We're going to into how biomechanics impacts exercise form, but if you haven't read the anatomy and physiology intro yet, do that first. This one takes into account that you already know all of that. Okay, on with biomechanics and form...
Range of motion. Obviously range of motion is a large part of your form during exercise. The range of motion you use dictates the amount of tension the muscle will endure. Greater tension means greater muscular damage. More damage throughout the muscle and muscle-tendon junction means more fibroblasts are sent to the area. The more fibroblasts in the area, the more muscular development. So using a range of motion that produces more tension, while under control, is better.
One of the most well known properties related to working muscle is the length-tension relationship. This states that the longer a muscle is stretched, the greater the tension produced. This is not force produced, it's tension. Tension is produced both by force and length. We'll example this one: you're flexing your arm. That's creating force. However much force you're creating (based on how hard you're flexing), you're creating that exact same amount of tension. Additionally though, the longer your muscle is stretched, the more tension is produced- added to however much force you're producing. Does that make sense? So if you're flexing your muscle in a more stretched position, you're creating more tension in the muscle and muscle-tendon junction than flexing your muscle in a less-stretched state.
That's the general summary. It does
get a little more complicated though due to the fact that your muscle is weaker at longer lengths (less cross-bridges linking up and creating force). At the mid-point in the range of motion, you have the optimal number of contractile proteins linked up pulling toward each other. If you're contracted too far, they're kind of cramped up and unable to connect all their hands. If you're overly stretched, not all of them can reach each other. So there is
an optimal length for strength and your stretched muscle is not in it. But as the muscle is stretched, the length-tension relationship accounts for the creation of more tension than that lost by weakening force output. So basically, the more stretched your muscle is, the more tension it experiences, and thus, greater hypertrophy.
Point: use a full range of motion. Squatting the upper 20% of it allows you to use tons more weight so that you can mess up your joints while getting nowhere near a high level of muscular tension. So basically, the force output is higher, the joint pressure is much higher, and the overall muscular tension is lower, so effectively you're putting on less muscle with more damage to your body. Usually you're going to get better results with a full range of motion regardless of how much weight you can handle by cutting out the lower half. The same rule applies to almost every exercise. There are
a couple exceptions though.
Overhead presses- don't do too much of the lower half here. Mechanical issue. Your deltoids extend over the tip of your shoulder (called your acromion) and down to your humerus. That formation makes the variable angle that it works through. When you're doing shoulder presses and you're pretty low in the range of motion, the angle that's created for your deltoid makes it so that regardless of how strongly it's contracting, you still have no leverage. So your rotator cuff has to pick up the tab. This isn't so good because it's not at all what your rotator cuff is supposed to be for. So don't overdo the stretch in overhead press exercises.
Squats. Leg presses. Hak squats. Be careful on the stretch on these ones. Your knees and back are what becomes vulnerable. It's not worth explaining, so I'll barely summarize. Basically, your knees are in the more mechanically unstable and vulnerable position when they're bent. It's called the "open" position. And when you're pretty far into it, you don't want to unevenly produce tons of force around your knees. Then if you go too low, especially in leg press, your pelvis has a tendency to rotate in a way that causes your low back muscles to be stretched. So more tension goes there. This is not the kind of tension you're hoping for though. It's similar, but not in the area you want. So be careful with these movements in using larger ranges of motion.
Lastly, on chest, the bottom half of exercises where your arms are straight-ish aren't so hot for the shoulders. Basically fly-type exercises. So bend your elbows or don't do the lower half. And if you're worried about bent elbows taking away from the chest results, don't be. It's how far back your elbows are that dictates the range of motion of your chest, not your hands. It might be slightly lighter if you bend your elbows during the lower half though. Physics. The weight is closer to the joint. Change your weight.
Pretty much all other exercises, use a full range of motion.
Okay, moving on to the next subcategory of form: speed of your reps. This one conjures up another fun biomechanical rule, kind of like the length-tension curve- but called the force-velocity relationship. It states that the faster you contract your muscle in the concentric portion of the movement, the less force is required to move the weight, so the less tension you create. The slower you contract, the more tension you end up generating. And when you contract slow enough to eventually stop moving entirely (isometric exercise), you're using more force and therefore creating more tension than any speed in the concentric portion of the movement. However, as soon as you begin the eccentric portion of the exercise, your force is at its highest point, larger than concentric or isometric is capable of producing.
I'll put it into an example for you. Standing barbell curls. You're holding the barbell in your hands. The faster you curl the weight up, the less force is required to do so. Newton's laws. Inertia, object in motion stays in motion stuff. The slower you curl the weight up, the more force is generated. If you hold the bar stationary, you're generating more force than any speed of curling the weight up. But after you're done curling the weight up, letting it down under control generates the most
So the implication on speed here is this: some people say you should lift slowly and controlled, others say fast, others say train isometrically. All of those have some level of justification, and really, you should incorporate all of them so that your muscle doesn't end up adapting strictly to a single one. But depending on your goal, you should stick with one of them more predominantly. For example, if your goal is muscular development, remember that hypertrophy is a result of muscular tension and tension is a product of length and force. So moving eccentrically through the full range of motion is key. You can go up fast to get the concentric over with, then control it slowly all the way down. This will give you the maximal amount of time under the maximal amount of tension.
But if your goal is something else, the eccentric may be less important. But I'm not going to example you on any of those because I wrote about them separately. So read the other sections for the application of these principles to your goals.