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Lower repetitions and more weight (i.e. 4-10 reps) help to build strength and mass, with the real low reps more for strength, and slightly higher reps for muscle mass.
also make sure you're not just using low reps, but increasing the weight so that 4-6 reps is all you can actually do! That's the other important thing about building up strength and mass. It's not just the reps but the weight.