Joint Actions

The vocabulary of movement — understanding what joints do and how muscles produce motion

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🎯 Overview

Joint actions are the fundamental movements that occur at each joint in your body. Understanding this vocabulary is essential for:

• Exercise selection — knowing what movements work which muscles
• Program design — ensuring balanced training across all actions
• Communication — speaking the language of movement with precision
• Muscle targeting — choosing exercises based on specific joint actions

Why this matters:

• Every exercise can be described by the joint actions it involves
• Muscles that perform the same action work together (synergists)
• Opposite actions require opposite muscles (antagonists)
• Balanced training means training opposing actions equally

How to use this page:

1. Learn the primary actions (flexion, extension, etc.)
2. Understand which actions occur at each joint
3. Apply this knowledge to exercise selection
4. Use action pairs to ensure balanced training

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🔄 Primary Joint Actions

Flexion & Extension

The most fundamental movement pair — occurs at nearly every joint in the body.

Flexion:

• Definition: Decreasing the angle at a joint (bending)
• Exception: At the ankle, "dorsiflexion" points the toes up (technically increasing the angle)

Joint	Flexion Movement	Primary Muscles	Exercise Examples
Shoulder	Bringing arm forward/up	Anterior deltoid, Pectoralis major (clavicular)	Front raises, Overhead press (bottom to top)
Elbow	Bending the arm	Biceps brachii, Brachialis, Brachioradialis	Bicep curls, Chin-ups, Rows
Wrist	Bending wrist palm-side	Wrist flexors (forearm)	Wrist curls (palm up)
Hip	Bringing knee toward chest	Iliopsoas, Rectus femoris, Sartorius	Leg raises, Knee raises, High knees
Knee	Bending the knee	Hamstrings, Gastrocnemius	Leg curls, Nordic curls
Ankle	Pointing toes up (dorsiflexion)	Tibialis anterior	Toe raises, Ankle pumps
Spine	Bending forward	Rectus abdominis, Hip flexors	Crunches, Sit-ups

Extension:

• Definition: Increasing the angle at a joint (straightening)
• Hyperextension: Extension beyond the neutral anatomical position

Joint	Extension Movement	Primary Muscles	Exercise Examples
Shoulder	Bringing arm backward	Latissimus dorsi, Posterior deltoid, Teres major	Pull-ups, Lat pulldowns, Straight-arm pulldowns
Elbow	Straightening the arm	Triceps brachii	Tricep extensions, Pushdowns, Dips, Bench press
Wrist	Bending wrist back-side	Wrist extensors (forearm)	Reverse wrist curls
Hip	Moving thigh backward	Glutes, Hamstrings	Deadlifts, Hip thrusts, Kickbacks
Knee	Straightening the knee	Quadriceps	Squats, Leg press, Leg extensions
Ankle	Pointing toes down (plantarflexion)	Gastrocnemius, Soleus	Calf raises, Jumping
Spine	Bending backward	Erector spinae, Multifidus	Back extensions, Deadlifts

Training Balance

For joint health and balanced development, train flexion and extension with roughly equal volume. If you do 12 sets of elbow flexion (bicep work), you should do ~12 sets of elbow extension (tricep work).

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Abduction & Adduction

Moving away from or toward the body's midline.

Abduction:

• Definition: Moving a body part away from the midline
• At the shoulder/hip: Lifting the limb out to the side
• At the fingers/toes: Spreading them apart

Joint	Abduction Movement	Primary Muscles	Exercise Examples
Shoulder	Lifting arm out to side	Middle deltoid, Supraspinatus	Lateral raises, Upright rows, Wide-grip pull-ups
Hip	Lifting leg out to side	Glute medius, Glute minimus, Tensor fasciae latae	Side leg raises, Fire hydrants, Cable hip abduction
Scapula	Moving shoulder blade away from spine	Serratus anterior	Push-ups (protraction), Punching movements
Fingers	Spreading fingers apart	Dorsal interossei	Finger band spreads
Spine	Lateral flexion (side bending)	Obliques, Quadratus lumborum	Side bends, Suitcase carries

Adduction:

• Definition: Moving a body part toward the midline
• At the shoulder/hip: Bringing the limb back to the side of the body
• At the fingers/toes: Bringing them together

