Movement Biomechanics of Human Gait

1
Movement Biomechanicsof Human Gait

• SP2004N Biomechanics
• http//homepages.unl.ac.uk/woodwarc

2
Plan

• Basic terms and definitions
• Overview
• Forces weight, GRF and muscle
• Determinants of gait
• Energy considerations
• Running vs walking
• Self-directed learning supplement
• Joint movements and muscles

3
Gait

• movements that produces locomotion
• including, for humans
• walking,
• running,
• swimming,
• cycling, etc
• characteristics
• energy-economical, particularly walking
• flexibility to cope with different speeds,
  terrains etc.
• sophisticated control mechanisms (bipedal gait
  inherently unstable)

4
Basic terms

• Stride a complete gait cycle, measured from one
  heelstrike to next heelstrike of the same foot
• Step (pace) interval from heelstrike of one
  foot to subsequent heelstrike of the other foot
• Therefore 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

5

• Note
• Alternating periods of double and single support
• About 7030 split between single and double
  support in normal walking

6

• Cadence steps taken per minute
• Cycle time ( stride time) stride duration in
  seconds
• Therefore, cycle time stride duration
• 2 x step duration
• 2 x 60/cadence 120/cadence
• For young adult males
• Cadence Cycle Stride Speed
• Time (s) length (m) (m s-1)
• 90-135 0.9-1.3 1.2-1.8 1.1-1.8
• Natural walking speeds, and stride lengths, are
  close to the optimum for energy efficiency
• Walking speeds higher in towns than in rural
  environments

7
Walking and running efficiency
This diagram shows the metabolic power (i.e.
energy used per second) to support walking (blue)
and running (red) at different speeds. Note the
disproportionate increase in energy usage at fast
walking speeds. So slow walking is very
economical, because the blue line stays close to
the x-axis up to about 2 m/s. But faster walking
and running are not economical.
(a) Metabolic power (W)
8
Walking and running efficiency
This slide shows the metabolic energy usage per
metre of movement (contrast with energy usage per
second on the previous slide). Bluewalking
redrunning Note the minimum energy usage at
intermediate walking speeds, indicating optimum
efficiency for gait on a per metre basis
(b) Economy (J m-1)
9
Forces

• The principal forces are
• body weight (BW)
• ground reaction force (GRF)
• muscle force (MF)
• 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.

10
Vitally important point

• Muscle forces can only influence the movement of
  the body as a whole indirectly, by their effects
  on the GRF

11
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

12
Walking as a controlled fall

• One way of beginning to understand the mechanics
  of walking is to view the movements as a
  controlled fall
• When starting a walk, we lean forward,
  overbalancing from the equilibrium position.
• This gives the upper part of the body forwards
  (and downwards) motion
• As the body falls forwards, a leg is extended
  forwards and halts the fall
• At the same time, the other leg kicks off 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

13
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

14
Body weight

• Always acts vertically downwards from the CoM
• 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

15
Ground reaction force

• Action force
• Push exerted on ground by foot
• Results from the sum of the following
• Body weight
• impact force of foot on ground (at
  footstrike only)
• pushing force from contraction of extensor
• muscles (towards end of stance phase)
• Reaction force
• Push exerted by ground on foot, as a consequence
  of Newtons 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

16
Muscle force

• 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 eccentric or
  isometric, rather than concentric

17
Combined effects of muscle force and BW on GRF
when standing

• When standing still, the sum of the two GRF
  forces (one acting on each foot) is equal and
  opposite to BW. So equilibrium prevails and there
  is no movement
• When the individual squats down, GRF magnitude
  decreases below that of BW during the phase of
  downwards movement.
• This happens because flexion at the leg joints
  prevents the full force of body weight from being
  transmitted down through the contact points
  between the feet and the floor
• So there is a net downwards force acting on the
  body this is what causes the downwards movement.
• When the individual is at the low point of the
  squat, the sum of the two GRF forces (one acting
  on each foot) is again equal and opposite to BW.
  So equilibrium prevails and there is
  momentarily - no movement
• When the individual rises up from the squat
  position, GRF magnitude increases above that of
  BW during the phases of upwards movement.
• This happens because extension at the leg joints
  increases the action force pushing on the floor
  above that of body weight
• So there is a net upwards force acting on the
  body this is what causes the upwards movement.

