Machines in the body (wk6)

Question 1

Describe the different types of application of force:

Answer 1

Centric force = External force directed through an object’s centre of gravity (or axis of rotation). They also produce a change in linear motion of an object .
Force couple = Forces that are equal in size but opposite in direction and do not act in the same line.
Eccentric force = An external force not directed through an object’s centre of gravity. Its going to produce a linear motion, but also some angular motion to an object to rotate it.

Question 2

Describe torque

Answer 2

-Torque -> Is the rotatory force when the line of action of a force does not pass through the axis of rotation.
-Moment arm -> The perpendicular distance between the line of action of the force and a parallel line passing through the object’s axis of rotation
-Torque = Force x moment arm (m)
-Rotation in a clockwise manner -> positive torque
-Rotation in an anticlockwise manner -> negative torque

Question 3

Describe why torques are produced within the human body

Answer 3

-Torque is the force applied by the muscles through a movement arm
-Muscle torque is calculated by the interaction of 3 factors: the actual force generated by the muscles themselves, the length of the limb or muscle group applying the force and the moment arm
-Torque is about generating force with your muscles to support your joints, ligaments, and fascia, while simultaneously exerting that force upon an object

Question 4

Describe the basics of lever systems

Answer 4

-A lever consists of a rigid bar (bone) that rotates about a fulcrum (joint-axis of rotation)
-Levers also involve ‘effort’ forces (muscles), that act to counteract ‘resistance’ forces (weight/ external pushes)
-Lever systems are used to generate torques onto the joints and rotate our limbs
-All lever system have 4 main components;
* The lever arms (bones)
* The fulcrum or axis of rotation (A)
* The force of the muscles (F)
* The resistance of body weight (R)

Question 5

Describe a 1st class lever
-Nodding head on neck

Answer 5

Example: Nodding head on neck;
* Lever – skull
* Axis of rotation – Atlanto-occipital joint (where skull articulated with neck)
* Resistance – Weight of skull (COM in front of pivot)
* Force – Contraction of posterior neck muscles

Question 6

Describe a 2nd class lever
-Plantar flexion

Answer 6

Example: Plantar flexion
* Lever – interlocked bones of foot
* Axis of rotation – 1st metatarsophalangeal joint (ball of big toe)
* Resistance – weight of body (COM behind pivot)
* Force – Contraction of posterior calf muscles

Question 7

Describe a 3rd class lever - Elbow flexion

Answer 7

Example: Elbow flexion
* Lever – bones of the forearm
* Axis of rotation – elbow joint
* Resistance – weight of forearm and hand (plus anything carried in hand)
* Force – contraction of biceps brachii

Question 8

Explain why the mechanical advantage differs in each lever system

Answer 8

-Mechanical advantage = length of FA (force x fulcrum) / length of RA (resistance x fulcrum) OR output force/ input force
-1st class -> Mechanical advantage may be high or low, depending on the length or the force arm in relation to the resistance arm
-2nd class -> Always have a high mechanical advantage because the force arm is longer than the resistance arm
-3rd class -> Always have a low mechanical advantage because the force arm is shorter than the resistance arm

Question 9

Describe what static equilibrium means in terms of forces and torques

Answer 9

-Static equilibrium -> For an object to be in static equilibrium both the external forces and external torques, about the axis, must sum to zero
-Static equilibrium equation broken down -> Sum of (force of A, multiplied by moment arm of A) + (force of B multiplied by moment arm of B) = 0
-Movement changes -> The moment arm changes as the angle of the joint are changed. Decreased moment arm means muscle is required to generate more force to the same torque. This explains why muscles are stronger in some joint positions than others.
-Equation is from lecture (29/10)

Question 10

Describe the static equilibrium equation for an isometric contraction

Answer 10

Sum of = (force of m (muscle), multiplied by moment arm of m (muscle) + (force of a (arm), multiplied by moment of a (arm) = 0
-Its on the lecture notes from 29/10