Deck 13: Equilibrium and Human Movement

Question 1

For one joint of the lower extremity, explain why eccentric torque occurs during gait.

Answer 1

Eccentric torques occur when a muscle contracts but is lengthened by an opposing force, either the action of an antagonist muscle or some other form of resistance. During running, both the hamstrings and the quadriceps muscles must be active in order to keep the leg moving forward and stabilize the gait as the rest of the body follows through. This results in discrepancies between the angular velocity of the joint and the resultant joint torque as can be seen in figure 13-7.

Question 2

Why does a force directed through an axis of rotation not cause rotation at the axis

Answer 2

The rotary effect created by an applied force is known as torque. Torque is the angular equivalent of linear force, and algebraically is related to force by the equation
, where T is torque, F is the applied force, and
is the force's moment arm, or the perpendicular distance from the force's line of action to the axis of rotation. Considering this equation, it become easy to see why a force applied through an axis of rotation does not cause rotation about the axis: because there is no lever arm available, the force has nothing to act on and cannot produce a torque, which in turn means that no movement will occur.

Question 3

Select one human motor skill with which you are familiar, and sketch a graph showing how you would expect CG height to change during that skill.

Answer 3

Question 4

Why does the orientation of a force acting on a body affect the amount of torque it generates at an axis of rotation within the body

Question 5

A 35 N hand and forearm are held at a 45° angle to the vertically oriented humerus, as shown in the text. The CG of the forearm and hand is located at a distance of 15 cm from the joint center at the elbow, and the elbow flexor muscles attach at an average distance of 3 cm from the joint center. (Assume that the muscles attach at an angle of 45° to the bones.) (a) How much force must be exerted by the forearm flexors to maintain this position (b) How much force must the forearm flexors exert if a 50 N weight is held in the hand at a distance along the arm of 25 cm (a. 175 N, b. 591.7 N)

Question 6

A 23 kg boy sits 1.5 m from the axis of rotation of a seesaw. At what distance from the axis of rotation must a 21 kg boy be positioned on the other side of the axis to balance the seesaw

Question 7

A hand exerts a force of 90 N on a scale at 32 cm from the joint center at the elbow. If the triceps attach to the ulna at a 90° angle and at a distance of 3 cm from the elbow joint center, and if the weight of the forearm and hand is 40 N with the hand/forearm CG located 17 cm from the elbow joint center, how much force is being exerted by the triceps (733.3 N)

Question 8

How much force must be produced by the biceps brachii at a perpendicular distance of 3 cm from the axis of rotation at the elbow to support a weight of 200 N at a perpendicular distance of 25 cm from the elbow

Question 9

A patient rehabilitating a knee injury performs knee extension exercises wearing a 15 N weight boot. Calculate the amount of torque generated at the knee by the weight boot for the four positions shown, given a distance of 0.4 m between the weight boot's CG and the joint center at the knee. (a. 0, b. 3 N-m, c. 5.2 N-m, d. 6 N-m)

Question 10

Two people push on opposite sides of a swinging door. If A exerts a force of 40 N at a perpendicular distance of 20 cm from the hinge and B exerts a force of 30 N at a perpendicular distance of 25 cm from the hinge, what is the resultant torque acting at the hinge, and which way will the door swing

Question 11

A 600 N person picks up a 180 N suitcase which is positioned so that the suitcase's CG is 20 cm lateral to the location of the person's CG before picking up the suitcase. If the person does not lean or compensate for the added load in any way, where is the combined CG location for person and suitcase with respect to the person's original CG location (shifted 4.6 cm toward the suitcase)

Question 12

To which lever classes do a golf club, a swinging door, and a broom belong Explain your answers, including free body diagrams.

Question 13

A worker leans over and picks up a 90 N box at a distance of 0.7 m from the axis of rotation in her spine. Neglecting the effect of body weight, how much added force is required of the low back muscles with an average moment arm of 6 cm to stabilize the box in the position shown in the text (1050 N)

Question 14

Is the mechanical advantage of a first class lever greater than, less than, or equal to one Explain.

Question 15

A man carries a 3 m, 32 N board over his shoulder. If the board extends 1.8 m behind the shoulder and 1.2 m in front of the shoulder, how much force must the man apply vertically downward with his hand, that rests on the board 0.2 m in front of the shoulder to stabilize the board in this position (Assume that the weight of the board is evenly distributed throughout its length). (48 N)

Question 16

Using a diagram, identify the magnitudes of the rotary and stabilizing components of a 100-N muscle force that acts at an angle of 20° to a bone.

Question 17

A therapist applies a lateral force of 80 N to the forearm at a distance of 25 cm from the axis of rotation at the elbow. The biceps attaches to the radius at a 90° angle and at a distance of 3 cm from the elbow joint center. (a) How much force is required of the biceps to stabilize the arm in this position (b) What is the magnitude of the reaction force exerted by the humerus on the ulna (a. 666.7 N, b. 586.7 N)

Question 18

A 10 kg block sits motionless on a table in spite of an applied horizontal force of 2 N. What are the magnitudes of the reaction force and friction force acting on the block

Question 19

Tendon forces T a and T b are exerted on the patella. The femur exerts force F on the patella. If the magnitude of T b is 80 N, what are the magnitudes of T a and F, if no motion is occurring at the joint (T a = 44.8 N, F = 86.1 N)

Question 20

Given the following data for the reaction board procedure, calculate the distance from the platform support to the subject's CG: RF 2 = 400 N, 1 = 2.5 m, wt = 600 N