Test 2 Flashcards
Mechanical stress
The internal force divided by the cross-sectional area of the surface on which the internal force acts
Ability of an object to develop a resistance to internal loading forces and to resist deformation caused by those forces
Tensile stress
Axial stress as a result of a force or load that tends to pull apart the molecules bonding the object together at the analysis plane
Compressive stress
Axial stress as a result of a force or load that tends to push or squash the molecules bonding the object together at the analysis plane
Shear stress
Transverse stress that acts parallel to the analysis plane as result of forces acting parallel to this plane
Internally resists sliding movement of one parallel layer of a material relative to the other
Uniaxial mechanical loads
Tensile
Compressive
Shear
Complex mechanical loads
Bending
Torsion
Combined loads
Bending
Counteracting tensile and compressive loads
Torsion
Torques acting about the long axis of the object at each end causing an internal torque created by the shear force btwn the molecules
Combined loads
Combination of loading configurations (uniaxial, complex)
Mechanical strain
Quantification of the deformation of a material from stress
Stress vs strain
stress measures the deforming force per unit area of the object, whereas strain measures the relative change in length caused by a deforming force
Linear strain
change in length as a result of tensile or compressive stress
Shear strain
change in orientation of adjacent molecules as a result of these molecules slipping past each other due to shear stress
Stress-strain relationship (elastic or young’s modulus)
Ratio of stress to strain (rise over run)
Elastic behaviour
occurs if an object stretches under a tensile load but returns to its original shape when the load is removed
Linear elastic behaviour
As stress increases, strain increases by a proportional amount
ex rubber band
Plastic behaviour
When a permanent deformation of an object occurs under a load
ex paper clip
Material strength
Max stress/strain a material is able to withstand before failure (breakage)
Yield point
Point of stress-strain curve where further stress will cause permanent deformation
Elastic region before, plastic region after
Yield strength
stress at the elastic limit of a material’s stress-strain curve
Ultimate strength
max stress material is capable of withstanding
Failure strength
stress where failure actually occurs
Ductile materials
large failure strains
Brittle materials
small failure strains
Hard materials
large failure stresses
Soft materials
small failure stresses
Toughness
Ability to absorb energy; area under stress-strain curve
Tougher= more energy required to break/reach failure
Viscoelastic materials
any material that exhibits both viscous and elastic characteristics (behaves as a liquid and a solid)
- bone, tendon, ligament, cartilage, muscle
Viscoelastic properties
Strain rate dependency
Stress relaxation
Creep
Hysteresis
Strain rate dependency
The rate at which you deform/strain a tissue will effect the stress it feels
Faster loading rate = more stress created
Stress relaxation
Decrease in stress under constant strain
(length held constant)
Creep
Increase in plastic strain under constant stress
(load held constant)
Hysteresis
the amount of energy dissipated as a result of internal friction during mechanical loading and unloading
Ex tendon hysteresis is imp for efficiency of locomotion
Active element of musculoskeletal system
Muscle tissue
Passive elements of musculoskeletal system
Connective tissue (bone, cartilage, ligament, tendon)
Anisotropic
having diff mechanical properties depending on the direction of load
Isotropic
having the same mechanical properties in every direction
Stiffness
measure of the resistance of elastic materials to deformation caused by stress
how much load a tissue can take before it deforms
slope of stress-strain curve
Two factors affecting mechanical properties of tissues
- Activity: strength of connective tissues increases w regular use
- Age: Connective tissues increase in ultimate strength w age until the third decade of life where strength decreases, bones become more brittle, tendons and ligaments become less stiff
Connective tissue
Composed of living cells and extracellular components (collagen, elastin, ground substance, minerals, water)
Collagen
stiff (brittle)
high tensile strength (hard)
unable to resist compression
Bone
Strongest and stiffest
- strongest in compression, then tensile, weakest in shear
-30-35% collagen (high tensile strength)
- 45% minerals (high compressive strength)
Cortical bone
dense and hard outer layer
can handle high stress, low strain
Elastin
pliant (soft)
very extensible (ductile)
Cancellous/trabecular/spongey bone
Less dense, porous inner portion
can handle low stress, high strain
Cartilage
can withstand compressive, tensile and shear loads
1. Hyaline: 10-30% collagen, 60-80% water
2. Fibrous: joint cavities, intervertebral discs
3. Elastic: external ear, organs
Tendons/ligaments
