Try this experiment: Grab a piece of rope, hold one end with both hands, and pull.
What happened? A whole lotta nothing. Why? Because you can only transfer force through the rope if the other end is tied to something. In other words, you can’t play tug of war with yourself.
According to our best theoretical model, muscles contract when their component parts slide in towards one another - when muscles shorten they pull on both ends.
Take a look at this picture of biceps brachii:
When the biceps contracts, what does it do? Most people say, flex the elbow, and they’d be right. But that’s not all - it also flexes the shoulder (raises your arm). And in fact, it can do both at the same time. What’s different in each case is which end of the muscle is fixed. When flexing the elbow, the shoulder end of the biceps is fixed, so when the biceps shortens, it moves the forearm. When flexing the shoulder, the shortening fibers pull the entire arm up over your head.
It all depends on where your rope is tied and which end you pull from.
What does this mean to you? That stabilization matters. Here’s something else to consider: All exercises - open-chain, closed-chain, or lifting chains - involve stabilization of your torso (and likely, stabilization of your limbs).
“Anchor yourself” when lifting a weight, and you’ll be able to lift it better. And lifting more weight better is the name of the game.
Two quick examples:
When performing a bench press or chest press, stabilize your shoulders by pulling them down and slightly back. By activating your lats and upper back muscles, you simulaneously increase your pushing power and decrease your chances of injuring your rotator cuff muscles.
Tighten your abs when performing a lat pulldown. If you’re using an apparatus that has a seatbelt, actively push your hips into the belt (performing a reverse crunch into the belt, essentially); this will better anchor your torso and give you more stability when pulling with your arms/lats/ serratus.
A note, especially to beginners: “Anchoring” your muscles will make the exercise feel a lot harder. This is because more of your energy is focused on directly resisting the weight (i.e., performing the task) instead of merely “working.” Notice that although the exercise feels “harder”, the weight feels easier. You should feel the sensation of a stronger push.