Pick an exercise. Pick an expert. Wrong. Every time.
Every piece of advice about good form is wrong for someone or even most people. There are two very simple categorical reasons: anthropometry and muscle action. Anthropometry is the scientific study of the measurements and proportions of the human body. No two skeletons are exactly the same, with the same lever lengths, or the same ratios. As such, no two people are even close on how they perform any movement. What we call "run," "jump," "squat," "push-up," or any movement, is a broad approximation which no two skeletons can perform the exact same way. In many cases, two different people can't even look kind of like they're performing the same exercise. On top of that, no two people have the exact same point of origin and insertion with muscles, tendons, and ligaments. On top of that, there is something called muscle action, which showcases much more dynamically than can fit in a tidy one-size-fits-all recommendation.
This is no small problem. It leads to various debates, different training systems, and divergent schools of thought about exercise, fitness, and human performance. So I can't tackle each one without a book deal. But I can give you a few examples to help you understand how everything you've ever learned about form and technique is AT LEAST sometimes wrong for most people or always wrong for some people.
Anthropometry And The Squat
*big breath in*
Ok. Where can we begin that most people agree? Well, probably most will agree that a squat involves concurrent hip/knee/ankle flexion followed by concurrent hip/knee/ankle extension. After that, everyone fends for themselves. Some people advocate narrow foot stance, some medium, some wide. Some people insist on parallel feet, some toe out, a few toe in. Recent debates center around femoral rotation, torso position, and "butt wink." We could go on all day. But let's try to abbreviate this with a simple thought experiment.
Imagine creating a robotic arm comprised of three segments (each representing shin, femur, spine) on a base (representing foot). No matter what, the three vertical segments will be 18 inches tall. The obvious iteration is three 6 inch vertical segments. But now imagine 3:5:10 versus 10:5:3 and so on. You'll soon discover that the exact same height in variegated distribution dictates different angles when you bend at the two fulcra. This IS anthropometry. With three men and three women who are all 5'7", we will have SIX different configurations of lever lengths. This will make one or two perfectly suited for any form or technique that you just arbitrarily make up while four to five will be wholly incapable of the same.
Cut out sheets of paper with the same idea and test for yourself. Two people of the exact same height but different lever ratios CANNOT squat with the same body position. So everything you've ever heard about "keep chest up" or "don't let knees past the toes" is 100% untrue for many people all of the time. Make the top and bottom segment short with the middle segment long, and you'll find that a "deep" squat doesn't work out so well for people with disproportionately long femurs. The center of gravity falls completely behind the base, which, of course, is mechanically impossible for a human.
I will go toe-to-toe with any guru, any expert, any Olympic coach, any famous authority who disagrees. Scientists have collected tons of data on normal variance of human bone lengths and ratios, and there is a BROAD variance. And that's just POSITION. Once you punch in Newtonian physics, you find that some force production is humanly impossible without the right levers. Simply, some people are built for some movements; some people aren't. Thus, one recommendation of form or technique does not suit all individuals.
Muscle Action And The Triceps Extension
It's ostensibly simpler than the squat. But that's where muscle action complicates the matter. I've heard many coaches over the years demand that athletes or clients keep the elbow in a fixed position while training triceps (or biceps for that matter). Oops. It looks like they didn't consult basic physiology 101.
The long head of the triceps originates above the shoulder joint. Yes, the muscle action of the triceps isn't just elbow extension. It's also SHOULDER extension. This actually means that if you don't move your elbow you aren't training the long head of the triceps through its action.
Flip this around, and more or less the same (but inverted) can be said about the biceps. The action of the long head of the biceps is SHOULDER flexion. If the elbow doesn't move during a curl, you aren't training the biceps through its action.
Overlay this understanding onto... well... the whole body. Yep. Everything you've heard is a lie.
Anthropometry And The Kettlebell Swing
There's actually quite a bit which should be addressed with kettlebells and ignorance; but I'm going to keep the discussion relegated to just the swing and just one school of thought.
There's an authoritative organization, group of advanced instructors, and even a certification whose proclamations about proper swing form are inexcusably wrong. I won't name them. I won't go through all of their faults. But I will completely and utterly destroy one of their beliefs.
