Loading of the Musculoskeletal System.

Injury occurs when the load applied to a tissue exceeds its failure tolerance (Bartlett, 1999).

• The load – the sum of the forces and moments of force acting.
• The characteristics of the loaded structures.

Results from repeated overloads with insufficient time for recovery.

Results from a single or a few repeated episodes (Watkins,1999).

Overtraining may result in injury

boredom, burnout.

Types of load:

Magnitude of load

Load rate

Frequency of load repetition and number of repetitions

tensile

compressive

shear

torsion

bending

How much loading occurs

• Rate of change of force with time
• (Δf/Δt)

Overuse injury.

Onset of fatigue.

Tension

Compression

Shear

Bending

Torsion

The deformation as a proportion of the dimensions of the material when unloaded

Strain is calculated as a percentage of change.

E

Load per unit cross-sectional area:

(s = Force / Area).

• 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
• deformed.

• Stress is a measure of how much the material resists
• this deformation.

The ratio between Stress and Strain

The stiffness of the material.

age

nutrition

fitness level

Resilience is the amount of energy returned as a percentage of the amount of energy stored.

coefficient of restitution

97%

93%

76%

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.

The energy absorbed during one loading-unloading cycle.

Peripheral

Brittle

Weakest when loaded in tension.

Ductile - osteons and fibres pulled apart

Brittle - cracks run across surface (similar trabecular - crack s propagate along length)

Spongy, porous core: absorbs energy, lower strength

Trabeculae orientated to compressive and tensile stresses (obeys Wolff’s law).

Trabecular - orientation trabeculae ~orthogonal

• more densely packed where
• greatest stress must be transmitted

internal struts

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

In the femur, tibia, and fibula - compact 80 -150 MPa,

Trabecular: lower than this (Nigg and Grimston, 1994).

1 MPa = 100 N / cm2

Compact: 106 - 224 MPa (Nigg and Grimston, 1994).

Trabecular: 1 - 26 MPa (van Audekercke and Martens, 1984).

Direction loading

• cracks propagate more easily in transverse cf
• longitudinal direction

Thin layer hydrated soft tissue covering ends of bones of diarthrodial joints

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

viscoelastic behaviour

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).

elasticity and contractility.

Elasticity is due mainly to the sarcolemma and connective tissue sheath surrounding the muscle fibres.

Muscle stretches up to 160% of resting length before rupture.

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.

extensibility and excitability

Elastic fibres in connective tissue cause shortening after stretching ceases collagen fibres protect against over stretching, breaking stress much less than for tendon.

• muscle
• - muscle fibre - myofibril - actin (thin filament) and
• myosin (thick filament)

Concentric

Eccentric

• They can both theoretically be:
• Isotonic (constant tension)
• Isokinetic (constant speed)

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.

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.

ultimate tensile stress of between 45 and 125 MPa (Woo, 1986).

Tendons are biologically strong.

They have a stronger tensile stress than bone in many cases, and can cause avulsion fractures.

An avulsionfracture occurs when a tendon or ligament pulls off a piece of bone from wherethey are attached.

Most tendon pain arises not from the tendon itself but from the surrounding paratenon.

Tendons are relatively stiff, with similar changes with load to those for ligaments.

Tendons are also largely avascular and without nerve endings

Between 8 and 10% (Herzog and Loitz,1994).

They have limited viscoelastic behaviour, with only 2.5 to 20% hysteresis (Herzog and Loitz, 1994).

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).

The achilles tendon (tendo calcaneus) is an important site for energy storage prior to jumping in athletes.

Powerful calf muscles and long achilles tendons.

• Ligaments: decrease in failure strength and energy absorption before failure, which leads to an increase in
• joint stiffness and injury susceptibility.

Bone: atrophy. Mass and size decrease through loss of equal proportions of matrix and mineral content.

Bones need to be loaded to stay strong.

• 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.

muscle strength

mobility and flexibility

coordination.