BIOMECHANICS OF HUMAN MOVEMENT:
FORCES, LEVERS, AND MOTION
Force Distance (Force Arm)
The straight-line distance from where force is applied to the axis
of rotation.
Effect of Moment Arm Length
A longer moment arm increases torque production
A shorter moment arm reduces torque
Patients must produce more muscle force if the moment
arm is shorter
Force Angle (Angle of Application)
The angle at which force is applied relative to a body segment or
object.
When Angle < 90°
The moment arm decreases; compressive force increases, while
rotational effect reduces.
When Angle > 90°
The moment arm decreases; traction (pulling apart) force
increases, while rotational effect reduces.
Newton’s Laws (Reworded)
Law of Inertia (Newton’s First Law)
An object remains at rest or continues moving at a constant
velocity unless acted upon by an external force.
,Law of Acceleration (Newton’s Second Law)
Acceleration depends on force and mass: more force increases
acceleration, while greater mass reduces it.
Example: A sprinter pushing harder off the blocks accelerates
faster.
Action-Reaction Principle (Newton’s Third Law)
Every applied force has an equal and opposite opposing force.
Example: Jumping on a trampoline.
Balance & Body Mechanics
State of Balance (Equilibrium)
Occurs when all forces acting on an object are balanced,
resulting in no movement.
Mass Center Point (Center of Mass – COM)
The point where the total body mass is evenly distributed.
Gravity Point (Center of Gravity – COG)
The point where gravitational force acts on the body.
Support Area (Base of Support – BOS)
The area beneath the body that is in contact with the ground or
support surface.
Gravity Line (Line of Gravity – LOG)
An imaginary vertical line passing through the center of gravity
toward the ground.
,Uniform Motion (Linear / Translatory Motion)
Movement where all parts of a body move the same distance, in
the same direction, at the same time.
Types of Motion
Curved Path Movement (Curvilinear Motion)
Motion that follows a curved trajectory but does not form a full
circle.
Rotational Movement (Angular Motion)
Movement in which a body turns around a fixed axis or pivot
point.
Straight-Line Movement (Rectilinear Motion)
Motion that occurs along a direct, straight path.
Combined Motion (Linear & Angular Movement)
A combination where a body translates and rotates at the same
time.
Simple Machines & Levers
Mechanical Aid (Simple Machine)
A device that makes work easier by altering the size or direction
of force, or both.
Rigid Bar System (Lever)
A solid structure that rotates around a fixed point (fulcrum)
when effort is applied.
, The pivot point acts as the axis of rotation
Levers contain two arms: effort arm and resistance arm
Effort = applied force
Resistance = load to be moved
Key Principle
Joints act as axes, bones function as levers, and muscles or
external loads provide force.
Lever Arms
Effort Distance (Force Arm)
The distance between where effort is applied and the axis.
Load Distance (Resistance Arm)
The distance between the load and the axis.
Classes of Levers (Reworded)
Type I Lever (First-Class Lever)
The pivot lies between effort and load (E–A–L or L–A–E).
Improves balance and may reduce effort needed
Example: Head balanced on the neck
Type II Lever (Second-Class Lever)
The load lies between pivot and effort (A–L–E or E–L–A).
Reduces effort required by increasing mechanical
advantage
Example: Standing on tiptoes
FORCES, LEVERS, AND MOTION
Force Distance (Force Arm)
The straight-line distance from where force is applied to the axis
of rotation.
Effect of Moment Arm Length
A longer moment arm increases torque production
A shorter moment arm reduces torque
Patients must produce more muscle force if the moment
arm is shorter
Force Angle (Angle of Application)
The angle at which force is applied relative to a body segment or
object.
When Angle < 90°
The moment arm decreases; compressive force increases, while
rotational effect reduces.
When Angle > 90°
The moment arm decreases; traction (pulling apart) force
increases, while rotational effect reduces.
Newton’s Laws (Reworded)
Law of Inertia (Newton’s First Law)
An object remains at rest or continues moving at a constant
velocity unless acted upon by an external force.
,Law of Acceleration (Newton’s Second Law)
Acceleration depends on force and mass: more force increases
acceleration, while greater mass reduces it.
Example: A sprinter pushing harder off the blocks accelerates
faster.
Action-Reaction Principle (Newton’s Third Law)
Every applied force has an equal and opposite opposing force.
Example: Jumping on a trampoline.
Balance & Body Mechanics
State of Balance (Equilibrium)
Occurs when all forces acting on an object are balanced,
resulting in no movement.
Mass Center Point (Center of Mass – COM)
The point where the total body mass is evenly distributed.
Gravity Point (Center of Gravity – COG)
The point where gravitational force acts on the body.
Support Area (Base of Support – BOS)
The area beneath the body that is in contact with the ground or
support surface.
Gravity Line (Line of Gravity – LOG)
An imaginary vertical line passing through the center of gravity
toward the ground.
,Uniform Motion (Linear / Translatory Motion)
Movement where all parts of a body move the same distance, in
the same direction, at the same time.
Types of Motion
Curved Path Movement (Curvilinear Motion)
Motion that follows a curved trajectory but does not form a full
circle.
Rotational Movement (Angular Motion)
Movement in which a body turns around a fixed axis or pivot
point.
Straight-Line Movement (Rectilinear Motion)
Motion that occurs along a direct, straight path.
Combined Motion (Linear & Angular Movement)
A combination where a body translates and rotates at the same
time.
Simple Machines & Levers
Mechanical Aid (Simple Machine)
A device that makes work easier by altering the size or direction
of force, or both.
Rigid Bar System (Lever)
A solid structure that rotates around a fixed point (fulcrum)
when effort is applied.
, The pivot point acts as the axis of rotation
Levers contain two arms: effort arm and resistance arm
Effort = applied force
Resistance = load to be moved
Key Principle
Joints act as axes, bones function as levers, and muscles or
external loads provide force.
Lever Arms
Effort Distance (Force Arm)
The distance between where effort is applied and the axis.
Load Distance (Resistance Arm)
The distance between the load and the axis.
Classes of Levers (Reworded)
Type I Lever (First-Class Lever)
The pivot lies between effort and load (E–A–L or L–A–E).
Improves balance and may reduce effort needed
Example: Head balanced on the neck
Type II Lever (Second-Class Lever)
The load lies between pivot and effort (A–L–E or E–L–A).
Reduces effort required by increasing mechanical
advantage
Example: Standing on tiptoes
