IPHY 4540 Biomechanics
• Objective: to learn how to quantitatively analyze the
mechanical function of the human musculoskeletal system
using principles of physics and physiology.
• Topics:
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Functional Anatomy
Tissue Biomechanics (muscle, tendon, bone)
Movement (kinematics)
Movement dynamics (kinetics)
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Whole body, muscle and joint forces
Impulse, Momentum
energy, power
torque
Outline
• Functional Anatomy
• Muscle actions
• Lever systems in the body
Functional Anatomy
• the study of body components needed to achieve or
perform a human movement or functions.
– Care about movement produced by muscle group, not
muscle location
Functional Anatomy
Hammill J, Biomechanical Basis of Human Movement
• Terms
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superior
Inferior
anterior
posterior
medial
lateral
proximal
distal
superficial
deep
unilateral
ipsilateral
contralateral
bilateral
Hammill J, Biomechanical Basis of Human Movement
Functional Anatomy
• Reference System
– origin
– axes
– planes
• Planes
– Frontal
• Vertical
• Anterior/posterior parts
– Sagittal
• Vertical
• Left/right parts
– Transverse
• Horizontal
• Superior/inferior
Hammill J, Biomechanical Basis of Human Movement
Functional Anatomy
• Reference System
– origin
– axes
– planes
• Planes
– Frontal
• Vertical
• Anterior/posterior parts
– Sagittal
• Vertical
• Left/right parts
– Transverse
• Horizontal
• Upper/lower
Hammill J, Biomechanical Basis of Human Movement
• Axes:
– Longitudinal (Twist):
• head to toe
– Mediolateral (Somersault)
• Left to right
– Anteroposterior (Cartwheel)
• Front to back
The knee
Muscle Actions
• Motors that move our limbs
• Example:
– Push-up
• Which muscle?
• Which joint?
• Flexion/extension?
– Leg curl
Muscle Actions
Muscles:
- only exert pulling forces
- 3 actions: can exert force and
-shorten (shortening/concentric)
-stay the same length (isometric)
-be lengthened or resist stretching
(lengthening/eccentric)
- when not activated, muscles can be passively lengthened
by other forces
Shortening Muscle Actions
Isotonic = constant force
(artificial)
Isovelocity = constant
velocity (artificial)
Neither, i.e. variable force
and velocity
(most common)
Isometric Muscle Actions
Muscle produces force, but
does not overcome external
load, so no movement
develops.
Very common: examples?
Lengthening Muscle Actions
Muscles only exert pulling
force,
But the opposing force can
be greater than the
muscle force, forcibly
stretching the muscle.
Also very common.
High Risk for injury
Examples:
Leg press example
• What is the primary muscle involved?
• Describe its actions throughout one cycle.
Muscle Actions
- only exert pulling forces
- 3 actions: can exert force and
-shorten (shortening/concentric): force>load
-stay the same length (isometric): force=load
-be lengthened or resist stretching (lengthening/eccentric) force<load
Force > load
Force
Load
Force = load
Force < load
Outline
• Functional Anatomy
• Muscle actions
• Lever systems in the body
Levers
• A method of transmitting force
– Amplify force
– Amplify velocity
– Amplify movement distance
• Muscles use leverage to generate movement
Levers
• 4 components
– Lever (bone)
– Fulcrum (pivot point, joint))
– Effort force (muscle force)
– Load force (external forces)
effort
load
lever
fulcrum
Levers: The Law
• Law of Levers:
– Fload x rload = Feffort x reffort
– r is the distance to the forces
“Give me a lever and I shall move the earth” – Archimedes
Mechanical Advantage: Fload/Feffort > 1, rload < reffort
Mechanical Disadvantage: Fload/Feffort < 1, rload > reffort
Levers: The Law
• Fload rload = Feffort reffort
A vs B: Mechanical Advantage or Disadvantage? Fload/Feffort = ?
100kg
F
A
rload=1m
Feffort=my laptop (2kg)
B
Levers: 3 classes
Don’t worry about first, second, third class
Do understand:
fulcrum = joint = pivot
The relative positions of muscle force, joint and
load are important.
What is being amplified? Force or velocity
3 classes of levers
1.
Classes of Levers
1st Class— see-saw
2.
2nd Class—Wheelbarrow
3.
3rd Class—Tweezers
Levers: Force and Velocity
– Fload x rload = Feffort x reffort
• Class 1
– Fload x rload = Feffort x reffort
• If rload>reffort
• Fload< Feffort velocity amplifier
• If rload<reffort
• Fload>Feffort  force amplifier
1st Class Levers
Muscle joint load (MJL)
Load
joint Muscle (LJM)
1st Class Lever
Muscle joint load
e.g. triceps acting at the elbow
Velocity or force magnifier?
1st Class Levers
Muscle joint load
e.g. your head is tilting forward (e.g. sleeping)
Muscles on the back of the neck pull
1st Class Levers (MJL) in the body
Amplify velocity of joint extension
1st Class Levers (MJL) tools
Can amplify force or velocity depending on
length of the lever
e.g. tool that amps force using 1st class lever
1st Class Levers (MJL) tools
Can amplify force or velocity depending on
length of the lever
e.g. tool that amps velocity using 1st class lever
Class 3 levers (JML)
– Fload x rload = Feffort x reffort
• Class 3
– Fload x rload = Feffort x reffort
• rload>reffort
• Fload< Feffort velocity amplifier
Joint
Muscle Load
3rd Class Levers
joint
muscle load
3rd Class Levers
joint
muscle load
e.g. hold weight in your hand, biceps pulls up
3rd Class Levers (JML) tools
Can only amplify velocity
e.g. field hockey stick, golf putter, shovel
Class 2 levers (JLM)
• Class 2
– Fload x rload = Feffort x reffort
• rload<reffort
• Fload> Feffort force amplifier
Rare in the body
Jaw joint (using molars) is a good
example
Any others?
Joint
Load Muscle
2nd Class Levers (JLM) tools
Can only amplify force
e.g. nut cracker
Summary
Classes of Levers
1. 1st Class— see-saw, push-up
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Muscle-Joint-Load
Force or Velocity amplifier,
depending on the lever arm
2. 2nd Class—Wheelbarrow
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Joint-Load-Muscle
Force Amplifier
1. 3rd Class—Tweezers, biceps-curl
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Joint-Muscle-Load
Velocity Amplifier
• Why do we usually see 3rd class levers in the
body?
OR
• Why do we have short muscle arms?
Anatomical Terms
• Joint Position
– To discuss joint position we
define joint angle
– joint angle: relative angle
between two segments
• Joint Movement
– Flexion: relative angle (joint
angle) decreases
– Extension: relative angle
increases (straightening
movement)
Hammill J, Biomechanical Basis of Human Movement
More muscle terms
agonist
muscles creating the same joint movement
antagonist
muscles opposing or producing the opposite joint
movement
must relax to allow a movement to occur OR
contract to slow a movement down
example: kicking
co-contraction
1 joint vs. 2 joint muscles
Examples of 1 joint muscles:
Examples of 2 joint (biarticular) muscles:
Biarticular muscles
• Advantages
How do we stand up from a chair?
Joint actions:
What muscles act across the hip?
What muscles act across the knee?
What muscles act across the ankle?
All 1-joint muscles? All 2-joint muscles?