Zastria what you suggested is also correct. The additional physical work that is required when constantly changing increases by several orders of magnitude the metabolic load. It then adds to exercises such as shoulder taps, leg raises and kicks additional layers of challenge that have to do with aerobic fitness, endurance and so on. Again, this comes down to specific fitness goals and, obviously, fitness levels. Kicking, for example, when done interchangeably so you're constantly changing sides and kicking leg with each rep increases the amount of oxygen required to perform each rep because the movement recruits more muscles. The proviso is that you maintain the intensity by making the change of position not just accurate but also fast. The same principle applies to almost every other exercise.

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