Reps-in-reserve (RIR) is a way to approach your workouts in a way that both promotes improvement, safety, and sustainability. This means that for any capacity of lifting at various loads, a lifter can always complete a given number of reps above what is programmed, or stops their set a number of reps shy of failure. For example, if a program calls for: 3x10 (three sets of 10 reps) a lifter warms up to a weight that challenges them for 10 reps with generally proficient technique, but if they really wanted to, might be able to complete 12-14 reps with that same loading before their technique starts to significantly deteriorate, or they simply can’t complete another rep (failure). There are various ways to measure output, or intensity, but some require significantly more experience, and often, ego can get in the way of accuracy, and sustainability. Using percentage-based lifting is not always very accurate because less experienced people don’t know what they’re capable of, or, are not very proficient lifters--meaning they can lift a much higher percentage of their max for more volume than others.
Heavy is a relative (not absolute) term, something that is often misunderstood. A weight that is challenging for a lifter to maintain proficient, good quality reps for a 10RM is going to be very different than a weight that a lifter can maintain quality, and proficiency for only one rep. However, the weight will still be relatively “heavy” for the number of reps being performed within the set. The absolute weight (total weight lifted) will be very different between the two, rep maxes. The 10RM load will be “light” in comparison to the 1RM load, yet still feel “heavy” to complete 10 good quality reps with. That being said, for the purpose of strength and muscle growth, does it really matter if you are supposed to complete 10 reps of a given exercise, and you are only able to complete eight reps with good form? No. Does it mean you failed? NO! But, you need to keep track of that in order to make measurable progress over the longer term.
As a general rule, if you always keep 2-4 (RIR) you will for the most part guarantee measurable, safe, and sustainable progress. Many times people get caught up in the thought that they should always be able to do just a little more weight. This can often turn into the lifter over-shooting what they are capable of handling for a given rep range, and letting their ego get in the way. For instance, all that matters to some people occasionally if they see this;
Back Squat 70%x10x3, is hitting those sets of 10 reps that are programmed at a given weight, no matter what. Paying more attention to your ability to maintain proper positioning, and technique, rather than the amount of weight on the bar will be equally (if not more) effective, and considerably more sustainable. Some days, 70% can feel more like 80% and that needs to be taken into account. So what do you do? There really are two options, both of which are probably equally effective in terms of the progress they'll lead to. Getting back to the squat...one, reduce the load to something you can do 3x10 with, but which really feels like you could complete 12 good quality reps/ set with. This will essentially guarantee you can add weight the following week. Or, stay at the prescribed weight, and as soon as you feel like things are going to start to fall apart in another couple reps, that’s where you end the set, example; 70%x9,8,6. Then, the following week, based on feel, try to hit the prescribed weight & rep range with that same weight (70%x10,10,10) rather than adding more weight. Continue this pattern as intensity increases and volume drops, and you’ll likely get much better results.