Final Flashcards
Define stress. How is it calculated?
Stress is the external forces resisted by internal forces causing deformation of a body. (Stress= Internal Force/Cross Sectional Area)
What kind of biological structures undergo stress?
Cartilage, tendons, ligaments, bones, and muscle
What are the three principle stresses?
Tension, Compression and Shear Stress
How does the size of CSA effect how a bone handles tension?
When the CSA is larger, stress is smaller and the bone is stronger, when the CSA is smaller, stress is larger and the bone is weaker.
What kind of deformation do tensile loads cause?
Typical deformation is stretching/elongating. Elongation is proportional to the magnitude of the stress.
What kind of injuries do tensile loads cause?
In humans, very large tensile loads may sprain/rupture ligament and tendons, tear muscle and cartilage and fracture bone
What kind of deformation do compressive loads cause?
Typical deformation results in shortening in the direction of these external forces.
What kind of injuries do compressive loads cause?
In humans, compressive loads may cause bruising of soft tissue and crushing fractures of bones.
How do these stresses act in relation to the analysis plane? Tension, Compression, Shear
Tension and Compression act perpendicular, Shear acts parallel
How is shear stress calculated?
shear stress= shear force/cross sectional area at analysis plane
What kind of deformation do shear loads cause?
Typical deformation causes a change in orientation of the sides of the object, or a skewing
What kind of injuries do shear loads cause?
In humans, shear loads cause blistering of the skin, Large shear loads acting on the extremities may cause joint dislocation or shear fractures of bones
Define a bending load.
A force couple would be created by the molecules near the top of the right piece pulling on the molecules at the top of the left piece, and by the molecules near the bottom of the right piece pushing on the molecules near the bottom of the left piece. Tensile stress occurs at the upper half of the plane while compressive stress occurs on the lower half
Do the stresses become larger or smaller as the distance away from the center line increases?
The stress becomes larger as the distance away from the center line increases
In a bending scenario, what limits the moment arm of an object? How does this effect the internal forces?
The objects depth limits the moment arm. When the moment arm is small, the internal forces must be large to create a big enough counter torque. An object with greater depth is able to withstand greater bending loads because it has a larger moment arm.
What is the relationship between moment arm and torsion?
The moment arm between the internal shear forces on either side of the longitudinal axis is limited by the diameter of the object under torsion. For the pencil, this moment arm is small, so the internal forces (and stresses) must be large to create a large enough countering-torque (refer to the equation for torque.)
If the stresses become too large, the pencil will break. An object with a larger diameter is able to withstand greater torsional loads since the shear stresses are smaller as result of the larger diameter.
Define Strain. How is linear strain calculated?
A property that quantifies the deformation of a material. (strain- change in length/original length)
What causes linear strain
Strain is caused by compression and tension
What is shear strain?
Shear strain occurs with a change in the orientation of adjacent molecules as a result of these molecules slipping past each other.
How does Poisson’s Ratio relate to shear strain?
Poisson’s ratio refers to specific ratio of strain in the axial direction to strain in the transverse direction. (rubber band ball)
Define Elastic Behavior.
When an object stretches under a tensile load, but returns to its original shape when the load is removed.
What is the elastic modulus? How is it calculated?
The ratio of stress to strain (the slope of a stress-strain curve) elastic modulus= change in stress/change in strain
Draw a stress strain curve for a stiff material versus a pliant material.
Stiff is at a higher angle than pliant
What is plastic behavior? What marks the transition from elastic behavior to plastic behavior on a stress-strain curve?
If the load exceeds a certain magnitude, some permanent deformation of the object may occur. The yield point/elastic limit marks the transition
Yield Strength
Stress at the elastic limit of the stress-strain curve; no breakage/rupture, but permanent changes in the dimension of the material occurs
Ultimate Strength
Max stress that a material is capable of handling; highest point on a stress-strain curve; measures the load
Failure Strength
Stress where failure actually occurs; failure in breakage or rupture; usually has the same value as ultimate strength
What is toughness? How is it measured?
Toughness is the ability of a material to absorb energy. It is measured by the area underneath of a materials stress-strain curve
isotropic Properties
Material has the same mechanical properties in each direction.
Anisotropic Properties
Materials that have different mechanical properties depending on the direction of the load
What are the properties of the Collagen? Isotropic or anisotropic?
Most abundant substance in all connective tissue, 8-10% failure strain, high tensile strength and no compression. strength (isotropic)
What are the properties of the Elastin? Isotropic or anisotropic?
Fibrous protein that is pliant and very flexible (anisotropic)
How are mechanical properties of all connective tissues affected by age and activity?
