Resting longer, such as 2- 3 minutes between sets, allows you to complete more reps with a given weight, leading to greater gains in muscle size and strength. However, resting for shorter periods, such as 1 minute, leads to a higher increase in levels of insulin-like growth factor-I, which instigates muscle growth.
Weight-trained men performed 4 sets of bench presses and 4 sets of squats using 85% of their one-rep max (a weight with which they could get about 6 reps) and resting either 60, 90 or 120 seconds between sets. IGF-I levels increased only when they rested 1 minute between sets. So be sure to occasionally drop your rest down to 1 minute between sets, while keeping the weight heavy and reps low to moderate (about 6-8 per set) to boost IGF-I levels and new muscle growth.