Exam 3 Flashcards
1
Q
The principle or law used to describe lift is called?
A
Bernoulli’s principle
2
Q
What shape is most conductive to optimizing lift?
A
Foil
3
Q
When we consider the dynamic fluid forces acting on an object, we must take into account the velocity of the object as well as the velocity of the fluid itself.
A
True
4
Q
The ratio of the weight of an object to the weight of an equal volume of water is …
A
Specific Gravity
5
Q
The size of the surface drag is affected by
A
All of the above:
-Coefficient of drag
-Density of the fluid
-Cross-sectional area of the object
-Square of the relative velocity
6
Q
Water pressure ___with depth. Pressure is defined as ____.
A
increases linearly; force per unit area
7
Q
The vertical force that always acts upward is …
A
Buoyant force
8
Q
Buoyant force is present whether the object is at rest or is moving relative to the fluid.
A
True
9
Q
The concept that explains how a spinning baseball changes direction is called ?
A
The Magnus effect
10
Q
Which component of bone is primarily responsible for its compressive strength
A
minerals
11
Q
To obtain the least pressure
A
apply a small force spread over a large area
12
Q
When stretched material returns to its original shape, it is called …
A
Elastic
13
Q
At 90 degrees of knee flexion, the mechanical stress at the tibiofemoral joint is 821.43 N/cm^2. If the compressive force is 1150 N, the joint contact area (in cm^2) must be …
A
1.40
14
Q
identify the true statement
A
increasing the surface area over which a force is distributed decreases the amount of stress acting on the surface
15
Q
ligaments are less stiff and slightly weaker than tendons because
A
ligaments have more elastin and less aligned collagen fibers
16
Q
At 1cm long section of the Achilles tendon stretches to 1.005 cm when it is subjected to a tensile force of 20.000 N. What is the strain in this segment of tendon?
A
0.5%
17
Q
Fluid
A
Most sport biomechanics are either
* Land-based: performed in air
* Aquatic-based: performed in water
Liquids and gases are called fluids because they can flow and easily change shape.
18
Q
Fluid forces
A
-Sometimes ignored (projectile motion)
-Sometimes life-saving (skydiving)
19
Q
Buoyant force
A
Upward directed force that a fluid exerts on an object that is partially or completely immersed in the fluid.
Vector: Magnitude & direction
20
Q
Buoyant Force Magnitude
A
Equal to weight of the volume of fluid displaced by the object (Archimedes’ principle)
-More water displaced = greater buoyant force
- If weight of object > weight of water displaced, sinks
-If weight of object = weight of water displaced, floats
-If weight of object < weight of water displace, rises
21
Q
Specific Gravity and Density
A
ratio; object’s weight to the weight of equal volume of water, helping determine if it sinks or floats.
If Specific gravity ≤ 1.0, object floats
If Specific gravity > 1.0, object sinks
Density : Ratio of mass to volume
22
Q
Buoyancy of the Human Body
A
The human body’s buoyancy depends on tissue density:
- Bone & muscle are denser than water (>1.0), so they sink.
- Fat is less dense than water (<1.0), so it floats.
(This concept is used in underwater weighing to measure body fat.)
air in the lungs reduces the body’s specific density, making it less dense than water. This helps the body float, as the air adds buoyancy.
23
Q
Floating Horizontally
A
When the body is horizontal in the water, the weight and buoyant forces aren’t aligned, causing a torque that tries to rotate the body into a vertical position.
When the legs sink, the weight and buoyant force align, meaning there’s no torque, and the body stabilizes in a more vertical position.
24
Q
Key to floating:
A
weight and buoyant force must align before angular momentum becomes too great
25
Dynamic Fluid Force: Force Due to Relative Motion
- Occurs when an object moves through a fluid or when fluid moves past an object.
Fluid forces depend on:
- Density of the fluid
- Square of the relative velocity between the object and the fluid
26
Dynamic Fluid Force: Force Due to Relative Motion Mathematically
Present when an object moves within a fluid
Mathematically: F pAV^2
Where:
* F = dynamic fluid force
* ⍴ = fluid density
* A = surface area of the object
* v = relative velocity of the object to the fluid
27
Dynamic Fluid Force: Force Due to Relative Motion Mathematically explained
Fluid density (⍴) and surface area (A) are linear factors in dynamic fluid force. This means:
- Doubling the fluid density (⍴) or surface area (A) results in twice the dynamic fluid force.
