Sir Isaac Newton on barbell jump squats

“I prefer kettlebells when it comes to power development. Makes me think of a basket of apples.” ~ S.I. Newton, Principia & Plyometrics, Vol. 3, 1684.[1]

Newton’s second law of motion is why I don’t let people do barbell jump squats at Custom Strength.

F=ma,

where F is force, m is mass, and a is acceleration.

Simple.

What goes up, must come down

When you jump squat with a barbell on your back, both you and the barbell accelerate upward, with the acceleration determined by the ground reaction force you generate minus gravity.

At the peak of your jump, both you and the bar reach a point of zero motion, and from there, you both accelerate back toward the ground.

On the way down, the ground reaction force is gone, and your acceleration is completely due to gravity, meaning you accelerate toward the ground at 9.8m/s2, or 10m/s2 for sake of simplicity. (m/s2 is meters per second squared, and is the standard metric unit for acceleration).

Despite what you learned watching cartoons, people and objects (from feathers to anvils and even barbells) travel to the ground at the same speed – the 10 m/s2) noted above. So if you accelerate at the same speed, why am I concerned about jump squats with a barbell on the back?

The problem is in the fine details. When you jump up, even though the bar is on your back, it is not a part of you. You jump up by pushing the ground away from you, and the bar on your back goes with you. But there is a slight delay. You start to accelerate upwards ever so slightly before the bar does. The result of this delay is that the barbell will get to the peak a touch later than you do, which means that it also starts it’s acceleration back to earth a touch later than you do. Since you both travel back to earth at the same rate, you will hit the ground first, and then the bar will hit you.

If you don’t believe me, grab something with a bit of heft to it, like the physics textbook I was just reviewing, and hold it in your hands. Now jump up. Notice anything? Maybe you noticed that at some point, the book feels almost weightless in your hands? And that when you start and finish the jump, the object feels a touch heavier? That’s the result of the delay between when you start to accelerate and when the book – or in the case of the squat jump, the barbell – starts to accelerate.

What this means is that when you land, the bar hits your upper back (or neck depending on your position) with a force equal to ten times the weight on the bar (remember that 10m/s2number?). Now, if it’s any consolation, that acceleration value is metric, so the force on your back is 10 times the mass in kg, or just shy of 5 times the weight in pounds (1 kg = 2.2 pounds). So if you’re in the US, the bar only hits your upper back with a fore of five times the weight of the bar.

Some people will claim that you will use your hands and arms to dampen the force and cradle the bar as it lands on your back. Absolutely true. But what if your timing isn’t great? What if you’re jump squatting with a weight you’ve never used before and don’t quite know how much to cradle? What if you’re tired?

Will you dampen half the weight? Three-quarters of it? Even if you dampen 90% of a 100 pound jump squat, you’ll have a force equal to 50 pounds hitting your upper back or neck (100 lbs x 5 x 10%). That is why we don’t do barbell jump squats.

There is one version of the jump squat that I like less than barbell jump squats: kettlebell jump squats with the KB handle held behind your head and the bell hanging down. Think about what I just wrote, and imagine what would happen here. Yes, the bell part of the kettlebell will continue to rise up after you hit your peak height long enough to rise up a bit, and when you land, the bell part crashes down hard on your upper spine. I saw this exercise being taught at a reputable seminar for personal trainers. Scary.

So what should you do instead?

We hold a KB or DB in the hands. It’s true that you can’t lift as much this way, but that’s a trade I will accept. We used to use weight vests as they distribute the load a bit more than the bar,but I still don’t like it. When (sometimes if) my clients get to the point where their ability to hold the weight limits their power development, we move to Olympic lifting, Kettlebell swings, Kettlebell cleans, or kettlebell snatches.

Wait, doesn’t Olympic lifting have the same problem?

In the case of Olympic lifts, you are “throwing the bar”, but the difference is that you are catching it when it is moving with the least force. Remember how in the jump squat example above, at the peak of your jump you stop moving as you transition to moving back to the ground? Same goes with the bar here. In Olympic lifting you catch the bar toward the apex of the lift, where its force is the smallest.

Is Olympic lifting a good option for everyone?

Despite their high awesome factor, Olympic lifts are not for everyone. For one thing, there’s a learning curve, and if you don’t have someone who can teach it to you, it isn’t much of an option. And for those with shoulder and/or hip mobility limitations the conventional Olympic lifts are probably not ideal, although there are four alternative versions:
  • Split stance snatch. I learned this when I took the Canadian National Coaching Certification Program (NCCP) weightlifting course. The split stance snatch used to be the standard, until the sport evolved and athletes realized they could snatch more with a squat snatch. The technique evolved because the squat catch is more stable and it also allows you to catch lower, meaning you don’t have to get the bar up as high. But guess what? For developing power generation as training for sport, a split stance snatch is plenty! And it really opens up the Olympic lifts for the mobility challenged. Not convinced? Grab a dowel, and first try doing an overhead squat, and then try doing an overhead split squat. Easier right? That is true of the split stance snatch as well.
  • KB clean and KB snatch. Kettlebells offer a nice alternative to the barbell for Olympic lifts. They still require instruction, as it is a complex movement with a learning curve, but they require less mobility – particularly the KB clean – than the barbell version. If you decide you want to try this, I suggest you hire a certified kettlebell professional (StrongFirst is the best option in my opinion) and take the time to learn it properly.
  • KB swings. The Kettlebell swing is such a great movement. Like the KB clean and snatch, it is a great power development exercise without the hip and shoulder mobility demands of the barbell Olympic lifts. But alas, it too is a complex movement with a learning curve. As with the KB snatch and cleans, do yourself a favour and take the time to learn it properly.

Each of these alternatives to the barbell jump squat is safer without any loss of power development potential. The downside is that there is more of a learning curve. Hopefully those of you who use the barbell jump squat will consider what I’ve written above and re-evaluate whether it really belongs in your program.

 

Elsbeth Vaino, B.Sc., CSCS, is a personal trainer in Ottawa Canada, who jumps (see what I did there) at any opportunity to include physics in her blog.

 

[1] You didn’t really believe Sir Isaac Newton has a quote about jump squats and kettlebells, right? And you know that he didn’t write a book about plyometrics, but you just checked this endnote to confirm your assumption that this is made up. Right?

 

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