Locomotion Flashcards
What is gait
Movements that produces locomotion for humans:
- walking,
– running,
– swimming,
– cycling,
Characteristics of gait:
- energy-economical, particularly walking
– Robust systems, flexibility to cope with different speeds,
terrains, injuries etc.
– sophisticated control mechanisms (bipedal gait inherently
unstable)
Stride:
a complete gait cycle, measured from one heel strike to next heel strike of the same foot
Step (=pace)
interval from heel strike of one foot to
subsequent heel strike of the other foot
how many steps equal 1 stride?
1 stride = 2 steps
The terms “stride” and “step/pace” may refer to any of
the following properties of the relevant movement:
- time duration
- distance covered
- number
Cadence
steps taken per minute
Cycle time (=stride time)
stride duration in seconds
For young adult males:
Cadence = 90-135
Cycle Time (s) = 0.9-1.3
Stride length (m) = 1.2-1.8
Speed (m s^ -1) = 1.1-1.8
Note:
* Natural walking speeds, and stride lengths, are close to the optimum for energy efficiency
* Walking speeds higher in towns than in rural environments
The principal forces are:
– body weight (BW)
– ground reaction force (GRF)
– muscle force (MF)
Which forces are external forces? What does this mean regarding the movement of the centre of mass?
BW and GRF are external forces; so the movement of the centre of mass (CoM) can be predicted from them alone.
MF must be examined however if we wish to consider either of the following:
– movements of individual limbs or body segments,
– why GRF changes in magnitude and direction during the gait cycle.
Vitally important point:
Muscle forces can only influence
the movement of the body as a
whole indirectly, by their effects
on the GRF
The gait mechanism: an overview
- Walking is a precise, co-ordinated set of movements involving multiple joints and body segments
- It comprises a pattern of alternating action of the two lower limbs
- Pendulum-like movements of the limbs give rise to two distinct phases: swing and support (or stance)
- In walking, but not running, the support phases of the two legs overlap
Walking as a controlled fall: forces involved
- When starting to move, we lean forward (MF)
- As the body starts to fall (BW), a leg is extended forwards and halts the fall (MF; GRF)
- At the same time, the other leg “kicks off, upwards and forwards” (MF; GRF) in order to keep the body moving forwards.
- This forward momentum carries the body forward into the next forward fall, i.e. the start of the next step
Body weight
- Always acts vertically downwards from the C of M
- If its line of action does not pass through a joint, it will produce a torque about that joint
- The torque will cause rotation at the joint unless it is opposed by another force (e.g. muscle, or ligament)
- BW contributes to GRF
Ground reaction force: Action Force
- Push exerted on ground by foot
Ground reaction force: Reaction Force
- Push exerted by ground on foot, as a consequence of Newton’s
3rd Law (Equal magnitude, opposite direction, same point of application
as action force) - If line of the reaction force does not pass through a joint, it will
produce a torque about that joint
In gait, as in all human movement, muscle activation generates:
internal joint moments (torques) that:
– Contribute to ground reaction force
– Ensure balance
– Increase energy economy
– Allow flexible gait patterns
– Slow down and/or prevent limb movements
Much muscle activity during gait is _________ or __________, rather than __________
Much muscle activity during gait is eccentric or isometric, rather than concentric
It’s obvious, from the previous slide, that GRF varies, through the stance phase, in terms of all three aspects namely:
– Magnitude
– Direction
– Point of action (= centre of pressure)
* We can understand GRF more readily if we resolve it into components that act vertically and horizontally
Both the horizontal and vertical components of the GRF vary during the _______ phase
stance
The direction of the horizontal component (i.e. forwards or backwards) tells us:
whether the body is accelerating or decelerating in its forwards movement at that moment of time
The magnitude of the vertical component (and specifically whether it is greater or less than body weight) tells us:
what is happening to the vertical
movement of the body
GRF during the contact phase
- Initially GRF acts diagonally backwards and upwards, from the heel. The horizontal component acts backwards, and the vertical component is greater than that of body weight. GRF at this moment therefore:
– stops the “controlled downwards fall” of the body
– exerts a braking, or slowing, effect on forward movement - During the middle of the stance phase the GRF:
– remains > body weight and therefore the CoM is lifted up slightly in
midstance.
– point of action moves forward from the heel.
– line of action becomes more nearly vertical and therefore the braking/slowing effect disappears - After the midpoint of the stance phase the vertical component of GRF
falls (< body weight) as the leg passes the vertical position and the CoM moves downwards. - At the end of the stance phase, the GRF increases in magnitude again,
acting forwards and upwards. This gives the necessary propulsive force to stop downwards movement of the CoM, and to to keep the body moving forwards.