Stiff, high tensile strength, little resistance to compression or shear
- 70% water, 25% collagen, 5% ground substance and elastin
Ligaments have a larger elastic component and their collagen fibres are arranged only near parallel compared to tendons being parallel making them less stiff and weaker than tendons
Ligaments can carry non uniaxial loads
Muscle fibres
single muscle cells; encased in ct sheath (endomysium)
Fascicles
bundles of muscle fibres; encased in ct sheath (perimysium)
Whole muscles
bundles of fascicles; encased in ct sheath (epimysium)
Tendons and aponeurosis
connect muscle to bone
Myofibrils
thread like structures lying parallel to each other within muscle fibre; light and dark bands
Sarcomere
contractile unit of a muscle; repeating unit of myofibril btwn the stripes (z-bands)
Myofilaments
Thin: actin, troponin C, tropomyosin
Thick: myosin
I- band
contains actin and z-line; light band
A-band
contains actin and myosin; dark band
H-zone
region of A-band containing only myosin and M-line
M-line
transverse band that anchors myosin to each other
Z-line
transverse band anchoring actin to each other; mark beginning and end of sarcomere
Factors affecting muscle force production
- Length
- Velocity
- Physiological cross-sectional area
- Muscle geometry
- Activation
Muscle force-length relationship
Optimal sarcomere length for force generation= 120% resting length (plateau)
Sarcomere shorter than optimal length = opposing actin filaments begin to overlap and myosin cant attach= less force (ascending)
Sarcomere contracts even shorter = actin and myosin jammed against opposite Z-band= no force produced
Sarcomere stretches longer than optimal length= thick and thin filaments spread too far apart and cant slide over each other= less force (descending)
Passive tension
Developed in sarcomere and within whole muscle by stretching of connective tissues
As muscle fibre length increases and active force is decreasing, passive force component comes into play and increases total force production of muscle
Stretching theory
Stretching you muscle enhances your force generating capcity by adding the passive component to the active component
Muscle force-velocity relationship
the greater the shortening velocity of a muscle= the smaller the force produced
more cross bridges in release step of contraction cycle and each cross bridge spends a large proportion of the contraction time in release step so less tension is developed
ex barbell bench press
Muscle contraction
As the muscle shortens during a
contraction, the cross bridges attach to the actin myofilament, pull it toward them, release it, and then reattach to it farther along its length
Cross bridge formation: Attach, pull, release and then contract
Essentric
Lengthening of muscle fibres
Can produce more force than a muscle contracting concentrically (along w isometric)
Concentric
Shortening of muscle fibres
Physiological cross sectional area
Adding sarcomeres in parallel by increasing number of myofibrils will increase muscle diameter and cross- sectional area = STRONGER
Adding sarcomeres in series (end to end) increases length of myofibril and longer muscles can stretch and shorten over greater lengths
Muscle geometry
Longitudinal vs pennate
Pennate have large cross sectional area due to the angle of their fibres and therefore can produce more force
But they have shorter fibres which limits the distance over which they can shorten
Activation
Number of muscle fibres that are stimulated to contract at any given time
Fine/precise control
smaller number of motor fibres per motor neuron
better for muscles
Coarse control
larger number of muscle fibres per motor neuron
Motor unit
a single motor neuron and all the muscle fibres w which it synapses
How to increase activation
- Increase firing frequency of a given motor unit
- leads to a repeated series of twitches which cause a tetanic response from a single fibre (sustained contraction w plateaued tension) - Recruit more motor units
Henneman’s size principle
motor units are recruited from smallest (slow twitch) to largest (fast twitch)
Benefits of henneman’s size principle
Fatigue prevention: save fast twitch fibres for when needed
Fine and coarse control: allow us to apply smaller, finer forces then higher forces when needed
Stiffness
measure of the resistance of elastic materials to deformation caused by stress
how much load a tissue can take before it deforms
slope of stress-strain curve
Depth
An object with greater depth (and more cross-
sectional area farther from its neutral axis) is able to
withstand greater bending loads. The counteracting
tensile and compressive stresses are lower,
because they have a larger moment arm.
Under similar bending loads, the stresses in such an object
will be smaller.
Moment arm, forces and stress
Ex pencil
moment arm is small, so the internal forces (and stresses)
must be large to create a large enough countering torque
An
object with a larger diameter is able to withstand greater
torsional loads since the shear stresses are smaller as a
result of the larger diameter and large moment arm
