Each repetition of a movement, or “rep” should be at the same weight, with a certain number of reps bunched together in a given number of “sets.” “Skeletal muscles have two types of muscle fibers, slow-twitch and fast-twitch,” explains Leon Veal, head of R&D at STYRKR. To stimulate those slow-twitch fibres and build resistance, aim for 13-20 repetitions per set at a lower intensity (around 50-65 per cent of one-rep max).