They DICTATE that the only proper swing technique is with completely retracted shoulders. In addition to this demand, they require that the kettlebell pass BEYOND the pelvis at the bottom of the swing. Think about what this must mean for a minute. Then, consult the anthropometry tables: https://multisite.eos.ncsu.edu/www-ergocenter-ncsu-edu/wp-content/uploads/sites/18/2016/06/Anthropometric-Detailed-Data-Tables.pdf. From the shoulder to the wrist, human reach is 15-21 inches (page 30). As you retract the shoulders, you create a diagonal which effectively shortens the vertical reach, moving the origin (the shoulder) behind the body while the hands must hold the kettlebell in front of the body. You're left with a 13-19 inch tether. But here's the kicker, the average distance from the shoulder to the bottom of the pelvis is 19-26 inches (page 28). So, a 13-19 inch tether MUST pass beyond a 19-26 inch blockade? Only anthropometric statistical outliers can achieve this "form." In fact, they have to be double outliers. They need to be excessively short-torsoed while also being excessively long-reached. And, in that authoritative organization, we find exactly that: super short torsos and extremely long reaches. Their master instructors are all built the same. Their "form" recommendations are actually at odds with the overwhelming majority of skeletal ratios in humans. Most people CANNOT contort the body to even approximate.
Muscle Action And The Abdomen
This last one burns me up the most. Other than my immediate peers, employees, clients, I've never seen ONE person get this right. Tony Horton does planks incorrectly. The crazy lady with clown makeup from the Biggest Loser gets it wrong. EVERYONE gets it wrong. EVERYONE.
Watch a plank, a sit up, a leg raise, ANY core work by ANYONE, and I guarantee 100% that they are doing it wrong at least at the end of the effort, if not throughout. Why? Muscle Action.
We're going to leave aside the external obliques, internal obliques, and tranversus abdominis, because, frankly, people can't even get the first layer right. The rectus abdominis (or what everybody calls and thinks of as the "six pack" or "abs") has various roles - it's true. It braces. It stabilizes. But don't get confused. It has one action: flexion of the trunk. Flexion looks like a "scaredy cat back." It looks like bad posture. It looks like slumped humpback. Go ahead: search your favorite fitness icon and all of google images of planks and core work. Any flexed spines? No? No surprise. AT BEST, if you really dig hard in order to disprove me, you may find someone close to neutral through the spine. But as you sift through thousands of images to uncover that one, you're going to see ample examples of EXTENDED trunks, EXTENDED torsos, EXTENDED spines, and stretched/distended abs. We don't have time for it here; but this one miss alone is responsible for most exercise-related back pain and aggravation (but that's another can of worms for another time).
People get confused by tension. I can tense the "abs" without working them through their action. This tension sensation is part of what reinforced the triceps and biceps mistakes for so many people as well. Artificial tension, or stretched tension is not action.
To help with clarity, think of holding a heavy weight in your your hand without moving your arm at all. You will eventually feel tension in the biceps. But you have failed to work the biceps through its action. Likewise, you can plank all you want, sit up all you want, and so on; but, if you do not flex at the trunk, you have failed to work the rectus abdominis through its action. And, as I said before, that's not even beginning to discuss the complications of the other layers of the abdominal wall and related structures.
Returning briefly to the thesis sentiment, even I am wrong, many many times over. I have come across so many different body types which challenge and stretch my beliefs. Every time I think, "I cracked the code," I get a client with a serious disease, a neurological disorder, a missing limb, a crushed/deformed shoulder. Now what? Everything I learned in 41,000 hours of coaching experiences goes out the window. Injuries change muscle action and alter effective anthropometry. Therefore, I've even discovered that some people cannot move the same way as THEMSELVES after certain orthopedic injuries.
I can even challenge a piece of my own argument about trunk flexion. The takeaway most people would have (and rightly so) is that the trunk better be flexed in a plank. I'm not revising that. But now add in anthropometry. If someone has a long torso, he has to produce much more force than someone with a short torso in order to nail this. That said, one person's "proper" plank may be kneeling, while another's is with knees off the ground, while yet another isn't quite strong enough to flex the trunk fully with any load (including while kneeling).
Various coaches, trainers, and instructors have long intuited that there are some movements which aren't a fit for all people. But too often they've laid the blame on muscle tightness or inflexibility. Those are real concerns - I agree. But you cannot foam roll your way to a shorter femur. You cannot lacrosse ball your way to a running style which is inefficient or impossible for your skeleton to adopt. Sometimes your yoga pose looks different because you aren't built like the master yogis who have short torsos, short femurs, long shins and long wingspans. There's no amount of chiropractic adjustment or turmeric essential oil which is going to lengthen your shin. Sorry. Without referencing anthropometry and muscle action, any form or technique recommendation is categorically wrong. Sadly, almost all form and technique recommendations are coming from people who don't even know about the existence of anthropometry or muscle action.