Physics of Weight Training – Explosive vs Grinding

Yesterday I have discussed the fundamental concepts to consider when analysing the physics of weight training – force, time (under tension, “TUT”), work, effort, and power – and have posited that two of them – force and effort – will be the main determinants of training impact, the former by influence how many muscle fibers are trained, and the latter by to which extent they will be (over-)trained.

Today I want to look more specifically at lifting weights, and what the difference is between lifting them very slowly, at moderate pace, and explosively.

In order to do this we first have to make a small excursion to physics, and in particular Newtonian physics (the guys with the apple tree, and F=m*a). When we are looking at movements in a friction-free environment (which applies for all kinds of free weights and most weight machines, but not for Concept rowers or exercise bikes) then we need to start with a slightly counterintuitive fact:

Moving any object horizontally at a constant speed does not require any force

You need force to accelerate (and decelerate) but once you get the thing moving, the movement in itself does not require any force (and therefore no work either). This is interesting, but does not bring us much further. However, there is an interesting corollary to this, which is

Moving any object vertically at a constant speed does exactly require its weight force (ie the force required to holding it)

This sounds counterintuitive – clearly pushing up seems to require more force than merely holding it. This is true in principle – but the point is as soon as you push with the Newton equivalent of 100g more than the Newton equivalent of the 200kg you are lifting, the weight will move up. Of course it is much more work – we remember that work is Force x Displacement, so physically holding an object is not work, whilst lifting it is.

This allows us to analyse two of the modes we discussed earlier: moderate pace and slow pace. In terms of force applied they will be very much the same – granted, one needs a bit more force in the moderate pace, but this is negligible vis-a-vis the force of holding the thing. The difference here is in the TUT – if you so want, a very slow motion is a combination of a normal motion and a static hold. Or, to put it differently, a slow motion has a stronger isometric component. Now I have no particular view on the relative effectiveness of isometric vs eccentric vs concentric exercises (except that apparently concentric exercises “negatives” lead to greater muscle damage and hence to more hypertrophy) but my general view is that it is probably worth training multiple modes, ie sometimes very slow, and sometimes at moderate speed.

The physics start changing at “explosive speeds” however, because there really Newton’s law comes in to play:

F – F_static = m * a

What this formula says is that the excess force (over and beyond what is needed to hold the weight) leads to an acceleration of the weight (“a”), and this acceleration is inversely proportional to the weight itself (mass “m”) – ie if you pull a weight double the size with the same force it accelerates only half as much, or if you want to accelerate a weight double the size you need to pull twice as hard over and beyond the force needed to hold it.

I repeat this last part of the sentence because it is crucial: over and beyond the force needed to hold it. This is why for slow motion it does not matter: if you lift 2000N (200kg) then pulling with another 100N or 200N does not make a big difference. However, assume the weight is lighter, say 50kg (500N). If you pull with 1000N now your excess force will be 500N, and if you pull with 1500N it will be 1500N both of which will nicely accelerate the weight, the latter twice as much as the former.

The way to think about it is deadlift vs kettlebell swing: in the former (at least at your PR) the force will be just enough to keep the weight in the air, and then a tiny little bit more (but the weight might be say 200kg, or 2000N force). In the latter, the force will be applied only at the very beginning of the movement but it will accelerate the bell enough so that it flies all the way to eye-level (if your are Russian) or overhead (if you are American). For the sake of argument let’s assume you apply the same 2000N force, but only using a 50kg bell. Out of the 2000N, 500N are needed to stabilise the bell, so 1500N will accelerate it – the trick is obviously to pull just long enough that it eventually goes exactly as high as you want it to go.

Now it is easy enough to solve for the “trajectory” of the above “Angry Birds” equation, ie the relationship of the amount of force applied, the time it is applied, and how high the bell will fly. There is however a more elegant way of doing it: physically the weight will always have the same starting and ending height, meaning that the work required bring it from A to B will always be the same. So for the sake of argument let’s assume a 100kg squat with a range-of-motion (“RoM”) of 1m. The work required will be 1000N x 1m = 1kJ (~0.25kcal, which incidentally is the thermal energy of 16th of a gram of sugar). If you push with a force of 2000N then after only 50cm the work done will be 1kJ, meaning that instead of grinding 100kg up, you can push with 200kg equivalent until you are half-way up and then relax and let momentum carry you upwards.

So now finally we can compare moderate speeds and explosive. For both we assume that the static hold component is negligible, so according to our previous definition (physiological) effort ~ (physical) work, and the physical work is determined as

effort ~ work ~ number of reps x weight x range of motion

ie in particular the effort does not depend whether you lift a certain weight slowly or explosively. The force however depends on it. Say for the sake of argument that you apply force only for half of the movement (for “snatch”-type movements this will be much less – maybe 10-20%; however, those tend not to have a grinding equivalent; just try to slowly bring a 2pood kettlebell overhead, starting from the floor…) then the force will be double.

So if the maximum you can grind is 200kg, the maximum you can lift explosively in that manner is 100kg. This is not to say that those two are equivalent however: when you grind, the same force is applied throughout the whole range of motion, whilst for the explosive movement, it is only applied at the beginning. This of course is the reason why you can snatch a kettlebell (or a barbell for that matter) but you cant grind it: our body geometry is such that we can apply a lot of force at the beginning of the movement, but very little in the later stages.

A final point to make here is about max-effort (or max-force rather) and safety margins: explosive movements are more difficult to execute, and hence they are inherently less safe, especially for beginners or intermediate lifters. This means that those lifter (should) have a wider security margin vis-a-vis their 1RM max when they attempt those lifts, which in turn means that the maximum stimulation that can be safely achieved by a non-elite lifter should be higher in a grinding movement than in an explosive movement.

So what is the conclusion?

  • When only considering force and effort, it is possible to substitute heavy/grinding movements with lighter/explosive movements, so under this angle their impact would be the same
  • Further analysis however shows that the pattern of force application is different – throughout the full RoM in the grind, only at the beginning for the explosive movement – which almost certainly leads to a different physiological impact of the respective movements
  • Arguably explosive movements will always be further away from the maximum potential than grinding movements due to technical difficulties and safety reserves

Ultimately all three movement patterns have their place in diversified weight training as the training stimulus for all three will be markedly different. However, for maximum strength and safety it seems to me that the moderate-speed grind is the best option.

Physics of Weight Training Part I – Fundamentals
Physics of Weight Training Part II – Explosive vs Grinding


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