Tendons Don't Know the Difference Between Concentric and Eccentric — So Stop the Nonsense
For decades, rehab “professionals” have acted like tendons possess some magical ability to detect whether a muscle is shortening or lengthening.
"You have Achilles tendinopathy? Do eccentric heel drops."
"Patellar tendinopathy? Eccentric decline squats."
"Tennis elbow? Eccentric wrist extensions."
Somewhere along the way, an entire profession became convinced that the secret to treating tendons was finding ways to make muscles lengthen under load.
The problem?
Tendons don't know what an eccentric contraction is.
And they can't.
The Part Nobody Talks About
Let's start with basic anatomy.
Tendons are not contractile tissues.
Muscles contract.
Tendons do not.
Muscles generate force.
Tendons do not.
Muscles contain actin, myosin, motor units, and all the machinery required to produce movement.
Tendons are essentially passive connective tissue structures that transmit force between muscle and bone.
A tendon cannot decide to shorten.
A tendon cannot decide to lengthen.
A tendon cannot generate force.
It is simply acted upon by forces from the muscular and skeletal systems.
So when someone says:
"Tendons respond differently because this is an eccentric contraction."
You should immediately ask:
How exactly does the tendon know that?
The tendon isn't performing the contraction.
The tendon isn't controlling the movement.
The tendon isn't even aware of the movement.
It is simply experiencing mechanical forces.
What Tendons Actually Experience
A tendon experiences:
Tension
Compression
Shear
Strain
Rate of loading
Duration of loading
Frequency of loading
What it does not experience is:
Concentric contractions
Eccentric contractions
Isometric contractions
Those are descriptions of what the muscle is doing.
Not the tendon.
Imagine removing an Achilles tendon and placing it into a materials testing machine.
The machine repeatedly applies 2,000 Newtons of force.
Can the tendon determine whether that force originated from:
A calf raise?
A heel drop?
A jump landing?
A sprint?
A robotic testing device?
Of course not.
The tendon experiences load and deformation.
Nothing more.
Its cells respond to the mechanical environment—not exercise terminology.
How Rehab Got Lost
Back in the late 1990s and early 2000s, several influential tendinopathy studies used eccentric loading protocols.
The most famous work came from Håkan Alfredson and colleagues, who demonstrated excellent outcomes using eccentric heel-drop programs for Achilles tendinopathy.
The profession then committed one of its favorite mistakes:
It confused what worked with why it worked.
The conclusion should have been:
"Loading appears beneficial for tendinopathy."
Instead, rehab concluded:
"Eccentric loading is uniquely beneficial."
Those are not the same thing.
Not even close.
What Eccentrics Actually Did
Why did those programs work?
Because they loaded the tendon.
That's it.
Many patients with tendinopathy weren't exposing the tendon to meaningful loading before rehab.
Then suddenly they were performing hundreds of loaded repetitions every week.
The tendon adapted.
Not because it recognized eccentric contractions.
Because it experienced a substantial mechanical stimulus.
The programs also provided:
High time under tension
Progressive exposure to force
Increased tissue tolerance
Reduced fear of loading
Consistent exercise adherence
None of those are exclusive to eccentrics.
Then The Research Started Catching Up
As more studies accumulated, the superiority of eccentrics became harder to defend.
Researchers started finding positive outcomes with:
Heavy slow resistance
Combined concentric-eccentric training
Isometrics
Progressive resistance programs
Plyometric progressions
Energy storage exercises
An uncomfortable pattern emerged.
The common factor wasn't eccentric contractions.
The common factor was:
Appropriately dosed loading.
Turns out tendons respond well when you progressively challenge them.
Who would've guessed?
The Damage Done
Unfortunately, the eccentric obsession created an entire generation of therapists who treated contraction types instead of tissue capacity.
Patients became trapped doing endless:
Heel drops
Decline squats
Wrist eccentrics
Slow lowering exercises
Months later they still have symptoms because nobody progressed the things that actually matter:
Load magnitude
Force tolerance
Velocity
Power
Elastic energy storage
Sport-specific demands
A basketball player's Achilles tendon doesn't need to become world-class at slow heel drops.
