The Physics of the Human Body-Richard J. Ingebretsen.pdf

The Physics of the

Human Body

Companion Manual

Physics 3110

Autumn Semester 2002

Richard J. Ingebretsen, M.D., Ph.D.

INDEX

Chapter 3: Muscles and Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

The Rotator Cuff and Shoulder Impingement Syndrome, Michael Doleac . . . . . . . . . . . . . . . . . . . . .4

Phy sics of Arm Wrestling . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 5

High Velocity Injuries, Harland Hayes, University of Utah School of Medicine . . . . . . . . . . . . . . . . . 7

Chapter 4: Physics of the Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Origin of Stress Generated Potential in Bones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Chapter 5: Pressure in the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Chapter 7: Physics of the Lungs and Breathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Chapter 8: Physics of the Cardiovasculature System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

C hapter 11: Physics of the Ear and Hearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Cochlear Implants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Chapter 13: Medical Imaging and the Treatment of Cancer . . . . . . . . . . . . . . . . . . . . . . . 40

X-Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Using X-Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Gamma Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Particle Accelerators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Magnetic Resonance Imagine (MRI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Brachytherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Radiation Injury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Problems

Chapter 2: Energy, Heat, Work and Power of the Human Body . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Chapter 3: Muscles and Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Chapter 4: Physics of the Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Chapter 5: Pressure in the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66

Chapter 7: Physics of the Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Chapter 8: Physics of the Cardio vasculature System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68

Chapter 9: Electrical Signals within the Body. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Chapter 11: Physics of the Ear and Hearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Chapter 12: Physics of the Eye and Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Chapter 13: Physics of Medical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Chapter 3: Muscles and Forces

Rotator Cuff and Shoulder Impingement Syndrome

Michael Doleac

The rotator cuff is a very important part of the functional anatomy of the shoulder. It

consists of four muscles: the supraspinatus, the infraspinatus, the teres minor, and the

subscapularis. This group serves as abductors, lateral rotators, and medial rotators of the

shoulder, but they also have another very important function. They act as depressors of the

humerus. This function keeps the action of the deltoid from pulling the head of the humerus

upward into the subacromial space, which would cause impingement of the cuff. When this

happens, the shoulder impingement syndrome that results can turn into a self-promoting cycle.

Impingement is caused when the subacromial space, the area between the top of the

humerus and the bottom of the acromion, becomes closed off and pinches the rotator cuff.

Symptoms include pain, weakness, and loss of motion. Movements above the shoulder tend to

increase pain, and pain while sleeping is often encountered. Besides weakness of the rotator

cuff, impingement can be caused by abnormal acromions, acromioclavicular joint arthritis, and a

calcified coracoacromial ligament. When impingement occurs, the natural response of the body

is to the pain is the disuse of the muscles. This disuse causes the muscles and tendons to

weaken, leaving the action of the deltoid even more unopposed, and therefore reinforcing the

cycle.

Impingement syndrome may be classified in three stages. Stage one is usually associated

with an overuse injury by someone who is under the age of 25. The rotator cuff is weakened, but

the condition is often reversible. Stage two usually involves people from the ages of 25 to 40,

and the condition is more advanced, with some irreversible tendon damage beginning to arise.

Stage three is the result of years of build up, and often involves tendon rupture or tear. This is

seen mostly in patients over the age of 50.

Understanding the impingement syndrome is aided by understanding the physical

properties of the tendons, which make up the cuff. The strength and stiffness of tendons, which

give them their mechanical advantages, comes from collagen molecules. Collagen molecules

run parallel to each other and derive their characteristic strength from the cross-linkage between

them. New collagen has relatively few cross-links, but as it matures, the number of cross-links

grows, therefore increasing the strength and stiffness of the fiber. This maturation process

begins to level off with age however. In older patients, the number of stable cross-links being

formed between collagen molecules drops off considerably, therefore reducing the strength of

the tendon. Tendon strength is also affected by the demand placed on it. An active person who

exercises frequently will have much stronger tendons than an inactive person will. Disuse or

immobilization of tendons causes them to weaken considerably, as no demands are being placed

on them.

Impingement in a young patient is often reversible because of the properties of the

tendons. Treatment is rather conservative, consisting of rest and ice, followed by a strengthening

program and possibly some physical therapy. A sling is never used because immobilization of

the joint would just weaken the rotator cuff and add to the problem. The strengthening program

will lead to the development of more stable cross-links between the collagen fibers, increasing

the overall stability of the tendons and the entire rotator cuff. The more stable rotator cuff will

relieve the impingement by holding the humerus in its proper position.

Physics of Arm Wrestling

Of the muscles in the human body, there are three kinds, smooth, striated, and cardiac.

When talking about mechanical advantage in muscles the focus turns primarily to striated

muscle. Striated muscles are the voluntary work force behind all major body movements and

include; triceps, biceps, pectoral etc.

Under stress striated muscle can react in different ways according to conditioning. In a

drop-jump study of trained and untrained subjects it was revealed that a trained gastrocnemius

muscle contracts and relaxes at a faster and more efficient rate. In the untrained subject the

muscle appeared to tense up and not have as high a contraction

strength. Muscle contraction in striated muscle tissue can contract up to 15-20 percent of its

resting length. This is largely due to the electric force of attraction. When examined under an

electron microscope the muscles reveal smaller bundles called myofibrils, which break up into

smaller parts called filaments. These filaments consist of the proteins myosin (thick), and actin

(thin). It is at the microscopic level that the contraction begins. The filament parts slide together

and collapse on themselves and by working together can lift an arm with a load.

Although it is well accepted that muscle contractions occur as a result of electrochemical

interactions the contraction mechanism at this level is not completely understood, however,

attractive electrical forces must be involved, for they are the only known force available. Even

though electrical forces can also be repulsive, no muscles are able to push all they can do is pull.

To compensate for the muscle's ability to pull in only one direction the body is constructed with

opposing muscle groups that allow the limb to be moved back the opposing direction.

The mechanical advantage of a muscle and joint can be calculated. The elbow, a third

class lever is a good demonstration of mechanical work. The elbow as a fulcrum is near the

upward pulling force of the biceps, this distance is 7.5 times shorter than the weight-bearing

extremity. To calculate the work produced by the biceps would involve determining the weight

of the arm and the weight of the load in the hand.

M = 3.5 H + 7.5 W (for a 30 N load and a 14 N arm weight).

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