Rest is essential for muscle growth because exercise creates micro tears in your muscle tissue. During rest and with proper nutrition (and adequate protein intake), cells called fibroblasts repair that damaged tissue. When the tissue heals, your muscles grow and become stronger. Your muscles grow in the rest period between sessions, so taking a rest day is important. In fact, it is ideal to wait between 48-72 hours between working the same muscle group again for your body to completely recover. Overtraining can cause unusual soreness after a workout, sometimes even lasting days. Overtraining can also cause inability to train at a previous manageable level or at the same intensity, can cause delays in recovery, and can even cause a performance plateau or decline. Ultimately, building muscle can be a slow process and can take time to see results. You may see small results in about 3-4 weeks, but you’ll see some real results after about 12 weeks of training. The way you train, nourish your body, and recover all affect the way you see results, and you should cater all of these factors towards your goals.
 

Hypertrophy happens at any rep range south of 25 or 30.. IF YOU GO TO FAILURE.. and rep ranges are heavily 8nfluenced on how you do the reps.. example.. I do a full 3 seconds down.. slight pause.. press through the concentric for 1 second.. pause .. repeat.. now doing that I fail at 8 reps.. But if I explode through the concentric and just control the negative on the way down I'm doubt a 2 second repetition.. doing it this way I can get 15 reps.. same load.. so which rep range is better?? Another example.. say I do 8 reps.. 3 second eccentric.. 2 second concentric.. my rep takes 5 seconds.. 8 reps takes 40 seconds or thereabouts.. but my partner does 8 reps but does them 1 second down.. 1 second up for a total of 16 seconds total on set.. both of us did 8 reps .. but do we think for a second the stress was the same? Reps are arbitrary.. they allow one to gauge progress as long as the reps are done the same each time.. So load and rep ranges are a bit arbitrary.. they can be manipulated a number of ways.. go to failure or close and keep track of poundages.. if joints hurt then lessen load or slow down the movements to failure.. this will lessen load on joints.. there is no better rep range.. all reps are different in time under tension and performance..

If it takes you less time
On average
To complete one repetition of an exercise
Than to read this sentence
You are performing them too quickly

Myth: Training to failure is dangerous.

Truth: There is nothing dangerous about training to failure in and of itself, the problem is when people loosen their form as they get close to failure because they are focusing on the assumed objective (completing repetitions) instead of the real objective (efficient muscular inroading).

Myth: Training to failure is counterproductive for muscular strength and size gains, you should always keep one or two reps in reserve.

Truth: Training to failure is more effective but requires a reduction in volume and frequency for most people. The problem is many assume they need some volume and frequency of training and that they must adjust their training intensity to allow for it, when instead they should train as intensely as possible and adjust their volume and frequency to allow for that.

"Most go through their workouts mindlessly with little awareness of, much less strict attention to, many important details of exercise performance. But, the lack of focus isn't even the worst part—almost all focus on the wrong goal, and their performance is wrong as a result.
Nearly every action you perform during your workouts, every small step of every exercise, can be performed in a variety of ways, and most are wrong.

About everything you do ask yourself, "What is my goal, and what is the best method for that purpose?"
When properly performed, exercise is a mindful process, not a mindless one.

Move and contract deliberately and with intense focus.

Do not flop and flail about, yanking, jerking, swinging, heaving, thrusting, throwing, dropping, bouncing, or slamming your body or the weights."

We’ve all experienced some version of pain giving way to pleasure. Perhaps like Socrates, you’ve noticed an improved mood after a period of being ill, or felt a runner’s high after exercise, or took inexplicable pleasure in a scary movie. Just as pain is the price we pay for pleasure, so too is pleasure our reward for pain.