Biomechanics: Statics and Dynamics

• Biomechanics studies the structure/function of biological systems via mechanics principles
• It blends engineering mechanics with biological/physiological principles to understand the musculoskeletal system's workings

Statics

• Statics analyzes bodies at rest or in constant motion, i.e., equilibrium
• Newton’s First Law (inertia law) and equilibrium equations govern it (sum of forces = 0, sum of moments = 0)

Dynamics

• Dynamics analyzes bodies experiencing acceleration
• Newton’s Second Law (F = ma) relates forces to motion

Kinematics

• Kinematics describes motion without considering the forces causing it
• Displacement, velocity, and acceleration are involved
• Types of motion:
  • Linear: Straight-line motion
  • Angular: Rotation around an axis
  • General: Combination of linear and angular motion

Kinetics

• Kinetics relates forces to the motion they cause
• Newton’s laws of motion are employed to analyze forces, moments, and their effects on the body

Stress

• Force per unit area on a surface within a solid material
• Pascals (Pa) or pounds per square inch (psi) measure it
• Stress types:
  • Tensile: Stretching/pulling force
  • Compressive: Pushing/squeezing force
  • Shear: Force parallel to the surface

Strain

• Material deformation due to stress
• It is dimensionless, often as a % or decimal
• Strain types:
  • Tensile: Elongation from tensile stress
  • Compressive: Shortening from compressive stress
  • Shear: Angular deformation from shear stress

Material Properties

• Elasticity: Material's ability to return to original shape after stress removal
• Plasticity: Material's ability to undergo permanent deformation without fracture
• Viscoelasticity: Materials exhibiting viscous and elastic characteristics during deformation
• Strength: Material's ability to withstand stress without failure
• Stiffness: Material's resistance to deformation under load (high stiffness = less deformation)
• Toughness: Material's ability to absorb energy and plastically deform before fracturing

Biomechanical Levers

• Levers are rigid, pivot around a fulcrum to amplify force or increase motion range
• Three lever classes exist in the body:
  • First-class: Fulcrum between force and load (e.g., neck extension)
  • Second-class: Load between fulcrum and force (e.g., calf raise)
  • Third-class: Force between fulcrum and load (e.g., bicep curl)
• Mechanical advantage is the force arm to load arm ratio
  • It determines lever system efficiency

Free Body Diagrams

• A free body diagram (FBD) is a visual of forces on an object
• It simplifies force/moment analysis for biomechanical problems
• FBD creation steps:
  • Isolate the body of interest
  • Draw all external forces on the body
  • Indicate each force's direction and magnitude
  • Establish a coordinate system

Musculoskeletal System

• Bones provide structural support and act as levers
• Muscles generate force for joint motion
• Joints articulate between bones, enabling movement
• Ligaments connect bones, providing joint stability
• Tendons connect muscles to bones, transmitting forces

Gait Analysis

• Gait analysis is the systematic study of human walking
• Kinematic and kinetic measurements assess gait patterns
• Gait phases:
  • Stance phase: Foot contacts the ground
  • Swing phase: Foot does not contact the ground
• Gait parameters:
  • Stride length: Distance in one gait cycle
  • Step length: Distance between successive heel contacts of opposite feet
  • Cadence: Steps per minute
  • Velocity: Walking speed

Joint Biomechanics

• Joints are classified structurally (fibrous, cartilaginous, synovial) and functionally (synarthrosis, amphiarthrosis, diarthrosis)
• Range of Motion (ROM) is the extent of movement at a joint
• Joint stability depends on ligaments, muscles, and joint geometry
• Common joint injuries:
  • Sprains: Ligament injuries
  • Strains: Muscle/tendon injuries
  • Dislocations: Bone displacement from normal articulation

Fluid Mechanics in Biomechanics

• Blood flow is governed by fluid dynamics principles (viscosity, pressure, flow rate)
• Airflow in the lungs follows similar principles, affecting respiration
• Drag and lift forces affect movement through fluids (e.g., swimming, cycling)

