Physics and Lifting, Part One -- Willem van der Merwe

 

Physics and Lifting: Force

Those of you who don't want to know any science, please skip! For the rest: physics are indeed relevant to weight training, and knowledge of a bit of it may help you, or at least be somewhat interesting to you. Now I will not be going into any bullsheet here. I've seen so much of it . . . people who talk 'science' but don't actually know the real science involved. There are people who do calculations and say you can't argue with the physics or the maths, but then they miscalculate or leave important terms out of the formulae. Will you be able to catch them out, unless you also know a bit of physics? 

As for me, I did well in physics in school and studied it a bit at university. But for lifting we don't actually need to go beyond high school physics. 

Basically, there are three physical concepts involved in lifting: 

Force

Work

Power

You'll have seen all these words used in informal ways in articles about weight training . . . but how about learning their rigorous meanings? For this article, we're going to look only at force. You have to understand force first, before you can understand work or power.

First, force is basically a push or a pull . . . this intuitive idea suffices. In science, force is measured in Newton (abbreviated N). About 5N amounts to a pound of weight, Force and weight actually mean the same thing in science. So . . . when we are doing weight training, we're doing FORCE training. 

Force is what we want, it is the main thing we're working with. It is why we use heavy weights . . . we want to subject our muscles to high levels of force. A muscle that is strong, is a muscle that can exert a lot of force. We need strength because we live on a huge planet that attracts us and makes it hard for us to move. Strength helps us overcome gravity. The stronger we are, that is to say the more force we can exert, the easier we can move, and the more physical stresses we can withstand. (Just for interest's sake: the planet attracts us because of the gravitational force, the weakest of nature's four fundamental forces. The electromagnetic force is what makes our muscles work and is proportionately far, far stronger. Lift your little finger, and that small finger has overcome the gravitational attraction of the entire planet.)

You may have come across the formula F=ma . . . force equals mass times acceleration. Acceleration means the RATE at which objects INCREASE or DECREASE their speed. In science, both are called acceleration although we humans would call the latter deceleration. But whether a body accelerates or decelerates, there is always a force involved. 

The force of gravity by which the planet attracts everything also causes acceleration. The acceleration of gravity is about 10 meters per second per second. That means after 1 second of such acceleration, starting from a dead stop, an object will be moving at 10 meters per second; after 2 seconds, it will be moving at 20 meters per second, and so on. A mere 10 seconds of this acceleration would be enough to make an object move at 100 meters per second or 360 km per hour! In fact, a falling human body will reach a limit before then because at a great speed through the atmosphere, the body will experience so much air resistance that it will counter and stop the acceleration due to gravity. 

I hope you understand that even objects that are not in free fall are still being affected by the force of gravity and the 'acceleration' they would experience had they been free to fall (all bodies will immediately start falling if the ground somehow vanished from underneath them). Also, you may find it interesting that all objects on the surface of Earth are being accelerated at the same rate, 10 meters per second per second, irrespective of how heavy or light they are.

A kilogram is a unit of mass, not weight. In science, a 1 kg mass accelerated by the force of gravity, at an acceleration of 10 m per second, will experience a force of 10 Newton. See, we used the formula F=ma to work that out. Since a pound comes to about 5 N, we can see that 1 kg experiences a gravitational force of about 2 pounds (we're rounding off a lot here, you can look up the exact numbers if you like). Newton and pound are just two different units for measuring force. Note that pounds and kilograms obey the ratio of 2 to 1 only on the Earth. On the Moon, for instance, a kilogram would NOT correspond to 10 Newton or 2 pounds; the Moon's gravity is about a sixth of the Earth's and thus a kilogram there would weigh about a third of a pound or one-and-two-thirds Newton. In a satellite orbiting Earth, an object will 'feel' no weight because it is in free fall and its surroundings are falling with it at the same rate; it is still being accelerated by the force of gravity, but has stopped 'resisting' this.

So, a pound of weight lying on the ground (on Earth) exerts about five Newton on the ground surface. Now in physics, forces always come in twos: every force goes together with an equal and opposite reaction force. So, a pound of weight lying on the ground exerts weight on the ground, but the ground also exerts the exact same force on it. If both the weight and the ground could feel, the ground would feel the weight of the weight trying to fall, and the weight would feel the resistance of the ground that prevents it from falling. 

What about if it were falling from a height? The weight would then feel the Earth pulling it downward . . . and the Earth would feel the weight pulling it UPWARD by the exact same force! Because nothing was preventing them, they would be moving (accelerating) towards each other. The weight is so much smaller than the Earth that in practice, it will do virtually all of the 'moving' the Earth itself moving just a tiny, tiny distance.

Now, when you pick up a weight, you use a very complex system of levers, hinges and pulleys made from your bones and the muscles and sinews attached to them. How much force lifting any particular weight will subject your muscles and joints to, depends on leverage factors. 

There are two essential leverage factors that are intrinsic to your body. The first is how close to their pivot points (the joints around which your limbs bend) the muscle attaches. All else being equal, the longer a person's limb bones, the more force the muscles must exert to lift the same weight. A very tall person may therefore not squat as much weight as a shorter one, but actually the muscles may be exerting as much or more force. Unless the tall person has muscles that insert much further from their joints, in which case the muscles may be by with exerting less force.

While force depends on these factors, because you can't change them, you shouldn't concern yourself about them. If in any exercise you lift more weight, without changing the way you lift it, then you're exerting more force. Now if you lift any weight, the forces still come in pairs. If you're bench pressing 300 pounds, you are exerting an upward force on the bar while the bar is exerting a downward force on you. 