- The force increases proportionally with changes in either parameter.
Relative velocity (v) is a squared term in dynamic fluid force. This means:
- Doubling the relative velocity (v) results in a fourfold increase in dynamic fluid force (2² = 4).
- Velocity has the most significant impact on fluid force compared to fluid density (⍴) and surface area (A), which are linear factors.
28
Relative Velocity
Difference between the velocity of the object and the fluid
(Velocity of the object) − (Velocity of the fluid)
Ex:
You standing still, wind blowing against you at 5 m/s
* RV = (0 m/s) − (−5 m/s) = 5 m/s
* You running at 5 m/s, no wind
* RV = (5 m/s) − (0 m/s) = 5 m/s
* You running at 5 m/s, wind blowing against you at 5 m/s
* RV = (5 m/s) − (−5 m/s) = 10 m/s
29
Drag Force
The drag force is the component of the resultant dynamic fluid force that acts opposite to the relative motion of an object moving through a fluid.
Drag force tends to slow down an object
30
Drag force mathematically
Mathematically, FD = CD pAV 2
Where:
* FD = drag force
* CD = coefficient of drag
* ⍴ = fluid density
* A = cross-sectional area of the object perpendicular to the relative velocity
* v = relative velocity of the object
31
Drag Force is Produced in Two Ways:
1. Surface Drag (Friction Drag)
-Caused by friction between fluid molecules and the object's surface.
-Rougher surfaces increase friction, leading to greater surface drag.
-Smooth surfaces (tight-fitting suits in running, swimming, skiing, and cycling) help reduce drag.
2. Form Drag (Pressure Drag)
-Caused by impact forces between fluid molecules and the object's surface.
-Broad, flat leading surfaces create more resistance and higher form drag.
-Streamlining reduces this effect by shaping objects to allow fluid to flow smoothly around them, decreasing resistance.
Both types of drag impact motion in air and water
32
Laminar Flow & Turbulent Flow
If fluid molecules stay close to the object's surface:
-At the leading surface, form drag forces push towards the rear
- At the trailing surface, form drag forces push towards the front
Laminar flow: when molecules stay close to surface
Turbulent flow: when molecules separate from the surface
33
Tennis ball fuzz and golf ball dimples explanation
Tennis ball fuzz and golf ball dimples create turbulent airflow, which helps control the ball's movement:
-Golf ball dimples reduce drag by keeping air attached to ball longer, allowing it to travel farther.
-Tennis ball fuzz increases drag and spin effects, helping with better control and different shot types.
34
Reducing the Drag Coefficient
Making body surfaces, clothing, and equipment smooth helps reduce surface drag by minimizing friction between the fluid (air or water) and the object.
Streamlining involves shaping the body and equipment to reduce drag and improve performance:
-Athlete body position relative to fluid flow (a cyclist lowering their torso or a swimmer maintaining a horizontal position) minimizes resistance.
-Equipment should have rounded fronts to smoothly push through the fluid and elongated rears to allow the fluid to flow smoothly away, minimizing wake and drag.
Reducing the total surface area exposed to the flow helps minimize drag:
In swimming, one way to reduce drag is by choosing pools with warmer water. Warmer water is less dense, which reduces resistance compared to colder water.
35
Form Drag & Surface Drag contributions to total drag
The relative contributions of form drag, and surface drag depend on the velocity of the object moving through the fluid:
-Form drag greater at faster velocity
- Surface drag greater at slower velocity
There is a trade-off between surface area and streamlining. Smaller surface area decreases surface drag, but streamlining typically increases surface area
speeds less than 10-20 m/s (32 km/h) streamlining benefits & at lower speeds, reduced surface area is more beneficial
36
Strategies for Increasing Drag Force
Increased drag force can benefit some activities:
-Rowing: Paddles and oars create drag force, which helps propel the boat or shell forward by pushing against the water.
-Skydiving: Parachutes use drag force to slow down the descent, ensuring a safe landing by creating resistance in the air.
-Water exercise: Drag force useful in rehabilitation and cross-training because it provides resistance without impacting joints, helping to build strength and endurance in a low-impact environment.