Modeling and Simulation

• Biomechanical models simulate and analyze human movement
• These models can be:
  • Two-dimensional or three-dimensional
  • Rigid-body or deformable
• Simulation software helps:
  • Predict intervention effects (e.g., surgery, rehabilitation)
  • Optimize performance
  • Understand injury mechanisms

Instrumentation in Biomechanics

• Force plates measure ground reaction forces during movement
• Motion capture systems track body segment positions/orientations
• Electromyography (EMG) measures muscle activity
• Accelerometers measure acceleration
• Pressure sensors measure contact forces

Ergonomics

• Ergonomics studies designing workplaces/products/systems to fit users
• It applies biomechanical principles to reduce injury risk and improve performance
• Key considerations:
  • Posture
  • Repetitive motions
  • Force exertion
  • Environmental factors

Injury Biomechanics

• Injury biomechanics studies tissue injury mechanisms due to mechanical loading
• Injury risk factors:
  • Force magnitude and direction
  • Loading rate
  • Tissue material properties
  • Individual factors
• Common injury mechanisms:
  • Compression
  • Tension
  • Shear
  • Bending
  • Torsion

Rehabilitation Biomechanics

• Rehabilitation biomechanics applies biomechanical principles to restore function after injury/surgery
• It involves:
  • Assessing movement patterns
  • Designing exercise programs
  • Using assistive devices
  • Monitoring progress

Sports Biomechanics

• Sports biomechanics analyzes human movement mechanics in sports to improve performance/reduce injury risk
• The focus is on:
  • Technique optimization
  • Equipment design
  • Training strategies
  • Injury prevention

Implant Biomechanics

• Implant biomechanics studies mechanical interaction between implants and biological tissues
• It involves:
  • Designing implants with appropriate material properties and geometry
  • Analyzing stress distribution around implants
  • Evaluating implant fixation and stability
  • Assessing long-term implant performance

Bone Biomechanics

• Bone is a composite material with anisotropic properties
• Bone strength/stiffness depends on:
  • Bone density
  • Bone geometry
  • Loading direction
• Bone remodeling adapts bone to mechanical loading
• Osteoporosis reduces bone density and increases fracture risk

Cartilage Biomechanics

• Cartilage is a viscoelastic tissue providing low-friction surface in joints
• Cartilage mechanical properties depend on:
  • Composition
  • Hydration
  • Loading rate
• Osteoarthritis degrades cartilage and causes joint pain

Muscle Biomechanics

• Muscles generate force through the sliding filament mechanism
• Muscle force depends on:
  • Muscle length
  • Contraction velocity
  • Activation level
• Muscle fatigue reduces muscle force and endurance

Biomechanical Modeling

• Musculoskeletal models: Simulate mechanical behavior of muscles, bones, and joints during movement
• Finite element analysis (FEA): Predicts stress/strain distributions in biological tissues
• Computational fluid dynamics (CFD): Simulates fluid flow in biological systems

Data Acquisition

• Motion capture systems: Track body segment position/orientation using cameras/sensors
• Force plates: Measure ground reaction forces during standing, walking, or jumping
• Electromyography (EMG): Records muscle electrical activity to assess muscle activation patterns
• Pressure sensors: Measure contact forces (e.g., foot and ground)

Signal Processing

• Filtering: Removes noise from biomechanical signals
• Differentiation: Calculates velocity/acceleration from position data
• Integration: Calculates displacement from velocity data
• Fourier analysis: Decomposes signals into their frequency components

Applications of Biomechanics

• Clinical biomechanics: Assesses/treats movement disorders from injury, disease, or aging
• Sports biomechanics: Improves athletic performance and reduces injury risk
• Ergonomics: Designs workplaces/products to reduce musculoskeletal disorder risk
• Forensics: Biomechanical analysis determines accident causes
• Rehabilitation: Develops and evaluates rehabilitation interventions
• Product design: Optimizes product design to improve performance and safety