Mitochondria are often called the “powerhouses” of the cell because they generate the majority of the cell’s energy (ATP) through oxidative metabolism. In skeletal muscle, mitochondria play a crucial role in determining your endurance capacity and how efficiently your body produces and utilises energy.
Muscle fibres can be broadly split into three main types:
Type I (slow-twitch)
Type IIa (fast oxidative-glycolytic)
Type IIb (fast glycolytic)
Each fibre type is distinct in its function, metabolic pathway preference, and adaptability to different training stimuli.
Quick Overview of Muscle Fibre Types
Type I (Slow-Twitch) Fibres
Primary function: Endurance activities (long-distance running, cycling, swimming)
Metabolic pathway: Oxidative metabolism (i.e. uses oxygen to produce ATP)
Mitochondrial content: High density of mitochondria
Fatigue resistance: Very high (they take longer to fatigue)
Power output: Lower (compared to fast-twitch fibres), but they can sustain activity for extended periods
Type IIa (Fast Oxidative-Glycolytic) Fibres
Primary function: Activities requiring both power and endurance, e.g., middle-distance running, repeated high-intensity efforts (CrossFit, HIIT)
Metabolic pathway: Uses both oxidative and glycolytic pathways (a blend of aerobic and anaerobic)
Mitochondrial content: Moderate to high (can increase significantly with training)
Fatigue resistance: Moderate (better than Type IIb, but not as good as Type I)
Power output: Higher than Type I, but lower than Type IIb
Type IIb (Fast Glycolytic) Fibres
Primary function: Explosive, powerful movements (heavy weightlifting, sprints)
Metabolic pathway: Primarily glycolytic (anaerobic)
Mitochondrial content: Lower density of mitochondria (but can increase to some degree with training)
Fatigue resistance: Low (they fatigue quickly)
Power output: Highest power output among muscle fibre types, but for shorter bursts
Heavy Weightlifting (Powerlifting, Bodybuilding-Style)
Example exercises: Low-rep squats, deadlifts, bench presses with high weight and long rest intervals.
Muscle Fibre Activation
Primarily recruits Type II (fast-twitch) fibres—especially Type IIb—for maximum force production.
Mitochondrial Adaptations
Heavy lifting is largely anaerobic and relies on the phosphocreatine and glycolytic energy systems.
While there can be some mitochondrial biogenesis (new mitochondria formation) in Type II fibres with consistent heavy training, the main adaptation is an increase in muscle fibre size (hypertrophy) and neuromuscular efficiency rather than a massive boost in mitochondrial density.
Net result: Mitochondrial density may slightly increase in Type IIa and IIb fibres but often less so than with endurance-focused training.
Performance Outcome
You gain strength and muscle mass, particularly in the fast-twitch fibres that produce high force but fatigue quickly.
CrossFit WODs (Functional, High-Intensity Mixed Modalities)
Example workouts: A mix of gymnastics, Olympic lifting, and cardio in short, intense “workouts of the day”.
Muscle Fibre Activation
CrossFit demands a hybrid of strength and endurance, activating both Type I and Type II fibres.
Frequent repeated bouts at high intensity can transform some Type IIb fibres towards Type IIa characteristics (slightly more endurance-capable).
Mitochondrial Adaptations
High-intensity, repeated efforts drive mitochondrial biogenesis in Type IIa fibres.
The variety of modalities (sprints, rowing, skipping, barbell movements) helps improve both aerobic and anaerobic capacity.
Net result: Increased mitochondrial density in Type IIa fibres, moderate improvements in aerobic capacity, and some retention of fast-twitch power.
Performance Outcome
Improved overall fitness: better work capacity across broad time domains, enhanced ability to produce power repetitively, and improved muscular endurance.
HIIT (High-Intensity Interval Training)
Example workouts: 20 seconds of all-out work (e.g., burpees, sprints on the bike, kettlebell swings, ) followed by 10–40 seconds of rest, or 30 seconds to 2 minutes of running / paddling at aerobic capacity repeated for multiple intervals with rest ratio 1:1 to 1:0.5.
Muscle Fibre Activation
High-intensity intervals recruit mostly Type II fibres, but short rest periods encourage a sustained demand for oxidative metabolism, tapping into Type I fibres too.
Mitochondrial Adaptations
HIIT has been repeatedly shown to induce significant mitochondrial biogenesis—often comparable to endurance training but achieved in a fraction of the time.
