Chapter 13: Equilibrium and Human Movement
The segmental COM is the point of resistance (where weight vector is drawn) and therefore determines the length of the resistance arm. T/F
True
The center of gravity of an object is always located within the object. T/F
false
Changes in moment arm directly affect what?
joint muscle torque
Torque is the product of force and the perpendicular distance from the force to the axis, also known as the __________________________________.
moment arm
For torque to remain constant, more force must be produced when?
moment arm decreases
Moment arm is largest when what?
the angle of pull on a bone is 90°
When other factors remain constant, an increase in what will subsequently increase the object's stability?
the mass of an object
If force is applied directly to the axis of rotation, torque will equal what?
zero
The amount of torque which must by generated by the biceps brachii at the elbow in order to maintain the arm in a particular position is 50 Nm. If the biceps attaches to the radius at a perpendicular distance of 3 cm from the joint center at the elbow, how much force must the muscle produce? (*Convert cm to m)
-1667 N
A claw-tooth hammer being used to pry up a nail is an example of which class of lever?
-1st class
A seesaw is an example of which class of lever?
-1st class
A wheelbarrow is an example of which class of lever?
-2nd class
In the human body, most of the lever systems formed by muscles pulling on bones are what class?
-3rd class
The resistance arm (RA) will always be longer than the force arm (FA) in what class lever?
-3rd class
The potential to move without limitation defines ___________________.
-mobility
The perpendicular distance between the line of force and the axis defines what?
-moment arm
What quantity is zero for an object in static equilibrium?
-momentum -kinetic energy
If disruption of stability is the desired outcome, what should be observed?
-move COM to edge of BOS
What will benefit mobility?
-raise center of gravity
What is the proposed advantage of accommodating resistance (AR) equipment over free weights?
-resistance does not vary over the range of motion with free weights
A lever positioned with the applied resistance between the axis and the force is a _______________ class lever.
-second, 2nd
Third class lever systems favor what?
-speed -range of motion
What factors influence the amount of torque produced by muscles at a joint?
-the amounts of tension generated by the muscles -the pennate arrangement of the muscles around the joint
In analyzing the human body as a projectile, what quantity follows a parabolic path?
-the center of mass
What affects the force-generating capability of a muscle?
-the muscle length and cross-sectional area -the angle and distance from the joint center of the tendon attachment to bone -the velocity of muscle shortening
When a muscle contracts at a joint, what affects torque?
-the rotary component of muscle force
A 30 kg child sits on the right end of a see-saw 1 m from the axis. If a 20 kg child sits on the left end of the see-saw 2 m from the axis, what will happen?
-the see-saw will tilt to the left
When the elbow is extended and the elbow flexors contract, what force acts in addition to the rotary component?
-the stabilizing component of muscle force
What must be true for an object to be in a state of static equilibrium?
-the sum of all vertical forces acting on the object must be equal to zero -the sum of all horizontal forces acting on the object must be equal to zero -the sum of all torques acting on the object must be equal to zero
Why do skilled high jumpers lower their COM on the last step?
-to increase time of support between foot and ground -to increase vertical velocity at take-off -to increase impulse at take-off
What can increase base of support?
-widen stance -add more points of contact (i.e. 3-point stance to 4-point stance)
In what situation is maximal stability desirable?
-a Sumo wrestler's crouched defensive stance
What correctly identifies the parts of a lever within the human body?
-a bone is the rigid bar -a joint is the axis of rotation -a muscle insertion provides the point of force -COM of the moving segment is the point of resistance
The ability to control equilibrium defines what?
-balance
When net muscle torque and joint movement occur in the same direction, what type of contraction occurs?
-concentric
What is/are function(s) of antagonist muscles?
-control velocity of movement -enhance stability of the joint
Which of the following is/are true regarding a force couple?
-created by equal forces on opposite sides of an axis -created by forces acting in the same angular direction
What does NOT increase stability? What does?
-decrease mass Increase size of base of support, increase friction, and lower the COG increases stability.
A concept indicating a balance between applied forces and inertial forces for a body in motion is called ________________ __________________.
-dynamic equilibrium (D'Alembert's principles)
Torque is the angular analogue of what?
-force
What is/are true regarding center of mass?
-in a spherical ball with evenly distributed mass, it is in the exact center -it is the point around which a body's weight is equally balanced regardless of body position -it may be located outside an object or body
What will improve leverage?
-increase force arm -decrease resistance arm
Which type of equipment provides constant velocity?
-isokinetic
What is a characteristic of torque?
-it is a vector quantity -it is characterized by magnitude -it is characterized by direction
What will happen to the COM of a human body if the arms are raised up overhead?
-it will move up
The ratio of force arm to resistance arm defines _________________ ________________.
-mechanical advantage
The relative efficiency of a lever is proportional to what?
-mechanical advantage of the lever
