Also continuously modify / rotate the movement / exercise being done. Avoids the adaptation to a specific type of movement, all else being equal. [edit] But a

Also continuously modify / rotate the movement / exercise being done. Avoids the adaptation to a specific type of movement, all else being equal. [edit] But also, speaking from many years of experience with Crossfit and military high intensity circuit training... your explanation leaves out a little bit: High intensity can be achieved in different ways depending on how your workout is structured. If a given workout specifies the movements, how much weight to use, and how many reps / sets to do, then your independent variable becomes the time it takes to complete. To increase the intensity, complete the workout faster. Thus you see in many interval workouts, one's "score" is their time. Getting better over a period of time means decreasing the time for a given workout of this structure. Second, a given workout can specify movement(s), weight, and workout time, say 20 mins. And the independent variable becomes reps and/or sets. Your score, and axis to increase intensity then is doing more reps/sets. Third, workout specifies movement, time, rep/sets. Variable / score / axis for improvement is then what weight did you use. Or, if it is some kind of increasing weight ladder within the workout (work up to X rep max)... what weight did you work up to. Lastly, one can view selection of movement(s) themselves that , put together make up a given workout, holding weight, time, and reps generally constant, as a another axis along which one can move along to avoid a plateau / adaptation to a workout one would see from repeating the same workout structure over and over. Increases in overall work capacity are achieved by improving performance along all of these different axes, for multiple different exercises / movements, as well as collections of movements that together make up specific "workouts / circuits". While not necessarily randomizing, but at least varying workout structure along these axes deliver optimal efficiency (most bang for your buck... time and effort spent in the gym) and results when results are measured in terms of GPP[0] [0] https://en.wikipedia.org/wiki/General_physical_preparedness https://en.wikipedia.org/wiki/General_physical_preparedness These are the general principles that Crossfit and other circuit style workouts follow. The key insight these programs use is not falling into one repeated structure of workout done over and over without variation, that most people naively employ when working out in the gym, that is never modified and thus quickly becomes adapted to. (Go to gym and do X reps for Y sets at Z weight. Take your time, no sense of urgency or goal of completing in as little time as possible. Time thus becomes fixed, as well as the general rep/set structure. Plateau is reached and intensity of workout is low).