Achilles Tendon
| |
| Achilles Tendon | |
|---|---|
| Muscle Type | |
| Origin | Gastrocnemius and soleus muscle fibers (tendon begins mid-calf) |
| Insertion | Posterior surface of calcaneus (calcaneal tuberosity) |
| Action | Transmits force for plantarflexion of ankle; enables heel raise, jumping |
| Synergists | |
| Antagonists | |
| Spinal innervation | |
| Peripheral Innervation | Tibial nerve (S1–S2) – via innervation of muscle, tendon itself has sensory innervation |
| Vasculature | Posterior tibial artery (via sural branches); peroneal artery |
The Achilles tendon, or calcaneal tendon, is the thick tendon that attaches the gastrocnemius and soleus muscles (triceps surae) to the calcaneus (heel bone). It is the strongest tendon in the body and enables powerful plantarflexion of the foot (e.g., pushing off during gait, jumping). Despite its strength, it is prone to injury due to high stresses. Clinically, it is involved in Achilles tendinopathy and rupture. The tendon has no true synovial sheath, but rather a paratenon, and has relatively poor blood supply in its mid-portion, which is a common site of degeneration.
Structure
The Achilles tendon is a thick, fibrous connective tissue structure formed by the merging of the gastrocnemius and soleus tendons approximately in the mid-calf. It is about 15 cm long. The gastrocnemius fibers spiral (~90°) as they join the soleus fibers in the tendon, which may contribute to its elasticity and also to the common site of tears (watershed area ~2-6 cm above insertion). There is a paratenon around it instead of a synovial sheath. Two bursae relate to it: the subcutaneous calcaneal bursa and the retrocalcaneal bursa.
Origin
Not an origin in muscular sense, but the tendon begins where the muscle bellies of gastrocnemius and soleus taper and merge – usually around the mid-leg (about 6-8 cm below the knee). The soleus contributes the deeper part, gastrocnemius the superficial part of the tendon.
Insertion
The tendon inserts onto the middle of the posterior calcaneal surface (calcaneal tuberosity). The lower part of the tendon might fan out a bit on the heel. Collagen fibers from the tendon actually continue into the calcaneus bone (Sharpey’s fibers).
Action
It transmits the force of the triceps surae to plantarflex the ankle (pointing the foot down). This action is fundamental in standing on tiptoes, walking, running, and jumping. By connecting leg muscles to the foot, it allows one to raise the heel off the ground. It effectively multiplies the muscle's effect by acting as a lever arm. Since gastrocnemius also flexes the knee, the Achilles tendon also indirectly participates in knee motion coordination (e.g., when knee is extended and foot fixed, Achilles tension can help prevent knee hyperextension by pulling tibia back).
The Achilles also stores elastic energy during dorsiflexion (like when landing from a jump or mid-stance in gait) and releases it in push-off, improving efficiency of movement. This spring-like function is key in running and hopping.
Related Structure Action: It does not act alone but as a conjoined tendon of two major muscles:
- With knee extended, gastroc and soleus via Achilles produce maximum plantarflexion force (gastroc at optimal length, soleus active).
- With knee flexed, soleus still acts strongly via Achilles, while gastroc is shortened (less contribution). Achilles still transmits soleus force.
- Achilles tension also influences foot arch via the calcaneus connection: excessive tightness can cause more foot supination or early heel-off in gait, affecting arch mechanics.
Synergists
Tibialis posterior, flexor hallucis longus, flexor digitorum longus, peroneus longus and brevis all assist in plantarflexion, but none as powerfully as the Achilles. However, in events of Achilles rupture, these synergists are what allow some residual weak plantarflexion (so patient can still weakly move foot down but not stand on tiptoe on that leg).
During push-off, Achilles works with intrinsic foot muscles to stabilize foot (intrinsics maintain toes rigid so force goes to ground).
Achilles and plantar fascia work synergistically: dorsiflexing toes (via ground reaction in toe-off) winds plantar fascia (Windlass mechanism) which assists Achilles in lifting heel by stiffening arch.
Antagonists
Dorsiflexors of the foot (tibialis anterior, EDL, EHL) are antagonists. They lift the foot up, Achilles lowers it/pushes it down.
During gait, dorsiflexors contract as antagonists to control foot slap when Achilles/gastroc stop firing at initial contact. Conversely, at toe-off, dorsiflexors are quiet while Achilles vigorously contracts.
At the knee, the quadriceps are antagonists to the knee-flexion tendency of gastroc (though Achilles itself doesn't directly attach to knee).
