Page 151: Questions # 1 to 5 inclusive  [answers in bold print]

1.    What are the two requirements for producing a torque?

A torque must have a force and a force arm, that is, the line of action of the force must be eccentric to the axis of rotation.

2.    Figure 4.6 shows three boxes on a board with an axis (fulcrum) in the middle of it.  The weights of the boxes are given as well as the distance that each is located from the axis: B is 1 unit from the axis, C is 2 units from the axis, and D is 3 units from the axis.  How far does box A have to be from the axis to make the system balance if A weighs 100 units?  If A weighs 40 units.

net T = 0
A(d) + C(2) + (-B)(1) + (-D)(3)=0
100d + 75(2) + (-100)(1) + (-50)(3) = 0
d=1 foot, for A=100 lbs

If A weighs 40 units:
40d + 75(2) + (-100)(1) + (-50)(3) = 0
d=2.5 feet for A = 40 lbs

3.    If a bone-muscles system is in static equilibrium, what does that tell you about the torques acting on the system?

The sum of the torques (net T) must equal zero.

4.    If the shoulder joint is being abducted, what happens to the resistance torque of the arm's weight force as abduction takes place to the horizontal position?  What happens to the resistance from a 90-degree abduction to a position over the head?  Explain why.

In anatomical position, the line of action of the force of the arm's weight is through the axis of rotation.  As the arm abducts to the horizontal, the line of action is drawn farther away from the axis of rotation, thus increasing the force arm of the resistive torque.  It is maximum at 90 degrees abduction.  Beyond 90 degrees abduction, the line of action of the weight force again goes closer to the axis of rotation until at 180 degrees abduction, the force arm is zero and no resistive torque exists.

5.    If the person in Figure 4.17 (page 162) were going to lift the object shown, what could the person do to make the lift easier, and why?

Flex the elbow to bring the weight closer to the axis of rotation, thus decreasing the force arm, and therefor, the resistive torque.
 
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Pages 155-157: Questions # 1, 2, 5, 7 [answers in bold print]

1.    Which of the following angles of msucle pull has the greates rotary component: 40 degrees, 23 degrees, 35 degrees, 65 degrees, 100 degrees, or 140 degrees?

100 degrees.

2.    Which of the angles listed in question 1 has the greates stabilizing component?

23 degrees.

5.    The systems in Figure 4.11 msucle forces on bones.  For each system, draw the stabilizing or dislocating force and the rotary components of force. (The force parallel to the bone may be dislocating instead.)
            a.    How does the direction of the resultant force change with chainging angles of the joint?
            b.    How does the direction of the resultant force change the stabilizing, dislocating, and rotary components of the muscles's force?
            c.    In what position does one get 100% force out of the muscle's tension?
            d.    Complete this sentence:  If one were going to lift (resist) a heavy load, one would want to position the body segments doing the lifting so that ...
            e.    After observing the situation of the stabilizing and rotary components of a muscle's pull, what can you say about the body segment's trade-off between the stability of the articulations (i.e., working against dislocation) and its mobility (i.e., the ability to produce motion)?

a.    As the joint angle increases, the resultant muscle force angle decreases (a-e).  As the joint angle decreases, the resultant muscle force angle increases (f-j).

b.    As the muscle angle increases to 90 degrees, the rotary component increases and the stabilizing component decreases; beyond 90 degrees the discloacting component increases and the rotary component decreases.

c.    The muscle angle is closes to 90 degrees pull relative to the bone.

d.    The muscleangle is closest to 90 degrees pull relative to the bone.

e.    In order to decrease the rotary component, must decrease the stabilizing component.  If both components are equally important, a 45 degree angle of muscle pull would be best.

7.    In Figure 4.12 are musculoskeletal diagrams and stick diagrams.  On the stick diagrams, draw and label the following:
            a.     Resultant muscle force (R)
            b.    Motive force arm (MFA)
            c.    Rotary force component (Ro)
            d.    Stabilizing or dislocating component (Stable or Disl)
            e.    Resistive force (RF)
            f.    Resistive force arm (RFA)

a.    Beginning at the point of application of the distal attachment of the muscle, the student should draw line representing some arbitrary magnitude of force, directed along the tendon.

b.    A line should be drawn that is perpendicular to the force vector and to the axis of rotation.

c. and d.    Beginning at the point of application of the distal attachment, the student should draw a component that is perpendicular to the bone, and a component that is 90 degrees to the first component and parallel to the bone.  In these two cases, the parallel component is stabilizing.

e.    The resistive force is the weight of the leg and foot.  In diagram a, the weight force passes through the axis of rotation at the knee, and therefor there is no resistive torque.  In diagram b, the resistive torque is directed close to 90 degrees to the bone and therefore is near its maximum.

f.    The student will find that no force arm can be drawn for the resistive force in diagram a.  The resistive force arm in diagram b will be slightly above the bone line.

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Pages 166-167:  Questions # 1 to 9 inclusive [answers in bold print]

1.    Draw a diagram of a first-class lever system that allows very vast movement of a light resistance.  Show the axis of rotation, the magnitude of the force, the resistance, the force arm, and the resistance arm.

Students should draw an arrangement with the axis (fulcrum) between the motive and resistive torques, and with the motive force closer to the axis than the resistive force.

2.    Repeat the task in question 1, showing a machine that is able to move a heavy resistance with a light force.

Students should draw a machine arrangement in which the force arm for the motive force is considerably greater than the force arm for resistive force.

3.    Repeat the task in question 1, showing a machine that provides for the balancing of the two forces.

Students should draw a first class lever system in which the motive torque and resistive torque are equal.  (Not necessarily equal forces and equal force arms on either side)

4.    Assuming the force is applied perpendicular to the bar, examine Figure 4.22 to answer the following questions:
            a.  What class lever system is it?
            b.   In which position, 1,2, or 3, will the task be the easiest?
            c.    If it takes 100 force units to move the system at position 2, what will it take to lift it at positions 1 and 3.

a.    Lever of the first class

b.    Position 1

c.    50 force units in position 1

d.    200 force units in position 3

5.    Complete the following sentences:
            a.    In concentric muscular contractions against the force of gravity, ____________  is the motive force.
            b.    In concentric muscular contractions against the force of gravitiy, ____________  is the resistive force.
            c.    In eccentric tension of the muscle, _______ is the motive force.
            d.     In eccentric tension of the muscle, _______ is the resistive force.

a.    Muscle

b.    Gravity

c.    Gravity or another external force

d.    Muscle

6.    Musculoskeletal levers most generally have an advantage in what?

Speed of movement or range of motion.

7.    When a pulley-type mechanism is found in the human body, what is its main function?

Change the direction of the applied force.

8.    It takes 50 units of force to lift 10 units of resistance.
            a.    What is the advantage of this type of arrangement?
            b.    Calculate the MA.
            c.    What class of lever probably was used.

a.    Mechanical advantage in speed of movement or range of motion

b.    1:5 advantage in ROM or speed of resistance moved

c.    Third class lever (the force is closer to the axis)

9.    The motive force is 100 units.  The balanced resistance is 50 units.  The motive force arm is 0.6m.
            a.    What is the motive torque?
            b.    What is the MA?
            c.    What is the advantage to this type of arrangement?
            d.    What is the resistive force arm?

a.   60 torque units

b.   2:1 advantage in ROM or speed of resistance moved

c.   Mechanical advantage in range of motion/ speed of movement

d.   1.20m