In these activities, drag force is intentionally used to enhance performance or ensure safety.
37
Lift Force mathematically
The component of the resultant dynamic force acting perpendicular to the relative motion is known as the lift force
Mathematically, FL = CL pAV 2
Where:
* FL = lift force
* CL = coefficient of lift
* ⍴ = fluid density
* A = reference area (usually the cross-sectional area of the object
perpendicular to the relative motion)
* v = relative velocity of the object
38
Lift force
Lift force changes the direction of the relative motion of an object in a fluid
-Can be in any direction perpendicular to the fluid flow, not just in the direction of motion:
* Upward
* Downward
* Sideways
39
Which of the following terms is defined as deformation divided by original length?
strain
40
When a stretched material does not return to its original shape, it is called
plastic
41
Stress is defined as the
force per unit area
42
A 1cm long section of the Achilles tendon is 1cm square in cross section. What is the stress in this tendon if it is subjected to a tensile force of 20.000N ?
0.20GPa
43
For a submerged object. the buoyant force acts at the
center of volume
44
Lift Force and Newton
For every action (the object pushing on the fluid molecules), there is an equal and opposite reaction (the fluid molecules pushing back on the object), creating lift force.
Force acts perpendicular to the relative motion between the fluid and the object.
45
Lift Forces in Human Activity
Ski jumpers
* Wide skis and body position increase lift force to stay in air longer
Water
* Upper extremity position and motion create lift force to propel
Sailing
* Lift forces propel sailboats and
sailboards
46
Bernoulli’s Principle
According to bernoulli’s Principle, faster moving fluids produce less force laterally than do slower moving fluids
* An airfoil is an object with two surfaces aligned with fluid flow, where one surface is more curved than the other.
* Fluid moves faster over curved surface than flatter surface
47
Bernoulli’s Principle surfaces
fluid moves faster over the curved surface, creating less perpendicular force, while it moves slower over the flatter surface, generating more perpendicular force.
Lift force results from the difference in perpendicular forces, acting from the flatter surface toward the curved surface.
48
Spin and the Magnus Effect
The Magnus Effect causes lift force to act on a spinning ball due to pressure differences created by varying airflow speeds around it.
-In topspin the upper surface has forward velocity relative to center of ball creating lower relative velocity with respect to the air molecules.
-while the lower surface has backward velocity relative to center of ball creating higher relative velocity with respect to the air molecules.
As a result, there is a greater perpendicular force directed downward on the upper surface of the ball.
49
Spin and the Magnus Effect: backspin & topspin
The Magnus Effect causes the lift force to be the net result of perpendicular forces: it acts downward on a ball with topspin and upward on a ball with backspin.
Ex:
-A curveball in baseball uses sidespin to generate a sideways lift force on the ball. When combined with topspin or backspin, it creates unpredictable movement, confusing the batter.
-In golf, backspin causes the ball to stay in the air longer, while sidespin makes the ball hook or slice to the right or left.
50
Center of Pressure
The center of pressure is the point where the dynamic fluid force, resulting from both lift and drag forces, is applied to an object.
The center of pressure is the theoretical point where the resultant fluid force (lift and drag) is applied, similar to how the center of gravity (CG) represents the point where an object's weight is concentrated.
If center of pressure and cg are not aligned, torque is created (Object will wobble)
51
Dynamic fluid force
also known as dynamic pressure, is the pressure exerted by a fluid in motion, measured parallel to the direction of flow, and depends on the fluid's velocity.
52
Effect of Fluid Forces
Newton’s second law, ΣF = ma, explains how the total force (ΣF) acting on a body affects its motion, where "m" is the mass of the object and "a" is its acceleration.
If a 50 kg and a 70 kg runner are similar in size and shape, the wind will have a greater effect on the less massive runner. In a headwind, the lighter runner will slow down more, while in a tailwind, the lighter runner will speed up more.
Easier to throw curveball with a Nerf ball than a baseball
53
External Forces Impose Loads
External forces on a body affect motion
External forces also load a body and affect internal structures such as: Cartilage, Tendons Ligaments, Bones, & Muscle
Mechanical response of structures are related to injury
54
Mechanics of Materials
considers a segment as a deformable body, meaning it can undergo changes in shape or size under the influence of forces, rather than being treated as a rigid, unchanging object
External forces imposed on a body are resisted by internal forces, which result in the deformation of the body.
