What Your Tendon Actually Goes Through

Strength and ConditioningRehabPerformance Coaching
Written By Cat Hoh

Coaches drill eccentrics.. Clinicians reach for isometrics… But, how do both muscle contractions play apart in performance and rehab?

A physio meets a runner with a hot Achilles and reaches for eccentric heel drops, because that is what the last twenty years taught everyone to reach for. A strength coach meets the same runner two days out from a race and reaches for isometric holds, because somewhere online said isometrics switch tendon pain off. Both are choosing a type of muscle contraction as though the tendon can read the label on it. It mostly cannot. A tendon responds to how hard you load it, for how long, and how often. Contraction type, the thing clinicians argue about most, is close to the thing the tendon notices least.

That is not an argument for loading carelessly. It is an argument for loading the right variable. Most of the folklore around tendons attaches enormous meaning to the direction a muscle is moving while the tendon underneath quietly tracks something else entirely. Once you see which variable the tissue is actually listening to, a lot of clinical decisions get simpler, and a few popular ones stop making sense.

The day after counts more than the session

Start with the hours right after a hard session, because that window is where a surprising amount of clinical trouble actually lives. A tendon does not finish a heavy bout in the same mechanical state it started in.

Figure 01 / Interactive

What one hard bout does to the tendon, straight afterwards

Loading mode / tap to change

Max isometric holds Eccentric heel drops Running & hopping
Normal stiffness Session +6h +12h +18h +24h STIFFNESS TIME AFTER SESSION

Isometric holds: a moderate dip. Repeated maximal contractions of the calf leave the Achilles transiently less stiff, then it climbs back.

Back to baseline within hours in a healthy tendon. A window, not a verdict.

Eccentric heel drops: the deepest dip. Tendon thickness drops and free-tendon strain rises, a larger acute shift than the same work done concentrically.

Also recovers within a day. The point buried here: eccentric is not gentle on the tissue, it is a big load.

Running and hopping: barely a dip. A stretch-shortening cycle loads the tendon in the pattern it is built to shrug off.

Little to recover from. Not every hard session leaves a window behind it.

Schematic. Curve shapes, not measured values. Directions of acute change collated by Wang & Zhao (2022) from in vivo ultrasound studies: repeated maximal isometric contractions transiently reduce Achilles stiffness; eccentric heel drops reduce tendon thickness and raise free-tendon strain, more than concentric loading does; running and hopping produce no significant change. All resolve within roughly a day in a healthy tendon.

None of those acute changes is damage. In a healthy tendon they settle within a day. But a briefly softened tendon is a briefly more vulnerable one, and the tendon injuries clinicians actually see are rarely the first hard session. They are the second one dropped into the recovery window of the first, once the muscle has stopped feeling sore and the tissue underneath it has not yet caught up. Muscle soreness is a loud, misleading signal here. It fades on its own schedule, which is not the tendon's.

So the first real decision in loading a tendon is not which contraction to prescribe. It is whether last session's tissue has come back yet. Get the spacing wrong and no clever choice of exercise saves you. Get it right and the tendon tolerates a lot.

The tendon is close to blind to direction

Now the finding that quietly unsettles the eccentric orthodoxy. When Quinlan and colleagues trained young and older men for eight weeks on concentric-only or eccentric-only contractions, matched for relative intensity, the patellar tendon stiffened by the same amount either way. The same Young's modulus, the same stiffness, even though the eccentric group was shifting far heavier absolute loads. The tendon did not appear to care which direction the muscle was travelling.

Their data suggests the tendon may be blind to the direction of strain it is exposed to. Quinlan et al., GeroScience, 2021

A larger body of work reaches the same verdict from the other side. The meta-analysis that Wang and Zhao lean on found that loading magnitude is the element of a training programme that drives tendon adaptation, and that muscle contraction type is not. The tendon stiffens because of how much force you put through it, held for long enough, often enough, for long enough in weeks. High intensity, past twelve weeks. Everything else is detail.

Figure 02 / Interactive

Turn up the load, the tendon responds. The direction debate, it ignores.

Training load / tap a heavier load

40% 1RM 60% 1RM 80% 1RM
none large TENDON ADAPTATION 40% 60% 80% TRAINING LOAD (% OF 1-REP MAX)
40%of 1-rep max

Below roughly this load the tendon doesn't change, even when total work is matched. Too light to register.

60%of 1-rep max

Enough to drive a clear rise in stiffness and Young's modulus. Young tendons plateau by week 4, older ones keep climbing to week 8.

80%of 1-rep max

Heavy loading gives the largest gains, Young's modulus up 70 to 84%, and the same figure whether the reps are concentric or eccentric.