Joint	Adduction Movement	Primary Muscles	Exercise Examples
Shoulder	Bringing arm down/across body	Pectoralis major, Latissimus dorsi, Teres major	Flyes, Cable crossovers, Pull-ups (bringing arm down)
Hip	Bringing leg toward midline	Adductors (longus, brevis, magnus), Gracilis	Copenhagen plank, Cable hip adduction, Sumo squats
Scapula	Moving shoulder blade toward spine (retraction)	Rhomboids, Middle trapezius	Rows, Face pulls, Scapular squeezes
Fingers	Bringing fingers together	Palmar interossei	Grip exercises, Crushing movements

Functional Note

Hip abduction and adduction are crucial for lateral stability. Athletes in sports requiring cutting movements (basketball, soccer, tennis) need strong hip abductors/adductors to prevent knee injuries.

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Rotation

Turning or twisting around a joint's axis.

Internal (Medial) Rotation:

• Definition: Rotating toward the body's midline
• At the shoulder: Turning arm so thumb points inward
• At the hip: Turning thigh so toes point inward

Joint	Internal Rotation	Primary Muscles	Exercise Examples
Shoulder	Rotating arm inward	Subscapularis, Pectoralis major, Latissimus dorsi, Teres major	Internal rotation with band/cable
Hip	Rotating thigh inward	Glute medius/minimus (anterior fibers), TFL	Hip internal rotation exercises (rare in training)
Spine	Rotating torso	Obliques (opposite side)	Russian twists, Cable chops (rotation)

External (Lateral) Rotation:

• Definition: Rotating away from the body's midline
• At the shoulder: Turning arm so thumb points outward
• At the hip: Turning thigh so toes point outward

Joint	External Rotation	Primary Muscles	Exercise Examples
Shoulder	Rotating arm outward	Infraspinatus, Teres minor	Band pull-aparts, Face pulls, External rotation exercises
Hip	Rotating thigh outward	Glute maximus, Deep hip rotators (piriformis, etc.)	Clamshells, Fire hydrants, Pigeon pose
Spine	Rotating torso	Obliques (same side)	Rotational movements, Wood chops

Spinal Rotation (Left/Right):

• Left rotation: Turning torso to the left — right obliques contract
• Right rotation: Turning torso to the right — left obliques contract

Shoulder Health

The shoulder's rotator cuff muscles primarily control rotation. Most people are internally rotated from excessive pressing/slouching. Prioritize external rotation work (face pulls, band pull-aparts) for shoulder health.

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Circumduction

Circular movement that combines flexion, extension, abduction, and adduction.

• Definition: Moving a limb in a circular pattern
• Occurs at: Ball-and-socket joints (shoulder, hip)
• Does NOT involve rotation: The limb traces a cone shape but doesn't twist

Joint	Circumduction Movement	Example Movements
Shoulder	Arm circles	Shoulder circles, Swimming strokes
Hip	Leg circles	Leg swings in a circle, Skating movements
Trunk	Torso circles	Hula-hoop movements

Note: Circumduction is a combination movement, not a distinct muscle action. It's the result of muscles performing multiple actions in sequence.

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Special Actions

Certain joints have unique movements with specialized names:

Pronation & Supination

Forearm rotation (radioulnar joint)

Action	Movement	Position	Primary Muscles	Exercise Examples
Pronation	Rotating forearm so palm faces down	Palm down	Pronator teres, Pronator quadratus	Reverse curls, Overhand grip exercises
Supination	Rotating forearm so palm faces up	Palm up	Biceps brachii, Supinator	Regular curls, Underhand grip exercises

Training Application

The biceps is stronger during supination (palm up). That's why underhand chin-ups feel easier than overhand pull-ups — your biceps contribute more.

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Dorsiflexion & Plantarflexion

Ankle movement (talocrural joint)

Action	Movement	Primary Muscles	Exercise Examples
Dorsiflexion	Pointing toes toward shin (foot up)	Tibialis anterior	Toe taps, Walking on heels, Ankle pumps
Plantarflexion	Pointing toes away (foot down)	Gastrocnemius, Soleus	Calf raises, Jumping, Walking on toes

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Inversion & Eversion

Foot tilting (subtalar joint)

Action	Movement	Primary Muscles	Exercise Examples
Inversion	Tilting sole of foot inward (medially)	Tibialis anterior, Tibialis posterior	Single-leg balance (common compensation)
Eversion	Tilting sole of foot outward (laterally)	Peroneals (fibularis muscles)	Peroneal strengthening for ankle stability

Ankle Stability

Ankle sprains typically occur during excessive inversion. Strengthening the peroneals (eversion) helps prevent ankle injuries.