Changes in the Centre of Pressure
(CoP)
- CoP is initially near the lateral edge of the heel
- As the stance (support) phase progresses, it moves forwards
and medially, ending up under the big toe.
Note: balance is primarily managed under the ______ of the foot.
Note: balance is primarily managed under the “ball” of the foot.
Walking is very energy-efficient because:
little ATP is required
-This is because of various
mechanisms that ensure the
mechanical energy the body has is
passed on from one step to the
next
The two forms of mechanical
energy involved are:
*kinetic energy (energy due to
movement
*potential energy (energy due
to position)
Gait efficiency & pendulum action: What is a pendulum?
- A pendulum is an object, swinging from a fulcrum, under the influence of gravity.
- A pendulum has a natural frequency of swing that is dependent on its mass, and the distance from the fulcrum to its CoM.
- Both the upper and lower limbs of the human body can move with pendulum motion, with or without muscle assistance.
During the swing of a pendulum, potential and kinetic energy are ________ and therefore, overall, energy is _________.
During the swing of a pendulum, potential and kinetic energy are interconverted and therefore, overall, energy is conserved.
What happens with kinetic and potential energy as a pendulum swings?
- As the pendulum swings away from
the midpoint, in either direction, KE
is progressively converted into PE - At the extreme points in the swing,
there is no KE at all and all the
energy is present as PE
(in the middle of the pendulum, kinetic energy is high, but at the extreme points, potential energy is high)
Conventional pendulum action during the swing phase
- The legs move as conventional pendulums during the swing
phase (with a little assistance from the hip flexors). - This reduces the amount of muscle energy needed to move the
swinging leg forward - It also accounts for the “natural” frequency of gait that has
optimal energy efficiency (see slide 7) - Although the legs swing forwards much like pendulums, they are
prevented from swinging backwards by foot strike. - During the stance phase, the leg can be viewed as an “inverted
pendulum”. This action also involves inter-conversion of
potential and kinetic energy
An “inverted” pendulum
The pendulum “bounces” backwards and forwards, using the springs.
At the middle of the pendulum, potential energy is high
At the extremes, kinetic energy is high
“Inverted” pendulum action
during the stance phase
- During the stance phase, the leg can be viewed as an “inverted pendulum”.
- The forward momentum of the body gives it the necessary initial angular
velocity of rotation (taking the place of the “spring” on the previous slide). - When reaching the endpoint of its “inverted swing” the stance leg does not
swing back, as a real inverted pendulum would, because the foot is taken off the floor, the fulcrum transfers from the foot to the hip, and the leg swings again as a conventional pendulum.
Walking modelled as a
rolling lemon: mid-stance
- At the midstance point for either leg, the CoM of the whole body is relatively high
- Therefore PE for the whole body is relatively high, and KE (forwards movement velocity) relatively low
What is the main qualitative difference between walking and running?
the flight phase (i.e. period of no support) and the absence of a period of double support.
What is a quantitative difference between walking and running?
in running gait, the foot hits the ground less far in front of the body’s centre of gravity, compared with walking (i.e.
when we run, the forward swinging leg “sticks out” less far in front of the trunk at footstrike).
*This characteristic is more pronounced the faster the run.
The above two differences lead to the following consequences:
- When running, the body’s momentum alone has to carry it over the support foot, as the other foot is not in contact
with the ground. - The position of heel strike, relative to the CoM, helps with this (see previous slide), because it means that the CoM
is not lowered as much at foot strike. - The position of heel strike relative to the CoM also reduces the ‘braking effect’ of the GRF during the first part of the stance phase
During transition from walking to running…..
*the period of double support disappears
*a greater proportion of the pace time is spent in the swing phase
Stride rate and length
- As running speed increases, both stride rate and length become higher.
- Initially, at relatively low speeds, the changes are proportionally greater in length than in rate
- Near maximum speed, however, rate increases more than length.
- The explanation for this is in terms of energy efficiency
- In energy terms, it is more efficient to increase speed by taking longer paces rather than taking them more rapidly
GRF during running compared with walking:
- Initial ‘contact’ peak is smaller and is not angled back as far (less braking effect)
- Final ‘thrust’ peak is larger (need to project body into flight phase, faster speeds etc)
- Duration of contact phase is shorter
Energy considerations during running
- In running, both kinetic and potential energy are high during the flight phase.
- Energy storage in elastic tissues at the start of the support phase has a more prominent role in running
(By contrast, elastic energy storage during walking is smaller)
Energy during running: the bouncing
ball model
KE and PE are both high at the top of the “bounces” (equivalent to the middle of the flight phase)
During ground contact, KE and PE are lower, and energy is stored in elastic
tissues.
For running, we have to consider
interconversion between three different forms of energy:
PE, KE and elastic
