Hey Reader,
This week, we’re diving into a key factor that impacts your endurance, power, and recovery in CrossFit.
The three trainable systems: the cardiac, musculoskeletal, and respiratory systems.
Each of these determines how efficiently you move, how long you can sustain intensity, and how quickly you recover between efforts.
Sounds like CrossFit performance, yeah?
If one of these is underdeveloped, it could be the reason you’re hitting a wall in tough workouts.
In this week's deep dive, I’ll break down how each system contributes to performance, how to identify weaknesses, and the best ways to train them for better results in the gym.
This deep dive gets pretty technical so please reply to this email if you have questions!
This Week's Training
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Day 1 - 2/3/2025
A) Power Snatch + Hang Power Snatch - 1+1; Every 2:30 x 5 sets - building
B1) Front Squat - 20X1; 4 x 2 @ 85-87.5%; rest 1' and move to B1
B2) Couch Stretch - 3 x :30e; rest 1' and move to B1
C) Metcon
3 Rounds For Time
30 Calorie Row
30 Wall Balls 20/14# to 10/9'
10 Burpee To Plate 45/25#
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Be sure to snap a pic of your training and tag @WillMurtagh_DPT on social!
Deep Dive
The 3 Systems To Train To Improve Your Metcon Performance
​CrossFit challenges your body in ways that demand more than just strength and skill.
To excel, you need a well-developed cardiac system, musculoskeletal system, and respiratory system.
These systems work together to dictate how efficiently you deliver oxygen, sustain energy production, and maintain intensity across different training zones.
If you want to improve your performance, understanding how to identify and train these systems is key.
1. The Cardiac System
Your heart’s ability to pump oxygen-rich blood directly impacts how long you can sustain high-intensity efforts and how quickly you recover between sets.
CrossFit workouts often push you into higher intensities, where efficient oxygen delivery becomes the difference between pushing through or burning out.
The stronger your heart, the more blood it can pump per beat—also known as stroke volume—which improves your ability to maintain performance in endurance training and short bursts of anaerobic effort.
One of the most significant adaptations to CrossFit is an increase in left ventricular wall thickness and cavity diameter.
This expansion allows more blood to be pumped with each contraction, improving overall cardiovascular system efficiency.
Over time, these changes help delay your anaerobic threshold, meaning you can sustain a higher level of effort before lactic acid buildup forces you to slow down. (1)
Signs of a Cardiac Limiter
Recognizing a weak cardiac system is essential if you want to refine your training intensity distribution and maximize performance.
If you notice these issues, your heart might be limiting your output:
- You fatigue early in long, steady-state workouts or struggle to maintain consistent power output.
- Your maximum heart rate climbs quickly, even at moderate intensities, and takes longer than expected to return to normal resting levels.
- You recover slowly between rounds, making it harder to sustain performance across multiple training sessions.
The primary reason for these issues is a low stroke volume (blood pumped per beat), which forces your heart to work harder to supply oxygen.
Without an efficient pump, your muscles don’t get the oxygen they need, leading to early fatigue and reduced ability to sustain anaerobic energy pathways.
How to Improve Cardiac Function
If your heart is your limiting factor, you need to focus on structured endurance training that specifically targets stroke volume and oxygen delivery.
One of the most effective methods is interval work that challenges your functional threshold power and progressively increases ventilatory threshold.
Such as through Max Aerobic Power (MAP) intervals.
This type of training creates the necessary stress to stimulate left ventricular mass growth and improve your ability to sustain a higher intensity over time.
Studies have shown that high-intensity interval training can increase left ventricular mass by 10–20% in athletes, leading to improved aerobic efficiency and greater capacity to clear carbon dioxide during exertion. (1)
To apply this effectively, track your heart rate drift test results and monitor your recovery between intervals.
Over time, you’ll notice your heart rate stabilizing at lower intensities, allowing you to push harder while staying within optimal training intensities.
2. The Musculoskeletal System
Your muscles rely on oxygen to sustain high-rep workouts and long-duration AMRAPs, making capillary density a key component of endurance. (2)
Capillaries are responsible for delivering oxygen to your mitochondria, the powerhouse of energy production in your body.
