1.2 Planes of Motion & Joint Kinematics

1.2 Planes of Motion & Joint Kinematics

Kinematics is the branch of classical mechanics that describes the motion of points, bodies, and systems of bodies without consideration of the forces that cause the motion. In exercise science, kinematics encompasses both osteokinematics (the gross, observable angular movement of bones through space) and arthrokinematics (the subtle, involuntary micro-movements occurring at articular surfaces within joint capsules).

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The Anatomical Position & Spatial Reference Systems

All anatomical descriptions and kinematic directional analyses assume the subject begins in the standard anatomical position:

• The individual stands completely erect, facing forward.
• Feet are parallel, flat on the floor, and pointed forward.
• Upper limbs hang relaxed at the sides.
• Palms face anteriorly (forward) with the fingers extended and the thumbs pointing outward (laterally).

Key directional terminology referenced in movement analysis includes:

• Superior / Inferior: Toward the head or upper part of a structure versus toward the feet or lower part.
• Anterior (Ventral) / Posterior (Dorsal): Toward the front of the body versus toward the back.
• Medial / Lateral: Toward the imaginary anatomical midline of the body versus away from the midline.
• Proximal / Distal: Closer to the point of attachment or trunk versus farther away along a limb.
• Superficial / Deep: Closer to the outer surface of the body versus deeper into internal tissues.
• Ipsilateral / Contralateral: Located on the same side of the body versus located on the opposite side.
• Unilateral / Bilateral: Pertaining to one limb or side versus involving both limbs or sides symmetrically.

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The Three Cardinal Planes & Orthogonal Axes of Rotation

Human movement occurs within three mutually perpendicular (orthogonal) cardinal reference planes that intersect at the body's theoretical center of gravity. A fundamental biomechanical law dictates that angular motion within any given plane always rotates perpendicularly around an orthogonal axis of rotation.

1. The Sagittal Plane & Mediolateral Axis

• Spatial Orientation: A vertical plane passing from anterior to posterior, dividing the body into right and left segments. The midsagittal or median plane divides the body into equal symmetrical halves.
• Orthogonal Axis of Rotation: The mediolateral (frontal-horizontal) axis, which runs horizontally from side to side.
• Osteokinematic Movements:
  • Flexion: Bending movement that decreases the angle between two articulating bones (e.g., bringing forearm toward bicep; bending knee posteriorly; tilting head forward).
  • Extension: Straightening movement that increases the angle between articulating bones, returning the segment toward anatomical position.
  • Hyperextension: Continuation of extension beyond the normal anatomical plane of reference (normal at the hip and shoulder; often pathological at the knee or elbow).
  • Dorsiflexion & Plantarflexion: Specific to the talocrural ankle joint. Dorsiflexion lifts the top of the foot superiorly toward the shin; plantarflexion pushes the foot downward away from the shin (standing on tiptoes).
• Representative Exercises: Barbell back squats, deadlifts, forward lunges, bicep curls, triceps skull crushers, running strides, cycling pedal strokes.

2. The Frontal (Coronal) Plane & Anteroposterior Axis

• Spatial Orientation: A vertical plane passing laterally from side to side, dividing the body into anterior (front) and posterior (back) sections.
• Orthogonal Axis of Rotation: The anteroposterior (sagittal-horizontal) axis, which pierces the body horizontally from front to back.
• Osteokinematic Movements:
  • Abduction: Movement of a limb or segment away from the anatomical midline of the body.
  • Adduction: Movement of a limb or segment toward the anatomical midline.
  • Lateral Flexion: Side-bending of the axial skeleton (cervical, thoracic, or lumbar spine) to the right or left.
  • Scapular Elevation & Depression: Upward vertical translation of the scapula (shrugging) versus downward return.
  • Inversion & Eversion: Subtalar foot kinematics. Inversion tilts the sole of the foot inward toward the midline; eversion tilts the sole outward away from the midline.
  • Radial & Ulnar Deviation: Lateral wrist abduction toward the thumb (radius) versus medial adduction toward the fifth digit (ulna).
• Representative Exercises: Dumbbell lateral raises, side lunges, side planks, jumping jacks, lateral monster band walks, side-lying hip abduction.

