The vast majority of lifters do 8 reps per set. Most exercise theorists will tell you it's because it's a good compromise between the supposed strength-building rep range and the supposed muscle-building rep range, but it's actually a case where tradition conveniently lines up with theory.
The earliest weight trainers often did 8 reps per set, probably because doing 6 or 7 didn't tucker them out enough and doing 9 and 10 reps got to be tedious.
It wasn't until years later, though, that weight-training science sorta-kinda confirmed the 8-rep standard as being a fair compromise between strength building and muscle building. Doing anything much beyond 8 reps, like 10, 12, or even 15 has been shown to be effective for hypertrophy, but most lifters largely avoid those "high rep" sets because their mind won't allow them to accept their efficacy. They seem too easy. The weight seems too light.
But maybe it's time to re-think that distaste for high reps because some researchers at McMaster University in Canada have done a study that shows that 20-30 rep sets are just as effective, and in some cases more effective, in building muscle than low or lower-rep sets.
The researchers recruited 15 weight-trained male students and had them do leg-extension workouts on three occasions.
- The first time they did 4 sets to failure using 90% of their 1RM (the most weight they could lift for one rep).
- The second time they did 4 sets using 30% of their 1RM but not to failure; just doing as many reps as was required to match the work effort of the 90% trial.
- The third time the subjects did 4 sets to failure, again using 30% of their 1RM.
Rep speed was strictly enforced. The researchers actually set up a metronome that enforced a 1-second lifting and a 1-second lowering rep speed. Each subject was allowed to rest three minutes between sets.
The scientists took small samples of muscle tissue from all study participants just before each of the workouts, 4 hours after the workouts, and then again 24 hours after the workouts. They then tested the samples for various measurements that indicate muscle signaling and growth.
The low-load, high volume (high rep) training had similar effects as the high-load, low volume training protocol in anabolic signaling molecules, MyoD and myogenin (proteins that regulate muscle differentiation) mRNA expression, and muscle protein synthesis.
However, the high rep training activated the anabolic signal protein 4E-BP1 more than the low rep workouts, prompting the researchers to conclude:
"These results suggest that low-load high volume resistance exercise is more effective in inducing acute muscle anabolism than high-load low volume or work matched resistance exercise modes."
This is just one study, and as provocative as it might be, few exercise physiologists would recommend that you give up heavier weight, fewer-rep training protocols and replace them exclusively with lower weight, higher-rep sets.
However, it seems like a good idea to throw in a few sets of higher rep (20 to 30) sets done to failure during hypertrophy-phase workouts. For instance, you might start your leg workout with 4 sets of heavy squats (6-8 reps), follow it up with 3-4 sets of narrow-stance leg presses (8-12 reps), and finish it off with 3 sets of leg extensions (20-30 reps).
That's a horribly generic leg workout, but it should give you an idea as to how to apply the conclusions arrived at by this study.
- Burd NA et al. Low-Load High Volume Resistance Exercise Stimulates Muscle Protein Synthesis More Than High-Load Low Volume Resistance Exercise in Young Men. PLoS One. 2010 Aug 9;5(8):e12033. PubMed.