INTRODUCTION TO BIOMECHANICS AND
KINESIOLOGY
WHAT IS BIOMECHANICS?
• Biomechanics is the study of how the human body moves and how forces act
on
it.
It applies the principles of physics and engineering to understand movement,
posture, and performance.
• In simple terms — biomechanics helps us understand how and why we move
the way we do.
EXAMPLE:
• When you jump, biomechanics explains how your muscles create force, how
gravity pulls you down, and how your joints and bones absorb the impact
when you land.
• When a physiotherapist observes a patient’s gait, they use biomechanics to
identify which muscles or joints are not functioning properly.
WHAT IS KINESIOLOGY?
• Kinesiology is the science of human movement — it studies
muscles, bones, and joints and how they work together to
produce motion.
• If biomechanics is about forces and motion, kinesiology focuses on
movement patterns and muscle function.
EXAMPLE:
•When you lift your arm overhead, kinesiology helps explain which
muscles (like the deltoid and rotator cuff) are involved and how they
coordinate.
•It tells us about types of muscle contractions (isometric, concentric,
eccentric) and movement planes (sagittal, frontal, transverse).
Basis of Difference
Biomechanics
Kinesiology
1. Definition
Study of forces and their effects on
human movement.
Study of human movement, focusing on
muscles, bones, and joints.
2. Main Focus
Emphasizes mechanical principles —
motion, force, torque, and equilibrium.
Emphasizes anatomical and
physiological aspects — muscle
action, joint function, and coordination.
3. Nature of Study
Quantitative — uses measurements,
equations, and analysis (physicsbased).
Qualitative — describes how
movement occurs and which muscles are
involved.
4. Key Concepts
Force, leverage, torque, center of
gravity, momentum, stability.
Muscle contractions, planes and axes of
motion, types of movements.
5. Tools & Methods Used
Motion analysis systems, force plates,
EMG, video analysis, mathematical
modeling.
Manual muscle testing, goniometry,
observation of movement patterns.
6. Example in Practice
Analyzing ground reaction forces during
walking to prevent injury.
Identifying which muscles activate
during shoulder abduction.
7. Application in Physiotherapy
Helps design mechanically efficient
Helps plan exercise programs by
movements and prevent injury by
knowing which muscles to strengthen or
understanding load and stress on joints.
stretch.
8. Type of Knowledge Required
Physics, engineering, and mathematics. Anatomy, physiology, and motor control.
9. Approach
Objective and analytical.
Descriptive and functional.
10. Goal
To improve performance and reduce
injury risk through mechanical
optimization.
To restore or enhance movement
through better muscle coordination and
control.
RELEVANCE TO PHYSIOTHERAPY
Understanding biomechanics and kinesiology is essential for every physiotherapist
because:
1. Assessment of Movement: Helps in identifying abnormal movement patterns — e.g.,
in stroke, cerebral palsy, or after an ankle sprain.
2. Treatment Planning: Knowing how muscles and joints move allows you to design
specific
exercises
that
restore
strength,
flexibility,
and
coordination.
Example: Correcting patellofemoral pain by training the vastus medialis oblique
(VMO) to improve knee tracking.
RELEVANCE TO PHYSIOTHERAPY
3. Injury Prevention: Helps in understanding how poor posture or faulty
mechanics lead to overuse injuries.
Example: Improper lifting mechanics can lead to low back pain — biomechanics
helps teach safe lifting techniques.
4. Rehabilitation & Performance: In sports physiotherapy, biomechanics is used to
analyze athletic techniques and improve performance (e.g., running, jumping,
throwing).
SIMPLE CLINICAL EXAMPLE
Case: A runner has knee pain.
Biomechanics view: Check ground reaction forces, joint angles, and alignment during
running.
Kinesiology view: Assess which muscles (like quadriceps, hamstrings, gluteus medius) are
weak or overactive.
Physiotherapy application: Design strengthening and movement retraining to correct the
cause.
INTRODUCTION TO
STRESS–STRAIN CURVE
WHAT IS STRESS?
• Stress is the internal force per unit area that develops within a material (or tissue) when an
external force is applied.
