Physics of Weight Lifting – Fundamentals


I have recently read a post that argued that one should either lift ultra-slow, or as fast a possible, with everything in-between a waste of time (I will link it if I find it again). I would not normally argue that with someone twice my muscles mass and half my body fat, but his argument was based on physics, and I know one or two things about this, so I thought I’d describe this in a series of posts.

Obviously one can make things as complicated as one wants to, but for starters I’d think that for strength training the following four physical quantities are the most important ones:

  1. Force: the force a muscle has to work against. It is generally measured in Newton, and 10N is the force needed to lift 1kg*. Also, 1N is the force that – when applied for one second – accelerates a weight of 1kg to 1m/s from zero.
  2. Time (under Tension, “TUT”): the time a force is applied.
  3. Work: defined as Force x Distance. This is the energy that is transferred to the physical system (eg the energy in the kettlebell when it is 2m above the floor, and that can be released to break the bones in your toes if you happen to let it fall down).
  4. Effort: this is more of a physiological quantity than a pure physical quantity, and it is the energy the body needed to perform a certain exercise. Obviously Effort = Work + Something, where Something will be related to inefficiencies in the body’s energy transformation, as well as to static holds (see below)
  5. Power: defined as Work per unit of time.

Now one can imagine that each of the above impacts the body in a different way, and hence different combinations will yield different training results. For example, TUT is an endurance concept, and a muscle has to use different energy systems depending on the TUT (first the anaerobic one, ultimately the aerobic one for very long TUT’s). Force on the other hand is a strength concept: the more force one needs to apply, the more muscle fibers need to be recruited.

Those two concepts are complimentary: firstly it is clear that at a given force the impact will be the bigger the longer TUT is and vice versa. It is also clear for everyone who has ever exercise that one is not a substitute for the other – if not try to hold 100kg in your hands for 1sec and compare this to holding 1kg for 100sec. Not the same…

Work is a physical concept, and therefore easily measurable, and it is a proxy for the amount of effort (or work, duh) you have put into your workout. It is not the whole story though. If you are standing with 100kg on your back in the bottom squat position (the one where hips are at the same level as the knees) than you are not doing any work – but it certainly is an effort. On the other hands, when you do squats on a regular cadence, bringing 100kg up and down in 1-2sec each then arguably the physical work is a good proxy for the effort you have put it.

So in order to measure Effort one has to somehow aggregate physical work (probably multiplied with a certain factor to account for inefficient energy usage) plus (a certain multiplier times) the TUT in the stress position times the weight in the stress position.

Finally power is the work done per unit of time. In my view it is the least useful of those concepts, at least from a strength point-of-view with free weights (friction-resistance machines like a Concept rower, or a stationary bike, are different – I might address this in a later post) where Force is very similar, but more useful. Where power comes in is in all kind of metcon sessions, either training cardio-vascular endurance, or muscle-specific strength endurance. However, due to the different aerobic and anaerobic energy systems the body provides, a measurement of power needs alway to be supplemented with a measurement of TUT – exercises powered by ATP/CP will have a significantly higher power output than those powered by the aerobic system.

So to simplify I would say that fundamentally the above five measures boil down to two when considering the physiological impact:

  • The force determines what is trained, ie the higher the force applied, the more muscle fibers are recruited, and the more muscle fibers are trained
  • The effort determines the training impact, ie the more effort is put in (both in the form of work and in form of static holds) the more adaptation will occur, within the constraints of the individual to optimally recover of course

*For sticklers: this number is not exact, but it is rather like 9.81N. And of course, on the moon it is very much lower. 10 is good enough for us though.

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