When you develop a higher capillary-to-fiber ratio, oxygen can diffuse into muscle cells more efficiently, reducing fatigue and increasing power output during training.
If your mitochondria struggle to extract enough oxygen, your body initiates compensatory angiogenesis, a natural process that increases capillary growth.
This adaptation is critical for CrossFit athletes since higher intensity efforts, such as interval workouts, demand a continuous oxygen supply.
Even if your anaerobic system kicks in during short, explosive movements, having a well-developed capillary network ensures faster recovery and better endurance over a long time.
Signs of a Muscular Limiter
If your legs feel like they’re filled with lactic acid halfway through a long chipper WOD, you may have a muscular limiter.
One major sign is early muscle fatigue, especially during steady-state workouts or long aerobic efforts.
If your quads or hamstrings burn out quickly during a long run or high-rep squats, your muscles may not be extracting enough oxygen.
Slow recovery between reps is another red flag.
If you struggle to sustain your training intensity distribution across multiple rounds, your mitochondria may not be producing ATP (energy) efficiently.
This issue is common in athletes who have excellent anaerobic capacity but struggle with aerobic efficiency.
If you feel strong in sprint-style workouts but fade fast in long rides or high-rep circuits, you likely need to improve your aerobic base training.
How to Improve Capillary Density and Mitochondrial Efficiency
One of the best ways to enhance capillary density is through high-rep endurance circuits, such as EMOM air bike sessions.
These workouts create capillary shear stress, triggering VEGF signaling, a process that stimulates new capillary growth.
By increasing your capillary network, your muscles become more efficient at using oxygen, delaying lactate inflection point and improving endurance.
Research shows that capillary density explains much of he differences in athletes' aerobic capacity.
Well-trained endurance athletes have significantly higher capillary-to-fiber ratios than untrained individuals, allowing them to sustain higher intensities for a long term without excessive fatigue. (2)
To maximize results, prioritize sustained, low-intensity work with high-intensity sessions sprinkled in.
3. The Respiratory System
Your ability to control your breathing directly impacts your power level, endurance, and overall training intensity distribution.
When you’re grinding through burpees or thrusters, your respiratory system works hard to manage gas exchange and remove carbon dioxide efficiently.
Well-trained CrossFit athletes have 30–40% higher maximal inspiratory and expiratory pressures than sedentary individuals, meaning they can take in more oxygen while minimizing respiratory fatigue. (3)
How to Improve Breathing Mechanics
One of the easiest ways to improve breathing efficiency is through breath-pacing drills.
These techniques help synchronize your breathing with movement, preventing shallow breaths that limit aerobic efficiency.
You can also use various devices to strengthen your breathing muscles.
Scientific research supports a strong correlation between maximal inspiratory pressure (MIP) and forced vital capacity (FVC) in CrossFit athletes. (3)
In other words, the stronger your respiratory muscles, the better your lungs can handle oxygen demands at higher intensity levels.
To enhance breathing control, focus on diaphragmatic breathing during both steady-state workouts and max-effort lifts.
Final Thoughts on The Three Trainable Systems
Improving your cardiac, musculoskeletal, and respiratory systems enhances endurance, power output, and recovery, allowing you to perform at higher intensities for a long time.
A structured training plan develops all three systems effectively.
If you'd like me to help identify which system may be holding you back and create a plan to improve it, book a free consultation here.
If you have questions about this week's deep dive or want a topic covered, reply to this email and let me know!
Read more about improving your aerobic conditioning for CrossFit (HERE)
This Week's Blog Post
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The 5 Best Glute Exercise Machine Options In Fitness
In this guide, you’ll discover five of the best glute exercise machines that can help you achieve your fitness goals.
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Talk to you soon, Reader
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Dr. Will Murtagh, PT, DPT, MS, CSCS, CISSN
Physical Therapist | Remote Fitness Coach
I help you train CrossFit without joint pain so that you can reach your highest potential possible.
Want help? Book your free consultation here​
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