At No Excuses CrossFit, we are a big believer in technical maxes, which is the most weight that someone can lift with minimal form breakdown. For most people this is the rep where they feel like they could do only one more rep, which you can also call having one ‘rep in reserve’.
Obviously, injury can play into this consideration, but we’d also argue that if your technique breaks down significantly then you’re not actually building strength, you’re building compensatory patterns and relying overly on bone and connective tissue to get the job done, rather than muscle.
As an example, if someone front squats 300 pounds, and maybe they have a slight hip shift and thoracic position is slightly compromised. Is it really worth going for another 5-10 pounds when we know that it’s not going to look good? That is literally the definition of ‘ego lifting’. They certainly aren’t doing their body any favors and it’s entirely to be at the top of the gym leaderboard.
We even have data to show that leaving reps in reserve can actually equate to greater strength gains than pushing ourselves past our technical max on a given day. This is also why we rarely use percentages for strength work, and encourage people to ‘Autoregulate’, which is a fancy way of saying scale their training intensity based on how they feel.
If you really want to geek out about autoregulation then checkout this article on The Journal of Strength and Conditioning Research. It shares the findings of greater strength improvements over a 12-week training program by leaving repetitions in reserve.