You’re on the trail, feeling strong. The pace is good, your pack isn’t weighing you down. You’ve been training for this every week. Then, the ground tilts upward.
Within 30 seconds, everything changes. Especially your breathing.
The sound of your own breathing explodes in your ears. Your legs, which felt fine moments ago, suddenly feel heavy, wooden, and weak. You’re not “out of shape”—you’re an athlete. Your $500 watch says your heart rate isn’t even maxed out. Yet, you’re forced to slow down, leaning on your poles, as you watch others pull ahead.
You’re gasping, but it’s more than that. You’re hitting a wall that feels like cardio, but the data doesn’t add up. You’ve told yourself the same stories: “I’m just not built for hills,” or “I need to train MORE.”
So you go back home and do more hill repeats. More Stairmaster. More running. You’ve adopted the “train harder” mentality, and all it’s given you is tired legs… and the same, familiar wall on the very next major uphill climb.
It’s one of the most maddening problems in endurance. Why do I struggle to run up hills?
Here’s the grounded truth: You’re not hitting a wall. You’re hitting a bottleneck. And that bottleneck isn’t in your legs. It’s in a single, overlooked muscle you haven’t been training: your diaphragm.
This is the hidden limiter for endurance athletes, runners, climbers, and anyone who moves on steep terrain. It’s the “weak link” that sabotages your performance – and no amount of leg strength can fix it.
The solution isn’t more grinding. It’s smarter training. It’s Respiratory Muscle Training (RMT).
The Engine That Couldn’t (Because You’re Not Training It)
What is the diaphragm, really?
We’ve been taught to think of the diaphragm in soft, abstract terms—something we “breathe into” during meditation. ”It’s that general area of your lower tummy.” This is a massive, performance-limiting misunderstanding.
Your diaphragm is a high-performance muscle. It is the primary engine of your breathing.
Picture a large, powerful parachute (or dome) of muscle and tendon at the base of your lungs, separating your chest (or thoracic) cavity from your abdomen. It’s one of the few muscles in your body that works 24/7, from your first breath to your last. Even at rest, it’s an endurance specialist, methodically contracting 15-20,000 times a day without you ever noticing.
But during exercise, it’s called on to be a power muscle. It must contract harder, faster, and deeper to move massive volumes of air. It works in tandem with your intercostal muscles (the muscles between your ribs) to do the physical work of pulling air into your lungs.
→ And the “pulling in” of air becomes that much more important at altitude, where the air pressure is lower. I.e. it takes your diaphragm much more force to pull the air into your lungs. So, it’s only natural that especially on an uphill at altitude, your diaphragm and other respiratory muscles will fatigue much faster.
But like any other skeletal muscle, it can be trained.
Why Hills Expose Your Weakest Link: The “Metaboreflex”
Let’s break down the precise physiological cascade that happens when you start running or hiking uphill.
- Metabolic Demand Skyrockets: Your big leg muscles (quads, glutes) are now fighting gravity. They are calling (loudly) for more oxygen and need to offload a massive amount of CO2.
- Breathing Rate Increases: To meet this demand, your breathing gets faster and deeper. Your diaphragm and intercostal muscles have to contract harder and more often, just to move the air. Their own oxygen consumption (the “work of breathing”) goes up dramatically as well.
- Core Demand Increases: To transfer power from your legs to your torso and to keep your posture upright on the incline, your body needs a rigid core. This demands that your diaphragm brace and stabilize.
- Lower Air Pressure: at high altitude, the air around you is less dense than at sea level (see calculator below). And it’s also less dense than the air inside your lungs. Since air moves from a HIGH pressure space → to a LOW pressure space, it requires more force to pull the outside air into your lungs.
And just like your quads after too many squats, your inspiratory muscles (the diaphragm and intercostals), without proper training, will fatigue.
The “Steal” That Kills Your Performance
This is the moment everything changes. When your diaphragm and inspiratory muscles begin to fatigue, your brain triggers a “red alert.”
Why? Because your brain can survive with leg fatigue. It’s unpleasant, but not life-threatening. Your brain cannot survive breathing failure.
In response, it activates a powerful, non-negotiable reflex called the Respiratory Metaboreflex.
This is physiological theft.
- This reflex constricts the blood vessels in your limbs—in your legs.
- It does this to redirect oxygen-rich blood away from your legs and to your fatiguing diaphragm and inspiratory muscles.
Read that again. The moment your breathing muscles trip over the fatigue threshold, your brain actively steals blood flow from your legs to save your diaphragm.
This is why your legs suddenly feel like they’ve been filled with lead. This is why you feel weak and powerless, even when you know you have more “cardio” in the tank. Your legs are “gassing out” not because they failed, but because your breathing system failed them. It’s like a bonk but not from glycogen depletion; from blood flow restriction.