Bone, cartilage, ligament, and tendon strengths increase with regular cycles of loading and unloading. Inactivity and immobilization decreases their strength. All connective tissue shows an increase in ultimate strength until the third decade of life (30s!). After this point, the strength decreases.
Bone
Carry almost all compressive loads of the body, are strong against tensile and shear loads as well (35% collagen, 20% water, 45% mineral) STRONGEST AND STIFFEST MATERIAL; graphs differ for each unique bone, the porosity of a bone storminess its strength and stiffness, bones are strongest in compression and weakest in shear
Cortical Bone
Found in the dense, hard outer layers of bone
Cancellous Bone
Spongy bone, less dense and more porous. Found deep and near the ends of long bones
What are the different effects between slow/fast loading on bone? Draw stress-strain curves to represent these differences
Bone is stronger and stiffer if a load is applied quickly, but weaker and less stiff is the load applied slowly. (fast loading rate curve above slow loading rate curve)
Cartilage (3 types!)
Hyaline Cartilage, Fibrous Cartilage, Elastic Cartilage; cartilage is able to withstand compressive, tensile, and shear loads (30% collagen, 70% water) hyaline fibers are aligned parallel to each other, deeper cartilage fibers seem to be arranged randomly
Articular Cartilage
transmits the compressive loads from bone to bone; fluid can be exuded from compression (this behavior causes creep and stress relaxation)
Hyaline Cartilage
Cartilage that covers the end of long bones at joints
Fibrous Cartilage
Is found within some joint cavities (the menisci of the knee), in the intervertebral discs (the annulus fibrosis), at the edges of some joint cavities, and at the insertions of tendons and ligaments into bone
Elastic Cartilage
Found in the external ear and in several other organs that are not part of the musculoskeletal system
What is creep?
Creep Rate: how quickly the cartilage reaches a constant strain. Depends on stress, thickness and permeability (fluid can be reabsorbed)
What is stress relaxation?
Constant strain doesn’t equal constant stress. Stress will decrease over time. As the articular cartilage contact area expands, the stress is reduced
Tendons and Ligaments
70% water, 25% collagen; toe region: represents the section of the curve where the straightening of the crimps in the collagen occurs; ALSO HAVE CREEP AND STRESS RELAXATION PROPERTIES
Tendons Collagen
Structured parallel to each other (more stiff!)
Ligaments Collagen
Fivers are nearly parallel (less stiff, can carry loads that are non axial!)
In the boxes, draw an example of tendon collagen fibers v. ligament collagen fibers.
Tendon: Lines straight up and down; Ligament: Criss cross pattern
Muscles
Muscle tissue is capable of actively contracting to produce tension within itself and the structures to which it attached; If a passive muscle is slowly stretched, there will be some resistance to this stretching and thus some stress developed; the stiffness of the passive contractile component caused by the filaments of the contractile element sliding past each other is very low, so the resistance to this movement is small as well.
Failure Strain for muscle is…
much larger than that of tendon/ligament (ultimate strength for a muscle is much lower than tendons/ligaments- tears occur much more easily)
What is the prevalence of ACL injuries in female athletes compared to male athletes?
Female adolescents who participate in pivoting and jumping sports suffer ACL injuries at a 4-6 fold greater rate than do male adolescents participating in the same sports
What variables did the researchers look at? Which of these are kinematic variables?
- Knee flexion/extension angle at initial ground contact
- Knee abduction/adduction angle at initial ground contact
- Maximum knee flexion/extension angle during stance phase
- Maximum knee abduction/adduction angle during stance phase
- Stance time
What variables did the researchers look at? Which of these are kinetic variables?
- Peak external hip adduction and knee abduction moments
- Peak external hip and knee flexion moments
- Peak vertical ground reaction force
Sensitivity
Ability of a test to correctly identify those with ACL injury (True Positive)
Specificity
Ability of a test to correctly identify those without ACL injury (True Negative)
Valgus Movement
Knee Abduction, creates a “knocked knee” position
Varus Movement
Knee Adduction, creates a “knees bucking out” position
Prospective Study
Study that uses the data it collects in the present
Retrospective Study
Study that uses the data collected in the past
(True or False) At the time this study was conducted, a successful method for screening and identifying athletes at increased risk of noncontact ACL injury was already available.
False
Of the total number of athletes that participated in this study, how many went on to suffer from a noncontact ACL injury during the 13 months following testing?
9
In what type of sporting situations to most noncontact ACL injuries occur for females?
Sudden deceleration maneuvers
Lateral pivoting maneuvers
Cutting and landing maneuvers
(All of the above)
What historical event is suggested to be partially responsible for the alarming increase in ACL injuries among female athletes?
The Passing of Title IX of the Educational Assistance Act
What was the total number of athletes that participated in this study?
205
From the list below, what were the two primary predictors of noncontact ACL injury risk?
