Why does my Achilles lag behind my fitness when I start running?
Its core barely renews after adolescence; it adapts to high strain more than moderate, and loses stiffness in 1-2 weeks of limb unloading. Ramp load slowly (practice).
Magnusson and Kjaer review how human tendon responds to loading, unloading, ageing and injury.
On turnover, they set two lines of evidence side by side: stable-isotope biopsy studies found collagen synthesis of about 1% per 24 h in one study and around 0.2% per 24 h in others, while bomb-pulse 14C dating showed that the core of the human Achilles tendon changes until about 17 years of age and is stable thereafter (REF-1356).
They reconcile them with a suggested two-pool model: a large stable pool formed in childhood and adolescence, and a much smaller pool with fast turnover for daily maintenance, whose location is unknown.
On loading, the cellular response is well documented in rodents and cell cultures, but in adult human tendon an acute rise in collagen synthesis seen in one study could not be robustly confirmed by several others; the authors suggest adult human tendon is far less responsive than that of growing rodents and that new tissue may be laid down at the periphery "analogous to growth of a tree".
Tendon does hypertrophy with long-term loading — the most convincing human evidence is a 20% larger tendon cross-section on the side of athletes who had a 22% side-to-side strength difference — and in healthy Achilles tendons loading at 90% of maximal voluntary contraction (~5% strain) raised stiffness and cross-section more than 55% (~3% strain) did; contraction mode (concentric or eccentric) did not matter.
Unloading works fast: 2-3 weeks of immobilisation cut the collagen synthesis rate by 80%, and 1-2 weeks of limb unloading lowered tendon stiffness with no change in its size. With age the tendon is not lost, and a lower modulus in older people disappeared when they were matched with young people for activity.
A surgically repaired Achilles tendon still showed raised glucose uptake 12 months after repair, and the authors conclude that the tendon's response to loading after injury is rather slow, not normalised until sometime after 6-12 months.
Our reading: this is the review behind "tendon is the slow tissue" — slow to renew and slow to heal — with an asymmetry worth keeping: stiffness is lost within one or two weeks of limb unloading or immobilisation, while the evidence for gains comes from training studies of several months; it does not give a time course for stiffness gains, and the authors note that studies of recovery between loading sessions are lacking.
Protocol
Key Results
| Question | What the review states | What it rests on |
|---|---|---|
| Collagen synthesis in human tendon | ~1% per 24 h in one study; around 0.2% per 24 h in others | stable-isotope infusion with tendon biopsy |
| Lifetime turnover of the core | Achilles core collagen changes until about 17 years, then stable; horse tendon half-life about 200 years (high-strain) vs ~34 years (low-strain) | 14C bomb pulse (REF-1356); racemization in horses |
| Acute loading → collagen synthesis in adult tendon | raised in one study; not robustly confirmed by several others using the same method and exercise; mid-tendon gene expression fell or did not change | human biopsy studies |
| Long-term loading → hypertrophy | a 22% side-to-side strength difference went with a 20% larger tendon cross-section on the stronger side ("perhaps currently some of the most convincing evidence") | athletes loading one side more for years |
| Strain dose | 90% MVC (~5% strain) raised stiffness and cross-section vs 55% MVC (~3% strain); long-duration loading (6 s cycle) more than faster 2 s cycles | training studies in healthy Achilles tendons |
| Contraction mode | 12 weeks of concentric or eccentric knee extension gave similar tendon hypertrophy; Achilles stretch is identical in a heel rise and a heel drop | human training and imaging studies |
| Unloading | 2-3 weeks of immobilisation: 80% lower collagen synthesis rate; 1-2 weeks of unloading: lower stiffness, no change in cross-section | human immobilisation studies |
| Ageing | no loss of tendon tissue with age; a lower modulus in old (66 y) than young (26 y) people disappeared when matched for activity | cross-sectional human studies |
| After rupture repair | glucose uptake 6x at 3 months, 3x at 6 months, 1.6x at 12 months that of intact tendon; normalised "sometime after 6–12 months" | human imaging study |
| Recovery between loading sessions | "Studies specifically addressing the effects of recovery and how it affects tendon adaptation are lacking" | — |
Summary table of the review (arrows = overall direction; anabolic/catabolic · cell number · tissue synthesis · cross-section · stiffness): loading ↑ · → · ↑ · ↑ · ↑; unloading ↓ · → · ↓ · → · ↓; ageing →↘ · ↓ (animal data only) · → · → · →↘; injury ↑ · ↓ · ↑ · ↑ · →↘ (tendinopathy only).
Mechanisms
Practical Application
Limitations
How this entry was checked and changed
an authoritative narrative review from the leading group on human tendon physiology (Institute of Sports Medicine Copenhagen), read from the full text; no systematic method, and much of the cellular and molecular evidence is from animals and cell cultures, while the human loading evidence it relies on is a few small training studies and cross-sectional comparisons (on fibril structure the authors call the human data "relatively scant" and "inconclusive")