When you think of a muscle contraction, you usually think of a muscle getting shorter, which is called “concentric” contraction — but that’s not always what happens. We can also do “isometric” contractions, in which there’s no change in length, AKA “clenching.” And then there’s the weird one, a mysterious but routine bit of muscle trickery known as “eccentric” contraction, and it is odd indeed: contraction while lengthening, also sometimes called a braking contraction.
How is this possible? How can that even be called a “contraction”?1 Good question! This is one of the classic examples of a small but persistent mystery of biology. In this age of science fiction body scans and custom-built medicinal molecules, no one really knows quite how eccentric contraction works. The theory of muscle contraction — the sarcomere model, more on this below — is impressive but inadequate.
|concentric contraction||=||contraction while shortening|
|isometric contraction||=||contraction without changing length (“clenching”)|
|eccentric contraction||=||contraction while lengthening (“braking”)|
What is an eccentric contraction used for?
Even if no one knows how it works, it’s easy to understand why you need eccentric contraction: we regularly need to control, slow-down the lengthening of a muscle, a “braking” contraction.
The simplest example of an eccentric contraction is lowering a barbell in a biceps curl. Obviously the biceps muscle contracts to lift the barbell up. But it’s also contracting as you lower the weight — if it weren’t, you would drop it pretty fast! The contraction is not quite strong enough to stop the lengthening of the muscle. The contraction is just strong enough to put the brakes on the lengthening of the muscle.
Here are three sneakier, less obvious examples:
- The tibialis anterior muscle (see Massage Therapy for Shin Splints) in the shin flexes the foot up when it contracts. But when walking or running, it contracts eccentrically to control the descent of the toes after heel strike. Without it, your feet would slap something awful with every step.
- The quadriceps muscle group contracts eccentrically as you descend stairs or a hill. The quadriceps are “anti-gravity” muscles when contracting concentrically, extending the knee powerfully to lift you up. But when you step down, your knee starts straight and then bends like a spring as your body follows: the quadriceps contract eccentrically to keep the knee from collapsing too fast or too far.
- The extensor muscle group on the back of your forearm (see Massage Therapy for Tennis Elbow and Wrist Pain) gets heavy eccentric use in raquet sports, where you are constantly swinging a heavy “weight” — the end of the racquet. That weight would drag your wrist into deep, floppy flexion with every swing … if not for eccentric contraction of the muscles on the back of the arm, which resist the flop and keep your wrist stable and reasonably straight. It still bends back, but it’s controlled and limited.
Notice that all three of these examples correspond to body parts that tend to get sore after exercising. Your shins hurt after your first hard-surface run in a while, your quadriceps hurt after climbing down a mountain, and the back of your forearm hurts after your first couple tennis games of the year.
A giant protein probably explains eccentric contraction
A muscle is made of microscopic contractile units arranged in series and bundles: the sarcomeres, tiny packages of proteins (actin and myosin, very famous molecules). Muscles contract because sarcomeres contract. Sarcomeres are little microscopic muscles-within-muscles. Micro muscles. These molecular machines are the best example of how life is chemistry. Although proteins have many impressive properties and do many dazzling things, none is more defining of living things than this ability to generate movement.2
It’s possible that eccentric contractions are a kind of hack, a software solution, a clever way of using the same protein hardware that all contractions use. Or a car metaphor: same motor, different way of driving.
But never bet against molecular machines. It’s always been clear that sarcomeres couldn’t explain everything about muscle behaviour with the actin and myosin proteins alone, and that muscle might well make use of other proteins with other properties. And indeed there is now much more evidence that another big organic molecule, titin, can explain the most puzzling properties of eccentric contraction.3
Basically, muscle behaves as if it’s elastic, and that is hard to explain if you only know about actin and myosin. But titin stiffens in proportion to the strength of muscle contraction, which fits the bill: titin is “loose” and allows lengthening when the muscle is relaxed, but as you power-up the muscle, it stiffens and resists elongation more and more as you clench. Clever.
Eccentric exercise and soreness!
Other than intellectual interest, this is the main reason you should care about eccentric contractions: because they hurt more! Which is both good and bad news. The pain is the cost of an interesting benefit: eccentric contractions are a more efficient way to exercise muscles than concentric. That is, they work the muscle harder with less energy than concentric contractions. But that also means it’s easy to overdo it.