- External applied load: stress
- Deformation caused: strain
55
Mechanical Stress
Mechanical stress refers to how an applied force is distributed over the body it acts on
Mathematically, σ =
Where
* σ = stress
* F = internal force (N)
* A = cross-sectional area of the internal surface (m2)
* Units: N/m2 (Pascal [Pa])
56
Internal Force Visualization
a stretched rubber band in equilibrium (ΣF = 0), the pulling force at the left end (P1) is equal to the pulling force at the right end (P2). If you cut the band through an analysis plane at A-A, each half of the band would no longer be in equilibrium unless forces are applied in the opposite directions of P1 and P2. The forces that must be applied at each end to maintain equilibrium are the internal forces.
57
Cross-Sectional Area
The force in the stress equation is the resultant of the many intermolecular bond forces across the imaginary cut plane. The cross-sectional area of this imaginary plane (denoted as A) is the area over which the internal force is applied, and this is used to calculate stress (σ = F/A).
58
Types of Mechanical Stress
The three principal types of mechanical stress, depending on the applied loading, are:
-Tension: Stress caused by forces that attempt to stretch or elongate an object.
-Compression: Stress caused by forces that attempt to compress or shorten an object.
-Shear: Stress caused by forces that cause sliding or twisting along a plane within the material.
Axial stress occurs when external forces act perpendicular (normal) to the analysis plane. It is also known as normal or longitudinal stress. In the case of a uniaxial load, the external forces are collinear, meaning they act along the same line.
59
Tension
Tension, or tensile stress, is an axial or normal stress that occurs at the analysis plane due to a force or load pulling the molecules apart at that plane. Tension causes the body to deform by stretching or elongating.
The bone's cross-sectional area depends on the plane of analysis. A larger cross-sectional area results in lower stress, while a smaller cross-sectional area leads to higher stress
60
Sample Tensile Stress Calculation
-Cross-sectional area: 1 cm2 (= 0.0001 m2)
-Tensile force applied: 700,000 N
* What is the maximum tensile stress?
σ = = (700,000 N) = = 7,000,000,000Pa
7,000,000,000 Pa = 7 GPa (Gigapascal)
61
Compression
Compression, or compressive stress, is an axial or normal stress that occurs at the analysis plane due to a load squeezing the molecules together. Compression causes the body to deform by shortening.
The bone's cross-sectional area depends on the plane of analysis. A larger cross-sectional area leads to lower stress, while a smaller cross-sectional area results in higher stress, as stress is inversely proportional to the area (σ = F/A).
62
Shear
Shear stress is a transverse stress that acts parallel to the analysis plane. It results from non-colinear forces that cause the molecules at that plane to slide. Shear causes the body to deform by skewing, changing the orientation of the sides of the object.
Shear stress represented with 𝞽
* 𝞽 = Shear force / area
63
Stresses and Injuries
Here’s how different types of stresses relate to injuries:
Tension:
- Sprains: Rupture of ligaments and tendons.
- Strains: Tears in muscles and cartilage.
Compression:
- Bruises: Damage to soft tissue due to compressive forces.
- Crushing fractures: Breaks in bones caused by compressive forces.
Shear:
- Blisters: Skin damage caused by shear forces.
- Dislocation: Joints are displaced due to shear stresses.
64
Mechanical Loading
When external forces are not uniaxial, more complex mechanical loading occurs, including:
Bending: Forces cause an object to curve, creating tension on one side and compression on the other.
Torsion: Twisting forces that cause an object to rotate about its axis.
Combined loads: A combination of different types of stresses (tension, compression, bending, torsion) acting on an object simultaneously.
-The complexity of loading reflects the number, direction, and location of external forces imposed on an object, as well as the shape of the object itself, which affects how forces are distributed and how the object deforms.
65
Bending
Bending load is a type of nonaxial loading that results in tension and compression at the analysis plane. This occurs when three or more forces create force couples at opposite ends of the object. Bending tends to rotate the ends of the body in opposite directions at the analysis plane, causing the object to curve.
Bending loads cause the body to deform by curving (bending). One side of the object is squeezed (compressed), while the opposite side is stretched (tensioned).
