What Is Biomechanics?

Biomechanics applies the principles of mechanics—kinematics, kinetics, materials, and fluids—to biological systems. It spans multiple scales:

• Molecular and cellular: cytoskeletal forces, membrane tension, mechanotransduction.
• Tissues and organs: muscle contraction, cartilage lubrication, bone remodeling, heart and lung mechanics.
• Whole-body movement: walking, running, lifting, swimming, flying.
• Populations and evolution: comparative biomechanics and bio-inspired design.

Core Mechanical Concepts

• Force: an interaction that changes motion or deformation.
• Stress (force per area): how load is distributed within a material.
• Strain (deformation relative to original size): a measure of stretch or compression.
• Stiffness and modulus: resistance to deformation; for small strains, modulus ≈ stress/strain.
• Viscosity: resistance to flow or rate-dependent deformation.
• Viscoelasticity: time-dependent behavior (creep, stress relaxation, hysteresis) common in biological tissues.
• Anisotropy: properties depend on direction (e.g., tendon is stronger along its fibers).
• Nonlinearity: many tissues have toe regions and stiffen as they stretch.
• Scaling: geometry and size alter mechanics (e.g., a bone’s strength scales with cross-sectional area more than length).

Facts About Tissues and Materials of Life

Bone: A Living Composite

• Bone is a composite of mineral (stiffness) and collagen (toughness), enabling high strength at relatively low weight.
• It remodels in response to load (Wolff’s law); regions under higher stress add material, while underused areas resorb, optimizing structure.
• Implants that are too stiff can cause stress shielding, where bone loses density due to reduced loading.

Muscle: Biological Actuators

• Force depends on fiber length (force–length curve) and contraction speed (force–velocity curve).
• Muscle is most powerful during rapid shortening but generates its highest forces when lengthening (eccentric contractions).
• Typical metabolic efficiency of skeletal muscle is about 20–25% for converting chemical to mechanical energy.
• Series elastic elements (e.g., tendons) store and return energy, reducing metabolic cost in bouncing gaits.

Tendon and Ligament: Load Pathways

• Highly aligned collagen makes them strong in tension but weak in shear or compression.
• They exhibit a toe region at low strain as crimped fibers straighten, then stiffen nonlinearly.
• Elastic tendons (like the Achilles) can recycle energy each step, improving running economy.

Cartilage and Joint Biotribology

• Articular cartilage is avascular and biphasic (solid matrix + interstitial fluid), with fluid pressurization supporting loads.
• Lubrication modes include boundary, fluid-film, and mixed; cartilage’s low friction rivals engineered bearings.
• Degeneration disrupts lubrication and load distribution, increasing wear and pain in osteoarthritis.

Movement Mechanics: From Gait to Jumps

• Walking resembles an inverted pendulum: the center of mass vaults over the stance leg; typical stance time is ~60% of the gait cycle.
• Running behaves like a spring–mass: tendons store and return elastic energy; stance time shortens and vertical stiffness increases with speed.
• Ground reaction forces during walking show a characteristic “double bump,” reflecting weight acceptance and push-off phases.
• Jump height is influenced by takeoff velocity, which depends on impulse (force × time) delivered by muscles and tendons.
• Energy trade-offs: Reducing vertical oscillation and stabilizing torso motion often improve running economy.

Control and Coordination

• The nervous system integrates feedforward commands with feedback from sensors and reflexes to stabilize movement.
• Central pattern generators produce rhythmic motions (e.g., stepping) even with limited sensory input.
• Skill involves optimizing the distribution of joint torques and muscle activations to meet task goals with minimal cost.

Fluid Biomechanics: Blood and Air

Hemodynamics (Blood Flow)

• In large arteries, flow can be pulsatile and transitional; in small vessels it tends to be laminar.
• Flow rate is highly sensitive to vessel radius (a small dilation can significantly increase flow).
• Endothelial cells sense wall shear stress and remodel vessels; disturbed flow patterns correlate with plaque formation.
• Arterial walls are compliant; elastic recoil helps maintain flow between heartbeats (Windkessel effect).

Respiratory Mechanics

• The lung’s compliance describes ease of inflation; elastance reflects the tendency to recoil.
• Surfactant in alveoli reduces surface tension, stabilizing small air sacs and reducing the work of breathing.
• Airflow is often turbulent in the trachea but becomes laminar in small bronchioles where velocities are lower.

Cellular and Molecular Mechanics

• Cells generate forces via the cytoskeleton (actin, myosin, microtubules) and transmit them through adhesions to the extracellular matrix.
• Mechanosensitive pathways (e.g., integrins, Piezo channels, YAP/TAZ) translate physical cues into gene expression changes.
• Substrate stiffness influences cell fate: softer matrices favor neuron-like phenotypes; stiffer ones encourage bone-like differentiation.
• Techniques like atomic force microscopy, traction force microscopy, and optical tweezers quantify forces at pico- to nano-Newton scales.

