The amount of weight and the number of reps are vital for success in training. You're at optimum level when you can feel the pull in your muscles without you risking injury. The ideal weight is about 70 to 85 percent of your maximum training intensity (one-rep max.). You don't need to experiment with heavier weights until you've found the maximum weight that you can do a single rep with. Get advice from a trainer or from someone you know who has a decent level of experience. With muscle building, load levels that you can lift between 8 - 12 reps. If you can't do more than 8, the weight is too heavy; if you can do more than 12, it's too light.