- Maximum/peak external knee abduction moment
- Maximim/peak knee abduction/adduction angle
One of the findings of this study was that those athletes who went on to rupture their ACL landed with an average vertical ground reaction force of 1266.1 N whereas noninjured athletes landed with an average ground reaction force of 1057.8 N. In addition, the methods tell us that injured athletes had an average body mass of 61.5 kg whereas the noninjured athletes had an average body mass of 59.1 N. Based on this information, what was the magnitude of the vertical ground reaction force in terms of bodyweights for the injured athletes? (Do not worry about including units. Round your answer to the nearest tenth of a body weight)
2.1
Dr. Kevin Ford, PhD is currently the Director of the Human Biomechanics and Physiology Laboratory here at High Point University. However, he was not affiliated with HPU at the time he co-authored the research article you just read. Who was Dr. Ford affiliated with at the time this study was published (i.e. 2005)?
Cincinnati Children’s Hospital Research Foundation
Be sure to review the results section of the article! Why was this study important?
?
Define Compression
Stress that results when a loaf tends to push or squash the molecules of a material more tightly together at the analysis plane (Uniaxial)
Compression Example in Human Body
During a pushup, forces push on each end of the humerus causing compression
Define Tension
Stress that occurs at the analysis plane as a result of a load that tends to pull apart the molecules bonding the object together at that plane (Uniaxial)
Tension Example in Human Body
When hanging from a bar, the humerus is under tension
Define Bending
Force couples cause one side of the object to experience tension, and the other side of the object to experience compression (Multi-axial)
Bending Example in Human Body
Neck of the femur is often subject to bending load. A foot bearing weight is an example of an anatomical beam that undergoes bending.
Define Torsion
Occurs when torques act about the long axis of the object at each end (Multi-axial)
Torsion Example in Human Body
Torsion loading in bones. When you step onto your foot and pivot, a torque is created around the longitudinal axis of your tibia, causing it to be torsionally loaded
Define Shear
Transverse stress that acts parallel to the analysis plane as a result of forces acting parallel to this place r. Forces tend to slide the molecules of the object past each other (Uniaxial)
Shear Example in Human Body
In humans, shear loads cause blistering of the skin, large shear loads can cause joint dislocations/shear fractures of bone
Define Combined Load
Combination of uniaxial tension, compression, shear loads, bending, and/or torsion that produce complex stress patterns (Multi-axial)
Combined Load Example in Human Body
Shaft of the femur during weight breaking, carries bending and compressive loads
- When an athletic trainer pulls laterally on your ankle while pushing medially on your knee, a bending load is placed on your lower extremity. Which side of your knee undergoes compressive stress, and which side of your knee undergoes tensile stress as a result of this manipulation?
The lateral side of your knee undergoes com- pressive stress, while the medial side undergoes tensile stress.
- A snowboarder catches an edge and falls. Her board twists in one direction as her body twists in the opposite direction. The torsional load places large shear stresses on her tibia and knee. If the load on the tibia is axial torsion only, where on a cross section of the tibia will the shear stress be greatest?
The shear stress will be greatest at the outer most surfaces of the bone.
- The shafts of two prosthetic legs have exactly the same surface area in cross section, but the shaft of leg A has a diameter 2% smaller than the shaft of leg B. The two shafts are made from the same material.
a. Under a uniaxial tensile load, which shaft is stronger?
b. Under a uniaxial compressive load, which shaft is stronger?
c. Under a torsional load, which shaft is stronger?
d. Under a bending load, which shaft is stronger?
The two leg shafts have the same strength
under a uniaxial tensile load.
b. The two leg shafts have the same strength
under a uniaxial compressive load.
c. Leg shaft B has greater strength under a torsional load.
d. Leg shaft B has greater strength under a bending load
- Which can withstand greater tensile stress—tendon or ligament?
Tendon
- Which can withstand greater tensile stress—bone or ligament?
Bone
- Which can withstand greater tensile strain—bone or ligament?
Ligament
- Which is stiffer—bone or ligament?
Bone
- Which more ductile—live bone or dry bone?
Live bone
- What type of stress is bone strongest in resisting? What type of stress is bone weakest in resisting?
a. The bone is strongest in resisting compressive
stress.
b. The bone is weakest in resisting shear stress.
- Describe the various measures of strength for biological materials.
Yield strength—stress at the elastic limit of the stress–strain curve
Ultimate strength—maximum stress that a mate- rial can withstand
Failure strength—stress achieved just before a material fails
Failure strain—strain achieved just before a mate- rial fails
Elastic modulus—slope of the stress–strain curve in the elastic region
Toughness—energy that a material can absorb before failure; the area under the stress–strain curve