Anyone who has ever exercised knows about that nasty soreness that comes on a few hours later. The muscle is weak and sensitive to contraction until you recover after two or three days. This phenomenon is called delayed-onset muscle soreness … and it’s much worse in muscles that have been worked hard eccentrically. That’s why your shins are sore after running hard on concrete, why your quadriceps are sore after climbing down a mountain, and why your forearms are sore after your first tennis match in a year.
Any kind of contraction can cause DOMS, but eccentric contractions will get you there quicker. And there’s no cure for it except to get it over with! Because no one really understands DOMS … although that may finally be starting to change, just in the last few years.4 For more information, see Post-Exercise, Delayed-Onset Muscle Soreness: The biology & treatment of “muscle fever,” the deep muscle soreness that surges 24-48 hours after an unfamiliar workout intensity
Eccentric exercise as a treatment for tendinopathy
The high “efficiency” of eccentric contractions makes them potentially useful for rehab.5 Eccentric exercise (EE) is often prescribed for tendinopathy — mainly Achilles tendinitis and tennis elbow — in particular, because it seems to “improve symptoms,”6 and could even be a rare example of truly evidence-based rehab.7
On the other hand, nothing is ever that tidy in sports medicine. Even if it is helping, recent evidence shows that EE may not actually be changing the tendon,8 which is probably what most people would hope and assume — a novel stimulus, with an interesting and useful tissue response, would be of great interest.
And some experts are just not impressed by eccentric exercise therapy, despite some positive studies: “There is no convincing clinical evidence to demonstrate that isolated eccentric loading exercise improves clinical outcomes more than other loading therapies.”9
I bet “it depends.” If exercising eccentrically is useful in rehab, it’s not clearly established yet.
Related: two ways to clench?
Isometric contraction is not as exotic as eccentric, but it has its mysteries as well. A 2017 study suggests that there are probably two ways to clench.10
Is there a difference between preventing an object from moving and pressing on a fixed object with the same force? You wouldn’t think so, if the amount of force required is truly the same, but apparently there is. The experiment clearly shows that a “holding” contraction is more exhausting than pushing equally hard on a stable object, demonstrating that “there are probably two forms of isometric muscle action.” Isometric contraction is contraction without movement. Both of these contractions are isometric — there’s no movement, and the forces are the same — but one of them is more tiring than the other.
It’s the difference between pushing on a wall versus trying to stop a moving wall… which sounds like a silly analogy, but the Star Wars trash compactor scene is a perfect example: “The walls are moving!”
Schaefer et al suggest some possible reasons for the difference, such as “complexity of neural control strategies” — in other words, adjusting your force to match an externally applied force may be more of a neuromuscular juggling act than applying a steady force to a stable object.
Interesting as this is, despit the measurable differences I think it might be overstating it to declare that there are two different “types” of contraction. This seems more like evidence that the same kind of contraction is simply more exhausting in one context than another.
About Paul Ingraham
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
This article is part of the Biological Literacy series — fun explorations of how the human body works, what I think of as “owner’s manual stuff.” Here are ten of the most popular articles on this theme:
- Why Do We Get Sick? — The curious and tangled connections between pain, poor health, and the lives we lead
- When To Worry About Shortness of Breath … and When Not To — Three minor causes of a scary symptom that might be treatable
- Micro Muscles and the Dance of the Sarcomeres — A mental picture of muscle knot physiology helps to explain four familiar features of muscle pain
- Does Fascia Matter? — A detailed critical analysis of the clinical relevance of fascia science and fascia properties
- Why Does Pain Hurt? — How an evolutionary wrong turn led to a biological glitch that condemned the animal kingdom — you included — to much louder, longer pain
- Organ Health Does Not Depend on Spinal Nerves! — One of the key selling points for chiropractic care is the anatomically impossible premise that your spinal nerve roots are important to your general health
- The Unstretchables — Eleven muscles you can’t actually stretch hard (but wish you could)
- Why Do Muscles Feel Stiff and Tight? — Maybe your range of motion is actually limited, or maybe it just feels that way
- Healing Time — Can healing be hurried? Would we even notice if it was?
- 28 Surprising Causes of Pain — Trying to understand pain when there is no obvious explanation
- We Are Full of Critters — The human body is a colony of ten trillion co-operating cells
What’s new in this article?
January — New section, a brief summary of “Eccentric contractions as a treatment for tendinopathy.”
2018 — Two new sections, about titin and isometric contractions, and numerous other minor improvements.
