# What IS human exertion?August 15, 2013 12:08 AM   Subscribe

Most machines are very predictable when in motion- their movement can be graphed in terms of speed, acceleration and jerk quite easily. Generally, the graph flattens out when you get to jerk. Can the same be done with humans? Or are our movements chaotic at a fundamental level? I know that ultimately we exert motion through force (=ma) but is it possible for us to increase the rate at which we increase the rate of force, and even increase the rate of that? Would we be able to register anything on the graph of m/s5?

I have a quick question that scientists/science students, mainly biophysicists and biochemists might be able to answer. Most machines are very predictable when in motion- their movement can be graphed in terms of speed, acceleration and jerk quite easily. Generally, the graph flattens out when you get to jerk. Can the same be done with humans? Or are our movements chaotic at a fundamental level? I know that ultimately we exert motion through force (=ma) but is it possible for us to increase the rate at which we increase the rate of force, and even increase the rate of that? Would we be able to register anything on the graph of m/s5? I'm just trying to figure out how deep my effort goes when pushing my body to the limit towards the end of a training session. I guess I want to know what the essence of human exertion is.
posted by Greener_pastures to Science & Nature (4 answers total)

Jerk is only really applicable in human motion when you're changing motions (ie, you lift something and then stop lifting it.) You're not going to see a measurable constant jerk within a motion. In fact human motion attempts to minimize jerk.
posted by empath at 1:12 AM on August 15, 2013 [1 favorite]

Imagine holding yourself from a bar with your elbows bent (like the middle of a pull up). No motion is happening; there is no net force. If you were a machine, you would be consuming no energy. Yet your muscles are working really, hard, and you are probably consuming so much energy that you can only maintain that position for a minute or less. This is because your muscles are fundamentally not like a motor; they are contracting and expanding hundreds of times per second at a cellular level even when you're staying still.
posted by miyabo at 6:14 AM on August 15, 2013

A constant force implies a constant acceleration. Anything that stops and starts and stops again has non-constant acceleration, and thus non-constant force (i.e. force changes from zero to non-zero). The question is, how does force change from zero to non-zero? Does it happen instantaneously? If so, you can apply an instantly acting, constant force on an object, resulting in an instant (and constant) acceleration, with zero jerk (or any higher order terms). But if force has to somehow change continuously from zero to non-zero, then you will at least measure jerk (e.g. a linearly increasing force results in a linearly increasing acceleration via a constant jerk), or possible higher order terms (depending on the precise nature of how the force increases from zero to non-zero).

It seems reasonable to assume that the human body is not capable of generating truly instantaneous changes in force, and thus there are most certainly higher order changes in position with respect to time occurring in your workouts. But as you hint at in your question, actually measuring these changes is non-trivial, and perhaps not possible at all.
posted by grog at 8:53 AM on August 15, 2013

Jerk is very dangerous to human joints, tendons, and muscles (human motion is inefficient, and jerk, so naturally we limit jerk instinctively. Our bodies functionally limit jerk, for this reason.

They go to 24-hour care facilities when they do not.
posted by IAmBroom at 12:05 PM on August 15, 2013

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