The repeated “all-out” nature, followed by brief recovery, forces your muscle cells to up-regulate oxidative enzymes and produce more mitochondria to meet energy demands quickly.
Net result: Increased mitochondrial density in both Type I and Type IIa fibres, improved aerobic and anaerobic capacity.
Performance Outcome
Enhanced VO2 max (aerobic capacity), improved lactate threshold, and better anaerobic power.
Helps retain or gain some muscle mass (especially Type IIa) due to the explosive aspect of intervals.
Sprint Intervals
Example workouts: 100–400 metre track sprints, repeated multiple times with long rests of 2–4 minutes (work: rest ratio of 1:8 to 1:4 depending on intensity)
Muscle Fibre Activation
Primarily Type IIb and IIa due to the explosive nature of sprinting.
Sprints require maximal effort, activating the fastest (but quickly fatiguing) fibres.
Mitochondrial Adaptations
While sprints are very anaerobic, repeated sprint training can increase mitochondrial content to a certain extent—especially in Type IIa fibres.
The high intensity stimulates adaptation in glycolytic capacity and also improves the muscle’s ability to clear lactate.
Net result: Noticeable boost in fast-twitch fibre efficiency. Mitochondrial density increases to handle repeated sprints, but not as prominently as with longer interval HIIT or steady-state endurance work.
Performance Outcome
Explosive speed, improved maximum velocity, and higher peak power output.
Excellent development for sports requiring short bursts of intensity.
Endurance Cardio (Long Steady-State Activities)
Example workouts: Long, moderate-effort running, cycling, rowing, or swimming for 30+ minutes at an even pace.
Muscle Fibre Activation
Primarily Type I fibres.
Type II fibres are minimally recruited at lower intensities (but they can be engaged during hill climbs or tempo surges).
Mitochondrial Adaptations
Significant boost in mitochondrial density and oxidative enzymes, but predominantly in Type I fibres.
The hallmark of endurance training is sustained oxygen demand, which signals the body to build more mitochondria for efficient ATP production.
Net result: Dramatic improvements in aerobic capacity, fatigue resistance, and efficient fat oxidation.
Performance Outcome
Enhanced ability to go for longer periods without fatigue, improved cardiovascular health, and better muscular endurance in slow-twitch fibres.
Summary of Mitochondrial Density Shifts
Heavy Weightlifting
Mild increase in mitochondrial density (mostly in Type II fibres), overshadowed by hypertrophy and neuromuscular gains.
CrossFit-Style Workouts
Moderate to high increase in mitochondrial density across both Type I and Type IIa fibres due to repeated high-intensity efforts and varied modalities.
HIIT
Known for a robust increase in mitochondrial biogenesis, affecting both Type I and Type IIa fibres significantly, enhancing both aerobic and anaerobic energy systems.
Sprint Intervals
Primarily affect Type IIb and IIa fibres; moderate increase in mitochondrial density, especially in Type IIa, but less than steady-state endurance or HIIT if the volume is low.
Endurance Cardio
Greatest overall boost in mitochondrial density in Type I fibres, significantly improving aerobic metabolic pathways and fatigue resistance.
Practical Takeaways
Balanced Fitness: If your goal is balanced fitness—strength, endurance, and power—incorporate elements of heavy weightlifting, interval training (HIIT/sprints), and some endurance work. This ensures that each muscle fibre type experiences sufficient stimulus.
Endurance Emphasis: Want to run a marathon or cycle long distances? Focus on long, steady-state sessions to maximise the mitochondrial density in Type I fibres.
Strength and Power: Looking for explosive power (e.g., powerlifting, Olympic lifting, sprinting)? Engage in heavier lifts and sprint intervals. You’ll develop high-powered Type II fibres, but you’ll see a smaller mitochondrial boost overall.
High-Intensity For Efficiency: Pressed for time? HIIT and CrossFit-style workouts provide substantial mitochondrial gains in shorter workouts and improve multiple energy systems.
Mitochondria are central to energy production, and different training methodologies place unique demands on muscle fibres, reshaping their metabolic capabilities. Incorporating a variety of training stimuli can help maximise overall fitness, as each fibre type adapts differently to different workout intensities and durations. Whether you’re looking for raw power, speed, or endless stamina, understanding how each fibre type and its mitochondria adapt to exercise can help you tailor your training for optimal results.
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