A peculiar bit of muscle physiology
Articles in the Biological Literacy series are fun explorations of how the human body works. See below for a complete listing of articles in the series.
When you think of a muscle contraction, you think of a muscle getting shorter, which is called “concentric” contraction — but that’s not always what happens. In fact, your muscles routinely have to pull off a trick known as “eccentric” contraction, and it is odd: contraction while lengthening, also sometimes called a braking contraction.
|concentric contraction ||= ||contraction while shortening
|eccentric contraction ||= ||contraction while lengthening?!
How is this possible? How can that even be called a “contraction”?
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 dominant theory of muscle contraction — the sarcomere model — cannot quite explain it.
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.
Eccentric contraction hurts!
Other than intellectual interest, this is why you should care about eccentric contractions: because they hurt more!
Anyone who has ever exercised knows about that nasty feeling you can get in your muscles. Usually it’s worst the day after, sometimes two days after. It feels like your muscles have been bruised inside and out. If it’s bad enough, it’s like you can feel individual molecules of air bashing into your skin. The muscle is weak and incredibly sensitive to contraction until you recover.
If it’s bad enough, it’s like you can feel individual molecules of air bashing into your skin.
This phenomenon is called delayed onset muscle soreness (DOMS) … 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.
Although concentric contraction can also cause DOMS, eccentric contractions are much worse. And — again — no one knows why. In fact, no one really understands DOMS, either. There’s no cure for it except to get it over with! See Delayed Onset Muscle Soreness (DOMS): The mysteries of muscle fever, nature’s little tax on exercise
How’s that for “biological literacy”? Now you can amaze your friends, team-mates, and running buddies with your knowledge of eccentric contractions! At least until they ask how it works, that is…
The Biological Literacy Series
- 7 Reasons Older Adults Don’t Stay in Exercise Classes — And 7 Reasons Why they Should
- An Introduction to Biological Literacy — Why you need to know more about your body
- An Open and Closed Case — An explanation for a strange duality of muscle sensation observed in massage therapy
- Dance of the Sarcomeres — A mental picture of muscle knot physiology explains four familiar features of muscle pain
- Eccentric Contraction — A peculiar bit of muscle physiology
- From Atoms to Elvis — A wide-angle look at the foundations of biology
- Getting On Your Nerves — Can you damage your nerves when self-massaging?
- How Many Muscles? — A (slightly tongue-in-cheek) tally of the body’s many muscles
- It’s the homeostasis, stupid — Vindication for an unconventional view of knee pain, and probably a lot of other common aches and pains
- Natural Imperfection — Evolution doesn’t care if you have back pain … just as long as you can breed
- Oh, a flow-induced system of mechanotransduction! Of course! — A century-old mystery of bone biology was solved just a little while ago
- The Respiration Connection — How dysfunctional breathing might be a root cause of a variety of common upper body pain problems and injuries
- Singing, Breath and Scalenes — Connections between singing, breathing and a strange group of muscles
- The Sixth Sense — Proprioception, the vital but mysterious sensation of position and movement
- Spinal Nerve Roots Do Not Hook Up to Organs! — 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
- Stiffness, tightness and limited range of motion are not always the same thing — An interesting little thing to understand about your body
- Ten Trillion Cells Walked Into a Bar — A humourous and unusual perspective on how, exactly, a person is even able to stand up, let alone walk into a bar
- Ugly Bags of Mostly Water — The chemical composition of human biology
- We Are Full of Critters — The human body is a colony of ten trillion co-operating cells
- When To Worry About Shortness of Breath … and When Not To — Some common, minor & treatable causes of a scary symptom explained
- Why Do We Get Sick? — The connections between poor health and the lives we lead
- Why Does Pain Hurt So Much? — How an evolutionary wrong turn led to a biological glitch that condemned the animal kingdom — you included — to much louder, longer pain
- Why Massage Makes You Tingle — The physiology of sensation in muscle tissue
- You Might Just Be Weird — The clinical significance of normal — and not so normal — anatomical variation
- The ‘Use It Or Lose It’ Principle — The importance of stimulation and movement in healing