Mind & Body · Sunday, 23 August 2026

01 · Briefing · what happened

Your tendons hold blood vessels out on purpose, and that is why they barely heal

Mind & Body 3 min 18 sources

A tendon is a great spring because almost nothing lives inside it. The same emptiness is why a torn one takes the better part of a year, and why the blood finally arriving is a sign of damage, not of repair.

43.8 mmHg

pressure inside a resting Achilles

past 100 under a modest pull, at which point blood flow stalls [2]

3-5x

body weight through the tendon when you run

engineering reviews put the peak nearer 12.5 times [1][4]

8-12 months

typical time back after a rupture

with 8 to 9 of the early weeks spent in a boot [8][9]

20%

elite footballers who never return to their old level

the figure is 25% in rugby and 35% in American football [1]

At a glance

A tendon is collagen packed tight and aligned one way, which makes it a spring that returns the energy you put in [3][13].
The packing leaves almost no room for cells or blood, and early medicine called tendons virtually dead during life [2].
The near-bloodless state is actively maintained: healthy tendon largely does not send the signal that calls vessels in [2].
So a tear has no route in for help, and heals as scar rather than as tendon [5][18].
Most Achilles ruptures happen 2 to 6 cm above the heel, the stretch with the poorest supply [6][7].
Most athletes are back eight to twelve months after a rupture, and up to one in five elite footballers never regain their old level [8][1].
Steroid injections help for about three to six weeks and repair nothing, and no single treatment for a rupture is agreed [15][17].
Vessels and nerves do grow in, but mainly as the tendon degenerates, and that is where the pain is thought to come from [2].

Forces in play

Packing for strength High→

dense aligned collagen is what makes the tendon a spring, and it is what leaves no room for anything else [3][13]

Blood held out High→

healthy tendon largely skips the signal that starved tissue uses to call vessels in, and load squeezes the rest shut [2]

Ability to repair Easing↓

few cells and poor supply mean slow healing, and the capacity drops further with age [3][5]

Vessels and nerves growing in Building↑

they arrive as the tissue degenerates, and are thought to carry the pain with them [2]

In play The Achilles tendon — the largest and strongest in the body, and the one most often torn Tenocytes — the sparse cells living inside the rope, the only repair workforce it has The stretch 2 to 6 cm above the heel — the poorest-supplied section, where most ruptures happen Nelly Andarawis-Puri — Cornell bioengineer working out when movement repairs a tendon and when it harms one

How it unfolded

The moment a push-off with nobody near you, and the tendon goes [1]
Weeks 1-9 eight to nine weeks in a boot while the ends knit [9]
After the boot the first eight weeks out are when the tendon is at its weakest [9]
8-12 months most athletes are back, though many never at their old level [8][1]

Where this points

Watch whether the loading research converges: teams are trying to pin down when movement repairs a tendon rather than damaging it, and the answer still differs person to person [3].

Full briefing

The bargain inside the rope

A tendon is collagen packed tight and lined up one way, a rope built to stretch and give the energy back [3]. That is why it works as a spring. It is also why animals, and the legged robots built to copy them, can be both efficient and steady on rough ground [13]. The packing leaves room for very little else.

Early medical writing called tendons “virtually dead during life”, because they hold few cells and little blood [2]. The strange part is that the emptiness is actively held. Starved tissue normally releases a signal that calls blood vessels in. Healthy tendon largely does not, and keeping a tissue near-bloodless means either muting that call or making the opposite one [2]. Load finishes the job. Pressure inside a resting Achilles averages about 43.8 mmHg, the unit blood pressure is measured in. Under a modest pull it climbs past 100, and the pressure pushing blood through the tendon falls below zero [2].

So an injury has almost no route in for help. Poor supply and few cells are why a torn tendon heals slowly, and heals as scar rather than tendon [5][18].

Where the sources disagree

How hard you pull on your own Achilles depends on who is counting. A clinician quoted by The Athletic puts running at three to five times body weight [1]; engineering reviews cite peaks near 12.5 times [4]. Different activities, and mostly modelled rather than measured. Rupture counts scatter the same way, from about 2 to 40 per 100,000 people a year [6][14]. That range says as much about which country and decade you count in as it does about tendons.

The emptiness itself is being revised. Fine X-ray scanning, called micro-CT, recently found more vessels running between the fibre bundles than the old dye studies had shown [11]. “Nearly bloodless” is a better description than “bloodless”.

What the numbers flatten

Hugo Ekitike pushed off, went down, and rolled over expecting to find the defender who had kicked him. Nobody was there [1]. The shape of it: no contact, no warning. Damage builds without hurting first. An overuse injury “can be silent for a long time”, says Cornell bioengineer Nelly Andarawis-Puri [3].

And pain does not track damage. Badly degenerated tendons are often painless, while many painful ones look normal under a microscope [2]. Current thinking is that the pain comes from nerves growing in alongside new vessels, which happens as the tissue breaks down [2].

