4.4 Kinesiology, Biomechanics & Movement Analysis

4.4 Kinesiology, Biomechanics & Movement Analysis

Quick Summary: Biomechanics applies principles of mechanical physics to human movement. Personal trainers must master anatomical planes of motion, axes of rotation, internal and external lever systems, torque mechanics, and kinetic chain classifications. This foundational knowledge allows trainers to evaluate exercise form, quantify mechanical stress, modify leverage to accommodate client needs, and prescribe safe, effective movement patterns.

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1. Anatomical Planes of Motion and Axes of Rotation

All human movement is analyzed relative to the standard Anatomical Position (standing upright, facing forward, arms at sides, palms facing forward). Movement occurs in three cardinal spatial Planes, each rotating around a corresponding perpendicular Axis of Rotation.

1. Sagittal Plane

• Description: Divides the body vertically into right and left halves.
• Axis of Rotation: Frontal (Coronal / Mediolateral) Axis, running horizontally from side to side.
• Joint Movements: Flexion (reducing joint angle), Extension (increasing joint angle), Hyperextension, Dorsiflexion, and Plantarflexion.
• Exercise Examples: Barbell squat, Forward lunge, Romanian deadlift, Biceps curl, Running, Triceps pushdown.

2. Frontal (Coronal) Plane

• Description: Divides the body vertically into anterior (front) and posterior (back) halves.
• Axis of Rotation: Sagittal (Anteroposterior) Axis, running horizontally from front to back.
• Joint Movements: Abduction (movement away from body midline), Adduction (movement toward body midline), Lateral flexion of the spine, Foot Inversion and Eversion, Scapular Elevation and Depression.
• Exercise Examples: Dumbbell lateral raise, Side lunge, Side plank, Standing calf raise (inversion/eversion control), Jumping jacks.

3. Transverse (Horizontal) Plane

• Description: Divides the body horizontally into superior (upper) and inferior (lower) halves.
• Axis of Rotation: Transverse (Longitudinal / Vertical) Axis, running vertically from top to bottom.
• Joint Movements: Internal (medial) rotation, External (lateral) rotation, Horizontal abduction, Horizontal adduction, Forearm Pronation and Supination, Spinal rotation.
• Exercise Examples: Seated chest fly, Cable woodchopper, Seated torso rotation, Dumbbell reverse fly, Bench press (shoulder horizontal adduction).

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2. Lever Systems in the Human Body

Biological movement is produced by anatomical lever systems. A lever is a rigid structure (bone) that turns about a pivot point or fulcrum (joint axis) when force (muscle contraction effort) is applied to overcome a load (resistance of body weight or external implement).

Levers are categorized into three classes based on the relative arrangement of the Fulcrum ($F$), Effort Force ($E$), and Resistance Load ($R$):

1st Class Lever:  [ Resistance Load (R) ] <--- [ Fulcrum (F) ] ---> [ Effort Force (E) ]
2nd Class Lever:  [ Fulcrum (F) ] <--- [ Resistance Load (R) ] ---> [ Effort Force (E) ]
3rd Class Lever:  [ Fulcrum (F) ] <--- [ Effort Force (E) ] ---> [ Resistance Load (R) ]

First-Class Levers ($R - F - E$)

• Structure: The Fulcrum lies between the Effort force and the Resistance load.
• Function: Serves primarily for balance, equilibrium, and changing the direction of applied force. Mechanical advantage can be $>1$, $=1$, or $<1$ depending on arm lengths.
• Anatomical Examples: The atlanto-occipital joint of the neck (cervical extensors supply effort, joint is fulcrum, head weight is resistance) and elbow triceps extension.

Second-Class Levers ($F - R - E$)

• Structure: The Resistance load lies between the Fulcrum and the Effort force. The effort arm is always longer than the resistance arm.
• Function: Force multiplication. Yields a Mechanical Advantage $>1$, allowing a small muscle force to move a heavy resistance, but sacrificing movement speed and range of motion.
• Anatomical Example: Standing Plantarflexion at the ankle (fulcrum is the metatarsophalangeal joints, body weight acts as resistance through the ankle joint in the middle, gastrocnemius/soleus effort is applied at the calcaneus insertion).

