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Achilles Tendinopathy: Understanding the Tendon and Rebuilding Capacity (PART 1 OF 3)
- 2 days ago
- 10 min read
The big picture
Achilles tendinopathy rehabilitation is no longer simply:
Rest → stretch → eccentric heel drops → wait for the pain to disappear.
The current approach is much more about load management and rebuilding capacity.
The 2024 JOSPT/AOPT Clinical Practice Guideline gives tendon-loading exercise a Grade A recommendation for midportion Achilles tendinopathy. Exercise should be performed at loads as high as tolerated and at least 3 times per week. Complete rest is generally not indicated; activity should instead be continued within pain tolerance.
This distinction is important: Reducing pain is not the same as restoring tendon capacity. Someone may feel substantially better before they have recovered their calf strength, Achilles stiffness, rate of force development, hopping ability, or tolerance for running and sprinting.
1. Understanding Achilles Tendinopathy
The Achilles tendon connects the gastrocnemius and soleus muscles to the calcaneus.
During walking, running, jumping, and sprinting, the calf–Achilles complex has to accept, store, transmit, and release very large forces.
Tendinopathy develops when there is a mismatch between the demands placed on the tendon and its current capacity to tolerate those demands. A classic example is a runner who suddenly adds: more mileage + hills + speed work after months of relatively stable training. The Achilles may have been capable of tolerating the previous training load but not the new one. That doesn't necessarily mean the tendon suddenly "tore." It means its current capacity was exceeded.
Tendinopathy vs tendinitis
The older term Achilles tendinitis implies that inflammation is the primary pathological process. We now know the condition is considerably more complicated.
Chronic Achilles tendinopathy can involve changes in:
extracellular matrix
collagen organization
tenocyte activity
tendon mechanical properties
tendon structure
pain processing
muscle function
tendon load tolerance
For this reason, Achilles tendinopathy is generally the preferred term.
Midportion vs insertional Achilles tendinopathy
This distinction matters because the rehabilitation programs shouldn't necessarily be identical. Midportion Achilles tendinopathy
Symptoms are generally located approximately 2–6 cm above the Achilles insertion.
Common findings include:
localized tendon pain
morning stiffness
tendon thickening
pain with calf raises
pain with running or jumping
stiffness when first beginning activity
Insertional Achilles tendinopathy
Pain occurs where the Achilles attaches to the calcaneus. This area has an additional consideration: compression. When the ankle moves into dorsiflexion, the insertional Achilles can experience compression against the calcaneus. This has important rehabilitation implications.
A major 2025 randomized clinical trial compared a rehabilitation program designed to reduce Achilles compressionwith a higher-compression program in people with insertional Achilles tendinopathy.
The lower-compression program limited dorsiflexion during exercise, removed calf stretching, and incorporated heel lifts. It produced significantly greater improvements in VISA-A at both 12 and 24 weeks.
This provides strong new evidence that insertional Achilles rehabilitation deserves its own loading strategy.
We'll cover that extensively in Part 2.
Typical symptoms
People commonly describe:
Morning stiffness
The first several steps after getting out of bed can be uncomfortable.
Start-up pain
Walking may initially hurt and then improve after several minutes.
Running pain
Symptoms may initially warm up during running before returning afterward.
Pain with jumping
Especially repeated jumping, hopping, or explosive movements.
Pain with calf raises
Particularly single-leg calf raises.
Pain after activity
Sometimes the Achilles feels relatively good during activity but becomes painful or stiff several hours later or the following morning. That delayed response is extremely important when deciding whether training load was appropriate.
Pain does not automatically mean damage
One of the most important concepts for patients to understand is: Pain ≠ tendon damage occurring every time the tendon hurts. Pain is still meaningful and should be monitored, but tendon pain is influenced by more than structural damage.
The relationship between: pain ↔ tendon structure ↔ function is imperfect.
Cook and colleagues specifically emphasized this when revisiting the tendon continuum model. This also explains why imaging can sometimes look abnormal in someone functioning very well. Conversely, someone can have substantial Achilles pain without imaging showing catastrophic tendon damage. We therefore treat the person's symptoms, capacity, and function, not simply their MRI or ultrasound.
Complete rest usually isn't the answer
An irritated tendon may initially benefit from reducing provocative loading.
But:
Load modification ≠ complete unloading. The goal is usually to remove or reduce the loads the tendon currently cannot tolerate while maintaining loads it can tolerate.