66
Torsion
Torsion is a nonaxial loading where shear stress acts parallel to the analysis plane due to opposing torques applied about the long axis of the body. Torsion causes the body to deform by twisting. The shear stress from the torsion load increases with greater distance from the axis of rotation.
Tubular and hollow cross-section of bones resists torsion
67
Combined Loading
Human bones experience complex loading from gravity, tendons, ligaments, other bones, and contact forces. This sustained loading typically involves a combination of:
Tension: Forces pulling apart the bone, such as those from tendons.
Compression: Forces pushing or squeezing the bone, such as from weight-bearing or contact forces.
Simple shear loads: Forces causing layers of bone to slide against each other.
Bending: Forces that cause the bone to curve, creating tension on one side and compression on the other.
Torsion: Twisting forces acting on the bone, creating shear stress along the length of the bone.
These combined stresses help bones adapt to various movements and loads, but they also increase the risk of injury if excessive forces are applied.
68
Strain
Strain quantifies the material's deformation in response to applied stress. There are two main types of strain:
Linear strain: The change in an object’s length, typically produced by tensile or compressive stress.
Shear strain: The change in the orientation of an object’s molecules, typically produced by shear stress.
Both types of strain describe how materials deform under different loading conditions.
69
Linear Strain
Linear strain is the deformation produced by tensile or compressive stress, resulting in a change in length. It can be reported in two ways:
Absolute terms: Describes the actual change in length, such as an intervertebral disc being compressed by 2 mm.
Relative terms: Describes the proportional length change using the formula: (Deformed Length – Undeformed Length) / Undeformed Length)
70
Linear Strain Mathematically
ε = (ℓ – ℓo ) / ℓo
Where:
* ε = linear strain
* ℓ = stretched length
* ℓo = original, undeformed length
* ℓ − ℓo = change in length
Linear strain is dimensionless because it is a ratio of the change in length to the original length. However, it is often reported as a percentage for easier interpretation: Strain(ε)×100
This gives a clearer sense of the proportional deformation, with the result expressed as a percentage.
71
Shear Strain
Shear strain is the deformation produced by shear stress, resulting in a change in the orientation of adjacent molecules.
Reported as the change in angle of a perpendicular plane
* △⍬
* Units: radians
72
Poisson’s Ratio
Poisson’s Ratio quantifies how an object’s width changes when it lengthens or shortens under stress. This effect is called transverse strain, where:
-When an object lengthens due to tensile stress, it gets narrower in width
-When an object shortens due to compressive stress, it gets wider in width
73
Types of Stress describe...
the internal resistance or deformation caused by these forces acting on the material
74
Poisson's Ratio equation
Poisson’s ratio = (Axial strain) / (Transverse strain)
It describes how materials deform in multiple directions under load.
Poisson’s Ratio (v) typically ranges from 0.1 to 0.5 (dimensionless), with most common materials falling between 0.25 and 0.35
75
The Stress–Strain Relationship
Describes the behavior of a material under load
Elastic Behavior
* Material deforms and returns to original length when unloaded
Plastic Behavior
* Material deforms but doesn’t return to original length (Permanent deformation; this is due to disruption of the internal structure)
76
Elastic Behavior
Elastic modulus (E), also called Young’s modulus, is the slope of the stress-strain curve, representing stiffness.
-Bulk modulus: slope describing compressive loading
-Shear modulus: slope describing shear loading
77
Elastic behavior: Stiff material/ pliant material
Stiff materials have a high elastic modulus (E), meaning they experience less strain per unit of stress (bone).
Pliant materials have a low elastic modulus (E), meaning they experience more strain per unit of stress (tendons and ligaments).
Bone is stiffer than tendons and ligaments, while tendons and ligaments are more pliant than bone, allowing for flexibility and movement.
78
Plastic Behavior
Plastic Behavior begins when material surpasses its elastic limit (yield point) on the stress-strain (σ/ε) curve.
-Elastic limit: maximum stress material can withstand while still returning to its original shape.
-Stress above this point causes permanent deformation, meaning the material will not fully recover when the load is removed.
Plastic behavior: material doesn’t return to original shape
The mechanical strength of a material refers to the maximum stress it can endure before failure
79
Types of Material Strength
Yield Strength: The stress at the elastic limit, beyond which the material undergoes permanent deformation and does not return to its original shape.
