Ever feel like you’re just punching a clock at the gym? You hit your three sets of ten, check the box, and head home. But then you see some guy on Instagram like Chris Beardsley talking about "stimulating reps" and Reps In Reserve (RIR), and suddenly your workout feels... hollow. Like you’re missing the actual signal that makes muscles grow.
The truth is, your body doesn't care about your rep counter. It cares about tension. Specifically, it cares about the mechanical tension on high-threshold motor units.
If you want to understand the Chris Beardsley stimulus by RIR model, you have to stop thinking about "work" and start thinking about "recruitment." Honestly, most people are leaving the best gains on the table because they don't understand the difference between a rep that feels hard and a rep that actually builds muscle.
The "Effective Reps" Concept Explained
Chris Beardsley is basically the godfather of the "stimulating reps" theory. His whole premise is that not all reps are created equal.
If you pick up a weight you can lift 12 times, the first 5 or 6 reps aren't doing much for hypertrophy. Why? Because your body is efficient. It’s lazy. It’s only going to use the small, low-threshold motor units to move that weight because they’re energy-efficient. These fibers don't grow much.
To get the big, juicy Type II fibers to kick in, you need one of two things:
- Massive load (lifting something so heavy you can only do 1–5 reps).
- Fatigue (lifting a moderate weight until the easy fibers quit and the big ones have to take over).
Beardsley argues that for hypertrophy, the "stimulus" only really happens in those last 5 reps before failure. Those are the "stimulating reps."
How RIR Changes the Stimulus-to-Fatigue Ratio
This is where Reps In Reserve (RIR) comes into play. If you go to absolute, soul-crushing failure on every set, you’re definitely getting the stimulus. You’ve checked the box for all 5 stimulating reps.
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But there’s a catch.
Failure is expensive. It fries your Central Nervous System (CNS). If you do your first set of bench press to 0 RIR (failure), your second and third sets are going to suffer. You might only get 2 or 3 stimulating reps in those subsequent sets because your nervous system is too "noisy" to recruit those high-threshold units effectively.
Basically, the Chris Beardsley stimulus by RIR approach suggests that staying at 1–2 RIR might be the "sweet spot." You get 3 or 4 stimulating reps per set, but you don't accumulate the massive systemic fatigue that prevents you from doing more high-quality sets later.
Total stimulating reps for the workout? Higher.
Risk of burnout? Lower.
The Math of Muscle Growth
Think about it this way. If you do 3 sets of 10 to failure, you might get:
- Set 1: 5 stimulating reps
- Set 2: 4 stimulating reps (fatigue-limited)
- Set 3: 3 stimulating reps
Total: 12 stimulating reps.
Now, if you do 4 sets of 10 at 2 RIR:
- Set 1: 3 stimulating reps
- Set 2: 3 stimulating reps
- Set 3: 3 stimulating reps
- Set 4: 3 stimulating reps
Total: 12 stimulating reps.
In the second scenario, you did the same amount of "growth work" but you probably feel way better. You aren't stumbling out of the gym like a zombie. That’s the magic of managing stimulus via RIR.
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Why Heavy Weights Feel Different
You’ve probably noticed that doing a heavy set of 5 feels different than a grueling set of 15. Beardsley explains this through the Force-Velocity relationship.
When a weight is heavy (85% of your max), your muscle fibers are forced to shorten slowly. This slow shortening allows more cross-bridges to form between the actin and myosin filaments in your muscles. More cross-bridges = more mechanical tension = more growth signal.
When you use light weights, the fibers shorten fast. Very few cross-bridges form. No stimulus. The only way to get a stimulus with light weights is to keep going until fatigue slows the contraction speed down for you.
The Fatigue Trap
Beardsley often points out that "peripheral fatigue" (the burn in the muscle) is actually your friend. It's what forces those high-threshold motor units to wake up.
However, "central fatigue" (the brain's inability to send a strong signal to the muscles) is the enemy. When you train with 5+ RIR (leaving lots of reps in the tank), you aren't creating enough peripheral fatigue to recruit the big fibers. You’re just burning calories and wasting time.
Sorta makes you rethink those "pump" sets with the pink dumbbells, doesn't it?
Putting the Beardsley Model into Practice
So, how do you actually use this without needing a PhD in biomechanics? It's simpler than it sounds.
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First, you have to be honest with yourself. Most lifters suck at gauging RIR. They think they're at 1 RIR when they actually have 4. To use the Chris Beardsley stimulus by RIR method effectively, you occasionally have to go to total failure just to "calibrate" your internal RPE (Rate of Perceived Exertion) scale.
Once you know what 0 feels like, aim for this:
- Compound lifts (Squats, Deadlifts): Keep it around 2 RIR. The risk-to-reward ratio for failure here is trash.
- Isolations (Bicep curls, Lateral raises): You can push these to 0–1 RIR. The systemic fatigue is low, so the cost of failure is minimal.
- Volume over intensity: Instead of trying to kill yourself in one set, focus on accumulating 15–20 stimulating reps per muscle group per session.
If you’re doing 3 sets of 8–12, and you’re finishing each set feeling like you could have done 2 more "clean" reps, you’re hitting the gold standard for long-term hypertrophy.
Moving Forward with RIR
Stop counting total reps. Start counting the reps that matter. If you’re not within that 0–3 RIR window, you’re basically just doing cardio with weights.
Focus on the speed of the bar. When it starts to slow down involuntarily—even though you’re pushing as hard as possible—that’s the sign that the "stimulating reps" have begun. Capture those, manage your fatigue, and you'll find that you can actually grow faster by doing slightly "easier" looking sets.
Actionable Next Steps:
- Test your RIR: Take one set of your favorite exercise to absolute mechanical failure today. Count how many reps you thought you had left vs. what you actually did.
- Adjust your logbook: Mark your RIR for every set. Aim for a total of 15–20 "stimulating reps" (the last 5 reps of a set taken to 0-2 RIR) per muscle per workout.
- Monitor recovery: If your strength drops in the next session, increase your RIR (leave more in the tank) to manage central fatigue better.