For example: Temporarily decrease:
sprinting
repeated jumping
hill running
high-volume running
aggressive plyometrics
while maintaining appropriate:
walking
cycling when tolerated
calf isometrics
calf raises
resistance training
general conditioning
Silbernagel and colleagues demonstrated that patients could continue tendon-loading sport activity during rehabilitation when using a pain-monitoring model without compromising outcomes compared with active rest.
Important: Make sure it actually is tendinopathy
Not every painful Achilles is Achilles tendinopathy.
A clinician should consider other diagnoses including:
Achilles rupture
partial Achilles tear
retrocalcaneal bursitis
paratenon disorders
plantaris-related pain
posterior ankle impingement
referred pain
sural nerve involvement
calcaneal pathology
Red flags
Seek medical assessment promptly for:
sudden "pop" or snapping sensation
sudden major loss of push-off strength
inability to perform a calf raise that was previously possible
palpable gap in the tendon
major bruising
substantial acute swelling
acute traumatic injury
rapidly worsening unexplained symptoms
significant redness/warmth accompanied by systemic symptoms
These findings can indicate something different from routine tendinopathy and may require imaging or medical management.
2. The Tendon Continuum
Cook and Purdam proposed one of the most influential models for understanding tendon pathology: the tendon continuum. It's useful clinically, but shouldn't be interpreted as three perfectly separated biological boxes.
Think of it as a framework.
Stage 1: Reactive tendon
Imagine someone normally runs 10 miles per week. Suddenly they run 25 miles and add hill repeats. The tendon experiences a load it isn't currently prepared for. The tendon may respond with changes involving the extracellular matrix and increased tendon thickness. Clinically, the tendon can become:
painful + sensitive + less tolerant of load.
The solution isn't necessarily to stop using the tendon.
Instead:
Remove the load spike.
Reduce the highest-strain activities temporarily while maintaining tolerable loading.
For example:
Temporarily remove: sprinting and jumping.
Maintain: controlled calf strengthening.
Stage 2: Tendon dysrepair
With persistent excessive loading and inadequate recovery, greater matrix disorganization can occur.
Changes may include:
altered collagen organization
increased matrix disorganization
structural changes
altered mechanical properties
At this point, simply "waiting for inflammation to go away" doesn't adequately address the problem. The tendon needs to progressively rebuild its capacity to handle force.
Stage 3: Degenerative tendon
Long-standing tendinopathy can contain areas of substantial structural abnormality.
But here's the important clinical point: Degenerative imaging does not automatically mean the tendon cannot become highly functional again. The tendon can improve its capacity even if imaging never looks completely "normal."
Reactive-on-degenerative
A chronically abnormal tendon can also experience a sudden increase in symptoms after a load spike. This is often described as: reactive-on-degenerative tendinopathy.
Someone may have had tendon thickening for years without major limitations. Then they suddenly increase: running + hills + jumping. The tendon becomes symptomatic.
This does not necessarily mean the tendon suddenly suffered massive new structural damage. It may represent an acute increase in irritability on top of chronic tendon changes.
3. How Tendons Adapt to Loading
Tendons are living tissues. They respond to mechanical stress. Cells within the tendon called tenocytes respond to mechanical loading through mechanotransduction.
Appropriate loading influences:
collagen synthesis
extracellular matrix remodeling
tendon mechanical properties
force transmission
tendon stiffness
muscle-tendon capacity
A systematic review and meta-analysis found mechanical loading can increase tendon stiffness and elastic modulus, with higher-strain resistance exercise producing greater adaptation than lower-strain loading.
A 2026 review further describes how appropriate mechanical loading influences tenocyte signaling, collagen synthesis, extracellular matrix remodeling, tendon stiffness, and mechanical strength.
This provides the biological rationale for progressive resistance training.
Your Achilles needs load.
But it needs the right amount of load at the right time.
Why rehabilitation takes time
Muscles can gain strength relatively quickly.
Tendons generally adapt more slowly.
That's one reason a patient may say:
"My Achilles doesn't really hurt anymore."
but still demonstrate:
weak single-leg calf raises
reduced heel-rise height
poor soleus strength
reduced hopping capacity
poor reactive strength
difficulty sprinting
Pain reduction should therefore not be the only rehabilitation goal.
