↓ Read top to bottom within each column · Columns = fibre types ↓
Henneman's
Size PrincipleRECRUITMENT ORDER
① TYPE I
Always first. Any force level.
→
② + TYPE IIa
Joins as intensity rises.
→
③ + TYPE IIx
Only at high/maximal force.
⚡ You cannot skip Type I and recruit Type II directly. The nervous system always escalates through the hierarchy.
To access IIx, you must first fatigue I and IIa — via very heavy loads, explosive speed, or high-rep fatigue.
This is why even light weights taken to failure build muscle: fatigue eventually forces IIx recruitment.
🟢
TYPE I
Slow Twitch
The Endurance Engine
🚛 Diesel truck — fuel-efficient, runs all day, low peak power output
Contraction speedSlow
Fatigue resistanceVery High
Mitochondria densityVery Dense
Myoglobin / O₂High — Red colour
Capillary supplyVery Dense
Force outputLow
Key enzymesCitrate synthase, SDH
RoleEndurance + posture
① Free Fatty Acids (Body Fat)
75%
Primary fuel. Type I is packed with mitochondria and fat-oxidising enzymes. Fat yields ~460 ATP per molecule — slow but enormous output over time. Stored body fat + dietary fat both contribute. This is why low-intensity exercise is ideal for fat loss.
Triglycerides → Free Fatty Acids
→ β-oxidation (mitochondria)
→ Acetyl-CoA → Krebs Cycle
→ Electron Transport Chain
→ ~460 ATP per fat molecule
Condition: Low intensity, adequate O₂ present, low insulin levels, fat-adapted state
② Blood Glucose + Glycogen
20%
Secondary fuel. Used alongside fat. As intensity rises, glucose contributes more. Goes fully aerobic in Type I — pyruvate enters Krebs cycle via Acetyl-CoA for full ATP yield (~32 ATP).
Glycogen → Glucose-6-Phosphate
→ Glycolysis → Pyruvate
→ Acetyl-CoA → Krebs Cycle → ~32 ATP
Condition: Moderate intensity range for this fibre, glycogen stores available
③ Lactate (Shuttle Fuel)
5%
Type I fibres are lactate CLEARERS — they absorb lactate produced by nearby Type II fibres and oxidise it aerobically. This is the Lactate Shuttle (Brooks). Type I is the clean-up crew for glycolytic activity.
Lactate (from Type II fibres)
→ Pyruvate (via LDH enzyme)
→ Acetyl-CoA → Krebs → ATP
Condition: High-intensity intervals nearby; Type II fibres producing lactate actively
🌿 Oxidative (Aerobic)
Duration: 2 min → hours · O₂ required: Yes
Completely oxygen-dependent. Fat and carbs fed into mitochondria via Krebs Cycle and Electron Transport Chain. Highest ATP yield of all three systems. Sustainable indefinitely if intensity is low enough. The engine that powers all endurance activities.
Fat: ~460 ATP/molecule · Glucose: ~32 ATP/molecule
Fat is more efficient — but requires oxygen and takes longer to mobilise. This is why low intensity = fat dominant.
activities that use this fibre
🚶Walking
Unlimited duration Very low intensity
Fuel: ~80% fat, ~20% carbs. Glycogen barely touched.
Best done fasted — pure fat oxidation mode. The simplest way to burn body fat daily.
🏃Easy Jog / Zone 2 Run
1–3+ hrs Low intensity
Fuel: ~65% fat, ~35% carbs. Zone 2 is the mitochondria-building zone.
This training zone upregulates fat oxidation enzymes over time. Consistent Zone 2 = better fat burner at all intensities.
🚴Cycling Zone 2
45–90 min Low–Moderate
Fuel: ~60% fat. Builds mitochondrial density and oxidative capacity.
Gold standard for metabolic health. Improves lactate clearance, fat oxidation, and cardiovascular efficiency simultaneously.
🧍Posture & Stability
All day Minimal intensity
Fuel: ~100% fat. Continuous low-level contraction all day long.
Type I works even when you're standing still. These fibres never truly rest while you're upright.
🏅Long Distance Running / Endurance
> 60 min Moderate
Fuel: Fat dominant early; shifts toward carbs near threshold. Glycogen depletion = "the wall".
Building Zone 2 base delays the crossover point — you burn more fat for longer before glycogen becomes critical.
✅Fasted training is fine — and beneficial. Fat oxidation is already dominant.
✅Low-carb or ketogenic diet works well for activities in this column.
