Understanding effect of repetitions on muscles and appearance

Question

According to the Guardian:

"How many reps should I do? The general rule is that using a weight that allows you to get through one to five reps builds pure strength, five to 12 helps with muscle, and anything over 12 builds muscular endurance – but there’s a fair bit of crossover."

This might be a simplification, but I considered the number of repetitions would affect whether we get fast-twitch muscle fibers (strength) or slow-twitch muscle fibers (resistance).

I find it difficult to understand hypertrophy without building muscle mass. What happen towards the physiological level? Why don't we see skinny Weightlifting champions?

So, if a person concentrated his squats, bench presses and deadlifts in the range of 1-5 repetitions, using the maximal weight he could move, would he increase his strength but not his size?

Answer 1

The guardian is wrong.

Research has consistently shown than rep range does not matter for hypertrophy, as long as sets are done sufficiently close to failure and the reps are not so ridiculously high (more than 30 reps per set) that it starts turning into cardio^{1, 2}. Sufficiently close to failure means that if you can lift 100kg for 15 reps, or 140kg for 5 reps, those two will have similar effects on hypertrophy, but lifting 100kg for 5 reps cannot be compared to lifting 140kg for 5 reps. Heavier weights for lower reps will get you better at producing maximal strength than lighter weights for higher reps, whereas lighter weights for higher reps will get you better at strength-endurance, but both will produce similar increases in muscle mass. Stronger By Science has a good write-up summarising this research.

Why don't we see skinny Weightlifting champions?

Because despite conventional gym wisdom, training with heavy weights for low repetition counts still produces increases in muscle mass.