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Flashcards in OCS: Biomechanics Deck (49):

Define: biomechanics

The study of the structure and function of biological systems by the methods of mechanics


Define: kinesiology

science of movement of the body


Define: kinematics

study of the geometry of motion without reference to the cause of motion


Osteokinematics vs Arthrokinematics

Osteo:motion of the bones around an axis ie - flexion, extension, abd, add, IR, ER Artho: motion between articular surfaces of two bones ie - spin,roll, and glide


What are the various types of levers?

Type I: see-saw, axis in the middle - ex. head and spinal column

Type II: resistance between axis and effort

Type III: effort between resistance and axis - ex. biceps


Explain the convex-concave rule

The convex surface of one bone moves on the fixed concave surface then the translation and rotation will occur in the opposite direction Concave moves on convex translation and rotation occur same direction


Is the convex and concave verified?

No, the shoulder contradicts it at times, while the knee has not been verified and pathological knees can also differ


Where is the location of the axis of rotation?

Non pathological: axis if typically in the convex portion of the joint Pathological joint: axis can shift outside of the joint


Why is it important to know the axis of rotation?

1.) knowing motion will occur in a cardinal plane only if the AOR is perpendicular to that plane, if it is not motion will occur in multiple planes 2.) Muscle function determined by line of pull to AOR 3.) knowing where to place the goniometer for ROM


Absolute vs Relative angle?


How is force related to strength

Force = mass x acceleration Strength is the ability to produce force


Can EMG signal quantify a muscle's force producing/absorbing capability?

No. This is determined by: type of contraction (ecc, con, iso), length of muscle, cross sectional area, number of active motor units, rate of motor unit activation, specific tension, prestretch/ contractile Hx


Why is it important to know the components of force?

It is important to know which forces will be bad or harmful to a joint, and which ones will be better at strengthening the joint.


Force in terms of angular motion is called:

Torque or the moment arm


What is impulse and how can it be manipulated in order to prevent injury?

Impulse = the change in momentum, F = mass x change of velocity/time Therefore if the time is increased the force will be decreased. If there is a thicker shoe, a flexed knee, or stretch of the calf muscle during impact it decreases the amount of force transmitted.


Provide examples of the concept of a moment arm:

Forward head posture effectively increasing the apparent weight of the head, low back body mechanics


What are the benefits of having three different types of muscle actions?

Concentric: provides movement/ produces force Eccentric: to decelerate and shock absorb/reduces force Isometric: to stabilize


Explain the force velocity curve:

The greatest force can be produced with eccentric contraction which is mostly independent of force. An isometric contraction with no velocity produces the next most force. A concentric is velocity dependent contraction that produces the most force with a slow velocity.


In an isometric contraction what is the power?

zero, as long as there is a net joint moment


Why is eccentric strength important in the prevention of injury?

Muscle-tendon complex has the greatest potential to safely absorb or distribute energy which is primarily accomplished by eccentric contractions. If the muscle is not strong enough more of the energy is absorbed by the bone and ligaments which is can cause injury.


Is joint instability consistently defined?

No. Three common definitions: 1.) excessive and/or uncontrolled ROM resulting in dislocation 2.) small abnormal movement in a relatively normal movement i.e. may result in pain from impingement 3.) low stiffness, joint moves easily


What factors determine if load or force will cause an injury?

Magnitude, rate, duration, frequency, variability, location, direction


Is patellofemoral pain related to pressure between the patella and femur?

yes: there is always some pressure between the femur and patella, this is determined by the quad contraction force and the amount of contact between the surfaces. The smaller the contact area the stronger relationship to symptoms with increased force.


Do human tissues respond to all stresses the same way, i.e. compressive, tension, shear, bending, torsional?

No: This variation base on role is called anisotropic. Tendons respond well to tension, not well to shear, and not at all to compression. Cartilage responds well to compression Bone: handles compression best, followed by tension, with a bending force (one side has compression force and other has tension force) tension will fail first; torsional force will typically cause the bone to fail due to shear forces


In orthopedics is pressure and stress the same?

Typically it is used interchangeably in orthopedics


What is the tissue response to a force and how is it measured?

Tissue deformation is a change in the size or shape of the tissue, or the "strain"


Can tissue response to stress be measured in vivo, and how is it accomplished?

