Muscle-centric vs. Joint-centric
Be a muscle-centric lifter, not a joint-centric lifter. A quick biology lesson:
Our muscles are the contractile units within our bodies that create movement; in other words, they are the engines that drive motion. When contracted, muscles pull on the various levers of the body -- a muscle pulls on an attached tendon, which in turn pulls on an attached bone -- to make us run, jump, and lift.
Our tendons and ligaments, on the other hand, are the fibrous tissues within our bodies that help hold everything together. Ligaments attach bone to bone, and tendons attach muscle to bone. These connective tissues help transfer energy from the muscles throughout the body.
So what do we mean by muscle-centric vs. joint-centric? Although we don't particularly care for the bicep curl, we'll use it as an example. You've seen Mr. Bro Science swinging his body while executing bicep curls, right? That's what we're talking about! He's using a weight that's too heavy, and he needs to contort his body and use momentum to lift the weights. Mr. Bro Science is relying on the leverage created from his joints to propel the weight. Not good!
One way to test and make sure you're being a muscle-centic lifter is to keep your joints stationary so that the only joint movement is the hinge. Being a muscle-centric lifter is better for performance, results, and longevity. Properly executing a lift, without swaying and improperly moving the bodies' joints, allows us to maximize potential and enables us to work out later on in life.
Use your muscles to control the weight, don't let the weight control you!