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Protraction & Retraction

Scapular (shoulder blade) movement

Action	Movement	Primary Muscles	Exercise Examples
Protraction	Moving shoulder blade away from spine (abduction)	Serratus anterior	Push-ups (plus), Punching movements, Protraction holds
Retraction	Moving shoulder blade toward spine (adduction)	Rhomboids, Middle trapezius	Rows, Face pulls, Scapular squeezes

Note: The scapula also performs elevation (shrugging), depression (pulling down), upward rotation (raising arm overhead), and downward rotation (lowering arm).

Posture & Shoulder Health

Most people are protracted (rounded shoulders) from desk work. Emphasize retraction exercises (rows, face pulls) to balance this out.

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Elevation & Depression

Vertical scapular movement

Action	Movement	Primary Muscles	Exercise Examples
Elevation	Raising the shoulder blade/shoulder	Upper trapezius, Levator scapulae	Shrugs, Overhead press
Depression	Lowering the shoulder blade/shoulder	Lower trapezius, Latissimus dorsi	Pull-ups, Lat pulldowns, Shoulder depressions

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Lateral Flexion

Spine side-bending

Action	Movement	Primary Muscles	Exercise Examples
Lateral flexion	Bending spine to the side	Obliques (same side), Quadratus lumborum, Erector spinae	Side bends, Suitcase carries, Side planks

Note: This is different from rotation. Lateral flexion is pure side-bending; rotation is twisting.

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🦴 Actions by Joint

Shoulder

Shoulder Joint Actions

The shoulder is the most mobile joint in the body, allowing movement in all three planes.

Action	Movement	Primary Muscles
Flexion	Arm forward/up	Anterior deltoid, Pectoralis major (clavicular)
Extension	Arm backward	Latissimus dorsi, Posterior deltoid, Teres major
Abduction	Arm out to side	Middle deltoid, Supraspinatus
Adduction	Arm down/across	Pectoralis major, Latissimus dorsi, Teres major
Horizontal Flexion	Arm across body (at shoulder height)	Pectoralis major, Anterior deltoid
Horizontal Extension	Arm backward (at shoulder height)	Posterior deltoid, Infraspinatus, Teres minor
Internal Rotation	Rotating arm inward	Subscapularis, Pectoralis major, Latissimus dorsi
External Rotation	Rotating arm outward	Infraspinatus, Teres minor
Circumduction	Circular arm movement	Combination of all actions

Training Note: The shoulder requires balanced work in all planes. Most people over-train internal rotation (pressing) and under-train external rotation (rowing, face pulls).

Elbow

Elbow Joint Actions

The elbow is a hinge joint with limited movement options.

Action	Movement	Primary Muscles
Flexion	Bending the arm	Biceps brachii, Brachialis, Brachioradialis
Extension	Straightening the arm	Triceps brachii
Pronation	Rotating forearm palm-down	Pronator teres, Pronator quadratus
Supination	Rotating forearm palm-up	Biceps brachii, Supinator

Training Note: The triceps has 3 heads and makes up 2/3 of arm mass. For balanced arm development, do 2:1 tricep:bicep volume.

Wrist

Wrist Joint Actions

The wrist allows movement in two planes.

Action	Movement	Primary Muscles
Flexion	Bending wrist palm-side	Flexor carpi radialis, Flexor carpi ulnaris
Extension	Bending wrist back-side	Extensor carpi radialis, Extensor carpi ulnaris
Radial Deviation	Bending wrist toward thumb	Flexor/Extensor carpi radialis
Ulnar Deviation	Bending wrist toward pinky	Flexor/Extensor carpi ulnaris

Training Note: Grip strength relies on wrist flexion. Balance with wrist extension work to prevent imbalances (e.g., reverse wrist curls).

Hip

Hip Joint Actions

The hip is a ball-and-socket joint allowing movement in all planes.

