When it comes to training, there are many variables we can play around with. Some of these include number of reps, amount of time, load, complexity, and load. Typically in a workout, you only want to change one variable at a time. You want your body to adapt to the variable and not overload it too much or else you will be unable to complete the exercise or progress successfully.
For example, if I want to progress my squat, I would choose either going up in weight or up in repetitions. If I decided to do both, then I may not be able to perform squats with good form. This can lead to compensations, imbalances, and even injury. Another example would be making the exercise more complex or changing the stance. If I want to make something complex then I would combine two exercises at once like going from a squat to press. As for stance or stability, I can go from a two legged stance to a single leg stance so now it’s more unstable and more challenging. I would not recommend changing both these variables at once unless you and your client are advanced.
A general rule of thumb – keep it simple! Based on your goals (whether it’s building strength or endurance) choose one variable to change and stick with it.