Some days you just feel better and some days you feel tired and week. For these scenarios let’s assume you had the same 1 day of rest the day before.
For weightlifting:
A) You are able to get through all of your sets with relative ease, struggling to just get your last rep of your last set for every exercise.
B) same number of exercises, sets, reps and weight, but the entire workout is a struggle. You feel like quitting every exercise, but you power through it.
For running:
A) An hour long run (let’s say 6 miles)where you are pushing yourself, but it feels easy. In the last mile your heart rate is 150.
B) An hour long run (still 6 miles) where the entire run feels like a struggle and in the last mile your heart rate is at 180.