Elite athletes in functional fitness are experiencing a striking divergence in physique.
Hyrox competitors are becoming progressively leaner and less muscular, while CrossFit athletes continue building strength and bulk.
Both disciplines share similar movements—burpees, wall balls, rowing—yet produce dramatically different body compositions.
In a recent video, senior scientist Gomar from ETH Zurich explored the fascinating biological constraints that explain why bigger muscles actually become disadvantageous for endurance performance.
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Understanding Modality Differences
Before diving into physiology, recognizing fundamental differences between these sports provides essential context. The comparison isn’t about which discipline offers better longevity or fitness outcomes.
Instead, examining what happens physiologically when athletes commit long-term to conditioning-based versus power-based functional training reveals profound insights.
Duration and Energy Systems
CrossFit events typically last 3 seconds to 20 minutes, emphasizing short-to-medium duration efforts. Hyrox competitions extend 50 to 100 minutes, demanding sustained endurance.
This time difference fundamentally alters which energy systems dominate performance. CrossFit relies heavily on creatine phosphate and glycolysis alongside aerobic metabolism, while Hyrox shifts predominantly toward aerobic phosphorylation.
Movement patterns differ significantly as well. CrossFit emphasizes vertical, sagittal plane movements, whereas Hyrox focuses on horizontal displacement across distances—excepting wall balls.
Body Composition Trends
Physical attributes reflect these demands. CrossFit athletes typically carry BMIs between 26-28, placing them in technical “overweight” categories due to substantial muscle mass. They tend toward shorter, stockier builds optimized for explosive power.
Hyrox athletes generally maintain BMIs below 24, with taller, longer-limbed physiques. Elite performers like Hunter McIntyre, Tim Vanish, and Jonathan Vitek exemplify this build—lean, rangy frames built for sustained effort rather than maximal strength.
Predictability matters too. CrossFit’s “unknown and unknowable” philosophy means competitors face surprise workouts, while Hyrox presents identical challenges at every competition.
The Biological Constraint of Muscle Size
A groundbreaking 2024 paper by Hans Degens analyzed 9,000 individual muscle fibers across species. The research uncovered crucial biological limits governing relationships between muscle fiber size, oxidative capacity, and capillary supply.
Elite endurance performance depends heavily on VO2 max and threshold speed. While heart size determines oxygen delivery systemically, muscles must also efficiently diffuse and utilize that oxygen at cellular levels.
Oxygen Diffusion Constraints
Muscle fibers sit surrounded by capillaries—tiny blood vessels delivering oxygen and removing waste products. Endurance training triggers increased capillary density around fibers, enhancing oxygen delivery to working muscles.
However, larger muscle fibers create diffusion problems. Oxygen must travel farther to reach central portions of bigger fibers compared to smaller ones. This size constraint hypothesis suggests inverse relationships between oxidative capacity and muscle fiber cross-sectional area.
Beyond oxygen, contracting muscles produce byproducts requiring removal—lactate, inorganic phosphates, hydrogen ions, ADP, reactive oxygen species, and heat. Capillaries must efficiently export these substances, which becomes increasingly difficult as fiber size expands.
The Capillary Density Ceiling
Degens’ research revealed striking patterns. Examining capillary density (capillaries per square millimeter) versus fiber cross-sectional area in rodents and humans showed a definitive ceiling.
Large fibers never achieved extremely high capillary density comparable to smaller fibers. Conversely, small fibers were never supplied by extremely low capillary density. Zero exceptions appeared in the data.
Interestingly, human samples included university students and bodybuilders who completed 9-10 week endurance programs designed to increase capillary density. Even after training adaptations, the relationship held firm.
Examining elite Hyrox athletes—who possess considerable muscle mass yet exceptional endurance capacity—could test whether this ceiling applies universally. Athletes like Hunter McIntyre or others combining size with endurance might break these patterns, though this remains unstudied.
Physical Constraints: The Square Cube Law
Beyond diffusion, pure physics imposes limitations. The square cube law dictates that as objects grow, volume increases faster than surface area.
Applied to muscle fibers, cross-sectional area (volume) increases quadratically while perimeter (surface contact with capillaries) grows only linearly. Bigger fibers literally run out of perimeter space for additional capillary support.
This physical constraint limits how many capillaries can surround enlarging muscle fibers, regardless of training adaptations or genetic potential.
Real-World Application: Body Composition Evolution
Willy George exemplifies remarkable athletic versatility across strength-endurance spectrum. He competed at CrossFit Games in 2018, 2019, and 2022 with strong placements.
