Blaž Hrastnik - Musclemania Pro says, "The most important factor of a successful workout is the amount of neurological stimulation you can exert on the muscles. More fibers get recruited resulting in more power and inevitably more muscle damage which is what we want. On a good day you will notice weights almost lift themselves, on a bad day however you will really struggle. Although the difference in number of repetitions will probably be negligible for an observer. You on the other hand will have to put a lot more effort into it."