3. The Transverse (Horizontal) Plane & Longitudinal Axis

• Spatial Orientation: A horizontal plane passing parallel to the horizon, dividing the body into superior (upper) and inferior (lower) portions.
• Orthogonal Axis of Rotation: The longitudinal (vertical) axis, which passes vertically from superior to inferior through the center of a joint.
• Osteokinematic Movements:
  • Internal (Medial) Rotation: Turning the anterior surface of a limb or bone inward toward the midline.
  • External (Lateral) Rotation: Turning the anterior surface of a limb or bone outward away from the midline.
  • Horizontal Adduction (Transverse Flexion): Movement of the arm or thigh in the transverse plane from an abducted position toward the anterior midline (e.g., chest fly).
  • Horizontal Abduction (Transverse Extension): Movement of the arm or thigh in the transverse plane away from the anterior midline (e.g., reverse rear delt fly).
  • Pronation & Supination: Radioulnar joint rotation. Pronation rotates the forearm so the palm faces posteriorly or downward; supination rotates the forearm so the palm faces anteriorly or upward.
  • Axial Trunk & Pelvic Rotation: Twisting the torso or pelvis to the right or left.
• Representative Exercises: Cable woodchoppers, seated dumbbell chest flyes, reverse pec-deck flyes, Russian twists, medicine ball rotational wall throws, golf swings.

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Multi-Planar Functional Movement

While isolated training often emphasizes a single cardinal plane, functional daily living and sports involve multi-planar motions that combine joint actions across all three planes simultaneously:

• Circumduction: A conical movement combining flexion, abduction, extension, and adduction sequentially at a triaxial or biaxial joint (e.g., shoulder arm circles), without true axial rotation.
• Multi-Planar Lunge with Rotation: A forward step (sagittal flexion) combined with trunk rotation (transverse plane) and lateral hip stabilization (frontal plane resistance).
• The Agility T-Drill: Requires rapid transitions from sagittal sprinting to frontal shuffling and transverse directional hip turning.

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Osteokinematics vs. Arthrokinematics

To safely prescribe resistance training, personal trainers must distinguish gross bone displacement from the micro-movements within the joint capsule:

• Osteokinematic Motion: The observable angular rotation of long bones around a joint axis, measured clinically in degrees using a goniometer or inclinometer (e.g., 0° to 135° of knee flexion).
• Arthrokinematic Motion: Involuntary, minute translatory movements occurring between the articular cartilage surfaces of diarthrodial joints. The three fundamental arthrokinematic motions are:
  1. Roll: New points on one joint surface continually contact new points on the opposing joint surface (analogous to a car tire rolling forward along a highway).
  2. Slide (Glide): A single fixed point on one moving surface contacts multiple consecutive points on the stationary surface (analogous to a locked car tire skidding across ice).
  3. Spin: A single point on one surface rotates continuously around a stationary longitudinal axis on the opposing surface (analogous to a child's toy top spinning on a tabletop).

The Convex-Concave Rule

The geometrical relationship of articulating joint surfaces dictates the direction of arthrokinematic slide relative to bone roll, known clinically as the Convex-Concave Rule:

• Convex Moving on Concave: When a convex joint surface moves on a stable, stationary concave surface, the arthrokinematic roll and slide occur in OPPOSITE directions.
  • Glenohumeral Abduction: The spherical convex humeral head rolls superiorly while simultaneously sliding inferiorly within the concave glenoid fossa. If the inferior slide is impaired (e.g., due to rotator cuff fatigue or capsule tightness), the humeral head impinges against the subacromial arch.
  • Femoral Movement in CKC Squat: When squatting down, the convex femoral condyles roll posteriorly on the tibial plateau while sliding anteriorly.
• Concave Moving on Convex: When a concave joint surface moves on a stable, stationary convex surface, the arthrokinematic roll and slide occur in the SAME direction.
  • Knee Extension in OKC Leg Extension: The concave tibial plateau rolls anteriorly and slides anteriorly on the fixed convex femoral condyles as the knee straightens.