It needs to tolerate repeated jumping, landing, sprinting, and cutting.
A volleyball player's patellar tendon doesn't care about decline squats.
It cares whether it can absorb and release force efficiently during thousands of jumps.
Real life isn't eccentric.
Real life is variable.
The Rehab Industry Loves Labels
Physical therapy has a tendency to become obsessed with categories.
Everything gets turned into a special exercise type:
Core exercises
Functional exercises
Stabilization exercises
Corrective exercises
Eccentric exercises
The body doesn't operate according to our billing categories.
Tissues respond to physiological and mechanical stimuli.
Not rehabilitation buzzwords.
The tendon doesn't receive a notification saying:
"Attention collagen fibers: this is now an eccentric repetition."
It receives force.
That's it.
What Tendons Actually Need
Most successful modern tendinopathy programs eventually include some combination of:
Isometrics
Heavy resistance training
Concentric-eccentric loading
Energy storage exercises
Plyometrics
Return-to-sport loading
Notice something?
Not one of those requires eccentrics to be magical.
Eccentrics are simply one loading strategy among many.
Useful?
Absolutely.
Special?
Probably not.
The Takeaway
The tendon doesn't know whether the muscle is shortening, lengthening, or holding.
It doesn't possess contractile tissue.
It doesn't generate force.
It doesn't understand exercise classifications.
It is a passive connective tissue structure being acted upon by forces generated from muscles and transmitted through the skeleton.
The tendon knows:
Force
Strain
Loading rate
Workload
That's it.
Eccentric exercises aren't bad.
The problem is that rehab elevated them from "one useful loading method" to "the magical cure for tendinopathy."
The real lesson should have been far simpler:
Load the tendon.
Progress the load.
Build capacity.
Prepare the tissue for the demands of life and sport.
Because tendons don't care what you call the exercise.
And they certainly don't know the difference between concentric and eccentric contractions. They just know whether you've given them an appropriate reason to adapt.
Clinical Reality:
Eccentrics work, but they are not clearly superior to other progressive loading strategies.
The mechanism is likely loading and capacity development, not some magical property of eccentric contractions.
Key Research Papers:
1. Malliaras et al. 2013
Achilles and Patellar Tendinopathy Loading Programmes: A Systematic Review Comparing Clinical Outcomes and Identifying Potential Mechanisms for Effectiveness
Peter Malliaras et al., 2013
Key finding:
Limited and conflicting evidence that eccentric loading is superior to other loading programs.
They specifically questioned the entrenched belief that eccentric loading should be the default treatment and found equivalent or stronger evidence for other loading strategies in some situations.
2. Beyer et al. 2015
Heavy Slow Resistance Versus Eccentric Training as Treatment for Achilles Tendinopathy: A Randomized Controlled Trial
Key finding:
Heavy slow resistance and eccentric training produced equally good long-term outcomes.
This is the paper that really starts poking holes in the "eccentrics are special" narrative.
The HSR group often had greater patient satisfaction early on despite not using a pure eccentric protocol.
3. Lim & Wong 2018
Effects of Isometric, Eccentric, or Heavy Slow Resistance Exercises on Pain and Function in Individuals With Patellar Tendinopathy: A Systematic Review
Key finding:
Various loading approaches improve pain and function. The review does not support a clear superiority of eccentrics across the board.
4. Li et al. 2024
Mixed Comparison of Intervention With Eccentric, Isometric, and Heavy Slow Resistance Training for Patellar Tendinopathy
Key finding:
Eccentric, HSR, and isometric loading all improve symptoms, while programs including isometrics or HSR may outperform eccentric loading alone for some outcomes.
5. Jayaseelan et al. 2019
Eccentric Exercise for Achilles Tendinopathy
This is actually useful because it comes from the opposite side.
The authors acknowledge that eccentrics can be effective but explicitly state that other loading approaches may be equally useful.
That supports a balanced argument rather than sounding like you're just attacking eccentrics.
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