You can’t “push through” this. You can’t “out-tough” a hard-wired survival reflex. You can only fix the root cause.
→ Also, note this isn’t just a “hill problem.” This same reflex is what causes you to “hit the wall” in the final mile of a 5k, in the last round of a high-intensity circuit, or during any sustained push at high altitude. Hills just get you there faster and more obviously.
What is Respiratory Muscle Training (RMT)?
This is where we shift from “training harder” to “training smarter.”
Respiratory Muscle Training (RMT) is, quite simply, strength and/or endurance training for your breathing muscles. It’s the most specific and effective way to strengthen your diaphragm and intercostals to make them more powerful and—critically—more fatigue-resistant.
You wouldn’t try to squat 300 pounds by just doing air squats, right?? You’d perform targeted exercises with resistance. Probably quite a bit of resistance to build up to 300 lbs.
→ RMT applies that same logic to your respiratory muscles.
The most common and effective form of RMT for endurance is Inspiratory Muscle Training (IMT). As the name suggests, it focuses on strengthening the muscles you use to inhale (your diaphragm and inspiratory intercostals). It’s more common because inhaling takes more force. In most cases, your exhale happens fairly naturally and passively as your intrathoracic pressure goes from higher than the ambient air outside your lungs to equal to it. This doesn’t require much force, and therefore we don’t need to focus as much on training it (except in preparation for pressure breathing, which we’ll get into in a future blog post).
How RMT Unlocks Your Uphill Endurance
RMT works by having you breathe against resistance. You can use a device that creates resistance to the airflow into your lungs – actling like a “dumbbell for your diaphragm.” For example, the SportsMask by Oxygen Advantage, AiroFit, O2 Trainer by Bas Rutten, or others. By forcing your inspiratory muscles to work against this resistance, you trigger physiological adaptations, just like lifting weights.
You can also use a resistance ‘belt’ around your rib cage (like the Buteyko Belt from Oxygen Advantage) that gives physical resistance, making it more difficult for your diaphragm to flatten out and ribcage to expand.
The benefits for uphill endurance are direct and profound:
- Increases Diaphragm Strength: Your diaphragm gets stronger. This is the primary goal.
- Delays Respiratory Muscle Fatigue: A stronger muscle takes longer to fatigue. This is the key.
- Prevents the “Steal”: By preventing your diaphragm from fatiguing, you prevent the metaboreflex from ever being triggered.
- The Result: Blood and oxygen stay in your legs, where you need them. Your “uphill wall” disappears, or rather, you push it further up the mountain.
But the benefits go even deeper. Studies on RMT have also shown:
- Reduced Perception of Effort: This is a huge psychological win. When your breathing is easier, your brain perceives the entire exercise as less strenuous, even when you’re working at the same pace.
- Improved Breathing Efficiency: Stronger muscles are more efficient. They consume less oxygen themselves, leaving more oxygen in your blood for your working legs.
- More Oxygen Saturation at High Altitude: Training your inspiratory muscles has been shown to result in less of a drop in blood oxygen saturation at high altitude, which can be a life-saver for mountaineers.
This isn’t theory. This training is used by elite athletes, climbers tackling high-altitude peaks, and runners who want to find a new gear. They’re not just training their legs; they’re training their lungs.
How to Train Your Diaphragm for Running and Climbing
So, how do you train your diaphragm for running? It’s a two-part process. You can’t just jump into “heavy lifting” without proper form. First, you master the mechanics, then you build the strength.
Step 1: The Foundation (Diaphragmatic Breathing)
Is diaphragmatic breathing good for runners? Yes. It is the absolute foundation. You can’t strengthen a muscle you don’t know how to engage.
Many of us are “chest breathers”—we use the small, inefficient muscles in our neck and shoulders (your scalenes and sternocleidomastoids) for breathing. This is a low-power, high-stress pattern that guarantees you’ll hit the metaboreflex early.
Diaphragmatic breathing (or “belly breathing”) retrains this.
- Lie on your back, knees bent.
- Place one hand on your chest, one on your belly.
- Inhale slowly through your nose, focusing on pushing your belly hand up toward the ceiling. The hand on your chest should stay still. This is the part people get wrong—they “push” their stomach out with their abs. Instead, focus on the “umbrella” feeling of your lower ribs expanding out to the sides and your belly rising as the diaphragm flattens.
- Exhale slowly (through your nose or mouth) and let the belly fall.
- Practice this for 5-10 minutes a day. The best time is right when you wake up or before sleep. It’s about building the brain-muscle connection.
Step 2: The Strength (RMT / IMT)
This is the actual training. While diaphragmatic breathing is about how you breathe, RMT is about making that “how” more powerful.
