Physiology
How your muscles work — the science behind muscle function, contraction, and energy production
🎯 Overview
While anatomy tells you where muscles are and what they do, physiology explains how they work. This knowledge helps you optimize training for specific goals.
Why this matters:
- Muscle roles explain how muscles work together in exercises
- Fiber types determine your training response and optimal rep ranges
- Contraction types influence exercise tempo and muscle damage
- Energy systems dictate rest periods and training adaptations
📚 Concepts
- Muscle Roles
- Fiber Types
- Contraction Types
- Energy Systems
Muscle Roles
How muscles function together during movement.
| Role | Definition | Example (Bicep Curl) |
|---|---|---|
| Agonist | Primary mover | Biceps |
| Antagonist | Opposes movement | Triceps |
| Synergist | Assists agonist | Brachialis, brachioradialis |
| Stabilizer | Holds joints steady | Rotator cuff, core |
| Neutralizer | Cancels unwanted action | Prevents forearm rotation |
| Fixator | Anchors origin | Scapular muscles |
Understanding roles helps you identify weak links. If your synergists or stabilizers are weak, your agonist can't perform optimally.
Fiber Types
The different muscle fiber types and how to train them.
| Type | Characteristics | Best For | Rep Range |
|---|---|---|---|
| Type I (Slow-twitch) | Fatigue-resistant, aerobic | Endurance | 15-20+ |
| Type IIa (Fast-twitch oxidative) | Moderate power, some endurance | Hypertrophy | 8-12 |
| Type IIx (Fast-twitch glycolytic) | High power, fast fatigue | Strength/Power | 1-6 |
Your fiber type ratio is largely genetic but training can shift Type IIx toward IIa (or vice versa).
Contraction Types
How muscles generate force.
| Type | Definition | Example | Training Effect |
|---|---|---|---|
| Concentric | Muscle shortens under load | Lifting phase of curl | Strength, skill |
| Eccentric | Muscle lengthens under load | Lowering phase of curl | Hypertrophy, strength |
| Isometric | No length change | Holding a plank | Stability, rehab |
Eccentric contractions cause more muscle damage and are stronger than concentric. This is why slow negatives build muscle and why you can lower more than you can lift.
Energy Systems
How your body fuels different activities.
| System | Duration | Fuel Source | Activities |
|---|---|---|---|
| ATP-PC (Phosphagen) | 0-10 seconds | Stored ATP, creatine | 1RM, sprints, jumps |
| Glycolytic | 10s - 2 min | Glucose/glycogen | Sets of 8-15, 400m run |
| Aerobic (Oxidative) | 2+ minutes | Fat, glucose, oxygen | Cardio, endurance |
Understanding energy systems helps you structure rest periods and training for specific goals.
🚀 Quick Start
"I want to understand how exercises work"
→ Start with Muscle Roles. Learn agonist, antagonist, synergist, and stabilizer.
"I want to optimize my rep ranges"
→ Read Fiber Types. Match training to your goals and muscle fiber composition.
"I want to know how to use tempo"
→ Read Contraction Types. Understand concentric, eccentric, and isometric phases.
"I want to understand rest periods"
→ Read Energy Systems. Match rest to the energy system you're training.
🔗 How This Connects
| Section | Relationship |
|---|---|
| Muscles | Fiber type ratios vary by muscle |
| Movement | Muscle roles change based on the movement |
| Exercise Library | Exercise technique leverages contraction types |
| Programs | Energy systems determine workout structure |
📊 Section Status
| Page | Status | Notes |
|---|---|---|
| Muscle Roles | ✅ Complete | Agonist, antagonist, synergist, stabilizer |
| Fiber Types | ✅ Complete | Type I, IIa, IIx |
| Contraction Types | ✅ Complete | Concentric, eccentric, isometric |
| Energy Systems | ✅ Complete | ATP-PC, glycolytic, aerobic |
Use physiology concepts to explain why certain training approaches work. When prescribing rep ranges, reference fiber types. When explaining rest periods, reference energy systems. When analyzing exercise technique, reference contraction types and muscle roles.