Muscles

Your muscle encyclopedia — anatomy, actions, exercises, and troubleshooting for every major muscle group

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🎯 Overview

This section covers the 12 major muscle groups you train in the gym. Each page gives you everything you need to understand and train that muscle effectively.

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💪 Muscle Groups

Upper Body

Upper Body Muscles

Muscle	Location	Primary Actions	Key Exercises
Chest	Front of torso	Horizontal push, arm adduction	Bench Press, Flyes, Push-ups
Back	Posterior torso	Pulling, rowing, posture	Rows, Pull-ups, Deadlifts
Shoulders	Top of arm	Arm elevation, rotation	Overhead Press, Raises, Face Pulls
Arms	Upper arm	Elbow flexion/extension	Curls, Pushdowns, Dips

Training Balance

For healthy shoulders, maintain roughly equal volume for pushing (chest/front delt) and pulling (back/rear delt) movements.

Core

Core Muscles

Muscle	Location	Primary Actions	Key Exercises
Core	Midsection	Spinal flexion, rotation, stability	Planks, Crunches, Pallof Press

The core includes:

• Rectus Abdominis — the "six-pack" muscle
• Obliques — side abs (internal and external)
• Transverse Abdominis — deep stabilizer
• Erector Spinae — lower back muscles

Core Training

The core's primary job is stability, not movement. Include anti-movement exercises (planks, Pallof press) alongside crunches and rotations.

Lower Body

Lower Body Muscles

Muscle	Location	Primary Actions	Key Exercises
Quads	Front of thigh	Knee extension	Squats, Leg Press, Lunges
Hamstrings	Back of thigh	Knee flexion, hip extension	RDLs, Leg Curls, Good Mornings
Glutes	Buttocks	Hip extension, abduction	Hip Thrusts, Squats, Lunges
Hip Flexors	Front of hip	Hip flexion	Hanging Leg Raises, Psoas March
Adductors	Inner thigh	Hip adduction	Copenhagen Plank, Sumo Squats
Calves	Lower leg	Ankle plantarflexion	Calf Raises, Tibialis Raises

Quad/Hamstring Balance

Many lifters are quad-dominant. Include dedicated hamstring work (RDLs, leg curls) to maintain balance and protect the knees.

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🎭 Understanding Muscle Roles

Muscles don't work in isolation. In any exercise, muscles play different roles:

Role	Definition	Example (Bench Press)
Agonist	Primary mover — does the main work	Pectoralis major
Synergist	Assists the primary mover	Anterior deltoid, Triceps
Stabilizer	Holds joints steady	Rotator cuff, Core
Antagonist	Opposes the movement (must relax)	Back muscles

Why this matters:

• The same muscle is an agonist in one exercise and a synergist in another
• Synergists get trained even without direct work
• Stabilizers need attention for joint health
• Antagonist balance prevents imbalances

→ See Physiology → Muscle Roles for the complete guide.

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📊 What Each Muscle Page Includes

Every muscle page follows the same structure for easy navigation:

Section	What You'll Find	Default State
⚡ Quick Reference	At-a-glance facts (location, fiber type, primary action)	Expanded
🦴 Anatomy	Origin, insertion, fiber direction, structure	Expanded
🔗 Joints & Actions	What joints it crosses, what movements it performs	Expanded
🎭 Functional Roles	When it's agonist, synergist, stabilizer	Expanded
💪 Best Exercises	Top exercises with activation levels	Expanded
🧘 Stretches	How to lengthen the muscle	Collapsed
⚠️ Common Issues	Tightness, weakness, injuries	Collapsed
🌐 Myofascial Connections	Anatomy Trains — how it connects to other muscles	Collapsed
🔄 Related Muscles	Synergists, antagonists	Collapsed
📚 Sources	Research references	Collapsed

Navigation

Click any section header to expand/collapse. Core information is expanded by default; reference material is collapsed.

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🔥 Muscles by Training Priority

For Aesthetics (Bodybuilding)

Priority	Muscles	Why
🔴 High	Chest, Back, Shoulders, Quads, Glutes	Most visible, biggest impact
🟠 Medium	Arms, Hamstrings, Calves	Complete the look
🟡 Lower	Core, Hip Flexors, Adductors	Functional, less visible

For Strength (Powerlifting)

Priority	Muscles	Why
🔴 High	Quads, Glutes, Back, Chest	The big lifts
🟠 Medium	Hamstrings, Core, Shoulders	Supporting strength
🟡 Lower	Arms, Calves, Adductors	Accessory work

For Function (General Fitness)

Priority	Muscles	Why
🔴 High	Glutes, Core, Back	Posture, daily movement
🟠 Medium	All others equally	Balanced development

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⚡ Quick Muscle Finder

By Body Part

Front of Body:

• Chest → Chest
• Abs → Core
• Front of thigh → Quads
• Front of hip → Hip Flexors
• Front of arm → Arms (Biceps)

Back of Body:

• Upper back → Back
• Lower back → Core
• Back of thigh → Hamstrings
• Butt → Glutes
• Back of arm → Arms (Triceps)

Sides:

• Side of shoulder → Shoulders
• Side of hip → Glutes (Glute Medius)
• Inner thigh → Adductors
• Lower leg → Calves

By Exercise

If you're doing...	You're primarily working...
Bench Press	Chest, Shoulders, Arms
Rows	Back, Arms
Squats	Quads, Glutes
Deadlifts	Back, Glutes, Hamstrings
Overhead Press	Shoulders, Arms

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📊 Section Status

Muscle	Status	Notes
Chest	✅ Complete
Back	✅ Complete
Shoulders	✅ Complete
Arms	✅ Complete
Core	✅ Complete
Quads	✅ Complete
Hamstrings	✅ Complete
Glutes	✅ Complete
Hip Flexors	📋 Planned
Adductors	📋 Planned
Calves	📋 Planned
Neck	📋 Planned

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For Mo

When recommending exercises, reference this section to explain which muscles are worked and why. Use the activation levels to justify exercise selection. For users with pain or tightness, direct them to the "Common Issues" section of the relevant muscle page.