18
Muscle activity influences the GRF

• The cause of the altered GRF is extensor muscle
  relaxation (downwards/squat) phase and
  contraction (upwards/rise phase). The altered
  muscle activity affects GRF by changing the
  extent to which the foot presses against the
  support surface.
• So for example
• GRF equal and opposite force to that exerted
  by foot on ground (by definition)
• body weight any extensor muscle activity
• When the extensors relax, on the other hand, body
  weight cannot be effectively transmitted to the
  foot (because there is no longer a rigid body
  structure to transmit it. Hence
• GRF equal and opposite force to that exerted
  by foot on ground (by definition)
• lt body weight

19

• To summarise
• Upwards GRF BW
• The CoM of the body will remain at the same
  height (or remain moving at the same rate)
• Upwards GRF gt BW (e.g. jumping)
• The CoM of the body will move upwards (or
  downwards movement of the CoM will be slowed or
  halted)
• Upwards GRFlt BW (e.g. squatting)
• The CoM of the body will move downwards (or
  upwards movement of the CoM will be slowed or
  halted)

20
Static posture GRF equal and opposite to BW.
Downwards squat GRFltBW
Upwards movement GRFgtBW
21
Butterfly diagram- showing GRF through the stance
(support) phase
The lines represent GRF force vectors at
intervals off about 50 ms during the stance
phase The line at the extreme left hand end
represents the force vector at the moment the
foot touches the ground. The next one represent
GRF 50 ms later, and so on. The line at the
extreme right-hand end represents the GRF when
the toe leaves the ground.
Remember vector lines incorporate three aspects
of the force they represent magnitude (length
of line), point of action, and direction of action
22
Butterfly diagram- showing GRF through the stance
(support) phase

• Its 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

23
Resolving the GRF into vertical and horizontal
components

• A represents the GRF at the moment of
  footstrike (see slide 20).
• It is made up of a horizontal component (C) and a
  vertical component (B).
• In terms of force vectors, we can write
• A B C
• (Note that this is not normal arithmetic
  addition because it also takes account of the
  relative directions of A, B and C)
• Geometrically, the arrowhead ends of A, B, and C,
  together with the common point of origin of the
  four forces, form the corners of a rectangle.
• This fact enables us to calculate the magnitude
  of B and C, provided we know A.

24
Resolving the GRF into vertical and horizontal
components

• Both the horizontal and vertical components of
  the GRF vary during the stance phase
• 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

25
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 gt 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 (lt 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.

26
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.

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CoP scans Red line shows passage of CoP during a
single stance phase Colours denote peak pressures
achieved at different points on the foot.
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Determinants of Gait

• 6 specific features that increase the efficiency
  of walking and running gaits
• All reduce unnecessary movement of the upper
  body, either vertically, or horizontally in the
  lateral axis

29
DG1 Pelvic tilt

• Reduces the vertical movements of the upper body,
  and thereby increases energy efficiency.
• The pelvis slopes downwards laterally towards the
  leg which is in swing phase. i.e. rotation about
  an anterior-posterior axis
• Only anatomically possible if the swing leg can
  be shortened sufficiently (principally by knee
  flexion) to clear the ground.
• Where this is not possible (e.g. through injury),
  the absence of pelvic tilt and pronounced
  movements of the upper body are obvious.

30
DG2 Pelvic rotation

• Rotation about a vertical axis enables a given
  step length to be achieved with less vertical
  excursion of the trunk.
• Alternatively, longer step lengths can be
  achieved for the same vertical movement.

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DG3 Knee flexion in stance phase

• As the hip joint passes over the foot during the
  support phase, there is some flexion of the knee.
  