4. PHASE 1
Settle Symptoms Without Completely Unloading the Tendon
Goals
The first phase is designed to:
1. Reduce excessive tendon irritation
2. Identify and reduce provocative loads
3. Maintain calf and Achilles loading
4. Establish an exercise dose the tendon tolerates
5. Build confidence in loading the tendon
Phase 1 doesn't necessarily mean "week 1."
Someone with relatively low irritability may begin further along the continuum.
Exercise 1: Standing calf isometric
Stand holding a wall, rack, or countertop.
Rise onto the balls of the feet.
Hold the position.
Starting dose
4–5 sets × 30–45 seconds
Rest:
60–90 seconds
Frequency:
Daily or every other day initially depending on irritability and the rest of the loading program.
Progression
Double leg→ more weight toward affected side→ single leg→ weighted single leg
Regression
Reduce:
heel-rise height
hold duration
load
Exercise 2: Bent-knee soleus isometric
The soleus deserves considerable attention in Achilles rehabilitation.
Sit with the knee bent approximately 90°. Push through the forefoot as though performing a seated calf raise.
Hold against resistance.
Dose
4 × 30–45 seconds
Rest:
60–90 seconds
Resistance can come from:
body weight
dumbbells
weight plates
seated calf machine
Smith machine
Exercise 3: Slow double-leg calf raise
Stand with knees straight.
Slowly rise onto the toes.
Pause briefly.
Slowly lower.
Starting dose
3 × 10–15
Tempo:
approximately 2–3 seconds up / controlled pause / 2–3 seconds down
Start with body weight.
When this becomes easy, add external resistance.
Exercise 4: Seated calf raise
This provides greater emphasis on the soleus.
Sit with knees bent around 90°.
Place resistance across the thighs or use a seated calf machine.
Raise the heels.
Pause.
Slowly lower.
Starting dose
3 × 10–15
Progress by adding load.
Exercise 5: Standing calf raise isometric with knee slightly bent
This provides another way to challenge the soleus/Achilles complex in a weight-bearing position.
Hold:
3–4 × 30–45 seconds
Progress toward unilateral loading.
What about isometrics for pain relief?
Isometrics became popular in tendon rehabilitation partly because of research suggesting they could produce an immediate analgesic response. But this effect shouldn't be oversold.
A 2026 systematic review concluded that evidence supporting the superiority of isometrics for tendinopathy is limited. They appear tolerable and may help some patients, but long-term superiority over isotonic loading has not been demonstrated.
So:
Use isometrics because they are a useful loading option—not because they magically turn tendon pain off.
Pain Monitoring: The Traffic Light Approach
Some discomfort during Achilles rehabilitation is often acceptable.
The more useful question isn't simply:
"Did it hurt?"
It's: "How did the tendon respond to the dose?"
Silbernagel's pain-monitoring approach provides an important foundation for this concept.
🟢 GREEN LIGHT
Pain is mild and manageable.
Symptoms settle after exercise.
Walking remains normal.
The tendon feels similar or better the following morning.
Continue or gradually progress.
🟡 YELLOW LIGHT
Pain is noticeably higher.
Morning stiffness increases.
Symptoms remain elevated longer after exercise.
Performance is starting to decrease.
Maintain or reduce the dose.
Possible adjustments:
reduce weightreduce repetitionsreduce setsreduce running volumeincrease recovery time.
🔴 RED LIGHT
Substantial increase in symptoms.
Progressively worsening morning pain.
Major swelling.
Loss of function.
Sudden loss of strength.
New bruising or concern for rupture.
Stop and reassess.
A specific pain score such as "3/10 is safe but 4/10 isn't" should not be treated as a universal biological cutoff.
The 24-hour response matters.
5. PHASE 2
Build Strength and Tendon Capacity
Once basic loading is tolerated, rehabilitation should become increasingly focused on progressive resistance training.
The goal changes from:
"Can I load my Achilles?"
to:
"How much force can my calf–Achilles complex produce and tolerate?"
Both the gastrocnemius and soleus need attention.
Straight-knee loading
Greater emphasis on the gastrocnemius.
Examples:
Standing calf raise
Double leg↓assisted single leg↓single leg↓weighted single leg
Bent-knee loading
Greater emphasis on the soleus.