The gap between feeling better and being repaired is where treatments get oversold. Steroid injections work, but reviews put the useful window at roughly three to six weeks, with no lasting gain after that [15]. Repeated doses risk weakening the tendon [15]. There is still no agreed best treatment for a full rupture [17]. Cartilage made the bargain harder still: no blood vessels at all, almost no self-repair [16]. What any of it means for one person is a question for a clinician, not a briefing.

02 · Lesson · why it matters

What you take out to make a thing good is what would have fixed it

Strip a part down until it does one job perfectly, and you have usually stripped out whatever would have repaired it.

How it works

Pack collagen tight and aligned, and you get a rope that stores and returns energy
That packing leaves almost no room for cells, blood or nerves
The tissue holds vessels out, and loading squeezes the little supply shut
So an injury has no route in, and heals as scar rather than as tendon
Vessels and nerves arrive only as it breaks down, bringing the pain with them

The twist

The emptiness that makes a tendon an excellent spring is the same emptiness that leaves it unable to repair itself, and it is held that way on purpose.

Where you've seen this

Sealed-for-life bearings

closed against dirt, which also closes out any chance of ever re-greasing them

Electronics set in resin

the potting that makes a board survive heat and damp makes a single failed part unrepairable

A job nobody has had to think about

it runs untouched for years precisely because nothing was left around it that could fix it

The catch

Poor is not none. Loading does change a tendon over months, and a tendon still developing before birth repairs almost without scarring [10][12].

Full lesson

The injury with nobody in it

A striker pushes off to change direction. He goes down, rolls over, and looks for the defender who kicked him. There is no defender. There never was.

That is an Achilles rupture from the outside: a man alone on the grass, hurt by nothing. From the inside it is simpler. A rope that had been quietly running out of the ability to repair itself ran all the way out.

What a tendon actually is

A tendon is not really an organ. It is a rope. Collagen fibres, packed as tightly as the body can pack them, all lined up in one direction, and very little else in between.

That packing is the whole trick. Pull a tightly packed, aligned rope and it stretches a little and gives the energy back, like a new elastic band rather than an old stretched one. It is why walking is cheaper than it should be, and why engineers building legged robots copy the design.

But every fibre you pack in is space you do not have for anything else. A tendon has few living cells and almost no blood. Early medical writing had a phrase for tissue like this: virtually dead during life.

The emptiness is enforced, not neglected

If you starve most tissue of oxygen, it complains. It releases a chemical signal that calls blood vessels toward it, and the body grows them. That is the normal response, and it is why most of you is threaded with supply.

A healthy tendon mostly does not do this. Keeping it near-bloodless takes work: either muting the call for vessels or making the signal that pushes them away. Loading finishes the job mechanically. Pressure inside a resting Achilles is already high for a body tissue, and a fairly gentle pull raises it past the pressure of the blood trying to get in. The supply simply stops.

So the emptiness is not an oversight. It is maintained. The tissue holds out the very thing that would repair it, because the crowding that keeps it out is the same crowding that makes it a spring.

Which is why a torn one takes a year

Follow the consequence. When a tendon tears, there is no road for help to arrive on. The few cells inside work slowly. What eventually bridges the gap is scar, which is not tendon and does not behave like it.

The calendar shows it. Weeks in a boot. Then, months later, a stretch where the tendon is at its weakest, long after the pain and the drama have gone. Most people are back between eight and twelve months. A fifth of elite footballers never reach their old level again.

Bone made the opposite bargain. It is threaded with supply, rebuilt constantly, and it knits a clean break in weeks. Neither tissue is better. They bought different things.

The arrival of help is the sign of failure

Blood vessels do eventually grow into a tendon. So do nerves. But they arrive as the tissue degenerates and loosens, and current thinking is that they are where the pain comes from. A tendon that has never hurt has almost nothing in it to hurt with.

So the supply showing up is not the repair beginning. It is a reading on how far the structure has already gone. And a tendon that stays silent is not reporting that it is fine. It is reporting that it is still packed too tightly for anything to get in and complain.

Where else this bargain sits

Once you have the shape, it is not hard to find. A bearing sealed for life keeps grit out and keeps grease in, which also means nobody will ever open it. A circuit board set in solid resin survives heat and damp that would kill a bare one, and cannot be repaired when a single component dies. A job so dependable nobody has thought about it in ten years has, by that same dependability, ended up with nobody around it who knows how it works.

None of those is a mistake. Each bought something real. What is easy to forget is what was handed over at the counter.

We are standing on several of these bargains at once, most of them made long before us and none of them put to a vote. You are running on tendons you did not specify, that will not tell you how they are doing, and that were built that way for reasons that are good. The honest position is not that something should be fixed. It is that the parts you rely on most are often the ones with the least capacity to say anything at all.

03 · Lab · your turn

Build A Tendon

Spend a fixed cross-section on spring or on repair, then run a season and feel why the body chose the spring.

04 · Hope · carry this

A hundred years ago tendons were written off as dead cable. People kept looking anyway, which is how the quietest parts of us end up understood.