REF-1356 · Heinemeier et al., 2013 · The FASEB Journal · Paper · PubMed · Free full text

Does my Achilles tendon rebuild itself like muscle?

Not its core: in 28 autopsy samples the core carried carbon from the growth years, while muscle matched the last ~2 years. Adaptation may happen at the edge (authors' suggestion).

Main caveat: Unknown donors. Forensic cases without activity or injury history; no athletes.
limited evidenceMeasured on non-athletes and recreational people

Heinemeier and colleagues asked how fast human tendon tissue is replaced, because published estimates of tendon protein half-life ranged from about 2 months (stable-isotope incorporation, about 1% per day) to about 200 years (racemization in horses).

They used the bomb-pulse method: nuclear tests from 1955 to 1963 nearly doubled atmospheric 14C, which then fell exponentially, and a tissue that is not renewed keeps the 14C level of the time it was formed.

In March 2000 they took 28 samples from the core of the Achilles tendon, 2-5 cm above the heel bone, during routine forensic autopsies, plus 4 psoas muscle samples from four of the donors (birth years 1945-1983), and measured 14C by accelerator mass spectrometry.

In donors born during and after the bomb peak, tendon 14C clearly exceeded the level in the air at sampling, and most samples matched the atmosphere several decades earlier (the 1945 donor's tendon, 99.95 pMC, sat below the 2000 air level of about 109).

In the 19 donors born during or after the bomb peak (1962 onwards), tendon 14C matched the air about 8 years (median) after birth, and the data fitted "relatively well" a model in which tendon is built during the first 17 years of life (the average end of height growth) and is practically inert afterwards.

Muscle 14C matched the atmosphere about 2 years before death (1998 for three samples, 1996 for one), showing continuous renewal.

The authors conclude that renewal of adult core tendon tissue is extremely limited, which likely explains the tendon's poor regenerative capacity, and suggest that the high synthesis rates measured with stable isotopes reflect protein that is made and broken down without entering the collagen fibrils, or the more adaptable outer region of the tendon.

Our reading: a direct, elegant measurement that the bulk of the adult Achilles core is old tissue; it does not show that tendon cannot adapt to training — the authors themselves place adaptation at the periphery — and the three outliers and the wide spread between people of the same birth year leave room for periods of faster turnover in some individuals.

Protocol

Design: cross-sectional post-mortem study; tissue renewal estimated by comparing each sample's 14C with the atmospheric bomb-pulse curve for the years after the donor's birth.
Samples: n = 28 Achilles tendon core samples (diameter about 4 mm, 2-5 cm from the most proximal part of the calcaneus) and 4 psoas major muscle samples, from routine forensic autopsies at the Arkansas State Crime Laboratory, March 2000; donor birth years 1945-1983 (Table 1 lists sex, height and weight; 14 men and 13 women plus one entry marked "W"); no activity or injury history.
Measurement: about 0.5 mm of the outer layer discarded against contamination; freeze-dried; accelerator mass spectrometry (graphitisation and AMS at Accium Biosciences, Seattle, because the Aarhus accelerator was damaged); 14C given as percent modern carbon (pMC) corrected to δ13C −25‰; stable isotopes and carbon/nitrogen ratios measured at Aarhus.
Model: tendon 14C compared with a 0-17-year moving average of the atmospheric curve (tendon built during height growth, inert after).
Funding and interests: Danish Medical Research Council (11-107595), Danish Rheumatism Association (A1698), Weimann Foundation, Lundbeck Foundation, Novo Nordisk Foundation, Nordea Foundation (Healthy Ageing grant) and the Danish Research Council for Natural Sciences; "The authors declare no conflicts of interest". The radiocarbon measurements were bought as a service from a commercial laboratory (Accium Biosciences), which is acknowledged, not listed as a funder.

Key Results

MeasureResult
Tendon 14C vs the air at sampling (2000, about 109 pMC)clearly higher in donors born during and after the bomb peak (not in the 1945 donor, 99.95 pMC); most samples matched the atmosphere several decades before sampling
The 19 donors born 1962 or latertendon 14C matched the air 8 yr (median) after birth
Fit to a "built in the first 17 years, then inert" model"relatively well" (Fig. 2); three samples deviated, possibly from periods of higher turnover later in life
Muscle 14C (4 donors)matched the air ~2 yr before death (1998 for three, 1996 for one) — continuous renewal
Unexplained outliers3 of 28 (samples 25, 28, 30: 14C impossible for the stated birth year; 25 and 28 remeasured, same result)
Collagen content checkC/N ratio 3.36 ± 0.14 vs a theoretical 3.10 (type I) and 3.09 (type III) collagen — mostly collagen
Sensitivity to a fast-turnover subpool10% of carbon from a high-turnover pool would lower an inert sample of 140 pMC to only about 137