Action	Movement	Primary Muscles
Flexion	Knee toward chest	Iliopsoas, Rectus femoris, Sartorius
Extension	Thigh backward	Glutes, Hamstrings
Abduction	Leg out to side	Glute medius, Glute minimus, TFL
Adduction	Leg toward midline	Adductors (magnus, longus, brevis), Gracilis
Internal Rotation	Thigh/toes rotate inward	Glute medius/minimus (anterior fibers), TFL
External Rotation	Thigh/toes rotate outward	Glute maximus, Deep hip rotators (piriformis, etc.)
Circumduction	Circular leg movement	Combination of all actions

Training Note: Most gym training focuses on hip extension (squats, deadlifts). Don't neglect abduction/adduction for lateral stability and injury prevention.

Knee

Knee Joint Actions

The knee is primarily a hinge joint with one main action.

Action	Movement	Primary Muscles
Flexion	Bending the knee	Hamstrings, Gastrocnemius
Extension	Straightening the knee	Quadriceps
Internal Rotation	Slight rotation when knee is flexed	Popliteus, Semimembranosus, Semitendinosus
External Rotation	Slight rotation when knee is flexed	Biceps femoris

Training Note: The knee's primary job is extension (quads). Many lifters are quad-dominant; include dedicated hamstring work (leg curls, RDLs) for balance.

Ankle

Ankle & Foot Actions

The ankle complex allows multiple movements.

Action	Movement	Primary Muscles
Dorsiflexion	Toes toward shin	Tibialis anterior
Plantarflexion	Toes pointed down	Gastrocnemius, Soleus
Inversion	Sole of foot inward	Tibialis anterior, Tibialis posterior
Eversion	Sole of foot outward	Peroneals (fibularis muscles)

Training Note: Most people only train plantarflexion (calf raises). Train dorsiflexion (toe raises) and eversion (peroneal work) to prevent ankle sprains.

Spine

Spine Joint Actions

The spine moves in all three planes.

Action	Movement	Primary Muscles
Flexion	Bending forward	Rectus abdominis, External obliques
Extension	Bending backward	Erector spinae, Multifidus
Lateral Flexion	Side bending	Obliques (same side), Quadratus lumborum
Rotation	Twisting	Obliques (opposite side for left/right rotation)

Training Note: The spine's primary role is stability, not movement. Include anti-movement exercises (planks, carries) alongside flexion/rotation work.

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📝 Naming Exercise Actions

Every exercise can be described by the joint actions it involves. This helps you understand exactly what an exercise does and what muscles it works.

How to Name an Exercise Action

Format: [Joint] + [Action] + (optional: starting position or direction)

Examples:

Exercise	Joint Actions Description
Bicep Curl	Elbow flexion
Tricep Pushdown	Elbow extension
Lateral Raise	Shoulder abduction
Bench Press	Shoulder horizontal flexion + Elbow extension
Pull-up	Shoulder extension + Shoulder adduction + Elbow flexion
Squat	Hip flexion/extension + Knee flexion/extension + Ankle dorsiflexion
Romanian Deadlift	Hip flexion/extension (eccentric/concentric)
Cable Crossover	Shoulder horizontal flexion + Shoulder adduction
Face Pull	Shoulder horizontal extension + Shoulder external rotation + Scapular retraction
Russian Twist	Spinal rotation (alternating left/right)

Compound vs. Isolation Exercises

Isolation exercises involve movement at ONE joint:

• Bicep curl (elbow flexion only)
• Leg curl (knee flexion only)
• Lateral raise (shoulder abduction only)

Compound exercises involve movement at MULTIPLE joints:

• Bench press (shoulder + elbow)
• Squat (hip + knee + ankle)
• Pull-up (shoulder + elbow)

Exercise Selection

To target a specific muscle, choose exercises that involve the joint action(s) that muscle performs. For example, to isolate the biceps, choose an exercise that involves only elbow flexion (bicep curl), not shoulder movement.

Describing Movement Phases

Exercises have two phases:

1. Concentric — muscle shortening (the "lifting" phase)
2. Eccentric — muscle lengthening (the "lowering" phase)

Example: Bicep Curl

• Concentric: Elbow flexion (curling up)
• Eccentric: Elbow extension (lowering down)

Example: Romanian Deadlift

• Eccentric: Hip flexion (lowering the bar)
• Concentric: Hip extension (standing up)

Why this matters: Some exercises emphasize the eccentric phase (e.g., Nordic curls). Eccentric training builds strength and muscle differently than concentric work.

