Is RIR actually accurate?
It can be — but it isn't magic.
A 2024 study involving resistance-trained men and women found participants could estimate 1 and 3 RIR on the bench press with relatively high accuracy, with an average absolute error of less than one repetition in that particular experimental setting.
Other research shows the picture is more complicated.
Accuracy can vary with load, exercise and how close someone is to failure. Research examining several common resistance exercises found 2-RIR estimates were more accurate at heavier loads than lighter ones.
Another study found people's estimates generally became more accurate as they approached failure.
And very recent 2026 research suggests RIR estimation itself may be trainable: after six sessions, both younger and older adults improved their estimation accuracy in bench press and leg press testing.
So don't expect a new lifter's:
“Yeah mate, definitely two reps left.”
to be laboratory instrumentation.
RIR is a coaching tool.
It becomes more useful when people learn what different levels of effort actually feel like.
Does getting closer to failure build more muscle?
This is where things get interesting.
Muscle hypertrophy appears to benefit from sets being performed with sufficient effort, but the evidence does not show that every set must end in momentary muscular failure.
A systematic review and meta-analysis examining proximity to failure found no clear hypertrophy advantage for momentary muscular failure over non-failure resistance training.
Another meta-analysis found that when training volume was equated, failure training did not produce significantly greater muscle growth than non-failure training.
The practical lesson isn't:
Stay miles away from failure.
It's:
You can train very hard without needing every set to end at failure.
Why not just train every set to failure anyway?
Because failure has a cost.
Take this example:
Set 1: absolute failure
Set 2: absolute failure
Set 3: absolute failure
Set 4: existence itself now feels questionable
That fatigue can affect the amount and quality of work you can perform afterwards.
Failure training may make sense selectively, particularly where the consequences of a failed rep are small.
Taking a machine lateral raise to failure is one thing.
Grinding a technically deteriorating barbell squat until you physically cannot stand up with it is quite another.
Exercise selection matters.
Is failure more useful on isolation exercises?
Often, it can be easier to use safely and practically.
Exercises such as:
biceps curls
triceps extensions
leg extensions
lateral raises
machine-based exercises
generally make reaching failure easier to manage than technically demanding free-weight movements.
That doesn't mean isolation exercises must go to failure.
It means the cost-benefit calculation can be different.
A coach might deliberately use failure on the final set of a stable exercise while leaving one or two repetitions in reserve on a heavy compound movement.
That's programming.
Not cowardice.
What about strength?
Failure is not necessary for strength development either.
A 2021 meta-analysis found no overall advantage for failure training in maximal strength, and some analyses actually favoured non-failure training where training volume was not matched.
Strength training also involves more than exhausting muscle fibres.
Technique matters.
Movement quality matters.
Managing fatigue matters.
Practising the lift matters.
If every heavy squat session becomes an archaeological excavation to discover whether you can get yourself back out of the hole, training quality can deteriorate quickly.
Should beginners use RIR?
Yes, but with context.
A beginner may not initially know what 2 RIR feels like.
They may finish a set saying:
“That was definitely my limit.”
Then perform another six reps when encouraged.
That's normal.
Learning effort is part of learning to train.
Coaches can help clients calibrate RIR occasionally by allowing an appropriate exercise to continue further than the client's predicted stopping point.
Over time, the client starts connecting:
how the reps feel
how much velocity has slowed
how much effort is required
how many repetitions are genuinely left
Interestingly, current research doesn't support assuming that every experienced lifter automatically possesses perfect RIR perception either. Accuracy depends on context.
Can technology tell when you're near failure?
Sometimes it can provide useful information, but be careful with claims.
Velocity-based resistance training can track how quickly a bar or load is moving, and repetition velocity often slows as fatigue develops.
But translating velocity directly into an exact number of repetitions remaining isn't universally reliable.
A 2026 study examining hexagonal-bar deadlifts found that velocity-based RIR predictions did not meet the researchers' required accuracy and applicability criteria.
Earlier research has similarly found that the relationship can vary across exercises, loads, sets and individuals.
Other studies have found more promising results when individualised velocity relationships are used.
That's a lovely example of where fitness technology should be treated as information rather than an oracle.
How should a personal trainer prescribe effort?
Instead of writing:
3 × 10
and assuming that means the same thing to everybody, a coach can prescribe:
3 × 8–10 @ approximately 2 RIR
Now the client has information about both:
how much work to perform
and
how hard that work should feel.
That becomes especially useful when programmes are delivered remotely.
Strivonex Coach's Workout Studio supports RIR, tempo and individual exercise prescription, allowing coaches to include effort guidance directly within the programme rather than relying solely on a fixed repetition number.
The decision about the appropriate RIR still belongs to the coach.
The software simply helps communicate it.
So how close to failure should you train?
There isn't one mandatory RIR for every person, exercise and goal.
For muscle growth, sets generally need to provide a meaningful training stimulus, and getting reasonably close to failure can help ensure the set is sufficiently challenging.
But:
0 RIR isn't automatically superior to 1 RIR.
1 RIR isn't automatically superior to 2 RIR.
And 4 RIR isn't automatically a wasted set.
Programming depends on the exercise, load, training volume, experience, goal and where the set appears within the session or training week.
Sometimes failure is useful.
Sometimes leaving repetitions available is smarter.
Good resistance training uses both deliberately.
Quick answer
You do not need to train every set to muscular failure to build muscle or strength. Research comparing failure and non-failure resistance training has generally found no clear overall advantage to mandatory failure training, particularly when training volume is accounted for. Repetitions in reserve (RIR) can help prescribe how close a set should come to failure, although people's ability to estimate RIR varies with exercise, load and experience. Training hard matters; failing every set does not.
