PHYS-38 · Principle

My heart is fit but my Achilles hurts six weeks into running. Why?

Fitness can rise while the tendon stays unchanged: tendon stiffens under high strain over weeks to months of loading, and its core barely renews. Ramp running by the tendon, not by how easy it feels.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to anyone adding impact load, most of all new runners and mastersEstablished — tendon stiffens under high-strain loading in training studies lasting weeks to months (none times the gain; under 12 weeks did as well as longer, REF-1152); well supported — its core collagen barely…

Practical Application

My heart is fit but my Achilles hurts six weeks into running. Why? Fitness can rise while the tendon stays unchanged: tendon stiffens under high strain over weeks to months of loading, and its core barely renews. Ramp running by the tendon, not by how easy it feels.
What it means for training: progress new impact load by the slowest tissue (SYNTH-21, SYNTH-75); high loads, in any contraction type, are what stiffen tendon (REF-517).
What it does not mean: that a fixed weekly percentage is safe, since the dose stays practice (SYNTH-29), or that tendon should be spared: loading maintains it (REF-599).

Mechanism

In loading interventions of 8 weeks or more, tendon stiffness rose with load intensity, not contraction type, mostly through the material, not size (REF-517); high-strain protocols did better (REF-1152, REF-1355).
The adult Achilles core still carries carbon from the growth years, while muscle matched the last 2 years (REF-1356); that loading adds tissue at the periphery is the authors' suggestion (REF-1355, REF-1356); with age, tendon cells proliferate less in vitro and glycation cross-links build up, while exercise tends to counter ageing (REF-599).
Muscle is protected after one bout (PHYS-24); tendon's clock runs in months, yet 1-2 weeks of limb unloading cut its stiffness (REF-1355).

Common misreading

"It felt easy, so it was easy on my legs." Pain at rest, at night or worsening through a run goes to a clinician (SYNTH-73).

Backs

SYNTH-21 "The tissue that limits a hybrid athlete may not be muscle" · SYNTH-21 "Progress tendon on its own clock" · SYNTH-75 "the first 8-12 weeks of running are the tendon's adaptation window"

Sources

Bohm, Mersmann & Arampatzis 2015 REF-517 · Paper ↗
Magnusson & Kjaer 2019 REF-1355 (B+) · Paper ↗
Heinemeier et al. 2013 REF-1356 (B) · Paper ↗
Svensson et al. 2016 REF-599 · Paper ↗
REF-1366 (B+)
REF-1367 (A-)