This training requires a dedicated RMT device to provide calibrated resistance. “Breathing exercises” like yoga won’t do this; it’s not enough resistance to trigger a strength adaptation.
The protocol is simple and used by top athletes:
- The “Workout”: 3 sets of 10 deep and fast inspiratory breaths against the device’s resistance. It should feel hard, like the last few reps of a heavy set at the gym.
- The “Frequency”: 1-2 times per day.
- The “Time”: The entire session takes 3-5 min.
This isn’t a huge time commitment. It’s a small, precise “dose” of training that makes the 5-hour hike possible. You’re building a stronger, more resilient respiratory engine.
Breathing Rules, Gimmicks, and What Actually Works
The endurance and wellness world is full of “breathing hacks.” It’s crucial to know the difference between a coping strategy, a relaxation technique, and real training.
The “Pacing Hacks”
- What is the 80% rule in running? (or 80/20) This is a stellar cardiovascular training principle. It states that 80% of your running should be at an “easy” (Zone 2) pace, and 20% should be “hard.” This builds your aerobic base and endurance. It is not RMT. It’s how you train your heart, not your diaphragm strength.
- What are the 10-10-10 or 4 8 12 rules? These are coping strategies for hills. “Run 10 seconds, walk 10 seconds” or “run 4 steps, breathe 8 steps…” They are ways to manage a physiological limit. RMT is how you remove that limit.
The “Nose vs. Mouth” Debate
- Do elite runners breathe through their nose? Yes, and no. Elite runners (and climbers) are masters of both.
- They breathe through their nose during the “80%” easy/moderate effort. This warms the air, filters it, and helps build CO2 tolerance (a different but related limiter, which we cover in other posts).
- At peak effort (a summit push, a final kick), they breathe through their mouth and nose—whatever it takes to get the maximum volume of oxygen and exhale CO2.
- The goal of training (both RMT and CO2 tolerance) is to raise the threshold where you are forced to mouth-breathe. A stronger diaphragm (from RMT) allows you to pull in more air through your nose at a higher intensity, keeping you more efficient, for longer.
The “Wellness Hacks”
- What is the 4 7 8 rule for breathing? This is a parasympathetic (calm-down) technique. Inhale for 4, hold for 7, exhale for 8. It’s a fantastic tool for pre-sleep or de-stressing. It is a brake, not an engine builder.
- Is Wim Hof breathing good for runners? Wim Hof Method is a powerful protocol for acute stress inoculation and (potentially) immune support. It involves intentional hyperventilation. It is not endurance breathing and is the wrong tool for aerobic performance.
These “hacks” all have a purpose. But not one of them will build the raw inspiratory muscle power you need to stop the “metaboreflex” on a steep hill. Only RMT does that.
Your First Step: Stop Guessing, Start Measuring
So, is your diaphragm your hidden limiter?
It’s the right question. But the body is a complex system. Your uphill struggle could be:
- A. Inspiratory Muscle Weakness: (The “metaboreflex” we just discussed).
- B. Low CO₂ Tolerance: A “panicky” brain response to rising CO2 that makes you feel breathless, even when your muscles are fine.
- C. Poor Mechanics: Inefficient chest breathing that wastes energy.
This is why “generic” training fails. If you have weak inspiratory muscles, no amount of CO2 tolerance training will fix it. If you have low CO2 tolerance, RMT alone won’t solve your “air hunger.”
You wouldn’t start a marathon training plan without knowing your starting pace. You have a watch that tracks your heart rate, your sleep, and your steps. So why would you start breathing training by just… guessing?
You have to diagnose the weak link.
This is precisely why we built the Recal Breath Index Assessment tool. It’s a free, data-driven diagnostic that acts as a “check engine” light for your respiratory system.
In 5 minutes, it will test your specific physiological bottlenecks. It doesn’t just give you a “breath hold time.” It uses protocols to measure your CO₂ tolerance (your “panic” sensor) and, crucially, your breathing mechanics (what we call LOM/ROM – Location of Movement / Range of Motion).
This LOM/ROM test is a functional diagnostic. It’s the indispensable first step that stops the guesswork. It’s the only tool that can “pinpoint the weak link” before you invest time in training you might not even need.
Stop Letting Your Lungs Limit Your Legs
That feeling of hitting the wall on a hill is real. But it’s not a limit on your potential. It’s a symptom of a specific, solvable problem.
You can’t “out-run” a weak diaphragm. You can’t “push through” a survival reflex.
But you can train it. You can build a stronger, more resilient respiratory system. You can stop the “steal,” keep the oxygen in your legs, and turn your greatest limiter into a new source of strength.
Your legs are ready. Your mind is ready. It’s time to get your breathing on the same level.
Let’s get to work.
Start by finding out If Your Breathing is Holding You Back?