Mechanisms

Measured: tendon core 14C reflects the years of growth; muscle 14C reflects the last couple of years.
Authors' interpretation: the tendon core is formed during height growth and essentially not renewed thereafter; stable-isotope studies may have overstated replacement because they count all newly made protein, including protein that is broken down without entering the fibrils, and may sample the more adaptable outer tendon; the data "could indicate" that much of the adaptation to loading, and the hypertrophy seen after long-term training, happens in the outer region.
Authors' interpretation (speculative, their word): some individuals' deviations may reflect periods of high turnover, for example after overuse injury, of which there was no donor history; a minor subpool of tendon protein, in cells and perhaps between fibrils, very likely does turn over normally.
Inference: a tissue that renews this little cannot quickly replace damaged core collagen, which fits the slow healing of tendinopathy and the advice to progress impact load by the slowest tissue (SYNTH-21); it does not by itself set a rate of progression.

Practical Application

Does my Achilles tendon rebuild itself like muscle? Not its core: in 28 autopsy samples the core carried carbon from the growth years, while muscle matched the last ~2 years. Adaptation may happen at the edge (authors' suggestion).
Expect tendon to be the slow tissue: the core collagen of an adult Achilles is essentially the tissue built during growth (B, cross-sectional, 28 samples); the authors read this as the likely reason tendon heals poorly. Not from this paper; the base's own practice: progress new running or jumping load by the tendon, not by how easy the session felt (SYNTH-21, SYNTH-75).
It is not a reason to spare the tendon: the paper measures renewal of old tissue, not the response to training; loading does stiffen and thicken tendon, perhaps by adding tissue at the periphery (the authors' suggestion; REF-1355, REF-517).
What it is good for as a source: the direct evidence that adult tendon core barely renews. What it does not show: anything about trained athletes, other tendons, injury risk or how fast to progress.

Limitations

Unknown donors. Forensic cases without activity or injury history; no athletes; the authors note that undocumented overuse injury may have raised turnover in some.
One tendon, one region. Achilles core only, 2-5 cm above the heel bone; the periphery, where adaptation is thought to happen, and other tendons were not sampled.
Whole tissue, not collagen alone. Collagen was not separated (to avoid contamination); the authors argue from the C/N ratio and a dilution calculation that this changes little.
Outliers. 3 of 28 samples had 14C levels impossible for the recorded birth year (two remeasured, same result); contamination by diagnostic 14C or a wrong birth year are the authors' possible explanations.
The 17-year figure is a model. It is the average end of height growth, chosen before fitting; the data fit it "relatively well", with wide differences between donors of the same birth year.
Small and single-laboratory. 28 tendon and 4 muscle samples; the measurement is robust, but the population and the sampling are narrow.
Funding and interests: Danish public research councils and private foundations (Lundbeck, Novo Nordisk, Nordea, Weimann) and the Danish Rheumatism Association; no conflicts declared. No device maker, federation or data platform is named.
How this entry was checked and changed
Why this level

a cross-sectional measurement study of 28 tendon samples, graded by design and n; it measures the question itself (tissue renewal over a lifetime) rather than a marker of synthesis, which is why it settled a conflict between methods, but it has no loading, injury or performance outcome; donors' activity and injury history are unknown, three of the 28 samples are unexplained outliers, only the core of one tendon was sampled, collagen was not separated from the rest of the tissue, and the "first 17 years" is a model the data fit "relatively well", not a measured age. Read from the full text (open access, CC BY-NC 3.0 US)

Full text (checked 2026-10-28)
Levels of 14C were measured in 28 forensic samples of Achilles tendon core and 4 skeletal muscle samples (donor birth years 1945–1983) (full text)
Existing data provide diverging estimates of tendon protein half-life that range from 2 mo to 200 yr (full text)
19 persons born during this period (1962–1985): the 14C content in their tendons corresponded to the atmospheric level present 8 yr (median) after birth (full text)
Considering that height growth continues to an average of 17 yr in humans, tendon tissue is formed during the first ca. 17 yr of life and is thereafter practically inert; the data fit this curve relatively well (full text)
Skeletal muscle levels of 14C correspond to atmospheric levels ∼2 yr before the time of sampling (1998: samples 24, 26, and 28; 1996: sample 25) (full text)
A C/N ratio of 3.36 ± 0.14 was measured, compared to a theoretical C/N ratio of 3.10 for type I and 3.09 for type III collagen (full text)
10% carbon from a high-turnover pool would only reduce a completely inert collagen 14C content of 140 pMC to a reading of ∼137 (full text)
The dataset contained three clear outliers, indicated as #25, #28, and #30 (full text)
Financial support from the Danish Medical Research Council (11-107595), Danish Rheumatism Association (A1698), Weimann Foundation, Lundbeck Foundation, Novo Nordisk Foundation, Nordea Foundation; the authors declare no conflicts of interest (full text)