Chapter Four
Self-Study Questions
1. Draw a diagram of a motor neuron.
2. Explain how the sliding filament theory accounts for muscle
contraction.
3. What are the problems with using electromyography to
measure muscle contraction?
4. Name four classes of synovial joints.
5. What are the opposite of these joint movements: Flexion,
abduction, and medial rotation?
6. Describe pronation of the forearm.
7. How do concentric, eccentric, and isometric muscle
contractions differ?
8. State Newton’s three laws of motion and give examples of each
in sport and exercise.
9. Which of these are vector quantities and which are scalars?
a.
b.
c.
d.
e.
f.
Force
Area
Linear displacement
Angular velocity
Speed
Temperature
10.
Explain how the impulse-momentum relationship can be
illustrated by considering actions in the high jump takeoff.
11. Describe the three different classes of levers. Which are most
common in the human body and why?
12. What are the factors that determine the motion of an object
when it is thrown into the air?
13. Explain how the following factors affect the drag force on an
object moving through the air:
a.
b.
c.
d.
Shape
Frontal area
Velocity of the object
Density of the air
14. How can a gymnast generate enough angular momentum t allow
him/her to perform a somersault?
Chapter Four
Self-Study Questions
Answer Key
1. Draw a diagram of a motor neuron.
Structure of a Motor Neuron
2. Explain how the sliding filament theory accounts for muscle
contraction.
Huxley’s Sliding Filament Theory
The heads of the myosin molecules are shaped like golf clubs and it is
the ends of the heads that attach to the actin.
Attached to the myosin head is a chemical called adenosine
triphosphate (ATP…remember this???) and this is split chemically into
adenosine diphosphate (ADP….remember this???) and phosphate.
At the same time the head of the myosin “bend” sliding the actin along
a little relative to the myosin.
Huxley’s Sliding Filament Theory
The myosin is then released from the actin and fresh ATP can join the myosin
head.
If the electrical signal is still present, fresh calcium will also be available, and the
myosin heads can bind to the next sites on the actin.
This action allows actin to be pulled along parallel to the myosin.
This happens repeatedly while the neural signal is active, therefore the muscle
contracts by sliding of the actin relative to the myosin.
*Muscle fibers are made up of many thousands of actin and myosin molecules
and the sliding of all these make the muscle contract.
3. What are the problems with using electromyography to
measure muscle contraction?
A. Fat, fluid and skin have a filtering effect on the signal as it travels
between the muscle being studied and the surface electrodes.
This reduces the size of the signal and also spreads it out, making
it difficult to identify what is happening at the motor units.
B. The movement of the muscles under the skin in dynamic
contractions means that the EMG may be recording electrical
activity from different parts of the muscle or even from different
muscles as the body segments move.
C. Electrical signals from other devices (such as fluorescent lights)
and from motion of the cables used to transmit the EMG signal
that can interfere with the recordings, and make it difficult to
distinguish the required information from the electrical “noise”.
4. Name four classes of synovial joints.
A. Non-axial – In gliding joints (e.g. between the carpal bones in the
palm of the hand) the bones simply slide in relation to each other.
There are no axes of rotation in this type of joint.
B. Uniaxial – In hinge joints (e.g. elbow) and pivot joints (e.g. radioulnar joint) there is only one axis of rotation. This means that the
structure of the bones at the joint restricts rotation to movement
around one axis only.
C. Biaxial – At condylar joints (e.g. the knee) and saddle joints (e.g.
base of the thumb) there are two axes of rotation and therefore the
bones can move in two different ways.
D. Triaxial – Ball and socket joints such as the shoulder and hip
allow rotation around three axes. These bones permit the greatest
movement, as they allow the limbs attached at them to move
through a large volume of space.
5. What are the opposite of these joint movements: Flexion,
abduction, and medial rotation?
Flexion – extension (Sagittal Plane)
Abduction – adduction (Frontal Plane)
*Medial rotation – lateral rotation (also known as Abduction and
Adduction in the Horizontal Plane)
6. Describe pronation of the forearm.
Starting in anatomical position, as the forearm moves into
pronation, the radial bone rolls over the ulna bone in the forearm.
7. How do concentric, eccentric, and isometric muscle
contractions differ? I added isometric and isokinetic.
Isotonic contraction – shortening or lengthening of a muscle
against a movable resistance
a. Concentric contraction – is a type of isotonic contraction.
It is the shortening of a muscle as it gains tension to
overcome a resistance. Movement against gravity.
b. Eccentric contraction – is a type of isotonic contraction. It
is the lengthening of the muscle as it develops tension to
overcome an external resistance. Movement towards gravity.
Isometric contraction – static contraction against a given resistance
where the muscle length remains unchanged as the muscle
tension increases.
Isokinetic contraction – contraction in which the tension developed
by the muscle while moving at a constant speed is maximal
through the full range of movement.
8. State Newton’s three laws of motion and give examples of each
in sport and exercise.
Newton’s First Law – “An object will remain at rest or
continue with constant velocity unless acted on by an
unbalanced force.” Meaning if that bodies or objects stay
where they are or keep moving unless acted on by an
unbalanced force. This law is sometimes known as the law
of inertia.
