Original episode & show notes | Raw transcript
The conventional understanding of muscle fibers often simplifies them into two categories: Type I (“slow-twitch”) for endurance and fat-burning, and Type II (“fast-twitch”) for sprinting and carbohydrate-burning. While this provides a basic framework, it fails to capture the intricate reality of muscle physiology, especially in trained athletes. This document delves into the nuanced relationship between muscle fiber type, metabolic capacity, and recruitment, revealing that the body’s response to exercise is far more complex and adaptable than simple charts suggest.
Muscle fibers are not a simple dichotomy. Physiologists use several methods to classify them, each offering a different perspective on their function.
Twitch Characteristics: The most basic classification is based on the contraction speed of an isolated fiber when electrically stimulated. “Slow-twitch” fibers (Type I) take longer to reach peak force, while “fast-twitch” fibers (Type II) contract more rapidly and typically produce more force.
Myosin Heavy Chain (MHC) Isoforms: This is a more precise genetic classification. The “motor” part of the muscle protein myosin, called the myosin heavy chain, exists in different versions (isoforms). In humans, these are primarily:
Type I: The slowest contracting.
Type IIa: Fast-contracting and moderately oxidative.
Type IIx: The fastest-contracting fibers in humans. (Note: Older literature or studies on rodents may mention Type IIb, which is a faster isoform not typically found in human skeletal muscle).
Hybrid Fibers: Many fibers are not “pure” but express a combination of MHC isoforms (e.g., I/IIa or IIa/IIx), representing a continuum of characteristics.
Biochemical Properties (Metabolic Profile): This classification focuses on a fiber’s primary metabolic machinery.
Slow Oxidative (SO): High oxidative (aerobic) capacity.
Fast Oxidative Glycolytic (FOG): High oxidative and high glycolytic (anaerobic) capacity.
Fast Glycolytic (FG): High glycolytic capacity with lower oxidative potential.
Crucially, these classification systems do not always align perfectly, especially after extensive training. A fiber with Type IIa MHC (genetically “fast-twitch”) can, with training, develop an extremely high oxidative capacity, making it behave metabolically like a “slow-twitch” fiber.
The central theme of the podcast is that a fiber’s genetic “type” (its MHC) does not rigidly determine its metabolic function. Evidence for this comes from several key studies.
This study provides powerful evidence by comparing the arm and leg muscles of elite skiers—athletes who require both high force and high endurance from all four limbs.
Methodology: Researchers analyzed muscle biopsies from the triceps (arms) and vastus lateralis (legs), measuring:
MHC Composition: To determine the distribution of Type I and Type II fibers.
Citrate Synthase (CS) Activity: An enzyme in the Krebs cycle, used as a marker for a muscle’s total aerobic (oxidative) capacity.
HAD (3-hydroxyacyl-CoA dehydrogenase) Activity: An enzyme essential for beta-oxidation, used as a marker for the capacity to oxidize fats.
Key Findings:
Fiber Type vs. Total Aerobic Capacity: While the legs had a higher percentage of Type I fibers than the arms (58% vs. 40% on average), the total aerobic capacity (Citrate Synthase activity) was nearly identical between the arms and legs. This demonstrates that the Type II fibers in the arms were just as aerobically developed as the Type I-dominant legs.
Fiber Type vs. Fat Oxidation: The legs showed a significantly higher capacity for fat oxidation (45% higher HAD activity) than the arms. This suggests that while Type II fibers can become highly oxidative, Type I fibers retain a superior inherent ability to metabolize fat.
Conclusion: There is a “divergence between fiber-type pattern and aerobic metabolic capacity.” In highly trained individuals, Type II fibers can and do develop an oxidative capacity on par with Type I fibers. Therefore, you cannot equate “fast-twitch” with “anaerobic.”
A common misconception is that different exercise intensities or “training zones” recruit different fiber types (e.g., Zone 2 uses only Type I). This is incorrect. Fiber recruitment is governed by a fundamental neurological rule.
Motor units (a single nerve and all the muscle fibers it controls) are recruited in a specific order based on the force required for a task, from smallest to largest.
Low Force: Small motor units, which typically control Type I fibers, are recruited first.
Increasing Force: As more force is needed, progressively larger motor units are recruited. These larger motor units tend to control Type IIa and then Type IIx fibers.
“All Hands on Deck”: Crucially, the smaller motor units are not turned off when the larger ones are activated. For a maximal effort, virtually all motor units—and thus all fiber types—are active.
This classic study examined metabolite changes in individual muscle fibers during a ramp test.
Methodology: Subjects performed a graded exercise test with muscle biopsies taken at rest, at an intensity below lactate threshold, above lactate threshold, and at exhaustion. Researchers analyzed individual fibers for metabolites like lactate, phosphocreatine, and ATP.
Key Finding: Even at the lowest intensity (well below lactate threshold), there was clear evidence of metabolic activity (e.g., changes in ATP and phosphocreatine levels) in both Type I and Type II fibers.
Conclusion: The idea that low-intensity exercise isolates Type I fibers is false. Due to factors like fatigue and the uneven force profile of movements like pedaling, the body begins to recruit Type II fibers much earlier and at lower power outputs than is commonly believed. Force demand, not metabolic state, dictates recruitment.
Understanding these concepts has several practical takeaways:
Training Adaptations Trump Genetics: While your genetic fiber type distribution provides a baseline, training is the most powerful determinant of your muscles’ metabolic capabilities. Recruiting a fiber is the stimulus for its adaptation. If you consistently recruit Type II fibers during endurance training, they will become more oxidative.
Endurance Training is Not Just for “Slow-Twitch” Fibers: When you perform a long endurance ride, you are not just training your Type I fibers. As fatigue sets in, you recruit progressively larger motor units to maintain the same power output, thereby providing an aerobic stimulus to your Type II fibers.
Fuel Use is Not a Direct Proxy for Fiber Type: Just because you are burning a high percentage of fat does not mean you are only using Type I fibers. Well-trained Type II fibers also contribute significantly to fat oxidation. Conversely, during a sprint, your highly oxidative Type I fibers are also contracting maximally and burning glycogen at a high rate.
Fiber Type Distribution is a Spectrum: Even among elite athletes, there is a vast range in fiber type distribution (from ~35% to ~85% Type I in one study of cyclists). This highlights the importance of individual adaptation and demonstrates that there are multiple physiological paths to elite performance.
In summary, the relationship between muscle fiber type and metabolism is a dynamic interplay of genetics, force demand, and training history. The most reliable principle is that the body recruits fibers based on the force required and that any fiber recruited will adapt to the stimulus it is given.