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what is kinematic and kinetics? expalin with examples? for 15 marks

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Kinematics and Kinetics in Biomechanics

(15-Mark Exam Answer)

1. Introduction to Biomechanics

Biomechanics combines engineering mechanics with biology and physiology. It applies mechanical principles to the human body to understand the mechanical influences on bone and joint health. Within biomechanics, two major subdivisions exist: kinematics and kinetics.

"Kinematics is the study of motion without analyzing the forces causing the motion... and kinetics focuses on the forces that generate that motion."

Firestein & Kelley's Textbook of Rheumatology

2. KINEMATICS

Definition

Kinematics is the study of motion without reference to the forces that cause it. It describes HOW a body moves - in terms of position, displacement, velocity, acceleration, and time.

Miller's Review of Orthopaedics: "Kinematics: study of motion (displacement, velocity, and acceleration) without reference to forces."

Types of Motion in Kinematics

Type	Description	Example
Translation	All parts of the object move the same distance at the same time	Sliding a book across a table
Rectilinear Translation	Path is a straight line	A person walking in a straight corridor
Curvilinear Translation	Paths are curved	A ball thrown through the air
Rotation	All parts rotate through the same angle	Swinging of the forearm at the elbow
General Motion	Combination of translation + rotation	Most human body movements

Key Kinematic Variables

Position - location of a rigid body in space (3 translations + 3 rotations = 6 degrees of freedom in 3D)
Displacement - change in position (linear or angular)
Velocity - change in displacement over time (m/sec)
- Linear velocity = rate of translational displacement
- Angular velocity = rate of rotational displacement
Acceleration - change in velocity over time (m/sec²)

Types of Kinematic Analysis

2-D (Planar) Analysis - simplified analysis where motion is in one plane (e.g., sagittal plane walking)
3-D Analysis - full analysis in all three planes (most accurate for human movement)

Clinical Example 1 - Gait Analysis (Walking)

During normal walking, knee motion is primarily in the sagittal plane:

At initial contact: knee is near full extension (~5° flexion)
During loading response: knee flexes to ~15°
At mid-stance: knee moves back into extension
At toe-off (~60% of gait cycle): knee begins rapid flexion in swing phase

This is a purely kinematic description - it tells us WHAT the joint is doing, not why.

Clinical Example 2 - Shoulder Flexion

When a patient raises their arm overhead, kinematics describes the arc of motion: 0-180° of shoulder flexion, the associated scapular upward rotation (scapulohumeral rhythm), and the velocity of movement. No forces are considered - only angles and motion.

Spatiotemporal Parameters (also Kinematic)

Step length - distance between heel contact of one foot to heel contact of the opposite foot
Stride length - distance from one heel contact to the same foot's next heel contact (one full gait cycle)
Cadence - steps per minute
Walking speed - distance covered per unit time

3. KINETICS

Definition

Kinetics is the study of forces that cause, arrest, or modify motion in a system.

Miller's Review of Orthopaedics: "Kinetics: study of forces that cause motion."
Firestein & Kelley's Textbook of Rheumatology: "Kinetics is the study of the forces that cause motion of a rigid body. These forces can be classified as either external forces or internal forces."

Types of Forces in Kinetics

A. External Forces - forces from objects contacting the body:

Gravity (body weight acting downward)
Ground Reaction Forces (GRF) - the force the ground exerts on the foot during walking/running
Applied loads - forces from equipment, weights, splints

B. Internal Forces - the body's responses to external forces:

Muscle forces - generated by muscle contraction
Ligament forces - passive tension in ligaments
Joint contact forces - compressive and shear forces at joint surfaces

Newton's Laws Applied to Kinetics

Law	Statement	Clinical Relevance
1st Law (Inertia)	A body at rest stays at rest unless acted upon by a net force	A limb won't move unless muscles produce force
2nd Law (F = ma)	Force = mass × acceleration	Greater muscle force needed to accelerate a heavier limb
3rd Law (Action-Reaction)	Every force has an equal and opposite reaction force	Ground pushes up on the foot with the same force the foot pushes down

Key Kinetic Variables

Force (F) - mechanical push or pull; unit = Newton (N)
Torque / Moment - rotational force; Force × perpendicular distance (moment arm); unit = N·m
Joint Reaction Force - net force transmitted across a joint
Power - rate of doing work (Force × velocity); unit = Watts (W)
Work - Force × displacement; unit = Joules (J)

Clinical Example 1 - Patellofemoral Pain Syndrome

The joint contact force at the patellofemoral joint during stair climbing can be 3-4× body weight. This is a kinetic measure. The large quadriceps muscle force (internal force) combined with gravity (external force) generates this high joint contact force. Understanding this helps physiotherapists prescribe exercises that reduce patellofemoral loading.

Clinical Example 2 - Ground Reaction Forces in Running

During running, the ground reaction force can reach 2-3× body weight at heel strike. Excessive vertical loading rate (a kinetic variable) is linked to stress fractures. Physiotherapists use this kinetic information to modify running technique and footwear.

Clinical Example 3 - Muscle Torque at the Elbow

When holding a weight in hand, the biceps must generate enough torque to balance the torque created by the weight. If the load is 5 kg held at 30 cm from the elbow, the external torque = 5 × 9.8 × 0.3 = 14.7 N·m. The biceps must match this with its own torque. This is pure kinetics.

4. KEY DIFFERENCES: Kinematics vs. Kinetics

Feature	Kinematics	Kinetics
Focus	Description of motion	Forces causing motion
Variables	Position, velocity, acceleration, joint angles	Force, torque, power, work
Question answered	How does the body move?	Why does the body move?
Forces considered	No	Yes
Clinical tools	Goniometer, motion capture, video analysis	Force plates, EMG, dynamometers
Example	Knee flexes to 90° during sitting	Quadriceps generates 150 N·m torque to achieve that flexion

5. Physiotherapy Relevance

Kinematics is used in gait analysis, post-surgical movement assessment, and range of motion measurement. A physiotherapist analyzing a patient's walking pattern and recording joint angles is performing kinematic analysis.
Kinetics is used to assess muscle strength, joint loading, and forces during functional tasks. Isokinetic dynamometry and force plate analysis are kinetic assessments.
In clinical practice, both are needed together for a full picture - kinematics tells you what's happening, kinetics tells you why and how much force is involved.

Summary

	Kinematics	Kinetics
Definition	Study of motion without forces	Study of forces causing motion
Sub-types	Translation, rotation, general motion	External forces, internal forces
Key variables	Position, velocity, acceleration	Force, torque, power, work
Human example	Measuring ROM of the shoulder	Calculating biceps force during lifting
Gait example	Joint angles during walking	Ground reaction forces during walking

Sources: Firestein & Kelley's Textbook of Rheumatology; Miller's Review of Orthopaedics 9th Edition; Rheumatology 2-Volume Set (Elsevier, 2022)

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