Schematic dose-response. Shape and direction, not exact values at every load. Around 40% 1RM is too light to shift tendon properties (Grosset et al., via Quinlan et al. 2021); 60% drives clear adaptation (Quinlan et al. 2021); 80% raises Young's modulus roughly 70 to 84%, concentric or eccentric alike (Malliaras et al., via Quinlan). The gain is in the tendon's material, not its size: cross-sectional area did not move.

That reframes why eccentric protocols work when they do work. Alfredson's heel drops did not rescue a generation of Achilles tendons because lengthening under load is special. They worked because a slow, controlled eccentric is a convenient way to park a large, sustained force on a tendon. Substitute a heavy slow concentric that delivers the same load and, on current evidence, you get the same tendon back. The direction was never the active ingredient. The load was, and eccentric exercise was mostly a delivery mechanism for it.

Which is a problem for the isometrics-fix-pain story

This is where a fashionable clinical shortcut runs out of road. Isometric holds became popular as a way to pull tendon pain down before or during a season, largely off the back of one small patellar-tendon study. When Clifford and colleagues pooled the randomised trials, isometric exercise came out no better than isotonic exercise for chronic tendinopathy, on every outcome they measured, and no better than ice for an acutely irritable rotator cuff. The response swung wildly from person to person and from tendon to tendon.

That does not make isometrics useless. It makes them one tool inside a progressive loading plan rather than a switch that turns pain off. The honest answer to "will isometrics settle this tendon" is that they might, for this athlete, and you will only find out by trying and watching. Worth saying plainly: that review rated its own evidence weak, ten trials with seven of poor quality, so the ceiling on how much anyone should conclude from it is low. The reasonable read is not that isometrics fail. It is that they hold no special power over pain, and any clinician selling them as a reliable analgesic is ahead of the data.

Where the mode does earn its keep

So does contraction type ever matter? Yes, though not where people spend their arguments. It decides where in the muscle-tendon unit the adaptation lands, even when it leaves the tendon's stiffness untouched.

Figure 03 / Interactive

Same person, two legs: where the stiffness goes

Training mode / tap to change

Isometric holds Plyometric jumps

Tendon stiffness rises

Muscle stiffness flat

Isometric holds stiffen the tendon and leave the muscle much as it was. A better spring for transmitting force.

Tendon stiffness flat, more extensible

Muscle stiffness rises

Plyometric jumps leave tendon stiffness alone, make the tendon more extensible under fast load, and stiffen the muscle underneath. A better spring for storing and returning energy.

Schematic. After 12 weeks of unilateral plantar-flexor training in Kubo et al. (2017), one mode per leg: isometric training raised tendon stiffness during ramp and ballistic contractions with no change in muscle stiffness; plyometric training left tendon stiffness unchanged but raised active muscle stiffness and increased tendon elongation at higher forces. Bar lengths are illustrative, not measured values. Eleven untrained young men.

For a clinician rebuilding a tendon, that is a design choice rather than a footnote. Shortening and lengthening work also remodel the muscle in opposite directions while leaving the tendon the same: eccentric training lengthens muscle fascicles, concentric training builds them at a steeper angle. So you choose the contraction not to stiffen the tendon, since load handles that regardless, but to shape the muscle around it, to bias a joint angle, or to keep an athlete inside a pain and load ceiling they can actually tolerate that week. Which is close to the case Brumitt and Cuddeford make for programming by the tissue and the stage of healing rather than blitzing a whole limb with one favourite exercise.

Old tendons, and the honest edges of all this

Age changes the clock, not the rules. Older tendons still adapt, and can be brought most of the way back toward younger values, but they take the full eight weeks where a young tendon is nearly there by four, and light loads around forty percent of a maximum do not shift them at all. The dial is still magnitude and patience. Contraction type stays quiet.

Hold the whole picture loosely at its edges, because the edges are thin. The cleanest study here used only men, only healthy tendons, only the patellar tendon, at a submaximal load. The isometric verdict rests on ten trials, seven of them poor. The two-legged spring experiment is eleven untrained young men and their calves, not athletes at the sharp end of anything. And much of the load-beats-type consensus comes from a small, interconnected set of laboratories, which is how a field looks right up until a larger sample disagrees with it. Women are close to absent from all of it, even though their tendons synthesise collagen differently after exercise, which is not a small gap to be sitting on.

So the working rule for a coach or clinician is duller than the folklore and easier to defend. Pick the load and the time under tension first, because that is what the tendon is listening to. Give it twelve weeks and more, not three. Treat the day after a hard bout as seriously as the bout. Reach for contraction type to shape the muscle, steer a joint angle, or stay under someone's pain ceiling, not as your lever on the tendon itself. And read every number here as an opening bet on the person in front of you rather than a rule they owe you compliance with, because the evidence, honestly, is not yet strong enough to be a rule.

References

Cat Hoh
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