Loading of the Musculoskeletal System.
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What is Injury?
Injury occurs when the load applied to a tissue exceeds its failure tolerance (Bartlett, 1999).
What are 2 factors in how and where injury occurs?
- The load – the sum of the forces and moments of force acting.
- The characteristics of the loaded structures.
What is Chronic (overuse) injury?
Results from repeated overloads with insufficient time for recovery.
What is Acute (traumatic) injury?
Results from a single or a few repeated episodes (Watkins,1999).
What is the result of Overtraining?
Overtraining may result in injury
What are the 7 Characteristics of Injury?
What are the 4 types of load characteristics important to injury?
Types of load:
Magnitude of load
Frequency of load repetition and number of repetitions
What are the 5 types of load?
What is Magnitude of load?
How much loading occurs
What is Load rate?
- Rate of change of force with time
What does Frequency of load repetition and number of repetitions affect?
Onset of fatigue.
What type of Load is this?
What is the formula for Force and deformation?
What is the Symbol for stress and strain?
What is Strain?
The deformation as a proportion of the dimensions of the material when unloaded
How is Strain calculated?
Strain is calculated as a percentage of change.
What is the Symbol for Strain?
What is Stress?
Load per unit cross-sectional area:
(s = Force / Area).
How is Stress measured?
- The unit is N/cm2 or N/m2
- (1 N/m2 = 1 pascal).
- A pascal is relatively small, so we often use megapascals (MPa) instead.
- 1 MPa = 100 N/cm2
- Strain is a measure of how much a material is
- Stress is a measure of how much the material resists
- this deformation.
What is Young's modulus of elasticity (E) (elastic modulus)?
The ratio between Stress and Strain
What does the Young’s modulus of elasticity measure?
The stiffness of the material.
What do each letter represent in the Stress and Strain relationship diagram?
Young’s Elastic Modulus Diagram.
What does HR and A stand for?
What does Young’s Modulus (stiffness) of the biological materials depends on?
Young’s Modulus (stiffness) of the biological materials diagram (Watkins 1999).
What is Resilience (Hysteresis)?
Resilience is the amount of energy returned as a percentage of the amount of energy stored.
What are the values for resilience known as?
coefficient of restitution
What is the coefficient of restitution (resilience) of Resilin (found in insect wings)?
What is the coefficient of restitution (resilience) of Collagen (found intendons / ligaments)?
What is the resilience of Elastin (found in tendons / muscles)?
What is a Hysteresis loop?
Although, in the elastic component, the strain is recoverable, the stress-strain curve is not the same for loading and unloading. Such materials instead exhibit viscoelasticity, involving both elastic and viscous components, which at normal loading and unloading rates leads to a hysteresis loop.
What does a Hysteresis curve look like?
What does the area of the loop in the previous diagram show?
The energy absorbed during one loading-unloading cycle.
What are the 3 factors of Compact (cortical) bone?
Weakest when loaded in tension.
What are the 2 fracture mechanisms of compact bone?
Ductile - osteons and fibres pulled apart
Brittle - cracks run across surface (similar trabecular - crack s propagate along length)
What are the 2 factors of Trabecular (cancellous) bone?
Spongy, porous core: absorbs energy, lower strength
Trabeculae orientated to compressive and tensile stresses (obeys Wolff’s law).
Factors of Trabecular
Trabecular - orientation trabeculae ~orthogonal
- more densely packed where
- greatest stress must be transmitted
shape varies: rod or plate-like
max economy of structure in strength-weight ratio
narrow middle section long bone reduces bending stresses --> minimises chance fracture
What is the typical tensile strength of bone?
In the femur, tibia, and fibula - compact 80 -150 MPa,
Trabecular: lower than this (Nigg and Grimston, 1994).
What does 1 MPa equal?
1 MPa = 100 N / cm2
What is the typical compressive strength of bone?
Compact: 106 - 224 MPa (Nigg and Grimston, 1994).
Trabecular: 1 - 26 MPa (van Audekercke and Martens, 1984).
What does anisotropy - i.e. properties depend on?
- cracks propagate more easily in transverse cf
- longitudinal direction
What is Cartilage?
Thin layer hydrated soft tissue covering ends of bones of diarthrodial joints
5 Factors of cartilage?
Articular cartilage is the most severely exposed to stress of all connective tissues, leading to wear and tear.
It provides a smooth joint surface to distribute stress.