For a body to be at rest (standing still) on the earth, there
must be a force balancing the weight, acting in the opposite
direction (upwards). This force often comes from supports or
the ground and is called the reaction force. If an object is
travelling at constant speed in a constant direction, the force
on it must also be balanced.
A true understanding of a force is that it is something that
changes (or tries to change) a body or object’s motion and not
something that causes motion.
Newton’s Second Law – “The acceleration (for a body/object
of constant mass) is proportional to, and in the same
direction as the unbalanced force applied to it.” This law is
sometimes known as the law of acceleration. (Force is equal
to mass multiplied by acceleration; F=ma).
A body’s object’s change of motion is directly relatied to the
size of the (unbalanced) force causing the changes and wil
change motion in the direction of the applied force. The
change in motion is also inversely related to the mass of the
object. Therefore heavier objects will accelerate less for the
same force, and to accelerate heavy objects, a large force is
needed.
Acceleration is change in velocity divided by the time it taken
(F=m(v-u)/t). V and u are the final and initial velocities and t
is the time for the velocity change.
Newton’s Third Law – “When one body or object applies a
force to another, the second body or object will apply a force
equal in size, but opposite in direction to the first body or
object.” “For every action there is an equal and opposite
reaction.” This law is often called the law of reaction.
Important factors: First, the two forces are on two different
bodies or objects (not on the same body/object). Secondly,
the forces on the objects are exactly the same size,
regardless of the masses of the objects. The effects of those
forces may be different if the bodies/objects are different
masses – this is due to Newton’s second law of motion.
Thirdly, the force happens at exactly the same time – one
does not occur later in response to the other.
9. Which of these are vector quantities and which are scalars?
Force Area Linear displacement Angular velocity Speed Temperature -
10. Explain how the impulse-momentum relationship can be
illustrated by considering actions in the high jump takeoff.
11. Describe the three different classes of levers. Which are most
common in the human body and why?
Levers consist of a rigid rod, a fulcrum (axis), a resistance force
and an effort force. The distance at which the resistance acts from
the fulcrum is called the resistance arm, and the distance at which
the effort acts from the fulcrum is called the effort arm.
The lever has a mechanical advantage. This is how much the effort
force is multiplied by to overcome the resistance force and can be
calculated as the effort arm divided by the resistance arm.
Three classifications:
A. First Class Levers –have the effort force and the
resistance force on opposite sides of the fulcrum. The
effort arm may be smaller than, equal to or smaller than
the resistance arm. These are fairly rare in the human
body. Example: the muscles in the neck providing the
effort force to overcome the resistance force caused by
the weight of the head.
B. Second Class Levers – have the effort force and resistance
force on the same side of the fulcrum, but with the effort
arm longer than the resistance arm (i.e. the effort force is
further away from the fulcrum than the resistance force).
This means the mechanical advantage is greater than 1
and small effort force can overcome large resistance.
This type of lever is very rare in the human body.
Example: Calf muscles contract to provide the effort force
when standing in plantar flexion. Example two:
wheelbarrow.
C. Third Class Levers – have the effort and resistance forces
on the same side of the fulcrum, but the effort arm is
smaller than the resistance arm (the effort force is closer
to the fulcrum than the resistance force).The mechanical
advantage is less than 1, and might seem
counterproductive as large effort forces are required to
overcome small resistance forces. A small movement of
the lever near the fulcrum is magnified by the length of
the lever, so that the end of the lever moves through a
greater angle, and with the greater angular velocity. Thus
the advantage is in range of motion and speed. This type
of lever is very common in the human body. Example:
biceps providing effort force at the elbow joint to hold a
weight at the hand (resistance force).
This may be an easier way to understand the levers
A. First Class Levers – The fulcrum is located between the
force and resistance, causing the two lever arms to move
in opposite directions like a teeter-totter or a pair of
scissors.
Example: Neck hyperextension. The atlas is the fulcrum,
face is the resistance, and the neck muscles are the force.
B. Second Class Levers – The resistance is between the
fulcrum and the force, is like a wheelbarrow.
Example: Toe raises. The ball of the foot in contact with
the ground is the fulcrum, the weight at the ankle joint is
the resistance, and the calf muscles are the force.
C. Third Class Levers – The force between the fulcrum and
resistance like a screen door opening, or swinging a golf
club. This is the most common in the body.
Example: biceps providing effort force at the elbow joint
(fulcrum) to hold a weight at the hand (resistance force).
12. What are the factors that determine the motion of an object
when it is thrown into the air? (pages 98-99)
An object that is thrown in the air or dropped and is acted upon by
only the forces of gravity, air resistance, and lift known as projectile.
Important parameters at release or take-off are:
*Projection speed
*Projection angle to the horizontal
*Projection height
*The first two are often combined as the projection velocity
13. Explain how the following factors affect the drag force on an
object moving through the air:
Shape
Frontal area
Velocity of the object
Density of the air
14. How can a gymnast generate enough angular momentum to
allow him/her to perform a somersault?