Comparative Biomechanics and Bio‑Inspired Design

• Flight and swimming: Birds and fish manage vortices to gain lift or thrust; wing and fin shapes are tuned to speed and maneuverability.
• Insects operate at low Reynolds numbers where viscosity dominates; wing kinematics exploit clap-and-fling mechanisms.
• Gecko adhesion relies on van der Waals forces via microscopic setae; engineers mimic this for reversible tapes.
• Bone and shell architectures inspire lightweight lattices in 3D-printed structures with high strength-to-weight ratios.
• Shark skin riblets reduce drag; similar textures are used on swimsuits and aircraft surfaces to manage flow.

Sports and Performance

• Biomechanics optimizes technique (e.g., sprint start angles, jump approach speeds) and equipment (shoes, rackets, bikes) to enhance performance.
• Energy storage in footwear foams and plates can improve running economy, but benefits depend on the runner’s mechanics.
• Injury risk relates to load magnitude, rate, and repetition; managing training load and movement patterns reduces risk.
• Real-time wearable sensors and motion analysis inform coaching with objective kinematics and kinetics.

Clinical Biomechanics

• Prosthetics and orthotics: Alignment and stiffness tuning affect comfort, symmetry, and metabolic cost.
• Joint health: The knee adduction moment is a surrogate for medial compartment load; gait retraining can reduce it in osteoarthritis.
• Implants: Materials and geometry must balance wear resistance, stiffness, and fixation; wear particles can trigger inflammation.
• Rehabilitation: Robotics and exosuits provide assist-as-needed, promoting recovery while avoiding learned dependence.
• Gait labs combine motion capture, force plates, and EMG to diagnose movement disorders and guide interventions.

Modeling, Simulation, and Measurement

Modeling and Simulation

• Multibody dynamics: Represents segments as rigid bodies linked by joints; estimates joint loads via inverse dynamics.
• Musculoskeletal models: Incorporate muscle paths and properties; Hill-type models approximate force–length–velocity behavior.
• Finite element analysis: Resolves stress/strain in complex geometries (bones, cartilage, implants) with patient-specific meshes.
• Computational fluid dynamics: Simulates blood and airflow in anatomically realistic vessels and airways.
• EMG-driven models: Use muscle activation signals to estimate forces and personalize simulations.

Measurement Technologies

• Motion capture: Marker-based systems provide high-fidelity kinematics; markerless vision systems enable field assessments.
• Force measurement: Force plates capture ground reaction forces; instrumented treadmills and insoles extend measurement contexts.
• Wearables: IMUs estimate joint angles and loading surrogates; careful calibration mitigates drift.
• Imaging: Ultrasound measures muscle fascicle dynamics; elastography quantifies tissue stiffness in vivo.
• Fluid diagnostics: Doppler ultrasound and particle image velocimetry reveal flow fields and shear stresses.

Everyday Applications

• Ergonomics designs tools and workplaces to reduce musculoskeletal strain and improve efficiency.
• Vehicle safety uses injury biomechanics to optimize restraint systems and crash structures.
• Product design (from backpacks to keyboards) relies on load distribution and comfort metrics.
• Public health employs gait and balance assessments to prevent falls and maintain mobility in aging populations.

Quick Facts and Common Myths

• Fact: Force scales with cross-sectional area, but mass scales with volume; larger animals need disproportionately thicker bones.
• Fact: Tendons can return more than 90% of stored elastic energy in some species, powering efficient locomotion.
• Myth: “No pain, no gain.” Pain is a protective signal; progressive loading should respect tissue adaptation timescales.
• Myth: “More cushioning always prevents injury.” Over-cushioning can alter mechanics; fit and technique matter.
• Fact: Joint cartilage is nourished by cyclic loading; moderate movement promotes cartilage health.

Ethics, Limitations, and Good Practice

• Models simplify reality; assumptions (rigid segments, homogenous tissues) limit precision and must be validated.
• Inter-individual variability is large; personalized parameters improve predictions and clinical decisions.
• Data privacy and informed consent are essential for human and animal studies.
• Open data and reproducible workflows increase trust and accelerate progress.

Where Biomechanics Is Heading

• Digital twins: Personalized, continuously updated models for prevention, training, and clinical care.
• Soft robotics and exosuits: Lightweight, compliant devices that augment movement and reduce fatigue.
• Organ-on-chip: Microfluidic systems replicate tissue mechanics for drug testing and disease modeling.
• AI-fused sensing: Markerless motion capture and smart wearables deliver real-time feedback and risk prediction.
• Advanced materials: 3D-printed lattices, hydrogels, and bioresorbables tailor stiffness and function to the body.

Key Takeaways

• Biomechanics links physical laws to living systems, explaining how structure enables function.
• Life’s materials are optimized for competing demands: strength, toughness, weight, and adaptability.
• Movement is a dance between power (muscle), storage (tendon), control (nervous system), and environment (gravity, fluids).
• Applied wisely, biomechanics enhances performance, reduces injury, and guides medical innovation.