Forces/ Joint Reaction Forces
What is the definition of a force?
What is Newton's 2nd law?
Force is a push/pull that initiates, modifies, or stops a movement
Newton's 2nd law is F=ma.
Where is the center of mass of the body in an adult?
Anterior to S2
Draw the 3 different types of levers.
1st class: EF then fulcrum then IF
2nd class: fulcrum, EF, IF
3rd class: fulcrum, IR, EF
What are the 2 types of internal forces? Provide an anatomical example of both.
Active internal forces are generated by muscle.
Passive internal forces provide tension when tissues stretch. Examples include connective tissues, ligaments, and joint capsules.
If a person raises their arms above their head, in which direction does their COM move?
Superior
The human body is comprised mostly of 3rd class levers. Why is this advantageous?
3rd class levers allow us greater ROM and speed to complete functional tasks.
What is the most common external force that acts on the body?
Gravity
What happens when our COM moves outside of our base of support?
We will fall! Unless we move our body in a way to get our COM back within the base of support.
Define mechanical advantage and what type can be found in each class of levers.
Mechanical advantage is the ratio of Ima to Ema. Mechanical advantage = Ima/Ema
1st class lever mechanical advantage can be either =1, >1, <1
2nd class lever mechanical advantage is always >1
3rd class level mechanical advantage is always <1
What are joint reaction forces (JRF) and why they are important?
A JRF is a force that exists at a joint, developed in reaction to the net effect of internal and external forces.
JRF prevents segment translation to create a potential for rotation at joint.
True or False. Torque is generated if an external or internal force is applied directly at the axis of rotation.
False. There must be a moment arm to create the rotational direction of the force for torque to occur. Forces applied to the axis of rotation have no moment arm.
You are bicep curling a 45 pound dumbbell that is held in your hand 40 cm away from the elbow. The biceps inserts 10 cm from the elbow. Neglecting the weight of the forearm, how many pounds of force does your bicep have to exert in order for you to curl the dumbbell?
Over 180lbs of force!!
Name 3 ways physical therapists can decrease JRF's to decrease pain in their clients.
Decrease external forces by:
1. making a carried weight lighter (i.e. purse or bag)
2. decreasing the moment arm of an external weight (i.e. moving a purse closer to the elbow)
3. Aquatic therapy (buoyancy decreases the effects of gravity)
4. Using an assistive device (i.e. cane or walker)
Describe how we could reduce the JRF of the tibiofemoral joint when doing a seated open chain knee extension exercise with a cuff weight at the ankle.
Since we are not able to change the moment arm of a muscle insertion, we must change the EF or Ema. This can decreased by decreasing the weight of the cuff weight (decreasing external force) or moving the cuff weight closer to the knee (decreasing external moment arm).
Draw a 1st class lever. Describe the mechanical advantage. What conditions are required for it to be in static equilibrium?
The torques must be equal!