Phases of Contraction
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When most people think about muscle contraction, they think about the true definition of contraction—”the process of becoming shorter.” For muscular physiology, however, there are three phases of contraction. Granted, only one of these phases of contraction fit the actual definition of “the process of becoming shorter.”
Each of the three phases of contraction describes the muscle’s length in relationship to the resistance. A muscle can shorten against resistance (concentric), lengthen under resistance (eccentric), or maintain its length against resistance (isometric).
Resistance training routines that target muscle growth should utilize both concentric and eccentric phases of contraction. It is also extremely important to balance the volume of the concentric and eccentric contractions. Lack of either phase or inappropriately balancing these phases of contraction may increase your risk for various connective tissue injuries and ailments including tendinopathies, fascial densification, and more.
Concentric contraction
When a muscle becomes shorter while under tension, it is described as a concentric contraction. Examples of concentric contractions are all the “up-phases” of your exercises—the up-phase of a bench press, squat, deadlift, biceps curl, etc. For the lat pulldown, the concentric phase is pulling the bar downwards.
Another, perhaps easier way to understand concentric contraction is to view the selected resistance. For free weights, plate loaded machines, and selectorize machines (those with a weight stack and a pin), if the resistance is moving against gravity, the action being performed is a concentric contraction. For elastic resistance (bands and loops), if the band/loop is getting longer, the target muscle must be getting shorter—a concentric contraction.
In this video (tap/click here to view on YouTube), the target muscle is the quadriceps group. To elevate the resistance (shown by the up arrow), the quadriceps muscle is shortening (shown by the arrows pointing inwards).
Of the three phases of contraction, concentric is the only one that follows the literal definition of “the process of becoming shorter.”
The most common errors in concentric contraction are variations of “cheat repetitions” where the resistance is moved by something other than the target muscle shortening. Examples of cheat repetitions include:
Eccentric contraction
When a muscle becomes longer under resistance while controlling a joint movement, it is described as an eccentric contraction. Examples of eccentric contractions are all the “down-phases” of your exercises—lowering the weight to your chest on a bench press, the down-phase of a squat or deadlift, lowering a weight back towards your hip in a biceps curl, etc. For the lat pulldown, the eccentric phase is controlling the handle on its way upwards.
Let’s take a look at eccentric contraction from the perspective of the selected resistance. For free weights, plate loaded machines, and selectorize machines: if the resistance is moving with gravity back towards the floor, the action being performed is a eccentric contraction. For elastic resistance, if the band/loop is getting shorter, the target muscle must be getting longer—an eccentric contraction.
In this video (tap/click here to view on YouTube), the target muscle is the quadriceps group. To lower the resistance (shown by the down arrow), the quadriceps muscle is lengthening (shown by the arrows pointing outwards).
Although the eccentric phase is not “the process of becoming shorter,” it is called a contraction because the target muscle is controlling the movement of the joint by engaging with less force than the resistance requires. For example, if your muscles generate ~130lbs of force against a 135lbs barbell during a bench press, the bar will move downwards towards your chest.
Training for both safety and hypertrophy and proportion requires performing repetitions that include a smooth, consistently paced eccentric contraction.
A lack of appropriate eccentric contraction during exercise may contribute to the development of fascial densification and tendon adhesion formation. Additionally, excessively training in the eccentric phase (over-utilizing “negatives,” as they’re often called in the weight room) will increase the risk for significant muscle damage. Therefore, do not skip your concentric contractions, either!
The most common errors in eccentric contraction are simply not performing the phase or not appropriately decelerating the resistance.
Training for hypertrophy
There is debate within the exercise community (researchers and bodybuilders and everyone in between) over which of the phases of contraction is most valuable for muscle hypertrophy. Unfortunately, different research has revealed different findings.
In the article by Higbie, et al listed below, the authors briefly review many of these research findings; some research suggests eccentric is superior, others suggest concentric + eccentric is superior, and some have even shown concentric-only to be superior for growth. However, concentric-only is more likely to result in muscle and/or connective tissue damage.
For Art of Anatomy‘s training protocol, it is suggested to always perform both the concentric and eccentric phases of contraction for all exercises with control. You will learn more about these movement patterns in Program Design and the Art of Bodybuilding sections of this website.
It is worth noting that there is at least a collective agreement by consistent research findings that isometric is the least valuable of the three phases of contraction for muscle growth.
Isometric contraction
The final of the three phases of contraction is when a muscle maintains its length during a contraction against resistance. Examples of isometric contractions include a basic plank exercise, the wall-sit, and all other static-hold exercises.
If we view isometric contraction from the perspective of resistance as we did above for concentric and eccentric contraction, the resistance will not move during an isometric contraction. If the resistance is moving, it is not isometric.
When training for hypertrophy and proportion, the isometric contraction is not particularly useful. An isometric muscle contraction will not promote a significant change in muscle strength or mass. Instead, the isometric contraction improves endurance. Therefore, exercises utilizing isometric contractions should generally be reserved for stability training (e.g., lumbar spine [core] stability, shoulder joint stability, cervical spine stability, etc.).
To some people’s surprise, a plank exercise will not build your abdominal muscles; it will not improve your “six-pack” or provide any significant change to your physique. When performed correctly, it will improve spinal stability which is still extraordinarily valuable.
Can you move the resistance?
There are two functional variations of isometric contraction: one where you’re attempting to move your joint(s) against resistance but physically cannot and another where you could move your joint(s) against resistance but choose not to.
The first variation of isometric contraction is when you’re attempting to move an object that is, to your functional capacity, immovable (e.g., pressing against the wall of a building or trying to lift a truck).
The second functional variation of this muscle contraction is a voluntary static hold. Examples of static hold isometric are plank exercises, wall sits, or yoga poses. In each of these examples, you are physically able to move, but you’ve instead chosen to hold a position for a target benefit (e.g., muscular endurance or joint stability).
Observationally, these variations are the same; the target muscle is not changing length and the joint(s) are not moving. Subjectively, however, they are drastically different based on exertion. Both variations have their place in certain training regimens, but for bodybuilding and hypertrophy, isometric exercises are rarely utilized.