After retiring from CrossFit, George focused entirely on Olympic weightlifting, clean-and-jerking 185 kilos in June 2022. Then he pivoted to Hyrox, clocking 56:29 at Paris—elite 15 caliber performance.
His body weight transformations prove illuminating:
- CrossFit (2022): 90-92 kg (BMI 27-28)
- Weightlifting: 102-104 kg (peak mass for strength)
- Hyrox: 85-86 kg (lean for endurance)
George shed nearly 20 kilograms from his weightlifting peak to compete effectively in endurance-dominant Hyrox. His BMI will likely decrease further as he optimizes for the discipline.
Research on elite CrossFit athletes shows BMIs ranging 24-25 for women and 27-28 for men among Games competitors and semifinalists. As CrossFit becomes increasingly power-dominant at elite levels, these numbers may climb higher.
Elite Hyrox males currently average BMI around 24, slightly lower for females. Expect continued decreases as the sport matures and athletes optimize body composition specifically for 50-100 minute efforts.
Why Not Minimize Body Weight Completely?
Given diffusion and physical constraints favoring smaller muscles, why don’t Hyrox athletes simply become ultra-lean like marathon runners? Several factors prevent indefinite mass reduction.
Power Output Requirements
Hyrox isn’t pure endurance. Heavy sled pushes, sled pulls, and wall balls demand absolute power generation—not just watts per kilogram, but total wattage. Insufficient muscle mass compromises performance on strength-dependent stations.
Rowing and SkiErg similarly benefit from muscle mass capable of generating high sustained power outputs.
Glycogen Storage Capacity
Muscle tissue stores glycogen—the primary fuel for moderate-to-high intensity efforts lasting 50-100 minutes. Reduced muscle volume means diminished glycogen reserves, risking depletion before race completion.
Durability and Fatigue Resistance
Durability describes maintaining high power output with minimal fatigue accumulation throughout competition. Elite endurance athletes demonstrate exceptional durability—finishing strong rather than fading dramatically.
For Hyrox specifically, wall balls at race end separate podium finishers from also-rans. Athletes depleted of glycogen or lacking muscular resilience collapse during this final test.
Greater muscle mass and physical athleticism likely enhance durability over extended efforts, though this remains somewhat hypothetical pending additional research.
Elite cyclists like Tadej Pogačar and Demi Vollering or triathletes like Kristian Blummenfelt remain lean but not emaciated. Norwegian coach Olav Alexander Bu noted his athletes performed best avoiding extreme leanness, supporting balanced approaches to body composition.
The Pareto Optimum in Functional Fitness
Athletic development seeks Pareto optimum—the ideal trade-off between competing demands. For functional fitness, this means balancing endurance capacity against strength and power.
Everything below this optimal curve represents inefficiency. Points above exceed physiological possibility—like the capillary density ceiling relative to muscle fiber size.
CrossFit, with nearly three decades of evolution, approaches its Pareto optimum. Athletes exhibit muscular, powerful builds with sufficient aerobic and glycolytic capacity for varied demands.
Hyrox remains farther from its optimal trade-off. Athletes and coaches continue discovering ideal training methodologies, body compositions, and performance strategies. Performance improvements will likely accelerate until approaching the Pareto frontier, then progress incrementally harder.
Breaking time barriers—51, 50, 49 minutes—will become exponentially more difficult as elite Hyrox athletes converge on optimal physiques and training approaches.
Practical Implications
Understanding biological constraints between muscle size and endurance capacity informs intelligent programming decisions:
- CrossFit athletes: Continued emphasis on strength and power development makes physiological sense given short, varied, high-intensity demands
- Hyrox competitors: Prioritizing cardiovascular development while maintaining functional strength optimizes performance within biological constraints
- Hybrid athletes: Recognize trade-offs when pursuing both disciplines simultaneously—physique optimal for one compromises the other
BMI provides useful shorthand when body fat remains low. Higher BMI with minimal fat indicates greater muscle mass, which helps CrossFit performance but potentially hinders Hyrox endurance.
Athletes switching between disciplines should anticipate significant body composition changes. Willy George’s 20-kilogram swing from weightlifting to Hyrox illustrates the magnitude of adaptation required for elite performance.
Training periodization must account for these realities. Building excessive muscle for Hyrox proves counterproductive beyond functional minimums needed for sled work and wall balls. Conversely, insufficient strength development limits CrossFit potential regardless of cardiovascular capacity.
The science reveals fundamental biological truths: bigger muscles face oxygen diffusion challenges and physical space constraints limiting capillary support. These aren’t mere practical inconveniences—they’re immutable physiological barriers shaping what’s possible in human performance.