✅Zone 2 training repeatedly upregulates PGC-1α → more mitochondria → better fat burner.
✅Post-exercise: light protein sufficient. No urgent carb window needed.
💡Consistent low-intensity training + low insulin environment = the formula to become fat-adapted over weeks.
⚠️Eating refined carbs before this work is not harmful but unnecessary — it simply delays fat mobilisation.
🟡
TYPE IIa
Fast Twitch — Oxidative Glycolytic
The Hybrid Engine
🚗 Hybrid car — can run on either fuel, adapts toward Type I with endurance training or toward IIx with power training
Contraction speedFast
Fatigue resistanceModerate
Mitochondria densityModerate
Myoglobin / O₂Moderate — Pinkish
Capillary supplyModerate
Force outputModerate to High
Key enzymesPFK + citrate synthase
PlasticityHigh — adapts both ways
Primary fuel. Glycogen stored within the fibre itself is rapidly mobilised. At moderate intensity, pyruvate enters aerobic metabolism for full ~32 ATP yield. At high intensity, glycolysis outpaces the mitochondria and lactate is produced.
Glycogen (stored in fibre)
→ Glucose-6-Phosphate → Pyruvate
→ [aerobic] Acetyl-CoA → Krebs → ~32 ATP
→ [anaerobic] Lactate + 2 ATP
Condition: Moderate–high intensity. Glycogen stores must be loaded. Depletion = immediate performance drop.
Circulating glucose from liver glycogen and dietary carbs. Insulin-mediated GLUT4 transporter expression increases with training — trained muscle takes up glucose more efficiently. Important for sustained moderate efforts beyond 60 min.
Blood glucose → GLUT4 transporter
→ enters cell → Glycolysis
→ Pyruvate → Krebs → ATP
Condition: Fed state, moderate sustained effort, insulin present to drive uptake
IIa has SOME mitochondria — unlike IIx, it can burn a small amount of fat. At the lower end of its intensity range (e.g. moderate tempo work), fat contributes meaningfully. This is why endurance training (which targets IIa heavily) improves fat oxidation over time.
Fatty acids → β-oxidation (partial)
→ Acetyl-CoA → Krebs (possible but slower
than in Type I)
Condition: Lower intensity end for this fibre, fat-adapted athlete, aerobic training background
🔥 Glycolytic + Oxidative (Mixed)
Duration: 30 sec – 60 min · O₂: Partially adequate
Both systems running simultaneously. At moderate intensity: aerobic dominates, lactate produced but cleared. At higher intensity: glycolysis outpaces aerobic capacity — lactate accumulates above lactate threshold. This is the crossover zone where glycogen becomes the performance limiter.
The lactate threshold is the point where lactate production exceeds clearance. Training this fibre pushes the threshold higher — you can sustain higher intensities aerobically before lactate floods the system.
activities that use this fibre
🏃♂️Tempo Run
20–60 min Moderate–High
Fuel: ~50% carbs / 50% fat. IIa fibre's classic training zone. Trains lactate threshold.
This zone improves how fast you can run before lactate accumulates — the single most important predictor of race performance.
💪Hypertrophy Training (8–12 rep)
30–60 sec/set High intensity
Fuel: ~85% carbs — deeply glycolytic zone. This is where carb timing matters most.
Training fasted or low-carb significantly impairs rep quality, hypertrophic stimulus, and performance. Always carb-fuel this work.
⚡5K Race Pace
15–25 min High intensity
Fuel: ~75% carbs. IIa + early IIx recruitment. Glycogen availability directly limits pace.
Pre-race carbs are essential. Glycogen depletion mid-race causes an immediate, dramatic drop in speed.
⚽Team Sports (sustained)
60–90 min Mixed
Fuel: Mixed — repeated sprints drain glycogen cumulatively across the match.
Half-time carbs help restore glycogen for 2nd half performance. Players with better glycogen loading last longer at full intensity.
🚣Rowing / CrossFit WOD
5–30 min High
Fuel: Carbs dominant, small fat contribution. Full body IIa engagement throughout.
Glycogen is the bottleneck. Post-workout carb + protein refeeding is essential for next-day performance.
🍚Carbs 1–2 hrs before training significantly improve output. Glycogen is the fuel — arrive with full stores.
🍗Protein + carbs within 1–2 hrs post-workout. The anabolic window is real for this training type.
⚠️Fasted training impairs strength, rep quality, and hypertrophic stimulus. Avoid for this column's activities.
💡This fibre adapts — consistent endurance training pushes IIa toward Type I (more fat burning). Consistent strength/power work pushes it toward IIx.
