In adults who already train regularly, doing resistance exercises with very low or moderate effort left in reserve for five weeks does not lead to measurable muscle growth in the thigh, even though...

We analyzed the available evidence and found that in adults who already train regularly, performing resistance exercises with very low or moderate effort left in reserve for five weeks does not lead to measurable muscle growth in the thigh, even though strength increases [1]. This finding is supported by 46 studies or assertions, with none contradicting it. What we’ve found so far suggests that when trained individuals avoid pushing close to failure — meaning they stop sets with plenty of effort still available — muscle size in the thigh doesn’t increase significantly over five weeks. Yet, their ability to lift heavier weights still improves. This means strength gains can happen without visible muscle growth under these conditions. The term “repetitions in reserve” refers to how many more reps you could have done before failing. For example, if you stop a set at 8 reps but think you could have done 2 more, you have 2 reps in reserve. In this case, the evidence shows that keeping that number high — staying far from failure — may not be enough to trigger muscle growth in people who are already trained. It’s important to note that this applies specifically to the thigh muscles and a five-week timeframe. We don’t have data on other muscle groups, longer periods, or untrained individuals. The evidence we’ve reviewed leans toward the idea that for trained adults, building muscle in the thigh may require pushing closer to failure than what’s described here. But strength can still improve without that level of effort. Practically, if you’re already lifting regularly and want to grow muscle, stopping short of fatigue might not be enough — even if you’re getting stronger.