Third-Class Levers ($F - E - R$)

• Structure: The Effort force lies between the Fulcrum and the Resistance load. The resistance arm is always longer than the effort arm.
• Function: Speed and range of motion. Yields a Mechanical Advantage $<1$, requiring muscles to exert high forces, but moving distal loads rapidly through wide angular arcs.
• Prevalence: Most common lever system in the human body.
• Anatomical Examples: Biceps brachii flexing the elbow (elbow joint is fulcrum, biceps tendon insertion on radial tuberosity is effort, dumbbell in hand is resistance) and hamstrings flexing the knee.

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3. Torque, Moment Arm, and Mechanical Advantage

Torque Mechanics

Torque ($\tau$) is the rotary quantitative turning effect of a force applied around an axis of rotation. It is calculated as the product of force ($F$) and the perpendicular distance from the line of action of force to the axis of rotation, known as the Moment Arm ($d_{\perp}$):

$\tau = F \times d_{\perp}$

• Internal Torque: Product of muscle force and internal muscle moment arm.
• External Torque: Product of external resistance load and external resistance moment arm.

Dynamic Moment Arm Changes and Sticking Points

During exercise execution, the external resistance moment arm changes continuously throughout the range of motion. For example, during a standing dumbbell biceps curl:

• At $0^\circ$ full extension, the line of gravity acting on the dumbbell passes near the elbow axis ($d_{\perp} \approx 0$), producing minimal external resistance torque.
• At $90^\circ$ elbow flexion, the forearm is horizontal, maximizing the perpendicular distance ($d_{\perp}$) and generating peak external resistance torque.
• The exercise "sticking point" occurs at the specific joint angle where the external resistance moment arm is greatest relative to the muscle's mechanical capacity.

Mechanical Advantage

Mechanical Advantage ($MA$) is the ratio of effort arm length to resistance arm length ($MA = d_{\text{effort}} / d_{\text{resistance}}$). When $MA > 1$, force is amplified. When $MA < 1$, muscle force requirements are magnified, but distal limb velocity is prioritized.

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4. Muscle Actions and Activation Modalities

Muscle tension generates distinct mechanical work depending on external load interactions:

1. Concentric Muscle Action: Muscle develops active tension while shortening ($F_{\text{muscle}} > F_{\text{load}}$), performing positive work ($W = F \times +d$). Used to accelerate loads (e.g., upward lifting phase of squat or bench press).
2. Eccentric Muscle Action: Muscle develops active tension while lengthening ($F_{\text{muscle}} < F_{\text{load}}$), performing negative work ($W = F \times -d$). Eccentric actions act as a brake to decelerate loads, produce 20% to 30% higher maximal force capability than concentric actions, consume less oxygen per unit force, cause greater mechanical microtrauma, induce Delayed Onset Muscle Soreness (DOMS), and trigger potent hypertrophy and sarcomerogenesis (sarcomeres added in series).
3. Isometric Muscle Action: Muscle develops tension without changing overall length or joint angle ($F_{\text{muscle}} = F_{\text{load}}$). No external mechanical work is performed ($W = F \times 0$), though metabolic energy is consumed (e.g., plank hold, wall sit).
4. Isokinetic Muscle Action: Muscular contraction performed at a constant angular velocity governed by an electromechanical dynamometer, accommodating resistance across the entire range of motion.

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5. Kinetic Chain Movement Analysis: Open vs. Closed Chain

Movement patterns are categorized based on distal segment boundary conditions along the kinetic chain:

Open Kinetic Chain (OKC)

• Definition: Exercises where the distal segment of the limb moves freely in space without being fixed to an immovable surface.
• Characteristics: Isolates individual joint actions and single muscle groups. Subjects isolated joints to higher rotary shear forces.
• Examples: Seated leg extension, Biceps curl, Dumbbell chest fly, Seated leg curl, Lat pulldown.

Closed Kinetic Chain (CKC)

• Definition: Exercises where the distal segment of the limb is fixed against an unyielding surface (e.g., floor, footplate).
• Characteristics: Integrates multiple joints and muscle groups simultaneously, requiring co-contraction of synergist and antagonist muscles. Increases joint axial compression forces, enhancing joint stability and functional transfer to daily activities and sports.
• Examples: Barbell squat, Push-up, Pull-up, Deadlift, Lunges.