Lecture 11 - Balance and Stability

Question 1

what is equilibrium in physics?

Answer 1

• all forces and torques add to zero

Question 2

what is static equilibrium?

Answer 2

• no acceleration (isometric position)

Question 3

what is dynamic equilibrium?

Answer 3

• can have acceleration but needs to add up to zero
• includes an inertial force

Question 4

what is inertial force?

Answer 4

• mass doesn’t want to be accelerated so your body resists acceleration
• IF = -m x a
• pullback feeling of guts getting left behind

Question 5

what do inertial forces act on?

Answer 5

• they oppose any accelerations of the center of mass
• these are forces you can feel (ex: when you push left you feel yourself moving right)

Question 6

what is the difference between equilibrium and stability?

Answer 6

• stable in static equilibrium (but not necessarily dynamic)
• stable = how close you are to losing control/falling over/ wiping out

Question 7

what is the definition of wiping out?

Answer 7

• accelerating at a magnitude or direction that you don’t want (one you cannot control)
• often related to the size of the base of support

Question 8

what is the base of support?

Answer 8

• the region between the edges of the points of contact
• the larger the region, the more stable the system
• if the gravity line goes through the base of support, the system is in static equilibrium and stable

Question 9

why is walking stable but walking on ice unstable?

Answer 9

• unable to predict the friction force when on ice, causes you to lose stability as your gravity line leaves the base of support

Question 10

what is the relationship between mobility and stability?

Answer 10

• highly stable = lack mobility
• highly mobile = lack stability

Question 11

which is better for sports, mobility or stability?

Answer 11

• high-level athletes need to be at the edge of control to be the best, this is when you are the most mobile
• being unstable may cause accidents to occur (but rarely, so it is still better to have higher mobility)

Question 12

what is posture?

Answer 12

• maintaining an upright position so the force of gravity goes through the center of the base of support
• maintained by extensor muscle groups –> aka. anti-gravity muscles

Question 13

what happens to your anti-gravity muscles when you faint?

Answer 13

• buckle into flexion
• this is what gravity wants to do to the body, when you faint and the brain turns off, this is what happens

Question 14

what is postural sway?

Answer 14

• the body making slight “adjustments” to your posture

Question 15

what 4 systems help you know where the center of mass is?

Answer 15

1. pressure on the soles of the feet (shut down with cold feet)
2. proprioceptive system/joint sense (shuts down with alcohol/drugs)
3. vision (need to know where horizon is)
4. vestibular system of inner ears (gets denser with alcohol, causes bed spins)
   *nausea occurs when the systems disagree with each other

Question 16

why do you not fall over when starting/stopping a movement? (or landing/rapid stops)

Answer 16

• loss of control is dependent on the management of the ruling acceleration
• stopping = body wants to get rid of inertia, bending of legs etc. spreads the force so you don’t lose control
• turning/changing direction = tilt body to keep stability (reduce dependence on friction with banked corners) which helps migrate center of mass outside base of support