💡Carb periodisation strategy: high-carb on intense training days, lower-carb on rest/easy days. Matches fuel to demand.
✅Intra-workout carbs (30–60g/hr) help for sessions over 60–75 min where glycogen begins to deplete.
🔴
TYPE IIx
Fast Twitch — Glycolytic
The Explosive Sprinter
🚀 Nitro drag engine — maximum explosive power, burns through fuel in seconds, overheats fast, needs full recovery between efforts
Contraction speedVery Fast (fastest)
Fatigue resistanceVery Low
Mitochondria densityVery Few / Sparse
Myoglobin / O₂Low — Pale/White
Capillary supplySparse
Force outputMaximum
Key enzymePFK — glycolytic rate limiter
Fat burningZero — no mitochondria
① Phosphocreatine (PCr)
50%
First fuel used at maximal intensity. Pre-loaded in muscle as creatine phosphate. Instantly regenerates ATP with zero metabolic steps, zero oxygen, zero digestion. The body's emergency power bank. Powers the first 0–10 seconds of any maximal effort. Creatine supplementation directly increases PCr store size by 10–20%.
Creatine-Phosphate + ADP
→ Creatine + ATP (immediate)
No O₂ required · No metabolic steps
Depletes in ~8–10 sec of max effort
Replenishment: ~3–5 min full recovery
Condition: First 0–10 seconds of maximal effort only. Depletes completely. Must rest 3–5 min between max efforts.
② Muscle Glycogen (Anaerobic)
50%
After PCr depletes (~10 sec), glycolysis takes over explosively. Type IIx is packed with glycolytic enzymes — especially PFK (phosphofructokinase), the rate-limiting step. Pyruvate is produced faster than mitochondria can process it, so it converts to lactate. Only 2 ATP per glucose — wasteful but very fast. The burning sensation in your legs is this process.
Glycogen → Glucose-6-Phosphate
→ Glycolysis (fast) → Pyruvate
→ Lactate + 2 ATP (anaerobic)
O₂ NOT required
Lactate ≠ waste — it's a shuttle fuel
Condition: 10 sec – 2 min maximal effort. Lactate accumulates rapidly. Burning sensation = NAD+ regeneration keeping glycolysis running.
Cannot use fat. Type IIx has almost no mitochondria — fat oxidation (β-oxidation) is completely mitochondria-dependent. The contraction speed is simply too fast for the slow aerobic fat-burning pathway. This fibre is hardwired for explosive power, not metabolic efficiency. No exceptions.
N/A — fat oxidation requires mitochondria
Type IIx has almost none
Fat burning = impossible for this fibre
Never. Even if the athlete is fat-adapted, IIx cannot oxidise fat. This is a structural limitation, not a dietary one.
⚡ Phosphocreatine System
Duration: 0–10 seconds · O₂ required: No
Instant ATP regeneration. No metabolic steps. The only system fast enough to match the contraction rate of IIx. Fully depleted in 8–10 sec. Recovery time 3–5 minutes for 100% replenishment. Creatine supplementation increases store by 10–20%.
🔥 Glycolytic (Anaerobic)
Duration: 10 sec – 2 min · O₂ required: No
Takes over when PCr is gone. Produces ATP via glycolysis without oxygen. Fast but low yield (2 ATP/glucose). Lactate is the byproduct — not waste, but a fuel shuttled to Type I fibres, liver, and heart. The "burn" is real: it signals pyruvate→lactate conversion keeping glycolysis running by regenerating NAD⁺.
activities that use this fibre
💥100m / 200m Sprint
< 20 seconds Maximal
Fuel: PCr for first 8–10 sec, then glycolysis. Fat contributes 0%.
PCr depletes completely in ~10 sec — then glycolysis takes over. Rest 3–5 min between reps for full PCr replenishment and max quality output.
🏋️1RM / Heavy Compound Lift
< 10 sec/set Maximal
Fuel: ~70% PCr, ~30% glycogen per set. Cumulative glycogen depletion across many sets.
Creatine monohydrate is the most evidence-based supplement specifically for this work — directly increases PCr stores. Rest period = a training variable, not laziness.
🤸Plyometrics / Vertical Jump
< 3 sec Maximal
Fuel: Almost pure PCr. Duration too short for glycolysis to meaningfully contribute.
3 min between sets ensures full PCr recovery. Rushing rest = reduced power output = reduced neural adaptation to the movement.
🔥400m Run
45–60 sec Very High
Fuel: ~70% glycolytic, ~30% PCr. The most metabolically demanding event per unit time.
