Concentric versus eccentric versus isometric, isotonic, and isokinetic. At some point, every anatomy or kinesiology student needs to learn the difference between the major muscle contraction types. So in this video, I’ll teach you the difference between these terms, how we use them outside of the classroom, and some tips for remembering them more easily. If you’re new here, welcome, my name is Patrick and this channel is about anatomy and how we learn about it. As always, you can find the notes for this video linked in the description. So why do we care about these contraction types? Well, they have a bunch of different clinical applications, and if you’re designing an exercise or rehab program, you can accomplish different goals with your exercise prescription if you understand how these contractions work. For example, if you were rehabbing a rotator cuff injury, you’d probably introduce isometric exercises before concentric exercises since your patient will have a reduced active range of motion. Or if your client doesn’t quite have the strength to do a compound exercise, you can prescribe eccentric exercises as a way to develop skills and strength, like in this single leg squat example. We’ll come back to this. When you first learn how to differentiate between concentric, eccentric, and isometric contractions, you’ll pay attention to one main factor: the change in muscle length. Isometric contractions happen when the force produced by the muscle of interest is equal to its resistance. Because of this, the joint doesn’t move. That resistance can come from different places — like a dumbbell during a static hold exercise, gravity like during a plank, a surface like during wall presses, or by your own muscles. For instance, when the wrist flexors and extensors contract on their own, they move the wrist in opposing directions. One muscle is the agonist while the other is the antagonist. When you grip something in your hand, they contract at the same time in isometric contractions, producing the same amount of force as each other. As you grip harder, both sets of muscles contract harder, but the wrist joint still doesn’t move since they’re both producing the same amount of force as each other. Concentric contractions happen when the muscle produces more force than the load placed on it, causing the muscle to shorten while eccentric contractions happen when the muscle produces less force than the load placed on it, causing it to lengthen. For example, a chin up is a concentric contraction of your biceps, while lowering yourself down is an eccentric contraction, what fitness folks sometimes call a negative. Walking downhill is another common example of eccentric work — your quads and calves generate force so you don’t just stumble downhill. Does that mean that everytime a muscle lengthens, it’s an eccentric contraction? No. You can stretch your hamstring, which lengthens the hamstring. The difference between the muscle lengthening in a stretch and an eccentric contraction is whether the muscle is actively producing force or not. Now, those are your raw definitions — just look at the length of the muscle. But there are some key differences between these contraction types that you might use in a clinical setting. Number one: Each contraction type can exert different amounts of maximal force. Any given muscle can exert the smallest amount of maximal force concentrically, the largest amount eccentrically, and somewhere in the middle isometrically. This is how the single legged squat example from earlier works. Maybe my muscles aren’t strong enough to do a full up and down single legged squat yet, but since they can produce more force eccentrically, I can at least do the down part of the squat single legged. Then I use both legs to concentrically lift my body weight back up. But there’s another big difference between them: energy expenditure. Eccentric contractions both produce the most force /and/ cost the least amount of energy. To understand why both of these things are true, we need to see how muscle moves at the microscopic level. So I recruited my friend Peter to help explain and animate this one. Peter Muscle tissue at the microscopic level looks something like this, a collection of overlapping bands of proteins arranged into the functional unit of muscle called a sarcomere. The two proteins that get the most attention are actin and myosin, because their sliding action is what actually allows the sarcomere to shorten and the larger muscle to contract. If we zoom in even further, we’ll see /how/ they move — cross bridges. Little by little, these molecular actions cause actin and myosin to slide past each other in something called the sliding filament model. Now, their movement is powered by ATP, but there are a handful of proteins in the sarcomere that /don’t/ use energy during contraction. One of them is actually the largest known protein, titin, which connects the middle of the sarcomere all the way to the end. Titin is elastic, and is one of the reasons why you don’t rip your muscles apart when stretching. But it becomes a force producer when you actively contract your muscle thanks to calcium. During a muscle contraction, a bunch of calcium ions come into the muscle cell, which lets myosin and actin do their normal sliding filament thing, but it also causes titin to hook onto actin. As the muscle contracts, actin rotates, which causes titin to wind up around the actin protein, storing energy like a spring. Then when the muscle lengthens in an eccentric contraction, titin unwinds and contributes to the overall force production of the sarcomere, allowing your muscles to produce more force at the macro level. Since titin doesn’t require ATP /and/ contributes to force production, this giant protein explains why eccentric contractions are more energy efficient and why you can produce more force eccentrically. Patrick Before we move on, if you liked those animations and need biochemistry help, go check out Peter’s channel, This Glorious Clockwork. You won’t be disappointed. Now we’ve still got a few more contraction types to figure out: isokinetic and isotonic. And unlike the last three, these have nothing to do with muscle length, I’ll explain. The prefix Iso- means /same/ or /equal/. Isometric refers to the joint angle, it keeps the same joint measurement, or metric, throughout the contraction. Isotonic means same tone or tension, referring to a constant muscle tension throughout the contraction. Usually, concentric and eccentric contractions fall under the umbrella of isotonic contractions. Now, during these kinds of lifts, the joint moves at different speeds. If you’ve done a heavy squat before, you move slower at the bottom than at the top. So while the muscle tension stayed the same, the joint speed did not. So our last type, isokinetic contractions, use some kind of machine to keep the joint motion at a constant speed. You could go all out and really forcefully contract your muscles, or approach it super delicately and the machine matches your force and keeps your joint motion at the same speed. Those machines are cool too, they can be used like traditional concentric-eccentric machines, but can also be concentric in both directions. I’ve only used these kinds of machines in a research setting for my exercise physiology masters though, so I don’t know how often you’d be prescribing isokinetic exercises. In summary, isometric is the same joint angle. Isotonic is the same muscle tension. Isokinetic is the same movement speed. Now, when you’re analyzing someone’s movement, you’ll probably be asked which muscles are contracting in which style. And that’s where we need some extra nuance. If you looked at a leg press, an exercise that’s classically described as a quad and glute exercise, you’d find that other muscles of the knee and hip are also contracting. Like your hamstring is contracting eccentrically to stabilize your knee — you’d potentially damage your ACL without it. So when we analyze movements, we usually look at the big agonist movers of a movement, but for completeness, know that other muscles are indeed activated. Cool. Let’s do some examples. My pushup techniq ue has caught some heat already on this channel, so we might as well dissect it. When I push off the ground, can you label the muscles that are contracting concentrically? What about eccentrics? Remember, we’re looking for moving joints, and in this case, my elbows and shoulder joints are my only moving joints. In this case, my triceps and pec major contract concentrically on the way up and eccentrically on the way down. Plenty of muscles are also working together to keep my core and hips in a straight line, so everything from abs, posterior neck muscles, and quads are contracting isometrically during a pushup. One more example. The side plank is a static hold exercise. No joints are moving, so we know we’re looking for isometric contractions. But where? Personally, I like to imagine gravity pushing straight down, and think about which muscles would push against it if it were a barbell. My oblique muscles and gluteus medius keep my hips off the ground, my deltoid keeps my rib cage from collapsing into my arm, and my upper trap, scalenes, and SCM keep my head up. Earlier in the video Peter mentioned the sliding filament model, and if you want a more in depth explanation of that concept, check out this video I made a while back. Thanks again to Peter for lending his talents to my channel. I’d recommend checking out his video about hemoglobin, which you can find here. And thank you to my patrons on Patreon for your support. Have fun, be good, thanks for watching.