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Synovial Joint Classification & Degrees of Freedom

Joints are structurally classified into fibrous (immovable synarthroses, e.g., skull sutures), cartilaginous (slightly movable amphiarthroses, e.g., pubic symphysis), and synovial (freely movable diarthroses). Synovial joints contain a joint capsule, synovial fluid, and hyaline cartilage, and are sub-classified by structural geometry and degrees of freedom (DoF)—the number of orthogonal planes in which a joint can rotate:

1. Ball-and-Socket (Enarthrodial): Triaxial / 3 DoF. Allows flexion/extension, abduction/adduction, and internal/external rotation. Characterized by high mobility. Examples: Glenohumeral joint and acetabulofemoral (hip) joint.
2. Hinge (Ginglymus): Uniaxial / 1 DoF. Constrained to rotation in a single plane (flexion/extension). Examples: Humeroulnar (elbow) joint and interphalangeal joints. (Note: The tibiofemoral knee joint is functionally a modified hinge / bicondylar joint because it permits axial rotation when flexed, known as the screw-home mechanism during terminal extension).
3. Pivot (Trochoid): Uniaxial / 1 DoF. Permits rotation around a longitudinal axis. Examples: Atlantoaxial joint (C1–C2, allowing head shaking "no") and proximal radioulnar joint (forearm pronation/supination).
4. Condyloid / Ellipsoid: Biaxial / 2 DoF. An oval convex condyle fits into an elliptical concave socket, permitting flexion/extension and abduction/adduction (and circumduction), but restricting axial rotation. Examples: Radiocarpal (wrist) joint and metacarpophalangeal (MCP) joints 2–5.
5. Saddle (Sellar): Biaxial / 2 DoF. Each articular surface possesses both concave and convex curvatures fitting together like a rider in a saddle. Example: First carpometacarpal (CMC) joint of the thumb (enabling thumb opposition) and sternoclavicular (SC) joint.
6. Plane / Gliding (Arthrodial): Nonaxial or multiaxial gliding. Flat articular surfaces sliding across one another without a defined rotational axis. Examples: Intercarpal joints, intertarsal joints, acromioclavicular (AC) joint, and spinal zygapophyseal (facet) joints.

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Open Kinetic Chain (OKC) vs. Closed Kinetic Chain (CKC)

The kinetic chain concept, introduced to biomechanics by Franz Reuleaux and adapted to human movement by Dr. Arthur Steindler, classifies multi-joint movement based on boundary conditions at the distal extremity:

Open Kinetic Chain (OKC)

• Definition: The distal segment of the limb (foot or hand) is free to move unconstrained through space against external resistance.
• Biomechanical Profile:
  • Motion at one joint can occur independently without requiring simultaneous angular movement at adjacent joints.
  • Agonist muscle recruitment is highly isolated, with minimal antagonist co-contraction.
  • High shear forces are generated across the joint. For instance, seated open-chain knee extensions generate significant anterior tibial shear force, stressing the anterior cruciate ligament (ACL) during terminal extension (0°–30°).
  • Low functional transfer to upright ground-reaction locomotion.
• Examples: Seated quadriceps leg extension, seated hamstring leg curl, dumbbell bicep curl, lying dumbbell triceps extension, lat pulldown.

Closed Kinetic Chain (CKC)

• Definition: The distal segment of the limb is fixed against an immovable boundary (such as the floor, a wall, or an immovable barbell platform).
• Biomechanical Profile:
  • Movement at any single joint forces predictable, interdependent angular displacement at all other links along the kinetic chain (e.g., knee flexion cannot occur in a squat without simultaneous hip flexion and ankle dorsiflexion).
  • Generates pronounced joint co-contraction of agonists and antagonists (e.g., quadriceps and hamstrings co-contracting during a barbell squat). This co-contraction dramatically elevates axial compressive forces, seating the joint surfaces firmly together.
  • Decreases joint shear forces. Hamstring co-contraction in a squat applies a posterior vector to the proximal tibia, actively shielding the ACL from anterior shear.
  • Enhances proprioceptive input from joint mechanoreceptors, vestibular pathways, and cutaneous foot mechanoreceptors, maximizing functional transfer to human athletic tasks.
• Examples: Barbell back squats, deadlifts, push-ups, lunges, pull-ups, step-ups, dips.

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Joint Kinematic Summary Table