It shows how much load or pressure the tissue is experiencing.
• Formula:
Force
Stress =
N/m𝟐 or Pascal
Area
Example:
When you stretch a muscle or tendon during exercise, stress develops inside the tissue resisting that
pull.
WHAT IS STRAIN?
• Strain is the amount of deformation or change in shape or length that occurs
due to stress.
It shows how much the tissue has stretched or elongated.
• Formula:
Change in Length
Strain =
Original Length
Example:
If a ligament of 10 cm length stretches to 10.5 cm,
THE STRESS–STRAIN CURVE
• When we gradually apply stress to a soft tissue
(like a ligament, tendon, or muscle), it deforms in
different stages. This relationship is shown by the
stress–strain curve.
Courtesy: Downey PA , Siegel MI . Bone biology and the clinical implications for osteoporosis.
Phys Ther
CLINICAL RELEVANCE TO PHYSIOTHERAPY
Understanding the stress–strain curve helps physiotherapists to:
• Plan safe stretching and strengthening exercises → Stretch tissues only within the
elastic region to improve flexibility without injury.
• Know tissue limits during rehabilitation → Avoid the plastic or failure region,
especially in healing tissues (like tendons post-repair).
• Guide manual therapy techniques → Joint mobilizations are applied carefully
within elastic range to restore motion safely.
• Design progressive loading programs → Gradually increase stress to strengthen
tissues without causing damage.
CLINICAL EXAMPLES
Tissue
Application of Stress–Strain Concept
Muscle
During flexibility training – aim for
elastic range stretch to improve ROM
safely.
Ligament
In post-sprain rehab – gentle loading to
promote collagen alignment without
overstretching.
Tendon
In eccentric strengthening – controlled
stress to promote healing and
remodeling.
“A PHYSIOTHERAPIST MUST KNOW HOW MUCH
STRESS A TISSUE CAN TOLERATE BEFORE IT
DEFORMS OR FAILS — THIS IS THE FOUNDATION
OF SAFE AND EFFECTIVE REHABILITATION.”
TENSILE STRENGTH (STRESS BEFORE FAILURE) OF DIFFERENT
HUMAN TISSUES
Approx. Ultimate
Physiological Meaning /
Example in
Tissue Type
Tensile Strength (Before
Clinical Relevance
Physiotherapy Context
Failure)
Very strong under
Avoid excessive loading
compression and tension;
Cortical Bone
100–150 MPa
during weight-bearing in
fractures occur beyond
osteoporotic patients.
this.
Can withstand high
Safe eccentric training
Tendon (e.g., Achilles,
tensile forces; fails with
50–100 MPa
gradually increases load
patellar)
sudden overstretch or
tolerance.
excessive eccentric load.
Moderate strength; can In ankle or knee rehab,
Ligament (e.g., ACL,
20–50 MPa
elongate up to 10–15% stretching beyond safe
MCL)
before rupture.
limit causes tears.
Skeletal Muscle
0.3–1.0 MPa
Articular Cartilage
5–25 MPa
(compressive)
Much weaker than
During stretching,
tendon; fails before apply mild tension
tendon in extreme only to reach elastic
stretch.
range.
Resists compression;
Avoid high impact on
low tensile strength.
cartilage-damaged
Degeneration leads
joints (OA rehab).
to osteoarthritis.
Skin
Fascia
2–30 MPa
Elastic, moderate tensile
Important for wound care,
strength; varies by age and scar mobilization, and burn
location.
rehab.
2–15 MPa
Moderately strong
Myofascial release
connective tissue; transmits techniques target this tissue
forces between muscles.
safely.
Intervertebral Disc
(Annulus Fibrosus)
4–10 MPa
Meniscus (Knee)
8–12 MPa
Peripheral Nerve
0.5–0.7 MPa
Artery / Vessel Wall
1.0–2.0 MPa
Good tensile resistance;
Avoid end-range lumbar
failure causes disc
flexion in acute disc cases.
herniation.
Withstands shear and
Twisting or deep squatting
compression; fails under
post-injury risks re-tear.
twisting force.
Very low tolerance to
tension; elongation > 20% Neural mobilization must be
can cause irreversible
gentle and within safe limit.
damage.
