Today let's talk about two connected topics - structure, and strength.
Structure - Also known as posture, proper body position, body mechanics, etc. but the term used in my sport in structure. Having structure means being in the correct position to maintain your position, your balance, and your mobility without a high cost of muscular strength.
If you have structure, you need significantly less muscular strength to maintain your position or accomplish a movement.
A good example is the lockout position on a bench press or the top of a pushup. Once you're at the top, it's significantly easier to stay there than it was to get there or return the weight (or your body) to the bottom under control. Why?
Your structure is working for you here. You've offloaded most of the load from your muscles to your joints and bones. Not all of it, because it will still take some energy to keep yourself there or keep the bar there. You need some isometric strength, but this takes less energy and less force production (less exertion, less muscle) than putting yourself into the position.
Standing with good posture is like this - you can stand for hours, because your structure is such that you've offloaded a lot of the effort of standing up. The muscles that do keep you standing are well-equipped to handle this smaller load. If you're standing with bad posture, however, your skeletal muscles are handling more of the load and will express it in fatigue and/or pain. Think of holding yourself in a pushup position with one arm slightly bent - the load is unevenly distributed and your body will fail much sooner.
This type of structure is why people can carry baskets of rocks on their heads, stand up under a load of hundreds of kilos in the Olympics, hold a yoga position, or carry heavy weights for long periods.
Muscular Strength - for purposes of this article, you can divide muscular strength up into two forms - isometric (unmoving/static) and isotonic (moving/dynamic). (As a side note, there is also isokinetic strength, but it's not germane to this discussion.)
Isometric strength is keeping yourself in a static position. Combined with structure, you can hold positions easily. Compare standing erect vs. standing in a slight squat position. The first has structure on your side so you need to expend less energy to stay there.
Isotonic strength is movement around a joint or joints. Combined with structure, you can lift more and lift more easily. A good example of this is lifting a heavy weight off the floor. If you keep your abs tight, your back flat instead of hunching over the weight, squat down enough to get your hips involved, and get over the weight, it will come up relatively easily. If you lose your structure (aka proper form), you must expend much more energy and strength to lift the weight. This can result in injury as a load your body can easily handle statically with proper spine position is suddenly put on it in an improper position and muscles incapable of handling it must kick in and attempt to compensate.
You'll occasionally see these as an either-or thing, especially in books on posture. But in conjunction, structure plus muscular strength is where it is at. Olympic Weightlifting is just the expression of structure plus muscular strength under a load. Walking with a load on your head or shoulders is also structure plus muscular strength under a load. Whether you can do these successfully or not, with or without injury, depends on properly combining the correct structure with muscular strength. Without one, you can only get so far.