Gait, Slipping and Fall

Question 1

What is the definition of a slip?

Answer 1

• The sudden loss of grip, resulting in a sliding of the foot on a surface due too a lower coefficient of friction than that required for the momentary action

Question 2

At what sort of distance will a slip lead to a fall?

Answer 2

3-10cm slip may result in corrective recovery
A slip over 10cm usually results in uncontrolled forward movement and often a fall

Question 3

What is the definition of a fall?

Answer 3

A free descent governed by gravity that occurs when human balance is perturbed beyond a certain recoverable point

Question 4

At what point in a step phase does a slip occur?

Answer 4

at 2 stages:
1) Upon heel strike (touchdown)
2) Push off (takeoff)

Question 5

Why do slips occur at touchdown and takeoff?

Answer 5

These are the moments at which maximum anterior-posterior force from the ground happens

Question 6

What type of slip is more dangerous? Touchdown or take-off?

Answer 6

Touchdown
- This results in a posterior fall
- There is less you can do to protect yourself or recover from the fall

Question 7

Explain the process of touchdown slipping

Answer 7

• Within 100ms after touchdown, bodyweight has been transferred onto slipping foot
• Forward momentum causes the slip in which the foot translates anteriorly resulting in a fall if slippage is large enough

Question 8

Explain the process of a slip from take-off

Answer 8

• Upon push off the anterior force overcomes static friction resulting in a posterior slip
• Usually the opposing foot is leading and already holding some of the weight, so less likely to lead to a fall

Question 9

Which falls are more dangerous, posterior or anterior and what are they caused by?

Answer 9

Posterior are more dangerous
- Posterior slippage upon touchdown
- Anterior slippage upon Take off

Question 10

What do these graphs tell us

Answer 10

• In a single gait cycle there are 2 opposing peaks of shear force
• starting with the heel touchdown there is of course an impact peak and an anterior force peak (force is pushed forward by the foot into the ground)
• At takeoff this is a propulsive peak and also posterior force peak, where force is put backwards into the ground to push forward

Question 11

What happens if one of these anterior or posterior shear force peaks is greater than the coefficient of static friction?

Answer 11

Slip and potentially fall

Question 12

What does this graph tell us?

Answer 12

• The y-axis is a measure of Horizontal force/Vertical force
• The large peaks at both ends indicate that upon take off and touch down there is a large amount of Horizontal force in comparison to vertical force
• the general trend shows a transition from a forward force to a backward push off force

Question 13

What are the 2 types of friction and how do you calculate them?

Answer 13

Static friction = Vertical reaction force * Static coeff friction

Dynamic Friction = Vertical reaction force * Dynamic coeff friction

Question 14

What does this diagram tell us?

Answer 14

A-C, as the applied force increases, the frictional force opposing it increases linearly keeping the object motionless
At C the maximal frictional force is overcome by the applied force
At D the object begins moving and is resisted by a lesser dynamic friction

Question 15

What has the potential to be greater, static friction or dynamic friction?

Answer 15

• Static friction

Question 16

Explain the definitions of RCOF, SCOF and DCOF?

Answer 16

Required Coeff of Friction: Friction required for forward propulsion without slips
Static COF: Max available friction between 2 objects without motion between
Dynamic COF: Available friction between 2 sliding objects

Question 17

As humans what do we intrinsically do to mitigate risk of slipping?

Answer 17

We shorten step length and slow down to reduce vertical reaction force

Question 18

How does the heel horizontal velocity change throughout a gait cycle

Answer 18

• Heel vertical velocity rises after take=off, and reaches a peak during the swing phase before falling to zero again upon heel strike

Question 19

What method did Winter et al use to measure lower limb joint kinematics?

Answer 19

Using exoskeleton set up, in which rods were connected external to the body and sensors measures the angle changes at the joint sites

Question 20

List some environmental risk factors for walking

Answer 20

• Steepness
• Surface roughness
• Topography
• Sole tread

Question 21

List some human risk factors for walking

Answer 21

• Neuromuscular control
• Foot positioning
• Vision
• Alcohol intake
• Balance and proprioception
• Reaction time

Question 22

What does this theoretical framework tell us?

Answer 22

• In the case when available friction is greater than required friction for walking, no slipping occurs
• In the case when available friction is less than the required friction, we either adapt our walking style so that it lowers the required friction below that of the available friction or not and risk slipping

Question 23

Why do we adapt our walking style on highly slippery surfaces?

Answer 23

• Highly slippery surfaces have a low available friction, often this is below that of our required friction for walking
• when RCOF>ACOF we can easily slip
• By adapting our walking style we lower the RCOF to below that of the ACOF

Question 24

In what ways do we adapt our gait to avoid slipping?

Answer 24

• Longer stance, swing and stride time (slower)
• Shortened stride length
• Greater toe grip
• Gentle foot strike (flat foot landing and stiffer ankle joint)
• Slower propagation speed and heel velocity and foot strike

Question 25

What does this graph tell us?

Answer 25

• When we walk on surfaces with an available COF of more than 0.2 we walk the same
• When the available COF is less than ~0.2, we begin to adapt our gait to reduce the Required/utilised COF, to be less than the available/dynamic COF of the surface