Low tensile strength;
Important in post-cardiac or
ruptures if overstretched or
vascular rehab precautions.
weakened.
KEY INSIGHTS
Bone and tendons are strongest, while muscles, nerves, and skin are weakest under
tension.
Ligaments tolerate stretch only up to ~10–15% of their length — beyond that, partial or
complete tears occur.
Physiotherapy exercises (especially stretching and resistance training) must stay within
elastic limits to promote adaptation, not injury.
Healing tissues have 50–70% lower tensile strength, so load must be progressed
gradually during rehab.
Tissue
Analogy
Bone
Like a hard stick – strong but can snap
suddenly under high load.
Tendon
Like a thick rope – strong but less elastic.
Ligament
Like a rubber band – flexible but can tear
if overstretched.
Muscle
Like a soft elastic cloth – stretches easily
but can tear with over-pull.
Nerve
Like a thread – delicate and easily
damaged by too much tension.
BIOMECHANICAL APPLICATIONS TO JOINT
STRUCTURE AND KINEMATICS &
INTRODUCTION TO KINETICS
LEARNING OBJECTIVES
• Understand Osteokinematics and Arthrokinematics
• Learn how forces act on joints
• Differentiate Statics vs Dynamics
• Understand Torque, Moment Arms, and Force Components
• Apply these principles to clinical examples
WHAT IS BIOMECHANICS?
• Definition: Study of mechanical principles applied to
biological systems — especially the human body.
• Clinical Relevance: Foundation for physiotherapy — helps
design exercise, analyze gait, and improve function.
OSTEOKINEMATICS – MOVEMENT OF BONES IN
SPACE
Definition: Gross movement of bones (flexion, extension,
rotation, etc.)
Planes and Axes:
•Sagittal → Flexion/Extension (Frontal Axis)
•Frontal → Abduction/Adduction (Sagittal Axis)
•Transverse → Rotation (Vertical Axis)
ARTHROKINEMATICS
JOINT SURFACE MOTION
Definition: Small accessory motions between articular surfaces.
Types:
• Roll – New points contact new points
• Slide (Glide) – Same point contacts new points
• Spin – Rotation around stationary axis
• Distraction – 2 joint surfaces moving away from each other
• Compression – 2 joint surfaces pressing over each other
Motion
Description
Example
Roll
One point of one surface contacts new
points on the opposite surface
Femoral condyles rolling on tibia
during knee flexion
Slide (Glide)
One point of a surface contacts new
points on the opposite surface
Tibial plateau gliding on femoral
condyles
Spin
Rotation of one surface around a
stationary axis
Head of humerus spinning in
glenoid fossa during
pronation/supination
Compression
Force pushes surfaces together
Distraction
Force pulls surfaces apart
Standing, deltoid at < 90°
abduction
Manual traction, finger pulling
ROLL
SLIDE/GLIDE
SPIN
COMPRESSION
&
DISTRACTION
CONVEX–CONCAVE RULE
Moving Surface
Convex on Concave
Concave on Convex
Glide Direction
Opposite to bone
movement
Same as bone
movement
Example
Shoulder Abduction –
Humeral head glides
inferiorly
Knee Extension –
Tibia glides
anteriorly
JOINT STRUCTURE & STABILITY
• Degrees of freedom (DOF):
- Number of possible movement axes (e.g., shoulder has 3
DOF).
• Joint congruency:
- How well surfaces fit.
JOINT POSITIONS
Closed-Packed Position: Max congruency → Stability
(e.g., knee extension)
Open-Packed Position: Loose → Mobilization
(e.g., knee flexion)
** REMEMBER: extremes of joint ROM are closed pack and mid ranges are loose pack
INTRODUCTION TO KINETICS
Kinetics is the branch of biomechanics that studies forces
that cause or modify human movement. While kinematics
describes how the body moves (motion), kinetics explains
why the body moves in a particular way.
Understanding kinetics is essential for physiotherapists because it
allows us to:
• Analyze abnormal movement
• Identify the cause of dysfunction
• Design targeted therapeutic exercises
• Improve functional performance and prevent injury
DESCRIPTIONS OF MOTION
Key Features
Clinical / Practical
Example
Linear path; no rotation.