Ultimate Strength: The maximum stress a material can withstand before it starts to fail or break down.
Failure Strength: The stress level where total failure occurs, resulting in fracture or complete rupture of the material.
80
Failure Strain
Failure strain is the amount of strain a material undergoes at total failure (fracture or rupture).
Ductile materials (ligaments) exhibit large failure strain, meaning they deform significantly before breaking.
Brittle materials ( bone) exhibit low failure strain, meaning they break with minimal deformation.
Toughness: ability of a material to absorb energy before
failure (Ligaments and tendons are tougher than bone)
81
Connective Tissue
Components of the musculoskeletal system exhibit different properties
-Muscle: active element of connective tissue
-Passive elements of connective tissue: Bone, Cartilage, Ligament, Tendon
The composition of connective tissue varies based on function, with different proportions of: ground substance, minerals, water
82
Connective Tissue Components
Collagen: a fibrous protein
* Very stiff (failure strain of 8%-10%; deforms only slightly before breaking.)
Elastin: a fibrous protein
* Very pliant (failure strain up to 160%; can stretch significantly before breaking.)
83
Connective Tissue Composition and Loading Response
Isotropic: stiffness similar in different directions meaning their response to stress is uniform regardless of the direction of the applied load.
Anisotropic: stiffness dependent on load direction meaning they respond differently to loads depending on the direction of stress.
84
Mechanical Properties of bone
Composition: 45% mineral, 30%-35% collagen, 20% water
Strongest and stiffest material in musculoskeletal system
Bone is strongest when subjected to compressive forces, followed by tension, with shear being the weakest.
Bone strength varies with health, influenced by factors like mineral density and collagen quality.
Rate of loading also affects bone strength, with bones being stronger under rapid loading (impact forces) compared to slow, sustained loading.
85
Mechanical Properties of cartilage
Composition: 60%-80% water, 10%-30% collagen
Types of Cartilage:
Hyaline (Articular) Cartilage: Covers bone ends at joints, reducing friction and absorbing impact forces.
Fibrous Cartilage: Found in specialized joint structures like intervertebral discs and menisci, providing shock absorption and stability.
86
Mechanical Properties of Tendons and ligaments
Composition: 70% water, 25% collagen
Ligaments have more elastin than tendons
Collagen fibers are organized in bundles along the functional axis, providing high stiffness and resistance to tensile forces, making tendons and ligaments strong under stretching loads.
87
Mechanical Properties of muscle
Active Component: Sarcomeres (contractile units) determine muscle stiffness, adjusting resistance based on activation level.
Passive component (connective tissue sheaths) create high failure strain (stretch significantly before rupture.)
88
Quantitative Biomechanical Analysis
-Measures human movement and its underlying causes using numerical data.
-Quantification: Describes motion with measurable values ( force, velocity, acceleration).
Previously required specialized equipment and significant effort.
Smartphone apps now make quantification widely available
89
When to Use a Quantitative
Biomechanical Analysis
When biomechanical changes are too fast or too subtle to detect with the naked eye.
Sports: Tracks performance changes over a season or career.
Ergonomics & Human Factors: determines causes of overuse injuries
Clinical biomechanists determine effects of medical
interventions on rehabilitation
90
Biomechanical Variables Quantified
-Temporal: Measures timing of movements ( stride time/reaction time)
-Kinematic: Describes motion without considering forces, including position, displacement, velocity, and acceleration.
-Kinetic: Analyzes forces and energy involved in movement, including force, work, power, and energy.
91
Measurement Issues
Technology used must minimize influence on performance
-Noninvasive; Parameter measured must be valid
Accurate and precise
* Quantified with minimal measurement error
* Error least tolerable in research, more tolerable in clinical
or sport application
92
Laboratory Data Collection
Labs allow for careful control of the environment
-Control helps minimize measurement error But novelty of the environment may affect performance, so duplicate natural setting as much as possible
Conditions similar for each repeat of skill performance; Much of the measurement equipment is permanently set up, allowing for efficient and consistent data collection across multiple trials.
