Why Achilles injuries keep coming back
Achilles tendinopathy is the most common overuse injury in runners, accounting for roughly 11% of all running injuries (Kujala et al., 2005). The frustrating part isn't the first injury — it's the re-injury. Studies show that 27% of runners who return from Achilles tendinopathy get re-injured within the first 6 weeks back.
The reason is simple: tendons heal slower than muscles. When your calf feels strong enough to run, your Achilles tendon is still rebuilding collagen cross-links. It can handle the load of a single run, but not the cumulative load of a training week. This is why "listen to your body" fails — by the time your Achilles hurts again, the damage is already done.
Understanding ACWR: the number that predicts re-injury
The Acute-to-Chronic Workload Ratio (ACWR), introduced by Tim Gabbett in his landmark 2016 paper, compares your recent training load (last 7 days) to your baseline (last 28 days). An ACWR between 0.8 and 1.3 is the "sweet spot" — enough stimulus to build fitness, not enough to spike injury risk.
For Achilles tendinopathy returns, the research is even more specific. Silbernagel et al. (2007) found that runners who stayed below an ACWR of 1.2 during the first 8 weeks had a re-injury rate of just 4%, compared to 31% for those who exceeded 1.5. The takeaway: controlled, progressive loading isn't conservative — it's the fastest safe path back.
The 8-week return protocol
Weeks 1-2: Walk-run intervals Start with 3 sessions per week. Each session: 20-30 minutes of alternating 2 minutes running / 2 minutes walking. Total running volume should be 40% of your pre-injury average weekly distance. Keep all running at conversational pace (RPE 2-3).
Weeks 3-4: Continuous easy runs Progress to 3 continuous runs per week at 55% of pre-injury volume. Introduce one slightly longer run (your "long run" — which may only be 6-8km). No tempo or speed work. Monitor morning Achilles stiffness — if it lasts more than 10 minutes, reduce volume by 15%.
Weeks 5-6: The danger zone This is where most re-injuries happen. Tissue feels recovered but hasn't fully adapted. Increase to 70% of pre-injury volume. Add a 4th run if your ACWR stays below 1.2. Begin light fartlek (not structured intervals). Weekly RPE check-ins help catch problems early.
Weeks 7-8: Return to normal structure Build to 85-95% of pre-injury volume. Reintroduce tempo runs and structured workload. Your ACWR should naturally stay in the 0.9-1.2 range if you've been progressive. The Achilles tendon now has 8 weeks of adapted loading history.
The Second Injury Shield window
After completing a return-to-running protocol, the next 6 weeks are critical. Research by Hägglund et al. (2006) found that athletes who had recently returned from injury were 2-3x more likely to sustain a new injury during this window.
This is why injury.vision includes a "Second Injury Shield" — for 6 weeks after your RTR protocol completes, your risk thresholds are tightened. What would normally register as "moderate" risk triggers an amber warning. It's the safety net that generic training apps don't provide.
What your Garmin doesn't tell you
Garmin and Strava track volume, pace, and heart rate. They can estimate VO2max and training load. But they don't compute injury-specific risk. They don't know you had Achilles tendinopathy. They don't apply a detraining multiplier based on how long you were off. They don't tighten your thresholds during the re-injury window.
injury.vision fills this gap. It takes your Strava or Garmin data and computes a daily injury risk score using five components: session spike (ACWR), TSS spike, intensity risk, cumulative fatigue, and injury history. Each component is weighted, and the composite score maps to green (low), amber (moderate), or red (high) risk.
Eccentric loading: the evidence-based complement
The Alfredson protocol (heavy eccentric heel drops — 3 sets of 15, twice daily) remains the gold standard for Achilles tendon rehabilitation. A systematic review by Malliaras et al. (2013) confirmed that eccentric loading combined with progressive running produces better outcomes than either alone.
During weeks 1-4 of your return, perform eccentric heel drops daily. During weeks 5-8, transition to heavy slow resistance (HSR) training 3x per week. This builds tendon stiffness that complements the running load progression.
Frequently asked questions
- How long should I wait before running after Achilles tendinopathy?
- Most sports medicine guidelines recommend waiting until you can perform 3 sets of 15 single-leg heel raises without pain, walk for 30 minutes pain-free, and have less than 5 minutes of morning stiffness. This typically takes 6-12 weeks from initial onset, depending on severity.
- What is a safe ACWR for returning from Achilles tendinopathy?
- Research suggests keeping your ACWR below 1.2 for the first 8 weeks of return. The "sweet spot" for ongoing training is 0.8-1.3 (Gabbett, 2016). Spikes above 1.5 dramatically increase re-injury risk.
- Can I run through mild Achilles pain?
- The "traffic light" system is widely used: pain under 3/10 during running that settles within 24 hours (green) is generally acceptable. Pain 3-5/10 (amber) means reduce volume. Pain above 5/10 or pain that worsens during a run (red) means stop and reassess.
- How fast should I increase mileage after Achilles tendinopathy?
- The old "10% rule" is too simplistic. A better approach is to use ACWR to guide progression — increase weekly load only when your ACWR is below 1.2 and you have no morning stiffness lasting more than 10 minutes. This typically works out to 10-20% increases per week in the first 4 weeks, slowing to 5-10% in weeks 5-8.
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