There are three main types of muscle actions: isotonic (eccentric and concentric), isometric, and isokinetic.
Isotonic muscle action is constant muscle tension. Under this category you’ll find eccentric and concentric. Eccentric muscle action is when a muscle develops tension while lengthening. Force is produced, tension is developed, and movement occurs. Work is actually being done ON the muscle instead of the muscle doing the work. Eccentric motions move in the same direction as the resistance, it is like deceleration. During exercise this looks like landing a jump, or lowering the weight during resistance training. Eccentric motions are also known as “the negative”. Concentric muscle action is when the contractile force is greater than the resistive force, therefore shortening the muscle. This is like acceleration in the sense that the muscle is doing the work by producing the force.
Isometric muscle action is when the contractile force is equal to the resistive force, and shows no change in muscle length. You can see isometric motion when you pause (or hold) during resistance training in between the lifting and lowering phases.
Isokinetic muscle action is when the muscle shortens at a constant speed over the full range of motion. Tension in the muscle is at its maximum throughout the whole range of motion. This can improve strength, endurance, and neuromuscular efficiency. Typically these types of movements are seen in rehab clinics because of the required training equipment.
If you want to take your training to the next level you have to do more than just “lift things up and put them down” try incorporating some of these movements into your current routine!