It is avascular and tends to have few nerve endings, meaning injured cartilage shows little external symptoms and repairs poorly.
Its compressibility (squeeziness) is between 50 and 60% of its size, which allows it to increase the joint contact area and range of motion.
Distributes joint stress
What type of Behaviour does Cartilage demonstrate?
What are the 4 Cartilage Biomechanical Properties?
loading leads to rapid initial deformation followed by more gradual increase (see diagram)
returns to initial shape on load removal
prolonged standing causes creep of intervertebral discs (hence shortening of the body’s overall height).
- Its ultimate compressive stress is 5 MPa
- (Shrive and Frank, 1994).
What are the two main features of Muscle?
elasticity and contractility.
What is the cause of elasticity?
Elasticity is due mainly to the sarcolemma and connective tissue sheath surrounding the muscle fibres.
How much does resting muscle length stretch up to?
Muscle stretches up to 160% of resting length before rupture.
What is Muscle's property of contractility?
Muscles’ property of contractility means that they have the ability to shorten and produce movement, with the result that they can shorten to between 25 and 75% of resting length.
What are the other two properties of muscle?
extensibility and excitability
What is elasticity?
Elastic fibres in connective tissue cause shortening after stretching ceases collagen fibres protect against over stretching, breaking stress much less than for tendon.
What is muscle?
- - muscle fibre - myofibril - actin (thin filament) and
- myosin (thick filament)
What are types of muscular contraction?
- They can both theoretically be:
- Isotonic (constant tension)
- Isokinetic (constant speed)
What are the 3 Factors of Ligaments?
( Bartlett, 1999)
Ligaments initially have non-linear tensile properties, mainly as crimped pattern of collagen fibres becomes straightened.
Because of this de-crimping, the initial portion of a ligament’s stress-strain curve has a high deformation/low force characteristic known as the toe region.
Most everyday activities occur in the toe region of stress-strain curve, allowing easy movement.
4 More Ligament Factors
Following the toe region, there is a large linear strain region, up to 20-40% above resting length.
Stiffness increases with force applied, preventing bones from pulling apart.
- Ligaments have mechanoreceptors which may
- initiate tension in nearby muscles in order to assist in maintaining joint stability.
- Ligaments are often strained so much during
- vigorous exercise (e.g. stretches held in gymnastics) that they lose their viscoelastic properties entirely.
What does tendons ultimate tensile stress range from?
ultimate tensile stress of between 45 and 125 MPa (Woo, 1986).
Are Tendons Biologically stronger than bone? thus what does this cause?
Tendons are biologically strong.
They have a stronger tensile stress than bone in many cases, and can cause avulsion fractures.
When does avulsionfracture occur?
An avulsionfracture occurs when a tendon or ligament pulls off a piece of bone from wherethey are attached.
Where does tendon pain arise from?
Most tendon pain arises not from the tendon itself but from the surrounding paratenon.
What are two characteristics of tendons?
Tendons are relatively stiff, with similar changes with load to those for ligaments.
Tendons are also largely avascular and without nerve endings
What is the ultimate failure strain of tendons?
Between 8 and 10% (Herzog and Loitz,1994).
What are tendons hysteresis percentage?
They have limited viscoelastic behaviour, with only 2.5 to 20% hysteresis (Herzog and Loitz, 1994).
What are tendons considered to be the main site for?
They are considered the main site for elastic energy storage in the muscle-tendon unit; this is important at high load, especially eccentric ones (Huijing, 1992).
Which tendon is an important site for energy storage?
The achilles tendon (tendo calcaneus) is an important site for energy storage prior to jumping in athletes.
What do Basketballers and High jumpers tend to have?
Powerful calf muscles and long achilles tendons.
What happens to failure strength in ligaments?
- Ligaments: decrease in failure strength and energy absorption before failure, which leads to an increase in
- joint stiffness and injury susceptibility.
Immobilisation and disuse of the bone?
Bone: atrophy. Mass and size decrease through loss of equal proportions of matrix and mineral content.
Bones need to be loaded to stay strong.
Immobilisation and disuse graph
What effect does training and exercise have on muscles?
- Progressive exercise improves all mechanical
- and structural properties of bone and connective tissue, including ultimate strength and strain and energy to failure, and bone mass and density.
Training changes muscle strength through changes in mass, recruitment pattern, fibre orientation.
What does types of training does preventive training include?
mobility and flexibility
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