Examples:
Seated calf raise
Body weight/light resistance↓dumbbell/plates↓machine↓heavy seated calf raise
Example Phase 2 Session
A. Standing calf raise
3–4 × 8–12
B. Seated calf raise
3–4 × 8–12
C. Leg press calf raise
3 × 10–15
D. Bent-knee standing calf raise
3 × 10–15
Perform approximately 3 times per week, adjusting frequency and load according to irritability and other running/sport demands.
Progressive overload matters
If someone performs:
3 × 15 body-weight calf raises
for six months, the tendon eventually stops receiving a meaningful new training stimulus.
Progressive loading can come from increasing:
External resistance
20 lb → 30 lb → 40 lb → 50 lb
Exercise difficulty
Double leg → single leg
Range of motion
when appropriate
Velocity
later in rehabilitation
Energy-storage demand
eventually adding jumping and hopping.
What about eccentric heel drops?
Eccentric exercise absolutely has evidence behind it. The classic Alfredson program used high-volume eccentric calf loading and was highly influential in Achilles rehabilitation.
But:
Eccentric isn't synonymous with Achilles rehabilitation.
Beyer and colleagues compared traditional eccentric training with heavy slow resistance training in people with chronic midportion Achilles tendinopathy.
Both groups demonstrated substantial improvements that persisted at 52 weeks.
There was no significant difference in clinical or structural improvement between the programs, while compliance was higher in the heavy slow resistance group.
This helped reinforce an important modern principle:
The Achilles needs progressive loading—not necessarily one magical contraction type. A good rehabilitation program can include:
Isometric → isotonic → heavy resistance → faster loading → energy storage → plyometric loading.
A Simple Phase 2 Progression
Level 1
Double-leg calf raise
3 × 15
↓
Level 2
Assisted single-leg calf raise
3 × 10–12
↓
Level 3
Single-leg calf raise
3 × 8–12
↓
Level 4
Weighted single-leg calf raise
3–4 × 8–10
↓
Level 5
Heavy single-leg calf raise
3–5 × 5–8
The same progression can be applied to bent-knee/soleus loading.
Special Note for Insertional Achilles Tendinopathy
Do not automatically perform heel drops off a step. For insertional tendinopathy, initially perform calf raises: from the floor → neutral ankle position
rather than: heel hanging below a step → deep dorsiflexion.
The 2025 randomized trial by Pringels and colleagues provides particularly important new evidence here: reducing Achilles compression by limiting dorsiflexion, eliminating calf stretching initially, and using heel lifts produced clinically superior outcomes compared with a higher-compression rehabilitation program.
As symptoms and capacity improve, dorsiflexion can be progressively reintroduced.
This is one of the most important recent changes in how I would approach insertional Achilles rehabilitation.
Where We Are Going Next
At this stage the patient has progressed from:
Painful and load-sensitive
↓
Tolerating isometrics
↓
Tolerating calf raises
↓
Building meaningful gastrocnemius and soleus strength
But this still doesn't mean they're ready to sprint.
A heavy calf raise is relatively slow.
Running is fast.
Jumping is faster.
Sprinting exposes the Achilles to very high forces delivered very quickly.
That brings us to the next major rehabilitation target:
Rate of loading and energy storage.
In Part 2, I'll build the progression from:
heavy strength → pogos → hopping → plyometrics → running → acceleration → sprinting → sport and provide objective return-to-running and return-to-sport testing criteria. One especially important theme will be why strength symmetry alone isn't enough to clear an athlete for high-speed sport.
Evidence level so far
STRONG EVIDENCE: Progressive tendon-loading exercise for midportion Achilles tendinopathy.
STRONG/MODERATE EVIDENCE: Both eccentric and heavy slow resistance approaches can improve chronic midportion Achilles tendinopathy; no compelling evidence establishes eccentric-only training as universally superior.
EMERGING HIGH-QUALITY EVIDENCE: Reducing tendon compression during early rehabilitation appears particularly important for insertional Achilles tendinopathy.
LIMITED/MIXED EVIDENCE: Isometrics may be useful as a tolerable loading strategy and may reduce pain for some patients, but should not be presented as a universally superior analgesic intervention.
IMPORTANT 2026 UPDATE: A new systematic review and meta-analysis found no clinically meaningful benefit from shockwave therapy for Achilles tendinopathy and concluded that it should not currently be considered a routine treatment. This is a meaningful update from some earlier reviews that were more favorable toward ESWT.
Next: Part 2 — Heavy Strength, Plyometrics, Return to Running, Sprinting and Return-to-Sport Testing.
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