Muscles create movement by contracting and relaxing. Muscles have attachment points, and when they contract they produce a force that attempts to pull those attachment points closer together. (It’s important to note that muscles don’t attach to bone in the same way that you fasten a tire to a car. It’s more like branches on a tree in that muscles grow-out-of bone, but for simplicity we’re calling them attachments.) This is the first thing to understand about movement and muscles: muscles can only pull two points together, or contract, but they can never push two areas apart.
There are 3 types of muscle contractions that we can achieve: concentric, eccentric and isometric. In a concentric contraction the muscle length shortens as it creates force. In an eccentric contraction the muscle lengthens in response to a greater opposing force, and in an isometric contraction the muscle remains the same length while creating a force.
Think about a bicep curl. In a curl the concentric phase would be during the actual curling motion. The bicep is creating a force and shortens which cause the elbow to bend into flexion. The eccentric would be the opposite as the elbow begins to extend the bicep is lengthening all the way until we return to a straight arm. If at any point we were to pause and hold the weight at a particular position, that would be considered an isometric contraction. During that moment of the pause the bicep is neither lengthening or shortening but it is producing a force to hold the dumbbell in place.
Many of the movements we perform contain both concentric and eccentric contractions. In a squat for instance, the eccentric contraction is happening as we descend into the bottom using the quadriceps, hamstrings and glutes among other smaller muscles. When we travel from the bottom to standing these same muscles are concentrically contracting, or shortening, to extend the knees and hips. Most movements involve both the lengthening and shortening of muscles but occasionally we specifically target one over the other.
Interestingly, in eccentric contractions our muscles are actually stronger. They can produce more force when being lengthened than they can while being shortened. There is an explanation for this but it would require me to get more nerdy than I think you’re ready for, so for this post just understand that we’re stronger under eccentric phases. We use eccentrics to take advantage of this fact of being stronger and practice movements that we would otherwise not be able to do. A good example of this is the athlete that cannot do a pull-up, but can do a pull-up eccentric.
There is a price to be paid for this additional strength, and it comes as additional damage to our muscle fibers. This isn’t inherently bad, after all when working out we are creating some damage to our tissues so that as they repair they end up stronger. That is how we build muscle, literally. This is also why we’re often more sore 1 or even 2 days after a day of eccentric focused work, and it’s also why we never do eccentrics paired with intensity. The damage being done is already higher than normal so to throw-in some additional intensity in the mix is a recipe for more damage than our bodies can handle.
Some movements have very little or even no eccentric phase. These movements include sled pushes, drags, carries, runs, etc. With little to no time in the eccentric phase we limit the amount of damage we create in the muscles, and therefore also limit the amount of soreness experienced in the succeeding days. This is why we can completely exhaust ourselves on sled pushes, to the point where our legs are jello when we leave, but then feel fine the very next day.
Isometrics, because they lack any movement, create the least amount of damage to our tissues. We use these specifically to help build our neuromuscular connection and to strengthen joints in specific ranges.
In order for us to create movement first we have a thought in the brain which then goes to the nervous system which then sends signals along to the motor neurons in the muscles we need to use to create that movement. By using isometric holds we can help make these transmissions stronger because they allow us to get into positions that we may otherwise not be able to achieve.
The bottom of a pistol squat is a good example. If you don’t have the strength or balance to get down there on your own you may never stress your muscles in that position, which translates to not building strength in that position. Though scaling to a box target or something will allow you to build some strength, it’s not helping build strength in the area where we’re ultimately trying to get to. Unless we use isometric holds in those positions and then hold for time to stress the muscles and nervous system appropriately to illicit a response. In the same way this allows us to stress the joint itself in that end range position so when the day comes that we do have strength and skill to get down there we can actually produce some force.
This also serves as a safeguard against injuries. You can imagine that if you’ve never been in a specific position and then suddenly you find yourself there but have no strength to do anything, you’re in a very vulnerable position. Isometrics help us to build that area so we can respond and hopefully not find ourselves in a vulnerable place.
This is where the action itself happens. Concentric contractions, similar to eccentrics, create muscle damage that when repaired creates more muscle tissue. This tissue tends to be most noticeable in the belly of the muscle while eccentrics tend to build more tissue towards the ends, or tendons. This is the pump we often hear about. CHASE THE PUMP!
Concentric are also where we can build more power, so not just how much weight we can move but also how quickly and explosively. This goes back to the neuromuscular connection we discussed earlier. As weight and speed increases, we demand more response from the nervous system, thereby building more neuromuscular synapses to fire and bringing on board more motor units to create a bigger and stronger contraction. It’s like adding more fuel to a fire.
As far as damage and soreness these rank behind eccentrics, which is good since most of our day is powered by this phase. You can think of these as putting everything we’ve worked on with eccentrics and isometrics to create the full movement.
Wrappin it Up
There you have it, the 3 types of muscle contractions and a little background on why we use each. Like everything else in our programming we make sure that we work all 3 so we can build the most well-rounded fitness and mitigate injury. This variance is another tool we use to learn new skills that may at first seem unattainable, to build a more resilient body, and keep us mentally engaged so we keep coming back.