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⚖️ Opposite Actions (Antagonist Pairs)

For balanced training and joint health, train opposing actions with equal volume. Here are the primary antagonist action pairs:

Upper Body

Joint	Action	Antagonist Action	Balance Ratio
Shoulder	Flexion (front raises)	Extension (pulldowns)	1:1
Shoulder	Horizontal Flexion (pressing)	Horizontal Extension (rows)	1:1 to 1:2 (more pulling)
Shoulder	Internal Rotation	External Rotation	1:2 (more external rotation)
Elbow	Flexion (bicep curls)	Extension (tricep work)	1:2 (more tricep)
Scapula	Protraction (push-ups)	Retraction (rows)	1:1 to 1:2 (more retraction)
Scapula	Elevation (shrugs)	Depression (pulldowns)	1:1

Lower Body

Joint	Action	Antagonist Action	Balance Ratio
Hip	Flexion (leg raises)	Extension (hip thrusts)	1:3 (much more extension)
Hip	Abduction (side leg raises)	Adduction (Copenhagen plank)	1:1
Knee	Flexion (leg curls)	Extension (leg extensions)	1:1 to 2:3 (slightly more extension)
Ankle	Dorsiflexion (toe raises)	Plantarflexion (calf raises)	1:3 (more calf work)

Spine

Joint	Action	Antagonist Action	Balance Ratio
Spine	Flexion (crunches)	Extension (back extensions)	1:1
Spine	Rotation (left)	Rotation (right)	1:1 (train both sides equally)
Spine	Lateral Flexion (left)	Lateral Flexion (right)	1:1 (train both sides equally)

Common Imbalances

Most people have these imbalances:

• Too much pressing, not enough rowing (shoulder internal rotation dominance)
• Too much quad work, not enough hamstring work (quad dominance)
• Too much abs, not enough lower back (though the core's job is stability, not movement)
• Too much calf work, not enough tibialis work (plantarflexion dominance)

Check your program against these action pairs to ensure balance.

How to Use This

Step 1: List all exercises in your program Step 2: Identify the primary joint actions for each exercise Step 3: Count sets per action (e.g., total sets of elbow flexion vs. extension) Step 4: Adjust volume to match the recommended ratios

Example:

You do:

• Bench Press (4 sets) — shoulder horizontal flexion
• Overhead Press (3 sets) — shoulder flexion
• Flyes (3 sets) — shoulder horizontal flexion

Total shoulder horizontal flexion: 7 sets (bench + flyes) Total shoulder flexion: 3 sets (overhead press)

You should do:

• 7-14 sets of shoulder horizontal extension (rows)
• 3 sets of shoulder extension (pulldowns, pull-ups)

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📚 Sources

Academic Sources

• Neumann, D. A. (2017). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation (3rd ed.). Elsevier. — The definitive textbook on joint actions and muscle function.
• Hamill, J., & Knutzen, K. M. (2015). Biomechanical Basis of Human Movement (4th ed.). Wolters Kluwer. — Comprehensive biomechanics reference.
• Muscolino, J. E. (2017). Kinesiology: The Skeletal System and Muscle Function (3rd ed.). Elsevier. — Detailed muscle and joint action reference.

Applied Kinesiology

• Kendall, F. P., McCreary, E. K., Provance, P. G., Rodgers, M. M., & Romani, W. A. (2005). Muscles: Testing and Function with Posture and Pain (5th ed.). Lippincott Williams & Wilkins. — Classic reference for muscle actions and testing.

Online Resources

• ExRx.net — Exercise and muscle directory with detailed joint action information
• Visible Body — 3D anatomy platform showing joint movements
• GetBodySmart — Interactive tutorials on joint actions and kinesiology

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For Mo

Use joint action terminology when explaining exercises to users. For example, instead of saying "this works your biceps," say "this exercise involves elbow flexion, which is performed by the biceps." When helping users choose exercises, reference the joint actions that muscle performs. When checking program balance, use the antagonist action pairs to ensure they're training opposing movements equally.