• This reduces vertical movements at the hip, and
  therefore of the trunk and head.

32
DG4 Ankle mechanism

• At footstrike, the effective length of the leg is
  increased by the projection of the calcaneus
  behind the ankle.
• This is brought about by dorsiflexion at the ankle

33
DG5 Forefoot mechanism

• During the final part of the support phase, the
  forefoot serves to increase the effective length
  of the leg lever.
• This is brought about by plantarflexion at the
  ankle

34
DG6 Reduced lateral pelvic displacement

• Is minimised by having a narrow walking base i.e.
  feet closer together than are hips.
• Therefore less energy is used moving hip from
  side to side (less lateral movement needed to
  balance body over stance foot.
• Enabled by valgus angulation at the knee

35
Efficiency, and energy considerations

• Walking is very energy-efficient 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)

Economy (J m-1)
36
Gait efficiency pendulum action

• 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.
• During the swing of a pendulum, potential and
  kinetic energy are interconverted and therefore,
  overall, energy is conserved.
• Both the upper and lower limbs of the human body
  can move with pendulum motion, with or without
  muscle assistance.

37
A conventional pendulum energy interconversion
P.E. Potential energy K.E. Kinetic energy
Three points on a pendulum swing are
illustrated. 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
38
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 footstrike.
• During the stance phase, the leg can be viewed as
  an inverted pendulum. This action also involves
  inter-conversion of potential and kinetic energy

39
An inverted pendulum
The pendulum bounces backwards and forwards,
using the springs.
40
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).
• Inverted pendulum action also involves
  inter-conversion of potential and kinetic energy,
  but in this case (unlike a conventional pendulum)
  KE reaches a minimum at the midpoint of the
  motion, and PE is highest at that point.
• 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.

41
Walking modelled as a rolling lemon
Slow
Pendulum considerations help us understand energy
efficiency by concentrating on the individual
legs. But ultimately we need to consider the
energy of the whole body A simple model that
allows this is that of a rolling ellipse, with
the midpoint of the ellipse representing the CoM
of the body
Fast
42
Walking modelled as a rolling lemon midstance
At the midstance point for either leg, the CoM of
the whole body is relatively high (despite the
best efforts of the Determinants of Gait).
Therefore PE for the whole body is relatively
high, and KE (forwards movement velocity )
relatively low
43
Walking modelled as a rolling lemon early/late
stance
Towards the beginning or end of the stance phase
for either leg, the CoM is lower. Therefore the
PE for the whole body is reduced, and KE
(forwards movement velocity ) relatively high
44
Running

• The main qualitative difference between walking
  and running is the flight phase (i.e. period of
  no support) and the absence of a period of double
  support.
• An important quantitative difference is that, in
  running gait, the foot hits the ground less far
  in front of the bodys 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.

45

• The above two differences lead to the following
  consequences
• When running, the bodys momentum alone has to
  carry it over the support foot, as the other foot
  is not in contact with the ground.
• The position of heelstrike, relative to the CoM,
  helps with this (see previous slide), because it
  means that the CoM is not lowered as much at
  footstrike.
• The position of heelstrike relative to the CoM
  also reduces the braking effect of the GRF
  during the first part of the stance phase

46

• 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
• Activity Approx time on
• stance swing
• Slow walk 60 40
• Race walk 50 50
• Run 30 70
• Sprint 20 80

47
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

48
GRF Butterfly during running
49
GRF during running

• Compared with walking
• Initial contact peak is smaller and is not
  angled back as far (less braking effect see
  slide 45)
• Final thrust peak is larger (need to project
  body into flight phase, faster speeds etc)
• Duration of contact phase is shorter of course!

50
Energy considerations during running

• Energy usage differs fundamentally between
  running and walking
• 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 in fact we ignored it
  altogether when considering this topic earlier.