In static stance, tibialis anterior and soleus are a force couple: tib ant tries to cause forward leg sway (dorsiflexion), soleus (via Achilles) resists. They antagonize each other to maintain balance.
Innervation
No direct motor innervation except via muscle (tibial nerve S1–S2 for triceps surae). The tendon itself has proprioceptive fibers (Golgi tendon organs) sending info via tibial nerve to S1–S2 spinal segments. There’s also some sensory innervation from sural nerve branches to paratenon/bursal tissue.
Blood Supply
Blood supply to Achilles is modest. Posterior tibial artery via sural branches supplies upper and mid portion; peroneal artery via calcaneal branches supplies near insertion. There's a watershed area about 2-6 cm above calcaneus with poorest blood flow, correlating with common site of degenerative changes and rupture.
Clinical Relevance
Achilles Tendinopathy: Overuse can lead to degeneration (tendinosis) often in the mid-substance (watershed area). Presents as pain, thickening, morning stiffness in Achilles. Risk factors include tight calf, increase in activity, certain antibiotics (fluoroquinolones). Treatment: rest, eccentric calf exercises (slow heel drops, which have evidence of promoting tendon remodeling), calf flexibility, maybe heel lifts (to reduce tendon stress). Chronic cases may require casting or even surgery (debridement). It's crucial not to inject steroid into Achilles (risk of rupture) – if needed, injection is done around but not in tendon.
Achilles Rupture: Typically occurs during explosive push-off (e.g., jumping, sprinting) especially in males 30-50 with history of tendon degeneration. Feels like being shot/kicked in calf. Exam: positive Thompson test (squeeze calf, foot doesn't plantarflex). Often a gap may be palpable. Non-surgical treatment (casting in plantarflexion then gradual dorsiflexion) vs. surgical repair is debated; surgery slightly reduces re-rupture risk but has wound complication risk. Post rupture, patient cannot do toe-off or single-leg heel raise. After healing, aggressive rehab is needed to restore strength (and often strength never fully returns to pre-injury levels, minor deficits remain).
Short Achilles (Equinus): Congenital or acquired tight Achilles leads to limited dorsiflexion (equinus deformity). This causes compensations like toe walking, early heel rise, and can contribute to deformities such as plantarflexed first ray, metatarsalgia, knee hyperextension, etc. Treatment might be stretching, serial casting, or surgical lengthening (e.g., gastrocnemius recession if soleus is fine, or Achilles tendon lengthening if both are tight).
Haglund’s Deformity & Bursitis: Chronic Achilles pull can contribute to a bony enlargement on posterior calcaneus (Haglund deformity) which can irritate the retrocalcaneal bursa (pump bump). Also, shoes rubbing can inflame the subcutaneous calcaneal bursa. Treatment: shoe modifications (backless shoes), heel lifts, Achilles stretching, and if severe, resection of bump and debridement of tendon if needed.
Achilles Reflex: This is a deep tendon reflex testing primarily S1 (tibial nerve). Absence can indicate S1 radiculopathy or tibial nerve issue (or lack of patient relaxation). Very brisk reflex might indicate upper motor neuron issues. It's useful in neurologic exams.
Tendon xanthomas: Patients with familial hypercholesterolemia can get Achilles tendon thickening (cholesterol deposits). They present as a firm enlargement of Achilles.
Surgical Accessory Path: In some foot/ankle surgeries, Achilles tendon might be moved or partially cut to allow access (e.g., in certain calcaneus fractures, a postero-medial approach might detach partly the Achilles insertion). Surgeons must carefully repair it to not lose strength. If Achilles is shortened (like surgical repair overlapping fibers), slight loss of dorsiflexion can result. If lengthened too much (in cerebral palsy management), push-off power decreases (some dynamic lengthening procedures aim to lengthen gastroc aponeurosis while sparing soleus to minimize power loss).
Calf Strength Recovery: Achilles is slow to heal due to low blood supply; after rupture or surgery, strengthening must be gradual (initially weightbearing in equinus, then neutral). It can take 6-12 months to get near-normal function. Many rehab protocols emphasize eccentric loading as tendon remodels stronger.
Prosthetics and Achilles: In amputations (Syme's or transtibial), loss of Achilles means loss of active plantarflexion. Prosthetic feet incorporate energy-storing springs to mimic Achilles function.
Athletic Performance: A healthy, stiffer Achilles tendon can improve running economy by storing and releasing energy. Some high-performance training focuses on plyometrics to improve Achilles stiffness. But too stiff (without muscle control) might risk injury. Balanced training for Achilles is crucial for athletes (combining strengthening and flexibility).
