Chapter 12- Clinical Biomechanics

Brendan McElligott

Kimmi Dotseth

Joe Kotansky

Clinical Biomechanics?

• Biomechanics is the study of forces, and their effect on living organisms

• Clinical Biomechanics is defined as the application of biomechanics to the treatment of patients, e.g., by orthopedic specialists or physical therapists

Why does this pertain to us?

• Orthopedists, physical therapist’s, occupational therapists, and athletic trainers are health professionals who use biomechanical concepts to evaluate and treat patients

• Bioengineers, ergonominists, and human factors specialists use biomechanical concepts to understand how individuals physically interact with their environment

What this chapter entail?

• In this chapter, the book looks at both statics and dynamics, statics is when an object is primarily stationary, whereas dynamics is when there is movement especially when applied to sports and exercise.

The Scope of Clinical Biomechanics

• Based on content areas of anatomy, mathematics, physics, and clinical sciences

• Additional content areas include specific rehab techniques, wheelchair design, anthropology, specific tissue repair, surgical techniques, and architecture

Kinesiology in Biomechanics

• Kinesiology, the study of human movement, is an important content area within biomechanics

• Kinesiology involves the study of the skeletal system, including the major joint articulations and the major muscles and muscle groups that are prime movers during exercise

• This is essential to Exercise Science students because they need to know which produce movements and why

Focusing on science in Clinical Biomechanics

• The main property in biomechanics is force, which can be defined as a push or pull

• A force that is applied externally to an object is a load, when motion occurs, force is the factor that causes a mass to accelerate

Force continued….

• This is shown through the equation F= ma

• The exact definition of force however must consist of four things: point of application, line of application, direction of push or pull, and magnitude

• All applications of forces and motions on objects in biomechanics are subject to Newton’s laws of motion

Gravity

• Gravity is the mutual attraction between two objects

• The earths gravity on an object is called the object’s weight

• The earth’s pull on an object is what we consider “down”

Contact

• Whenever two object are in contact, a force acts between them

• This goes along with Newton’s 3rd law• Forces acting in the body can cause a few

different adverse things such as compression- the process in which 2 forces act along the same line in opposite directions toward each other and tension, the process in which 2 forces act along the same line in opposite directions away from each other. The forces tend to pull the object apart

Inertia

• “An object at rest tends to remain at rest, and an object in motion tends to remain in motion at a constant velocity unless acted on by an external force.” – This is inertia

Muscle

• It is important in biomechanics because it generates the bodies forces

• Different times of lifts/contractions mean different things in biomechanics

• Isometric contractions are when the muscle force is equal to the resistance offered and there is no change in length in the muscle

• Concentric contraction occurs if the muscle force exceeds the resistance offered and the distance between the attachments decreases

• A eccentric contraction occurs when the muscle force exceeds the resistance offered and the muscle increases in length

Elasticity

• Is defined as the capacity of an object to reform to its original size and shape once it has been deformed

• This can be seen through F=-kl, where k is the material and l is the amount of deformation

Composition and resolution of forces

• Combining forces is called the composition of forces

• The process of dividing forces is called resolution of forces

• When 2 or more forces are subjected on an object, the single force is called a resultant of the forces

• Because forces are vectors, most all forces are associated with arrows to which the forces are pushing or pulling

Resolution

• The process of resolution separates the force into two perpendicular components

• This can be done either graphically or mathematically, often found in geometry

Equilibrium

• When in equilibrium, the sum of the forces and torques equal zero

• Called “static equilibrium” when an object is at rest (Newton’s 1st law)

• First Condition:Ʃ F = 0 (sum of forces equal zero)

• Second Condition:ƳM = 0 (sum of torques equal zero)

Second Condition

• ƳM = 0• M= moment

• OR: application of a force at a distance from axis

• Since the force does not act through the pivot point, the object rotates

• In order to remain in static equilibrium (rest), what must happen?

• The ability to determine force components is essential to evaluate effects of moment on an object

First-Class Levers (EOR)

• Point of axis (O) between two forces, Effort (E) and Resistance (R)

• One force will tend to rotate the object clockwise, the other will tend to rotate the object counterclockwise

• The distance from the axis can determine the magnitude of force needed to keep equilibrium

• Axis force will equal the sum of effort and resistance

Second-Class Levers (ORE)

• Resistance is between the axis and effort

• Magnitude of effort is always less than resistance

• Magnitude of force at axis point will always be less than then force at resistance

• Example: wheelbarrow

Third-Class Levers (OER)

• Effort between resistance and axis

• Magnitude of effort is always greater than resistance

• Resistance will always move faster and farther than effort

• Force at axis will be less than at effort

• Works well for throwing or kicking a ball

Strength of Materials

• Strength of a material is an object’s ability to resist deformation when a load is placed on it

• Strain- the measure of the change in dimensions of an object

• Mechanical stress- the property of a material to resist deformation (units are force per unit area)

• Three principal stresses and strains: tension, compression, and shearing

Stresses and Strains

• Tension: two or more collinear forces act away from each other– Material ____________

• Compression: two or more collinear forces act towards each other– Material ____________

• Shearing: two or more non collinear, parallel forces pointed in opposite directions act on the material– Material ____________

Loads

• Cause stresses and strains to arise

• Axial, bending, and torsion• May occur alone or in combination

• Compression, torsion, and shearing usually all occur to some degree

Axial Load

• Loading along the axis of an object– EX: Intervertebral disc

• Will mainly have compression stress

• The widening of the disk suggests torsion stress as well

• Shearing occurs at 45 degree angle to the loads

Bending Load

• Forces act in coplanar manner, but not collinear– EX: a beam supported at both ends, or foot– EX: Cantilever

• Compression stress occurs in top part of beam

• Tension occurs in bottom part of beam• Shearing occurs parallel and

perpendicular to forces

Cantilever

• An eccentrically loaded beam– A horizontal beam is anchored at one end and loaded

at the other– EX: diving board, proximal end of femur

• Beam tends to bend• Compression occurs on lower side of beam• Tension occurs on upper part of beam• Shearing occurs perpendicular and parallel to

forces

Torsion Load

• Rod or shaft is loaded so that it twists around the long axis– EX: removing lid from jar, spiral fracture of

tibia

• Compression and Tension occur along spiraling lines

• Shearing occurs perpendicular and parallel to the rod

Effects of Loading on Biologic Tissue

• Wolff’s law: the ability of the bone to adapt (by changing size, shape, internal structure) depends on mechanical stresses

• Important in early development

• Too much or too little can be dangerous

Effects of Loading on Biologic Tissue

• Wolff’s law: the ability of the bone to adapt (by changing size, shape, internal structure) depends on mechanical stresses

• Important in early development

• Too much or too little can be dangerous