51
Energy during running the bouncing ball model
Contrast this with the rolling lemon model for
walking. Here, 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.
So, for running, we have to consider
interconversion between three different forms of
energy PE, KE and elastic
52
Walking and running energy costs
(b) Economy (J m-1)
(a) Metabolic power (W)
53
Elastic energy storage during running

• Total kinetic energy dissipation at footstrike
  100 J/pace (70 kg subject 4.5. m s-1)
• At start of support phase, elastic energy (from
  the foot impact) is stored in
• Achilles tendon 35J
• Patellar tendon 20J
• Arch of foot 17J
• TOTAL 72J
• Thus, almost ¾ of the kinetic energy that would
  otherwise be lost at footstrike, is instead
  stored as elastic energy, in ligaments and
  tendons, and recovered in kinetic form during the
  latter parts of the support phase
• Due to this elastic energy storage, muscles do
  not need to contract as far or as fast, and
  metabolic energy use, in the form of ATP, is
  reduced

54
Elastic properties of tendons

• Tendons can stretch up to 8 and recoil
  elastically
• gt93 of stored energy is recovered
• Both patellar and Achilles tendons are relatively
  long and thin. Hence they allow significant
  flexion while storing energy

55
Experimental data on elastic force and the
Achilles tendon

• Marathon runner (2h 37 min)
• Peak force on ball of foot 2.7BW (1900 N)
• Peak force in Achilles tendon 4700 N
• Minimum cross sectional area of tendon 90mm2
• ?Force per unit cross-section area 50 N mm-2
• This will stretch tendon by 6
• Margin of safety is about 100 i.e. tendon will
  rupture at about 100 N mm-2

56
Running shoes and elastic energy storage

• Heels of running shoes absorb a maximum of about
  7J, and return less than 66 of this.
• Therefore, they make only a small contribution to
  energy economy
• Lateral stability limits the degree of elasticity
  that can be incorporated into shoes.

57
References
Coverage of gait in biomechanics textbooks is
variable.The following list is a selection that
each approach the topic from different
angles Adrian MJ Cooper, JM (1995)
Biomechanics of Human Movement. Brown
Benchmark Enoka, RM (1994) Neuromechanical Basis
of Kinesiology. Human Kinetics Luttgens, K
Hamilton, N (1997) Kinesiology. Brown
Benchmark McNeill Alexander, R. (1992) The Human
Machine. Natural History Museum Publications.
58
Movements at different joints during the gait
cycle, and associated muscle activiesSupplementa
ry slides for lecture, showing involvement of the
Determinants of Gait (DG)
59
Swing phase spine pelvis

• Movement
• Rotation of pelvis towards support leg (i.e.
  non-support side goes forward) DG2
• Lateral tilt of pelvis towards unsupported leg
  DG1
• Movements at the vertebral joints of the spine
  aim to counteract the consequences of the above
  pelvic movements
• Upper spine rotates in opposite direction to
  pelvic rotation
• Lumbar spine flexes in opposite direction to
  pelvic tilt.
• These spinal movements ensure that the pelvic
  movements do not result in rotation and flexion
  of the entire upper body.
• Determinants of gait, numbered as in main
  lecture slides

60
Swing phase the hip

• Flexion is the main movement.
• Maximum hip flexion is reached about half way
  through the swing phase.
• Hip rotation and adduction/abduction are needed
  to ensure the swinging leg continues to point
  forwards, despite pelvic rotation.
• Iliopsoas is prime mover, but active only early
  on
• Swing phase is therefore partly a ballistic
  movement
• At the start of the swing phase, gravity and
  energy stored in the stretched hip ligaments also
  contribute to movement.

61
Hip flexion reaches maximum about half way
through swing phase
62
Swing phase the knee

• Flexion initially this assists ground clearance
  and is related to pelvic tilt (DG1)
• Then extension preparation for heel-strike
• Little muscle activity during swing. Knee
  movements are passive consequences of hip
  flexion. The leg acts as a double jointed
  pendulum.
• Hamstring activity, at end, halts extension
  (eccentric action). Quadriceps contraction
  prepares for heelstrike (isometric, stabilising
  effect)

63
Knee movements during swing phase flexion
initially, and then extension
64
Swing phase ankle and foot

• Movements
• Dorsiflexion (for ground clearance and for ankle
  mechanism - DG4).
• Muscle actions
• Tibialis anterior at start of swing (ground
  clearance, related to pelvic tilt)
• activity tapers off in mid-swing
• contracts again for footstrike (prevention of
  footslap stabilise heelstrike).

