Is Your Achilles Tendon Pain Due to Tendonitis or Tendinosis?

Pain along the back of the ankle or lower calf usually gets labeled "Achilles tendonitis" — but the actual cause could be tendonitis or the related but different condition, tendinosis. Getting the distinction right changes how it should be treated.

Tendonitis: an acute, inflammatory response

Tendonitis is the body's immediate inflammatory response to a recent injury or overuse, bringing redness, swelling, and sharp pain. It typically has a sudden onset tied to a specific activity spike, and symptoms tend to subside relatively quickly with proper rest and treatment — usually rest, ice, NSAIDs, and physical therapy once the acute pain settles.

Tendinosis: a chronic, degenerative process

Tendinosis is different at the tissue level: rather than inflammation, it's a gradual breakdown of the tendon's collagen fibers from long-term wear or inadequate healing after repeated minor injury. It develops slowly, often unnoticed until the discomfort becomes significant, and because the underlying issue is structural degeneration rather than inflammation, recovery tends to be slower and more involved.

Telling them apart

Tendonitis tends to bring sharp, acute pain during or after activity, noticeable swelling and redness, and a sense of warmth in the area. Tendinosis tends to bring a deep, dull, chronic ache that builds gradually, stiffness especially after inactivity, and a persistent ache that doesn't respond well to typical anti-inflammatory treatment. If pain lingers and standard treatment isn't helping, tendinosis is worth considering over a simple case of tendonitis.

Why the distinction changes treatment

For tendonitis, the focus is rest, ice, NSAIDs, and gentle stretching and strengthening once the initial pain subsides. For tendinosis, anti-inflammatory medication is less effective since inflammation isn't the primary problem — treatment shifts toward eccentric strengthening exercises that stimulate collagen repair, and advanced options like shockwave therapy or, in resistant cases, surgical intervention. Healing is typically slower and requires a longer-term, more tailored plan.

What contributes to either condition

High-impact activity and sudden increases in training load, poor foot mechanics like overpronation, muscle imbalances in the calf, and natural age-related tendon changes all raise the risk of both tendonitis and tendinosis. Prior Achilles injury also increases susceptibility to future issues, particularly tendinosis.

Keeping the tendon healthy

A proper warm-up and regular stretching, eccentric calf raises and balanced strength training, supportive footwear (with orthotics if biomechanics call for them), and simply listening to early warning signs rather than pushing through pain all reduce the risk of developing either condition in the first place.