If you're an athlete, chances are you want to become bigger, faster, and stronger. You know resistance training or lifting weights makes us better athletes, but do you know how?
Muscles adapt to stress in two ways. The first way is the muscle grows more stuff to pull. In nerd wording, the cross-sectional fibers (myofibrils or contractile proteins) of your muscles receive micro-tears from the stress of moving heavy loads, adapting and thickening the myofibrils (stuff that pull) by activating satellite cells (cells waiting to be used) and transforming them into new contractile filaments. This results in more connection points to pull with the next time you attempt to move a resistance. Your muscles are “calling for backup.” They are saying, "hey, we can't lift this on our own so let's hire more people to pull the load." This occurs during repetitions above 85% of your 1RM (repetition max).
The second way muscles adapt involves making a bigger fuel tank or increasing the sarcoplasmic reticulum. This fluid filled area surrounding the muscle becomes “flooded” with metabolic resources from high-volume, short-rest exercise using moderate resistance. Specific metabolic sources are ATP/CP, glucose, creatine, calcium, and water, and they are there to supply the muscle with energy and contraction/reaction enzymes.
In other words, your muscles run out of fuel during your set so they say, "let's hold more fuel for next time so we can go longer." This is what causes the pump/swelling effect in muscles when you are performing short rest/high volume hypertrophy and endurance-training workouts. Workouts affecting this process use rest intervals of less than 30 seconds between sets.
Those are the two means by which muscles are built. One mechanism enhances contractile ability and the other increases efficiency. By working both systems, heavy and low repetition with long rest and moderate and high repetition with short rest, we can enhance the athletic ability of both, making us faster and stronger athletes.