A physiologist wants to measure the intensity of the activity of a muscle or muscle group over a certain period of time. But the physiologist cannot measure what happens inside the the muscle itself (the strain on fibres, the blood flow and nutrient #Umsatz#, etc.), so the physiologist measures what the muscles do – the duration, force and frequency of contractions – and unifies these measures into a concept of "intensity of muscular activity" or "strain".
For example, think of an office worker clicking his or her mouse button. The physiologist conceptualizes the intensitiy of muscular activity as consisting of three partial aspects: the force with which the mouse button is pressed, the duration for which the mouse button is pressed, and the frequency of pressing the mouse button.
It is important to consider both frequency and duration, because there might be a difference between pressing a button for 30 seconds and then resting for 30 seconds, and pressing a button for one second, resting for one second, and then repeating the procedure for a minute. The same overall duration of muscular activity spread differently over the same period of time might "strain" the muscle differently.
So the physiologist measures these three aspects, and then calculates the latent construct "muscular activity" or "strain" from the three measures. But what formula would the physiologist use?
I'm pretty sure someone in sports, occupational or military medicine must have modelled this construct theoretically and/or validated it experimentally, but I cannot find anything in the literature.
Does anyone here know any models for calculating amount of muscular activity over time from external measures?