Let’s talk about lifting light weight as part of your strength training regimen. I recently got into a discussion (argument) with a couple people, which revealed to me that there is some serious misunderstanding in this area.

Newton’s 2nd law of motion is Force = mass x acceleration (F=ma). When lifting a heavier weight, the mass is larger, and the acceleration is lower. When lifting a lighter weight, the mass is smaller, and the acceleration is higher. When one goes up, the other goes down. Simple. No confusion there.

But the question is, “What happens to force?” (To be clear, we could be talking about the force you apply to the bar, the force you push into the ground, or the tensile force in your muscles. The discussion is the same for all three.) Here is the key point. The fact that acceleration goes up when mass goes down does not mean that they balance each other out and force stays the same. It does not mean that! I had people telling me that you can generate the same force regardless of how much weight you are lifting. That is false. If you lift less weight, you produce less force, assuming your effort level and the movement are the same.

Why is that?  It’s actually because of Newton’s 3rd law of motion, which states that every force has an equal and opposite reaction force. One of the implications of this law is that Object A cannot apply more force to Object B than Object B applies back to Object A. So the force that Object A applies is dependent on Object B.

This can be demonstrated with any object, but I’ll just give a couple examples. Let’s say your car is rolling down the street at 10 meters per second. You hit a construction barrel, and it does no visible damage to your car. The force the barrel applied to your car was not enough to cause damage. Again your car is rolling down the street at 10 meters per second. Now you hit a parked car, and it totally smashes up your front end. The force the parked car applied to your car was great enough to cause major damage. It is obvious that the parked car applied far more force to your car than the construction barrel did. Because of Newton’s 3rd law we also know that your car applied far more force to the parked car than it applied to the construction barrel. Nothing changed about your car. Same mass. Same velocity. But it applied drastically different amounts of force when interacting with a different object.

Another example. You wind up and kick a soccer ball as hard as you can. No problem, right? But let’s say there’s a bowling ball sitting on the ground. Are you going to wind up and kick it? No, because you instinctively know that doing so will hurt your foot. Why? The bowling ball will apply far more force to your foot than the soccer ball did. If the bowling ball applies far more force to your foot, we know that your foot also applies far more force to the bowling ball. You can do an identical wind up and kick, but your foot will apply drastically different amounts of force when it interacts with different objects.

Those are common sense examples. When it comes to lifting weights, this can be a bit counter-intuitive. If you get in the exact same position and give the same effort level, why would your force production be lower with lighter weight? Lighter weight cannot apply as much force to you, so you cannot apply as much force to it.

Just based on physics, we can know this to be true beyond the shadow of a doubt. But for those who need research to believe anything, here’s a graph of peak force during deadlift with different loads. It shows clearly that force increases as load increases. Deadlift-Load-Force-Relationships

It’s important to understand that this clear relationship between load and force is reliant on consistent execution of the movement. It shows up clearly in deadlift, a movement which maintains fairly uniform execution. But in any eccentric to concentric movement like squats or jump squats, execution tends to change with heavy weight, and that changes the force output. For example, someone may tend to bend their knees more with heavier weight in a jump squat. That is going to produce more muscle tension, but lower ground reaction force. Or when squatting 70% of max, someone might drop down fast, spring out of the hole, and accelerate all the way through the ascent. At 90% the squat might be more slow and controlled. Different execution of the movement impacts the physics outputs, which can make the load vs force picture less clear. But that does not change the physics. If everything else stays the same, greater load evokes higher force.

We produce less force when lifting lighter weights, so why should we do it? Generally proponents of barbell speed work give some unclear, pseudo-scientific explanation about the importance of acceleration. Yes, accelerating a given weight faster involves higher force production. We should use maximum effort when lifting and accelerate the weight as fast as possible. No one is arguing against that. The question is, “Why accelerate a lighter weight rather than a heavier weight?” From purely a physics standpoint, there is no reason.

All that being said, you absolutely should use light, fast lifting in your strength training. Here are the real reasons.

  • Technique. You need a lot of light, perfect reps to groove in correct technique. You cannot establish a perfect movement pattern if you are always struggling to finish reps. This is critical for beginner and intermediate lifters.
  • Reducing neural stress. If we take the force argument to the extreme, we conclude that only max attempts should be used in strength training, because anything less involves lower force production. But lower force production does not mean no force production. Lifting lighter weight still involves neural drive and muscle tension. It still trains strength, just not in as intense a fashion as heavy lifting. Using lighter lifting days is a way of managing the stress placed on the nervous system.
  • Transfer to faster movements. The first two reasons apply to anyone trying to get stronger. This reason applies specifically to athletes trying to use strength as a means to improve sprinting, jumping, throwing, etc. Strength developed with lighter, faster lifting slows down force production less and carries over better to explosive athletic performance. This is why the use of olympic lifts is popular among explosive athletes.

There are plenty of reasons to use light lifting, but none of them have anything to do with force production.