Joint distraction, anterior
tibial glide, sliding a
book.
Rotatory Motion
Movement occurs around
Occurs at joints; angular
a fixed axis; points
displacement.
follow arcs.
Elbow flexion, shoulder
rotation.
Angular Motion
Movement causing a
change in angle
between two segments
at a joint.
Combination of roll +
glide; measured in
degrees.
Knee flexion, hip
abduction.
Curvilinear Motion
Motion that occurs in a
curved path, combining
translation + rotation.
Most human movements
are curvilinear; center of
mass follows a curve.
Foot during gait,
throwing motion, lifting
an object from the floor.
Type of Motion
Definition
Translatory Motion
All points of an object
move the same distance
in the same direction.
INTRODUCTION TO FORCES
Definition:
A force is a push or pull that produces, modifies, or stops
motion.
Force = Mass × Acceleration (F = m × a)
FORCES IN HUMAN MOVEMENT
• Create movement
• Stop movement
• Modify movement
• Change the direction of movement
TYPES OF FORCES ACTING ON THE HUMAN BODY
A. Internal Forces
• Generated within the body.
• Muscle forces – primary internal forces
• Eg: Quadriceps force during knee extension
• Ligament forces – passive tension
• Eg: ACL resists anterior tibial translation
• Joint reaction forces
• Eg: Hip joint compressive force during walking
B. External Forces
• Originating outside the body.
• Gravity (most dominant external force)
• Acts downward through the Center of Gravity (COG)
• Eg: Shoulder abductors work against gravity during arm elevation
• Ground Reaction Force (GRF)
• Equal & opposite to the force the body exerts on the ground
• Eg: GRF peak at heel strike during gait
• Frictional forces
• Eg: Foot friction prevents slipping
• Resistance (weights, bands, manual resistance)
Characteristics of a Force
• Magnitude (how much)
• Direction (toward/away; rotational/linear)
• Line of Action
• Point of Application
STATICS VS DYNAMICS
Concept
Statics
Dynamics
Definition
Example
Body at rest or
Holding a dumbbell
moving at constant
still at 90° elbow
velocity (ΣF = 0)
flexion
Body under
Lifting a dumbbell
acceleration (ΣF ≠ 0)
(biceps concentric)
Understanding statics helps in postural analysis,
while dynamics helps in movement retraining.
TRANSLATORY MOTION IN FORCE SYSTEMS
• Linear Force System
All forces act along the same line (in same or opposite directions).
Example: Two people pulling a rope from opposite sides.
• Clinical Example:
Traction therapy on cervical spine = linear, collinear force.
• Concurrent Force System:
Forces act at a common point but along different lines.
Example: Deltoid muscle’s different fiber directions converging at the humerus.
• Resultant Force:
The combined effect of multiple forces acting concurrently.
Useful for understanding muscle synergy — e.g., trapezius and serratus anterior
combine for upward scapular rotation.
ADDITIONAL LINEAR FORCE CONSIDERATIONS
a. Equilibrium
• When sum of all forces and torques = 0
Example: Balanced posture during standing.
• b. Center of Gravity (COG)
• Point where body’s weight is evenly distributed.
• In standing: around S2 vertebra.
• In child: slightly higher (due to head size).
Physiotherapists use COG in balance training and gait retraining.
c. Base of Support (BOS)
• Wider BOS → more stability.
Used in rehabilitation (e.g., staggered stance for progressive balance
challenge).
TORQUE OR MOMENT OF FORCE
• Definition: Rotational effect of a force around an axis.
Torque = Force × Moment Arm
• Moment Arm: Perpendicular distance between force line and axis.
• Greater moment arm → greater torque.
• Examples:
Biceps produces maximum torque at 90° elbow flexion (moment arm longest). In
wheelchair propulsion, hand contact point farther from wheel hub increases
torque, making it easier to move.
• Clinical Relevance:
Helps determine manual resistance angle in strengthening exercises.
Guides joint protection in weak muscles (shorter lever = less torque demand).