Download Biomechanics Notes

Biomechanics Notes
07-02-98
Shoulder Joint:
1. The ball is bigger than the socket
2. Structurally unstable joint
3. Problems generally due to muscular problems (it takes a massive amount of muscle coordination for proper
shoulder movement)
Hip Joint:
1. Biomechanically, this is a classic ball and socket joint (the socket is actually bigger than the ball)
2. The hip has a smaller range of motion than the shoulder does but it does has global range of motion (in all
directions)
There are three important angles at the hip:
1. Neck To Shaft Angle (angle of the femoral neck)
(Reference figure 7-2, page 136 in the text)
Normal angle = 125 degrees + 5 degrees
Coxa Vara < 125 degrees (greater torque here)
Coxa Valga > 125 degrees
Not a good biomechanical model for strength. With this formation, the femur neck is more likely to break.
The advantages to this formation are twofold:
A. This allows us to have stronger leg muscles without having them rub against each other when we
walk (chaffing at the inner thigh!).
B. This also allows for easier child bearing in women.
2.
Q Angle (angle made by the quadriceps muscle)
The quads pull the patella laterally when they fire
Women have a bigger Q Angle due to their wider pelvis so women runners tend to have more knee problems
than do male runners
The gluteus medius attaches to the greater trochanter and it contracts so that the force on the neck of the femur
is compressive instead of tensile. (Reference Figure 1-32 page 17 in the text) Elderly people who don’t exercise
much get stress fractures of the femur because their g. medius is weak.
Regarding the Neck to Shaft Angle discussed above:
The g. medius has to work harder if you have a Coxa Vara angle
The g. medius has to work less if you have a Coxa Valga angle but this joint is also a greater risk
for developing osteoarthritis due to the greater amount of force being directed through the hip.
3.
Angle of Aversion (the intersection of the long axis of the femoral head and the transverse axis of the femoral
condyles)
(Reference figure 7-3, page 137 in the text)
This angle is normally 12 degrees in adults
Femoral torsion is usually:
40 degrees back as an infant
25 degrees back at 5 years old
If the angles are different on the left and right side, this adversely affects stability. 50% of all hip
osteoarthiritis has no known underlying cause (no history of trauma, etc.). Perhaps this may be due to difference
in the left and right angles of aversion. Today we can measure this with a CAT scan. This used to be
approximated by measuring from the greater trochanter to the table in a supine patient. Needless to say, this
wasn’t a very accurate method.
Forces at the Hip:
Mostly due to contraction of muscles that cross the joint. The primary muscle involved is the G.
Medius.
Question: What is the force on one femur head in a 150 pound person?
Answer: Assuming that 1/3rd of a person’s weight (50 pounds) is in the lower extremities, this
leaves us 100 pounds to work with so there are 50 pounds of weight on each hip while standing.
If the person is standing on one leg, there is now a 450 pound force through the hip. There is a shift of
weight which requires the G. Medius to fire more to maintain balance and therefore the force on the hip is
even greater.
The force on a bone increases with increased firing of muscles. In walking, women have less muscle force
just before toe-off than men do. This is thought to be due to women having a wider pelvis and a lower force
on the hips than men. In men, the force is 7 times their body weight. So men get a lot more osteoarthritis of
the hip and more fractures due to force here. (Reference Figure 7-15, page 146 in the text)
In running, there is a huge increase in the force through the joint.
In runners:
Women have more knee problems
Men have more hip problems
The hip is the 3rd most common site of osteoarthritis behind the spine and knee.