Parts of an exercise
A static strength training where the length of the muscle doesn’t change. An example of this would be a ‘plank’. In this case you get into a classic ‘pushup’ position, or rest on your forearms with your elbows under your shoulders, and contract your abs to hold your body in a ‘plank’ position with head, back and pelvis aligned for 30 to 60 seconds. You’re still working against gravity, but to keep your body in one position.
The definition of concentric exercise is a contraction that shortens a muscle, while eccentric exercise is a contraction that lengthens the muscle; yes it’s that simple. Most exercise is concentric focused. It’s the part of the exercise when the targeted muscle is working to perform the action.
An example is a leg extension machine which works the quads in the front of your thighs; the concentric portion of the exercise is the part where you extend your leg against the resistance. Even though your leg is extending, your quad muscles contract to straighten your knee. As you return to the bent-knee position, your quad muscles extend. Similarly the concentric motion of the bicep curl is when you bend your elbow and bring the weight towards your shoulder.
The return to the starting position is eccentric. Done properly, in a slow and controlled motion, your muscles work in the eccentric phase to slow the motion down. This both avoids injury and works your muscles in a different way. Eccentric training is sometimes used by elite athletes and lifters. Of course, the only way an exercise can be fully eccentric is if a partner hands you a weight at the top of an extension. Bringing the barbell or dumbbells down to shoulder height would be eccentric, in that case. The most common way to perform eccentrically focused exercise is to use explosive movement, making use of momentum in the concentric phase, and then returning slowly.