65
Swing phase ankle movementsdorsiflexion
66
Principal events during the stance phase

• 1. Heelstrike
• 2. Foot-flat (followed by opposite toe-off)
• 3. Heel-rise (followed by opposite heelstrike)
• 4. Toe-off

67
Principal events during the stance phase
heel-strike, foot-flat, heel-rise, toe-off
68
Stance phase the hip

• Main movement is extension from flexed starting
  point
• Hip rotation and adduction/abduction mediate the
  pelvic movements required for DG1 and DG2
• Gluteals and hamstrings active during early
  phase, tapering off during midstance as gravity
  takes over
• During single support phase, gluteus medius and
  tensor fascia lata (hip abductors) work
  isometrically/ eccentrically to maintain pelvic
  stability

69
The hip during stance phaseMain movement is
extension from flexed starting point
70
Stance phase the knee

• Slight flexion from heel-strike to midstance
  (DG3),
• Then extension after midstance to heel-lift.
• Quadriceps active during early phase (eccentric/
  stabilising effect as GRF line of action passes
  behind knee joint)
• This action stores energy which is recovered
  during subsequent concentric activity
• Once leg has passed vertical position
  (midstance), the knee locks (ie no need for
  extensors).
• Hamstrings active at start and end of the support
  phase (initiating flexion during stance and swing
  respectively).

71
Stance phase - the kneeslight flexion followed
by extension
72
Stance phase ankle movements

• Initially (at heelstrike) close to neutral
  position (DG4).
• Plantar flexion produces footflat
• Then slight dorsiflexion as upper leg and body
  swing forward.
• Prevention of further dorsiflexion, which body
  weight tends to cause.
• Plantar flexion at end of propulsive phase (DG5)

73
Stance phase the ankleneutral at foot-strike
then plantarflexion as foot goes flat
74
Stance phase the anklethen dorsiflexion as
upper body swings over ..and finally
plantarflexion as heel lifts off
75
Stance phase ankle muscle actions

• Tibialis anterior at heel strike (prevents
  footslap).
• Gastrocnemius and soleus from midstance to
  toe-off
• Along with the hip extensors, this activity by
  the triceps surae group generates propulsive
  force needed to maintain forward movement
• Initial activity is eccentric muscles become
  active while ankle is still dorsiflexing.
• Subsequently action is concentric during ankle
  plantarflexion
• This biphasic action generates a
  stretch-shortening cycle, giving rapid and
  powerful contraction

76
Running movements and muscle involvement

• Movements are in general similar to those for
  walking, but
• range of joint movement usually greater
• co-ordination between the legs is different (e.g.
  no double support phase).
• more muscle involvement, since emphasis is on
  speed rather than energy economy

77
Running swing phase

• Muscular rather than pendular motion at hip.
• Knee flexion, and ankle dorsiflexion, bring CoM
  of the leg closer to the hip. This reduces moment
  of inertia and increases angular velocity.
• Knee movements largely passive (i.e not due to
  muscle activity), and result from transfer of
  momentum from thigh.
• Depending on the speed of running, initial ground
  contact may be with heel, whole foot, or ball of
  foot.

78
Running support phase

• Hip slight flexion followed by extension.
  Gluteus maximus activity initially eccentric
• Knee degree of flexion increases with speed
  that of extension decreases. Quadriceps active
  at knee, initially eccentrically
• Ankle dorsiflexion followed by plantarflexion.
  Gastrocnemius and soleus active during whole
  phase, particularly so at the end.
• Stretch shortening/energy storage activity occurs
  at all three joints