Achilles tendinopathy is one of the most prevalent running injuries in endurance sport, accounting for approximately 8–15% of all running injuries. It is also one of the most mismanaged — commonly called "tendinitis," treated with rest and anti-inflammatories, and returned to training too soon, only to recur. The evidence on Achilles tendinopathy has transformed dramatically in the past 20 years, and the treatment protocol is the opposite of what most athletes expect.
What Achilles Tendinopathy Actually Is
The Achilles tendon connects the calf muscles to the heel bone and is the strongest tendon in the body — capable of withstanding forces of 6–8× body weight during running. Tendinopathy is a degenerative condition characterised by disorganised collagen, increased ground substance, and neovascularisation (new blood vessel ingrowth) — not primarily inflammation. Biopsies of tendons diagnosed as "tendinitis" consistently show degenerative, not inflammatory, changes. This matters enormously because it changes what treatment is effective.
Two distinct presentations exist:
- Mid-portion tendinopathy: Pain and thickening 2–7cm above the heel. The most common presentation in runners. Responds well to eccentric loading.
- Insertional tendinopathy: Pain at the heel bone attachment. Requires modified protocol — standard eccentric heel drops onto the step can aggravate this type.
Causes and Risk Factors
Achilles tendinopathy follows the core mechanism of all overuse injuries: load applied faster than the tendon can adapt. Key risk factors:
- Training load spikes: Mileage increases exceeding the tendon's adaptation rate — tendons lag behind cardiovascular fitness by weeks
- Calf weakness: Fatigued calf muscles transfer increasing load to the Achilles in the later kilometres of a long run
- Footwear changes: Transitioning rapidly to lower-drop or minimalist footwear dramatically increases Achilles loading
- Surface changes: More uphill running or harder surfaces increase tendon stress per stride
- Metabolic factors: Type 2 diabetes, obesity, and fluoroquinolone antibiotic use all impair tendon collagen quality and synthesis
The Evidence-Based Treatment: Heavy Slow Resistance
The most effective treatment for mid-portion Achilles tendinopathy is heavy, slow resistance exercise targeting the calf — specifically the Alfredson eccentric heel drop protocol, validated in multiple randomised trials and a Cochrane review.
The Alfredson Protocol
- Exercises: Straight-leg calf raise (targets gastrocnemius) + bent-knee calf raise (targets soleus), performed on a step with heel drop below the edge
- Dose: 3 sets of 15 reps, twice daily, 7 days/week, for 12 weeks
- Tempo: Deliberately slow — 3 seconds lowering, brief pause, use the uninjured leg to return. Lower eccentrically on the injured leg only.
- Progression: When bodyweight becomes comfortable, add load with a weighted backpack. Successful completion with 20–30kg is the target.
- Critical point: The protocol is performed through mild-to-moderate pain (≤5/10). Painless exercise is not the goal. The controlled load is the therapy.
Alfredson et al. (1998) reported 15 of 15 previously treatment-resistant runners recovered with this protocol. A Cochrane review confirmed eccentric loading produces superior long-term outcomes compared to rest, NSAIDs, and corticosteroid injections.
What Doesn't Work
- Complete rest: Removes the tensile loading that stimulates collagen remodelling. Symptoms typically return at the same training load when rest ends.
- Anti-inflammatories (NSAIDs): Reduce short-term pain but don't address degenerative pathology. Some evidence suggests interference with collagen synthesis.
- Corticosteroid injections: Provide short-term pain relief but are associated with increased long-term tendon rupture risk and worse outcomes at 1-year follow-up than loading programmes alone.
- Passive stretching: Static calf stretches don't load the tendon sufficiently to stimulate adaptation.
Can You Continue Running?
For mid-portion tendinopathy, continuing to run at reduced volume while performing the loading protocol is appropriate for most athletes — provided pain does not exceed 5/10 during running and returns to baseline within 24 hours. Use this traffic-light system to guide daily decisions:
- Green (continue training): Pain ≤3/10 during running, no significant morning stiffness
- Amber (reduce load 20–30%): Pain 4–5/10 during running, mild stiffness first thing
- Red (rest from running): Pain >5/10, pain at night, significant morning stiffness — continue loading exercises, seek physiotherapy assessment
Prevention: Building Tendon Resilience Before Problems Develop
Two sessions per week of heavy calf raises — 3 sets of 6–8 repetitions at 80% of maximum — is the minimum effective dose for tendon health maintenance in runners. Avoid simultaneous mileage and intensity increases in the same week: add one variable at a time. Athletes returning from any training interruption should treat tendon adaptation as the pacing constraint, not cardiovascular fitness. Use the NorthLine Training Load Calculator to monitor your acute-to-chronic workload ratio and catch spikes before they stress Achilles tissue beyond its adaptive capacity.