemius contributes to rapid, forceful movements.

Relative Contributions to Plantar Flexion

The soleus is primarily active during standing and slow walking, providing constant tension for balance and venous return. In contrast, the gastrocnemius is more active during activities that involve speed or power, such as sprinting or jumping. The combination of both muscles allows for a full range of plantar flexion capabilities, balancing endurance with strength.

Clinical Differentiation of Injuries

Differentiating between soleus and gastrocnemius injuries is important for accurate diagnosis and treatment. Pain localized deeper and more distal in the calf typically indicates soleus involvement, whereas pain higher up and more superficial suggests a gastrocnemius strain. Clinical tests such as resisted plantar flexion with the knee flexed isolate the soleus, while the same test with the knee extended engages the gastrocnemius, aiding in identification of the affected muscle.

References

1. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer; 2018. p. 618–621.
2. Drake RL, Vogl AW, Mitchell AWM. Gray’s Anatomy for Students. 5th ed. Elsevier; 2024. p. 775–778.
3. Standring S, editor. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2021. p. 1398–1401.
4. Neumann DA. Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 3rd ed. St. Louis: Elsevier; 2017. p. 546–550.
5. Delp SL, Loan JP, Hoy MG, et al. An interactive graphics-based model of the lower extremity to study orthopaedic surgical procedures. IEEE Trans Biomed Eng. 1990;37(8):757–767.
6. Payne AH, Berg HE, Jones DA. A comparison of muscle activity in the triceps surae during different calf exercises. J Sports Sci. 2001;19(12):879–886.
7. O’Brien M. The anatomy of the calf muscles: implications for Achilles tendon injury and repair. Clin Anat. 2005;18(5):403–410.
8. Giddings VL, Beaupré GS, Whalen RT, Carter DR. Calcaneal loading during walking and running. Med Sci Sports Exerc. 2000;32(3):627–634.
9. Hermens HJ, Freriks B, Disselhorst-Klug C, Rau G. Development of recommendations for SEMG sensors and sensor placement procedures. J Electromyogr Kinesiol. 2000;10(5):361–374.
10. Koulouris G, Connell D. Imaging of muscle injuries: MRI findings in the soleus and gastrocnemius complex. AJR Am J Roentgenol. 2006;187(3):616–623.