by Jonathon Sullivan MD, PhD, SSC, PBC
Stopping the bar and switching from thataway in the groove to thisaway in the groove is not something that just happens. This is something that you, the athlete, must do.
Momentum is an important physical quantity, and an important concept for those who still live in a world governed by laws and reason (a dwindling slice of the population to be sure, but it’s the where the cool people hang out). Momentum is the product of the mass and velocity of an object, or:
p = mv
Now, don’t get skeered. There’s nothing spooky about this. To start with, I’m writing this for you normal, human, decent folk, not for the engineers and physicists who’ve already started sneering in the wings. We all know those people have no life, no fun, and no joy. The only way these poor, lonely people can fill the void in their atrophic and pathetically pedantic souls is to accost the rest of this essay. Screw them. We never liked them anyway, and just to spite them this essay contains only one non-scary equation, the one above, and uses mechanical concepts very loosely but with suitable elucidation for our purposes. So let’s ignore those pocket-protector guys. Pity them, of course. But ignore them. Now, then. The above equation is simply read “p equals m times v” and it means “momentum is the product of mass and velocity.” Which is the same thing I just said a couple of paragraphs ago, only in the very elegant and succinct language of mathematics. The formatting of the characters is not an accident. Mass, or m, is in plain italics, indicating that it is what we call a scalar quantity: it is just the numerical value of a certain unit, say 10 kg or 22 lbs or 1.57 stone. Velocity or v, on the other hand, is what we call a vector quantity. It has both a magnitude and a direction. It’s not just 50 miles per hour, it’s 50 miles per hour thataway, or 200 kilometers per second thataway. Because one of the factors of momentum (v) is a vector, momentum (p) is also a vector. If a 1 kg mass is moving at 1 meter per second thataway, then it has a momentum of 1kg-m/s thataway. And if you want to change that momentum, you will have to apply a force to that object, because otherwise the object will just keep going thataway at 1 m/s. Allow me to give you an example. Let’s say you have a bar on your back and you’re in the eccentric phase of the squat. The bar weighs 200 lbs and it’s moving downward at 1 meter/second. It really, really, really wants to keep moving downward, and it will do so unless acted upon by an unbalanced force: Newton’s First Law of Motion. Now, let’s say you get to the bottom of the squat: knees go wide so they can'na slide, hips below parallel, everything tight, bar in the vertical slot, all that stuff you get tired of hearing your coach bitch about. Your hips stop for the briefest of intervals and then drive up. All good. Except for one thing: that bar wants to keep going down thataway, and it will do so unless acted upon by a force. Which is you. One way or another, you are going to resist the downward motion of that bar, because you are a physical object, and the bar cannot pass through you on its way to the center of the earth (thataway!). For our purposes, we consider two ways this can happen. In the first way, which is the way of pain and loser-ness, you passively change the momentum of the bar by simply existing as a physical object in its path, and by allowing your back angle to change under the load. In this case, the bar pushes you down and forward. The bar is now out of the groove, off the vertical, in front of the middle of the foot. This will tend to rotate the entire barbell lifter system forward, onto your toes, as the barrier presented by your body to the bar changes the thataway of the bar’s momentum to a new, non-vertical thataway. It’s a shambles. The lift will be ugly, and it will not be fun, and it may even fail. “Shame, and eternal shame. Nothing but shame.” The other way this can happen leads not to shame, but to great triumph and great joy. In this case, you stop the bar. This means that at the exact moment you apply the force to stop the descent of your hips, you also actively apply the force to stop the bar, in the groove, so that its momentum is precisely reversed, from thataway to ANTI-thataway, or more technically speaking, thisaway, from straight down over the middle of the foot to straight up over the middle of the foot. (Those physics nerds are all totally losing their shit right now. I love this job.)
Figure: Stopping the bar. Left: On the way down with the squat, bar over the midfoot, moving vertically at constant speed. Center: At the bottom, you stop your hips, but not the bar. Your upper back flexes. Your chest collapses, You are forward, the bar is outside the slot. "Shame, and eternal shame. Nothing but shame." Right: When you stop the hips, you stop the bar, so it stays in the slot. Triumph and grooviness. And there is much rejoicing.
Stopping the bar and switching from thataway in the groove to thisaway in the groove is not something that just happens. This is something that you, the athlete, must do. How? By doing all those things your pesky coach keeps yelling at you to do: holding that superman chest, fighting to maintain that back angle, focusing on control of the eccentric phase, not dive-bombing into the squat, and keeping the Master Cue firmly in the front of your mind. And, if all else fails, focus explicitly on that one physical objective: stop the bar. The two places where the bar gets out of the groove are at the top, when we injudiciously dive forward into the rep, and at the bottom, when we fail to stop the bar and turn it around in the slot. If you find that your squat goes well until you get to the bottom, and then all of a sudden you’re under a taco squat with the weight on your toes with the flailing and the grunting and the panic and the biting and the scratching, think about stopping the bar on your next rep. I’ve seen this simple cue work wonders, and it may do the same for you...even you slide-rule-and-pocket-protector types.