What about speed of movement? The formula F=ma involves acceleration, not speed. Something either not moving at all (relative to the surface of the Earth) or moving at a constant speed, would feel no additional force other than its own weight. If you lift a weight, then you start it by imparting a force a bit more than its own weight. The extra force, called the resultant force, would then start accelerating it by the equation F=ma. But of course this is not the only force your muscles will feel. They will feel the weight of the object bearing down as well as the bit of extra force which is accelerating it. In most lifting actions, you will accelerate the bar only for a short time; then the bar will be moving at a constant speed for another bit of time; then the bar will need to slow down to a stop at the top of the movement. For the little time that it is accelerating upward, it will feel a bit more force than its own weight coming from you; at a constant speed, it only feels as much as its own weight; and when slowing down, will feel a bit LESS than its own weight. Reversing the process to lower the bar; as you allow the bar to start moving downward, you still use a bit less than the weight of the bar; then it moves at a constant rate, the force being equal to its own weight; then you need to decelerate the bar again to bring to a stop at the bottom, and again the force must be a bit more than the bar's weight.

Because the acceleration from the Earth's gravity is quite substantial, in practice we will not be able to add much extra force by deliberately accelerating the bar. If you accelerated the bar upward at a rate to match gravity, that is, by 10 m per second squared, then after just 1 second the bar will be moving at 10 meters per second or 36 km/h (22.5 mph)! In this case the weight would feel DOUBLE the force of its own weight: as always, it feels its own weight, and then an additional force equivalent to that weight accelerating it upwards. (And because of action/reaction you would feel double the object's weight pushing down on you on your end.) If we could keep the same acceleration going for two seconds, the weight will be moving at 72 km/h (45 mph) which would make a heavy weight into a very dangerous projectile. Even in Olympic lifting, the weight never reaches this kind of speed. In essence, it means that either you don't accelerate the weight at any rate comparable to gravity, OR you can keep such acceleration up only for very brief periods.

So: you needn't bother much with figuring in acceleration when lifting weights. At the typical sports at which weights will be lifted in for instance the bench press, squat, deadlift, row or overhead press, there will not be enough time or space for accelerating the weight much. You will accelerate and decelerate the weights a bit at the start and finish of the movement but this component will hardly compare to the component due to the weight's own. So the force you will experience will be pretty much all dependent on the weight you're using rather than the speed. 

So what about Olympic lifting? The reason you use speed there, is because there are parts in the weight's range of motion where you can't effectively exert much force on it. So you impart a lot of speed to the bar so it can 'free fall' upwards over these parts of its range of motion (with the force of gravity decelerating it for a split-second from its initial speed to a standstill), which takes it the necessary distance so you can dip underneath it and catch it either at shoulder height or overhead and finish the lift. This only works because the Olympic lifts have very large ranges of motion -- from the height of the bar with the weight on the floor, to where it's held overhead at arms' length, so the total range of motion can be about two meters/6 feet or so for a tall lifter. And these movements happen very fast. You're not actually 'feeling' the extra force due to the acceleration for any longer than a fraction of a second during the initial part of the snatch or the clean & jerk. 

When you're doing more typical exercises, such as the bench press or curl, the range of motion is much too little for any really significant acceleration to be imparted . . . and even if you could, it could only be for a very brief time. The majority of time 'moving' the bar it will be done with the force being imparted to the bar being very close to just the weight of the bar. And the force 'felt' by your muscles will be determined basically by this weight.

The force experienced by the muscles will depend on the weight lifted and the precise configuration of the lever/pulley system made by the bones it attaches to and the joints they form. In typical exercises, the force exerted and experienced by the muscles will actually be far greater than the weights used, because muscles work at a leverage disadvantage on purpose: the bones translate a small movement of the muscle, into a large movement at the end of the limb. If you do a curl, you move the weight up by about one-and-one-half to two feet but the biceps muscles only shorten by about three inches at the elbow! This means that the arm flexing muscles will actually feel a force of six to eight times as much as the weight of the bar in the curl! 

A final thing needs to be said about the forces the muscles experience. To visualize the forces, one can think of a muscle as a piece of rope connecting two points: one where it originates, and one where it inserts. (This analogy breaks down a bit with muscles that originate over a broad area rather than at a point, but I'll speak about that later in, I hope, a future article.) If you imagine a suspended rope from which a weight is hanging, you have something similar to a muscle that is tensed under a load. Disregard the weight of the rope; the tension at the bottom of the rope will be the same as the tension at the top, and also the same as the tension in the middle and at all other points. (Tension is yet another word for force, typically when a force is stretching or tending to stretch something.) The same with muscles: a straight muscle tensed will experience the same stress aat its origin, its insertion, and all points in between. This suggests that muscles will distribute a force along their entire length, rather than the force being concentrated at one or the other length. This is a simplification . . . it seems that some muscles CAN be preferentially loaded at one or the other end, by arcane factors such as speed of motion; this is still being investigated; I'll try to stay abreast of this development but for now I think it would be unlikely that this factor could make a really significant difference; generally it would be be to focus on exercises that stress as much of a muscle as possible, as would be true of basic exercises such as barbell and dumbbell bench and overhead presses, chins and pulldowns, rows, curls, squats, deadlifts, leg presses and so on. 

So how do we apply the scientific concept of force to our training? 

Very easy! 

We simply train for strength. We use the heaviest weights we can, for the exercises and the rep count we're using, while using good form, and we try to improve. 

Progressive poundages in good form . . . 

there's the science behind the notion! 

This wraps up force for the moment. 
I will next write about work.

Enjoy Your Lifting!