Biomechanics

Intermediate

Biomechanics is the interdisciplinary science that applies the principles of mechanics to biological systems, including the human body, animals, plants, and individual cells. By combining knowledge from physics, engineering, anatomy, and physiology, biomechanics seeks to understand how living organisms generate and respond to forces. The field encompasses everything from the molecular mechanics of muscle contraction to the large-scale dynamics of human locomotion, providing a quantitative framework for analyzing movement, deformation, and fluid flow in biological tissues.

The roots of biomechanics trace back to antiquity, with Aristotle's observations on animal gait and Leonardo da Vinci's anatomical sketches of human movement. However, the modern discipline truly emerged in the twentieth century with advances in computing, imaging technology, and materials science. Giovanni Alfonso Borelli is often regarded as the father of biomechanics for his 1680 work 'De Motu Animalium,' which applied mathematical principles to animal movement. Today, tools such as motion capture systems, force plates, electromyography, and finite element analysis allow researchers to measure and model biological systems with remarkable precision.

Biomechanics has far-reaching practical applications across medicine, sports, ergonomics, and bioengineering. In orthopedics, biomechanical analysis guides the design of joint replacements and fracture fixation devices. In sports science, it helps athletes optimize technique and reduce injury risk. Forensic biomechanics reconstructs injury mechanisms in legal cases, while occupational biomechanics designs workplaces that minimize musculoskeletal disorders. As computational power and wearable sensor technology continue to advance, biomechanics is increasingly integrated into rehabilitation robotics, prosthetic limb design, and personalized medicine.

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Key Concepts

Newton's three laws of motion form the foundation of biomechanical analysis. The first law (inertia) explains why a body at rest stays at rest unless acted upon by an external force. The second law (F = ma) relates the net force on a body to its mass and acceleration. The third law (action-reaction) explains ground reaction forces during walking and running.

Example

When a sprinter pushes backward against the starting blocks (action), the blocks push the sprinter forward with an equal and opposite reaction force, propelling them down the track.

The branch of mechanics that describes the geometry of motion without reference to the forces causing it. In biomechanics, kinematics involves measuring joint angles, linear and angular velocities, and accelerations of body segments during movement using motion capture systems.

Example

A gait analysis lab uses infrared cameras to track reflective markers on a patient's legs, producing time-series data of hip, knee, and ankle joint angles throughout the walking cycle.

The study of forces and torques that cause or result from motion. In biomechanics, kinetics examines ground reaction forces, joint reaction forces, muscle forces, and joint moments. It answers why movement occurs, complementing the descriptive nature of kinematics.

Example

A force plate embedded in the floor measures the three-dimensional ground reaction forces when a basketball player lands from a jump, revealing peak impact forces of up to five times body weight.

Stress is the internal force per unit area within a material, while strain is the deformation (change in length relative to original length) that results. Biological tissues such as bone, cartilage, tendon, and ligament exhibit complex stress-strain behaviors including viscoelasticity, anisotropy, and nonlinearity.

Example

Cortical bone can withstand compressive stresses of approximately 170 MPa but only about 130 MPa in tension, which is why bones more commonly fracture under bending loads where tensile stresses develop on the convex side.

The Hill muscle model represents skeletal muscle as a three-component system: a contractile element (active force generation), a series elastic element (tendon), and a parallel elastic element (passive tissue). The force-velocity and force-length relationships of the contractile element govern how much force a muscle can produce under different conditions.

Example

A muscle contracting eccentrically (lengthening under load, as when lowering a heavy box) can produce greater force than during a concentric (shortening) contraction at the same speed, which is why eccentric exercise is effective for rehabilitation.

The center of mass (COM) is the point where the total mass of a body can be considered concentrated. For stable standing, the vertical projection of the COM must fall within the base of support. Biomechanists analyze COM trajectories to assess balance, stability, and energy expenditure during locomotion.

