Etiology of Achilles Tendonitis
Achilles tendonitis is primarily caused by repetitive eccentric loading that produces microtrauma to the tendon and surrounding structures, typically triggered by training errors, biomechanical abnormalities, or a combination of both. 1, 2
Primary Causative Mechanisms
Mechanical Overload
- Eccentric loading during activities like running, jumping, or exercises such as Bulgarian split squats is the fundamental mechanism, causing repetitive microtrauma that exceeds the tendon's capacity for repair 1, 2
- The injury results from load-induced stress that damages the normally highly arranged collagen fiber structure, leading to collagen disorientation and fiber separation 3
Training Errors (Most Common Precipitant)
- Excessive running mileage and training intensity are the most frequently responsible factors, present in approximately 75% of cases 2, 4
- Hill running, training on hard or uneven surfaces, and poorly designed footwear contribute significantly 2
- Sudden increases in activity level or initiation of new activities without adequate conditioning 3
Biomechanical Predisposing Factors
Muscle-Tendon Unit Dysfunction
- Gastrocnemius-soleus muscle weakness or inflexibility creates abnormal loading patterns that concentrate stress on the tendon 2, 4
- Fatigued muscle produces eccentric loading that the tendon cannot adequately absorb 4
Lower Extremity Malalignment
- Functional overpronation (present in approximately 56% of cases) produces a "whipping action" of the Achilles tendon during gait 4
- Hindfoot malalignment with foot hyperpronation creates conflicting rotatory forces on the tibia during simultaneous pronation and knee extension 2, 4
- This produces vascular blanching of the tendon in its critical hypovascular zone 5, 4
Pathophysiologic Evolution
Acute to Chronic Transition
- Most patients present after prolonged symptoms, by which time acute inflammation has subsided and been replaced by degenerative changes (tendinosis) 3
- The condition exists on a spectrum: paratenonitis (inflammation of surrounding tissue) → tendinosis (structural degeneration) → partial rupture → complete rupture 2, 6
Anatomic Vulnerability
- The critical hypovascular zone of the Achilles tendon (2-6 cm proximal to insertion) is particularly susceptible to injury due to reduced blood supply 5
- Insertional tendinopathy specifically affects the tendon's attachment to the calcaneus 1
Clinical Presentation Pattern
The typical presentation is insidious onset of load-related localized pain coinciding with increased activity, with pain initially present during activity but subsiding after warm-up, then gradually increasing in intensity and duration until present even at rest 3, 1
Key Distinguishing Features
- Pain and tenderness localized at the Achilles insertion on the heel (insertional type) or 2-6 cm proximal (mid-substance type) 1, 5
- Transient relief with stretching is characteristic 1
- Swelling, erythema, and asymmetry are commonly noted on examination 3
Important Clinical Caveat
Multiple symptomatic tendons should alert you to consider systemic rheumatic disease rather than simple overuse tendinopathy, warranting rheumatologic evaluation 3