Cartilage is not measurable in vivo, but musculotendinous units are. It can be accomplished by measuring the force with a dynamometer and measuring the deformation with ultrasound in the tissue.


Explain the different points of a stress strain curve:

Ultimate strength: the point of tissue failure Yield Point: point where permanent deformation occurs Elastic region: portion of curve before the yield point Plastic region: portion of the curve after the yield point Stiffness: the slope of the curve in the elastic region also known as "Young's Modulus" Energy: area under the curve


Will a tissue return to its original state after a force is removed?

As long as it stays within the elastic region and doesn't go beyond the yield point.


Give an example of the clinical implication of the stress strain curve:

Sprain of a ligament - Grade I: stretched within the elastic region, but not beyond the yield point Grade II: Exceeds the yield point, but stays with in the plastic region and does not reach the ultimate strength but has deformation of the ligament Grade III: Goes beyond the ultimate strength, complete failure of the ligament


Is tissue response to a submaximal stress time dependent?

yes - creep occurs with a sustained submaximal force.


What is hysteresis? Is high or low hysteresis desirable?

Tissue is loaded and subsequently unloaded the stress is less for any given strain; " a retardation of an effect when the forces acting upon a body are changed (as if from viscosity or internal friction)" Low hysteresis results from repeated loadings and stretching increasing tendon compliance and decreases hysteresis thereby increasing energy returned and improving performance while decreasing risk of injury.


Stretching has what two benefits?

Increased ROM and decreased hysteresis


Explain the length tension relationship of a muscle:

The maximum force can be produced at the resting length. The amount of force produced decreases both directions for lengthening and shortening.


Physical activity has an overall effect ___ and changes tendons and ligaments by:

Strengthens; decreases cross linking - increases glycosaminoglycan content, alignment of fibers


Disuse, aging, and corticosteriod use ___ tendons and ligaments:



What are the normal processes of joint lubrication?

Boundary lubrication: a layer of fluid prevents direct contact and decreases friction Fluid film lubrication: fluid separates two surfaces and distributes the loading


How does fluid film lubrication work?

- separating surfaces (hydrodynamic) - increased fluid pressure deforms the articular cartilage thereby increasing contact area (elastohydrodynamic) - pressure on articular cartilage forcing fluid onto the surface (weep)


What is friction? Is it good or bad?

A force parallel to the contact surface that opposes motion between two objects. Some friction is needed to prevent a person from slipping, but high and/or repetitive can cause injuries.


Do all tissues adapt at the same rate? What is the clinical implication?

No: example tendon adapts slower than muscle because it has fewer cells capable of adapting. Bone is also slower than muscle. The clinical implication is that muscle strength is not a good indicator of the rehabilitative process.


Describe the difference in spurt and shunt muscles?

Spurt: insertion is close to the joint - prime mover Shunt: origin is close to the joint - stabilizer


Biomechanical factors of joint implant?

Initial stability: surgery technique and implant design Late stability: bone growth around implant Stress shielding: a joint implant absorbs more of the force thereby weakening the bone surrounding the implant Wear of the implant: Wear debris: polyethylene wear can cause osteolysis Changing of anatomical alignment: from installation


Explain osteolysis in a joint replacement:

When the artificial material starts to wear away the body tries to clean up the debris and an autoimmune reaction is triggered where the body to starts to break down the bone. This is usually a progressive problem.


List factors that effect stability of an external fixator:

Pin diameter, number of pins used, distance from the surface to the bone, stiffness of the frame, number of fixation planes


What happens to the strength of an intermedullary rod when its diameter is increased?


How do holes in bone affect its strength?

A decreases strength by causing a stress concentration point, a hole size 20% of the bone will decrease the strength by about 50%


How long does it take for strength to return to normal levels after the removal of a screw?

4-12 months


List the types of metals that are closest biomechanically to bone?

Young's Modulus (stiffness): Aluminum, titanium, stainless steel, cobalt chromium Biocompatibility: Titanium, cobalt-chromium, stainless steel, aluminum


How much strength does a well-placed lag screw add to fracture fixation?

(Lag screw only has threading on the lower portion) approximately 40% increase in strength over plating alone