* Performer must become familiar
with the environment
93
In-the-Field Data Collection
Actual competition may be the best environment for athlete
* Familiarity with the setting
* High motivation and performance level
But not the best for data collection
* Technology not easily portable
* Instrumentation not easily mounted in competitive setting
* Limited control of the environment
* Restricted locations for recording instrumentation
* Crowd control issues
94
Analog Signals and Sampling Rate
Most biomechanical parameters vary with time
* Require measurement throughout the performance
* Many instruments record the variable in analog form
* Electronic voltage
* For processing via computer, converted to digital form
* Analog to digital convertor
* Measure voltage (signal) at discrete times
* Sampling rate or sampling frequency
* Video: frames per second (fps)
95
Video Sampling Frequencies
Typical consumer video camera: 30 to 60 fps
* Smartphones and digital cameras: 120 to 240 fps
* High speed video cameras: 1,000 fps (but lower resolution)
* For most analyses: less than 100 fps adequate
* Faster action and impacts requires higher fps
96
Tools for Measuring Biomechanical
Variables
From stopwatches to synchronized multicamera systems
* Constantly evolving instrumentation and techniques
* Two general categories
1. Tools for measuring kinematics
* Position, velocity, acceleration
2. Tools for measuring kinetics
* Force
97
Kinematics: Timing Devices
Simple watch appropriate for approximation of long events
* Automatic timing devices appropriate for more accuracy
* Pressure-sensitive mats trigger step-on and step-off
* Photogates use light beam to trigger on-off as beams broken
* Set triggers known distance apart provides average speed
* Speed = distance/time
98
Kinematics: Velocity Measuring
Systems
* Measure instantaneous speed
* Radar guns (police speed traps)
* Ball and other implement speeds
* LIDAR (light detection and ranging)
* More tightly focused than radar
* Useful to measure individual runners within a group
99
Kinematics: Optical Imaging Systems
Video cameras
* Provide sequential two-dimensional images at specific time
intervals
* Calibrated with object of known dimensions provides real-life units
* Position recorded in sequential images provides displacement
* Velocity = displacement / time
* Acceleration = velocity / time△ △
* Special software computes 3D coordinates from multi-cameras
* Automatic digitizing
* Light reflective markers tracked by cameras, auto-digitized with software
100
Kinematics: Accelerometers
Device for directly measuring acceleration
* Sample at high sampling rates (1,000 samples per second)
Light and small in size
* Attached to object of interest provides acceleration at point of attachment
* Triaxial accelerometer provides 3D acceleration measure
Useful for impact analysis
* Helmet evaluation
* Mount on headform dropped onto a rigid surface
101
Kinematics: Motion Capture Systems
MOCAP systems record 3D motion of the whole body
Typical system
* Six or more video cameras
* Marker system or set (> 50 markers, two or more per segment)
* Specialized software and hardware
Expensive but accurate and reliable
* Research labs
* Clinical gait analysis
* Digitizing performers for video games and movie animation
102
Kinetics: Force Platforms
Measure reaction forces in 3D
* Normal contact force (vertical)
* Friction force in anterior-posterior direction
* Friction force in medial-lateral direction
Resultant reaction force and point of application of the resultant
* Clinical gait analysis to evaluate disease progress or rehab
* Athletics to understand technique difference
* Shoe research labs to evaluate design and materials
103
Kinetics: Force Transducers
Devices to measure force
* Force platform includes multiple transducers
* Measures strain (changes in length divided by original
length)
* Sports: design gymnastics bars, oar in rowing
* Orthopedics
* Attached to artificial joint surfaces, ligaments, tendons
Measure loads in vivo
104
Kinetics: Pressure Sensors
typically thin mats with arrays of imbedded force sensors
* Quantify pressure on small areas under the foot
Gait analysis: effect of orthotics and shoes
* Important in treating foot disorders in diabetics
* Ski boot design: pressure on sole of foot
105
Electromyography
Measures electrical activity of a muscle
* Surface electrodes: placed on skin over muscle of interest
* Indwelling electrodes: inserted into muscle of interest
Indicates if muscle is active or not
* With special processing indicates relative strength of contraction
106
Computer Simulation and Modeling
Analysis tool more than a measurement tool
Model equations derived from Newton’s laws of motion
* Inertial properties of body segments
* Initial conditions at start of simulation
* Positions and velocity, from a kinematic data base
Time histories of the control functions
* Relative position of the limbs
* Muscle forces
* Resultant joint torque
* Allows exploration of “What if . . .” questions