Lactate floods the muscle. The burning sensation is a signal that pyruvate→lactate conversion is actively regenerating NAD⁺ to keep glycolysis alive. Not a sign to stop — it's the system working.
🌀HIIT Work Intervals
10–30 sec bursts Maximal
Fuel: PCr → glycolytic each interval. Shorter rest = less PCr replenished = declining quality.
Never do HIIT fasted. Glycogen depletion is rapid. Fasted HIIT = elevated cortisol + muscle breakdown + dramatically reduced output.
⚾Throwing / Striking / Jumping
< 2 sec Maximal
Fuel: Pure PCr. Explosive single-action movements entirely PCr-dependent.
Power athletes — pitchers, throwers, jumpers — benefit most dramatically from creatine loading. Repeated power output across a game depends on PCr availability.
💊Creatine monohydrate is the single most evidence-based supplement for this fibre. 3–5g/day directly increases PCr stores by 10–20%. More fuel in the tank = more reps at max output.
⏱️3–5 min rest between sets is non-negotiable for max effort work. PCr replenishment is ~70% at 2 min, 100% at 5 min. Rushing rest = less PCr = less power.
🍚Carbs essential before and after. Glycogen is the only backup fuel once PCr depletes. Arrive with full stores.
⚠️Never train this fibre fasted. Glycogen depletion causes immediate, dramatic performance drop after the first few reps.
🍗Post-workout: aggressive protein + carb refeed. IIx fibres sustain the most mechanical damage — repair demand is highest here.
💡The "burning sensation" in a 400m or heavy set is not a reason to stop — it is lactate production regenerating NAD⁺ to keep glycolysis running. Learn to interpret it correctly.
The Crossover Concept — How Fuel Shifts With Intensity
Where fat and carbohydrate swap dominance as your intensity climbs. The crossover point sits at roughly 65–70% VO₂ max in average individuals — fat-adapted athletes push this rightward.
REST ZONE 2 MODERATE THRESHOLD MAXIMAL
0%20%40%60%80%100% VO₂
🟢 Fat
80–90% → drops to ~5% at max
🟡 Carbs
10–20% → rises to ~95% at max
🟣 PCr
0% until maximal effort
Key Concepts to Memorise
PCr System
Duration: 0–10 seconds
Fuel: Creatine phosphate (pre-loaded)
O₂: Not required
ATP yield: Tiny but instant
Recovery: 3–5 min full replenishment
Supplement: Creatine monohydrate increases stores 10–20%
Examples: 1RM lift, jump, 40m sprint start
Glycolytic System
Duration: 10 sec – 2 min
Fuel: Glycogen → Glucose → Pyruvate → Lactate
O₂: Not required
ATP yield: 2 ATP/glucose (fast but wasteful)
Byproduct: Lactate — not waste, it's a fuel
Recovery: 30–60 min lactate clearance
Examples: 400m, HIIT, repeated sprints
Oxidative System
Duration: 2 min → hours
Fuel: Fat + carbs (ratio = intensity)
O₂: Completely required
ATP yield: Fat ~460 ATP · Glucose ~32 ATP
Location: Mitochondria
Limiter: Mitochondria density + O₂ delivery
Examples: All endurance activities, walking, Zone 2
Lactate — Not the Enemy
Lactate is not "lactic acid" and is not the cause of muscle soreness (that's DOMS — delayed onset, different mechanism). Lactate is a fuel molecule produced when glycolysis outpaces aerobic capacity. It is shuttled via the Lactate Shuttle (Brooks) to the heart, liver, and Type I muscle fibres to be oxidised aerobically. The burning sensation during high intensity = NAD⁺ regeneration keeping glycolysis running.
The Crossover Point
At low intensity (<50% VO₂ max), fat is dominant (60–80%). As intensity climbs, Type IIa and IIx are recruited — these fibres have few mitochondria and cannot burn fat efficiently. Glycolysis takes over. The crossover point (~65–70% VO₂ max) is where carbs overtake fat. Fat-adapted athletes (consistent Zone 2 training) push this point rightward — burning more fat at higher intensities, sparing glycogen.
Practical Daily Decision
Low intensity (walk, Zone 2): Fasted is fine. Burns fat.
Moderate–high (tempo, hypertrophy): Carbs 1–2 hrs before. Glycogen is the limiter.
Maximal (sprints, 1RM): Carbs + creatine. 3–5 min rest between sets.
Post-workout: Match carb intake to glycogen used — heavy work needs more carbs post.