I read many times here about repetition ranges but I never understood their idea. Why are there different ranges? What are they? And for what purpose should one do?
Also how much weight should one lift for each range?
People implement different rep ranges because they have different goals. An Olympic lifter who is only concerned with lifting the largest amount of weight possible would train in very low rep ranges (eg. multiple sets of 1-3 reps). Due to the low number of reps completed in each set, heavier weights can be used. This form of training is highly intensive on the Central Nervous System, and you are essentially training it to fire more efficiently. If your goal is to become faster/stronger/more explosive, heavier weight with lower rep range is the range for you. While gaining size is not the primary focus of this rep range it may occur incidentally, though not as dramatically as would happen with a higher rep range.
For muscular hypertrophy (growing the size of your muscles) more volume is required, and rep ranges of 8-10 reps are best for this goal. It might help to focus on really contracting your muscles with each rep, feeling the squeeze in the muscle group you are training. This rep range will also build strength, though not as rapidly as a lower rep range would.
An endurance athlete may perform 12-30+ reps of a certain exercise. Training in this range raises cortisol levels, and this can actually result in the loss of muscle mass. Less mass on your frame means less mass you have to move, which can be beneficial for long distance runners/triathletes/crossfitters.
If your goal is a mix of the above, by all means mix up the rep ranges implemented in your workout. It is not uncommon for people to perform low rep strength sets (e.g 5x5) followed by sets in the hypertrophy range as they are looking to gain strength and size simultaneously.
You should use a weight heavy enough so on the last few reps of your later sets your spotter has to help you. Don't sacrifice good form for reps. Good luck with your training!
cant yet comment but if you train for strength purposes you shouldn't train until failure, this zaps your CNS and that will take about 5x times longer than your musculature to bounce back.
basically 1-3 is for strength and produces more muscle activation 4-7 is combined strength and muscle building 8-12 is for building muscle (I believe making more muscle fibers) and above 12 is for endurance purposes.
The amount of reps usually also matches the weight, so lower reps higher weight en vice versa