Example

The Fosbury Flop high jump technique allows an athlete's body to clear the bar while their center of mass actually passes below it, making the technique mechanically more efficient than the straddle method.

A computational method that combines kinematic data (motion capture), kinetic data (force plates), and body segment parameters (mass, moment of inertia) to calculate the net forces and moments acting at each joint. This is the primary method for estimating internal joint loading during movement.

Example

Using inverse dynamics on gait data, clinicians can determine that a patient's knee experiences an abnormally high adduction moment during walking, which is a known risk factor for medial compartment osteoarthritis.

The property of biological materials that exhibit both viscous (time-dependent, energy-dissipating) and elastic (energy-storing) behavior when deformed. Viscoelastic tissues display creep (continued deformation under constant load), stress relaxation (decreasing stress under constant deformation), and hysteresis (energy loss during loading-unloading cycles).

Example

An intervertebral disc under sustained compressive load gradually loses height throughout the day due to creep, which is why people are measurably taller in the morning than in the evening.

Wolff's Law states that bone remodels in response to the mechanical loads placed upon it, becoming stronger where loads are high and weaker where loads are low. The underlying cellular process, mechanotransduction, converts mechanical stimuli into biochemical signals that regulate tissue growth, maintenance, and repair.

Example

Astronauts lose bone mineral density at a rate of approximately 1-2% per month during spaceflight because the absence of gravitational loading removes the mechanical stimulus that maintains bone density, illustrating Wolff's Law.

Biofluid mechanics applies principles of fluid dynamics to biological flows, including blood flow through the cardiovascular system, air flow in the respiratory system, and synovial fluid lubrication in joints. Key concepts include viscosity, laminar versus turbulent flow, and the Navier-Stokes equations.

Example

The development of atherosclerotic plaques preferentially occurs at arterial branch points where disturbed blood flow and low wall shear stress allow lipid accumulation, demonstrating how fluid mechanics directly influences cardiovascular disease.

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Curriculum alignment— Standards-aligned

Grade level

College+

Learning objectives

•Explain the principles of Newtonian mechanics as applied to biological tissues, joints, and movement systems
•Apply force analysis and kinematic methods to quantify human movement patterns during functional activities
•Analyze musculoskeletal loading conditions to assess injury risk and optimize athletic or rehabilitative performance
•Evaluate biomechanical research methodologies including motion capture, force plates, and computational modeling approaches

Recommended Resources

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Books

Biomechanics of Human Movement

by Joseph Hamill, Kathleen M. Knutzen & Timothy R. Derrick

Biomechanics: Mechanical Properties of Living Tissues

by Yuan-Cheng Fung

Introduction to Sports Biomechanics

by Roger Bartlett

Clinical Biomechanics of the Spine

by Augustus A. White III & Manohar M. Panjabi

Courses

Fundamentals of Biomechanics

CourseraEnroll

Biomechanics of Human Movement

edXEnroll

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STEM & Engineering

Physics

Physics is the study of matter, energy, and the fundamental forces of nature, encompassing everything from the motion of everyday objects to the behavior of subatomic particles and the structure of the cosmos.

Intermediate

STEM & Engineering

Biomedical Engineering

An interdisciplinary field applying engineering principles to biology and medicine to develop technologies that improve human health.

Intermediate

Biomechanics

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Cheat Sheet

Glossary

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Key Concepts

Newton's Laws Applied to Human Movement

Kinematics

Kinetics

Stress and Strain in Biological Tissues

Muscle Mechanics and the Hill Model

Center of Mass and Balance

Inverse Dynamics

Viscoelasticity

Wolff's Law and Mechanotransduction

Fluid Mechanics in Biological Systems

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Recommended Resources

Books

Biomechanics of Human Movement

Biomechanics: Mechanical Properties of Living Tissues

Introduction to Sports Biomechanics

Clinical Biomechanics of the Spine

Courses

Fundamentals of Biomechanics

Biomechanics of Human Movement

Related Topics

Physics

Biomedical Engineering