Chapter Four

Self-Study Questions

  1. Draw a diagram of a motorneuron.
  1. Explain how the sliding filament theory accounts for muscle contraction.
  1. What are the problems with using electromyography to measure muscle contraction?
  1. Name four classes of synovial joints.
  1. What are the opposite of these joint movements: Flexion, abduction, and medial rotation?
  1. Describe pronation of the forearm.
  1. How do concentric, eccentric, and isometric muscle contractions differ?
  1. State Newton’s three laws of motion and give examples of each in sport and exercise.
  1. Which of these are vector quantities and which are scalars?
  1. Force
  2. Area
  3. Linear displacement
  4. Angular velocity
  5. Speed
  6. Temperature
  1. 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:

  1. Shape
  2. Frontal area
  3. Velocity of the object
  4. 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 motorneuron.

  1. Explain how the sliding filament theory accounts for muscle contraction.

  1. What are the problems with using electromyography to measure muscle contraction?
  1. 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.
  2. 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.
  3. 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”.
  1. Name four classes of synovial joints.
  1. 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.
  2. Uniaxial – In hinge joints (e.g. elbow) and pivot joints (e.g. radio-ulnar 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.
  3. 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.
  4. 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.
  1. 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)

  1. 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.

  1. 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.

  1. 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.

  1. Which of these are vector quantities and which are scalars?

Force -

Area -

Linear displacement -

Angular velocity -

Speed -

Temperature -

  1. Explain how the impulse-momentum relationship can be illustrated by considering actions in the high jump takeoff.
  1. 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:

  1. 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.
  2. 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.
  3. 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

  1. 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.

  1. 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.

  1. 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).

  1. 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

  1. 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

  1. How can a gymnast generate enough angular momentum to allow him/her to perform a somersault?