- It is an oxymoron if you only use the conventional sense of “contraction,” but there is a technical meaning of “contraction” that is well established and makes more sense. BACK TO TEXT
- Hoffmann PM. Life's Ratchet: How molecular machines extract order from chaos. New York: Basic Books; 2012. A wonderful but difficult read about the dazzlingly complicated chemistry and nanoscale “machines” that are the most basic explanation for how living things work. As books go, it doesn’t get much more difficult or rewarding. Although the history of science will bore many readers, it’s impossible to appreciate what we know about know today without hearing the story of how we got here. It is amazing how much we figured out by inference decades, centuries, even millenia before we had the tools to actually examine these things. And, now that we can, they are still among the hardest things to understand that humans have ever grappled with. Chapter 7, “Twist and Route,” is about the molecular machinery of movement and muscle: the motor proteins kinesin, myosin, and dynein. “There is not one type of kinesin, myosin, or dynein doing one type of job. Instead, like a fleet of customizable trucks, there are superfamilies of molecular motors, with eighteen known classes of myosins, ten classes of kinesins, and two classes of dyneins.” This rabbit hole goes deep. BACK TO TEXT
- Hessel AL, Lindstedt SL, Nishikawa KC. Physiological Mechanisms of Eccentric Contraction and Its Applications: A Role for the Giant Titin Protein. Front Physiol. 2017;8:70. PubMed #28232805. ❐ PainSci #53120. ❐
BACK TO TEXT
Shortly after the sliding filament theory of muscle contraction was introduced, there was a reluctant recognition that muscle behaved as if it contained an elastic filament. … This additional filament, the giant titin protein, was identified several decades later, and its roles in muscle contraction are still being discovered. Recent research has demonstrated that, like activation of thin filaments by calcium, titin is also activated in muscle sarcomeres by mechanisms only now being elucidated. … Titin stiffness appears to increase with muscle force production, providing a mechanism that explains two fundamental properties of eccentric contractions: their high force and low energetic cost.
- Mizumura K, Taguchi T. Delayed onset muscle soreness: Involvement of neurotrophic factors. J Physiol Sci. 2016 Jan;66(1):43–52. PubMed #26467448. ❐
A series of Japanese studies since 2010 have showed that the pain may be related to neurotrophic factors: substances secreted by muscles cells that goose nerve growth. A simpler way to say this would just be nerve growing pains. Exercise develops our nerves, and that’s uncomfortable.BACK TO TEXT
- Hessel et al: “The high force and low energy cost of eccentric contractions makes them particularly well suited for athletic training and rehabilitation. Eccentric exercise is commonly prescribed for treatment of a variety of conditions including sarcopenia, osteoporosis, and tendinosis.” BACK TO TEXT
- Frizziero A, Vittadini F, Fusco A, Giombini A, Masiero S. Efficacy of eccentric exercise in lower limb tendinopathies in athletes. J Sports Med Phys Fitness. 2016 Nov;56(11):1352–1358. PubMed #26609968. ❐ “Eccentric exercise (EE) is considered a fundamental therapeutic resource, especially for the treatment of Achilles and patellar tendinopathies.” BACK TO TEXT
- Rees JD, Wolman RL, Wilson A. Eccentric exercises; why do they work, what are the problems and how can we improve them? Br J Sports Med. 2009 Apr;43(4):242–6. PubMed #18981040. ❐ “Eccentric exercises (EE) have proved successful in the management of chronic tendinopathy, particularly of the Achilles and patellar tendons, where they have been shown to be effective in controlled trials.” BACK TO TEXT
- Drew BT, Smith TO, Littlewood C, Sturrock B. Do structural changes (eg, collagen/matrix) explain the response to therapeutic exercises in tendinopathy: a systematic review. Br J Sports Med. 2014 Jun;48(12):966–72. PubMed #23118117. ❐ “The available literature does not support observable structural change as an explanation for the response of therapeutic exercise except for some support from heavy-slow resistance training.” It’s conceivable that moderate intensity eccentric contractions are just somewhat closer to the real goal of “heavy-slow resistance training” — a shortcut. BACK TO TEXT
- Couppé C, Svensson RB, Silbernagel KG, Langberg H, Magnusson SP. Eccentric or Concentric Exercises for the Treatment of Tendinopathies? J Orthop Sports Phys Ther. 2015 Nov;45(11):853–63. PubMed #26471850. ❐ BACK TO TEXT
- Schaefer LV, Bittmann FN. Are there two forms of isometric muscle action? Results of the experimental study support a distinction between a holding and a pushing isometric muscle function. BMC Sports Sci Med Rehabil. 2017;9:11. PubMed #28503309. ❐ PainSci #52864. ❐ BACK TO TEXT