Summary BBS2052 - Neuromuscular Control of Movement

Gait cycle
The gait cycle starts when one foot makes contact with the ground and ends when that same foot
contacts the ground again.

- Stance (60%)
o Initial double-leg support (10%)  both feet are in contact with ground.
o Single-leg support (40%)  opposite or contralateral foot is in the air.
o Terminal double-leg support (10%)  both feet are in contact with the ground.
- Swing (40%)

The cycle can also be divided according to functional phases:

- Initial contact (0-2%)  start of loading response of weight acceptance. Beginning of stance
and first part in double-leg support.
- Loading response (2-12%)  rest of double-leg support, complete weight acceptance.
- Midstance (12-31%)  first part of single-leg support. Base of support decreases and center
of gravity will move to its highest point through leg extension. Kinetic energy transfers to
potential energy.
- Terminal stage (31-50%)  second part of single-leg support. Heel strike of opposite foot,
center of gravity falls and potential energy transfers to kinetic energy.
- Preswing (50-60%)  terminal double-leg support. Maximum propulsion peak in terms of
anterior-posterior force.
- Initial swing (60-73%)  first part of swing period. Flexion decreases leg’s moment of inertia
and increases angular velocity of swinging leg.
- Midswing (74-87%)  second part of swing period, contralateral leg is in single support with
small base of support. Getting ready for upcoming foot contact at end of swing.
- Terminal swing (87-100%)  last part of swing and upcoming foot contact.




Energy
Centre of mass  reaches highest point when person starts standing on a single leg (midstance). The
potential energy is maximal, whereas the kinetic energy is on its lowest.

- During double-leg support the Centre of Mass reaches its lowest point, and hence the
potential energy is minimal.
- During both periods of time, the total energy is presumably equal and simply differ between
kinetic and potential counterparts.




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,Ground reaction force (GRF)  the force exerted by the
ground on a body in contact with it. When a person is
standing the GRF corresponds with the person’s weight.
When the body is moving, the GRF increases due to
acceleration forces.

The curve consists of two peaks:

- Passive (weight acceptance)  foot’s collision
with the ground.
- Active (push off)  active force applied by the foot into the ground as it pushes off.




Muscle activation
1. Heel strike
a. Tibialis anterior  plantar-flexion of ankle due to eccentric contraction.
b. Quadriceps  extends knee
c. Rectus femoris  hip flexion
2. Loading response  shock absorbed by foot rolling into pronation.
a. Adductor magnus  hip extension
b. Gluteus maximus  hip extension
c. Flexion of knee and further plantar-flexion of ankle.
3. Midstance  knee reaches maximal flexion and extends.
a. Rectus femoris  extension of hip
b. Triceps Surae  dorsiflexes and supinates ankle
4. Heel-off 
a. Hyperextension of hip and reverts back to flexion
b. Knee becomes flexed again
c. Ankle supinates and plantar flexes
5. Toe-off 
a. Hip becomes less extended
b. Knee flexes 35-40 degrees
c. Plantar flexion of ankle increases to 20 degrees

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, 6. Early swing 
a. Iliopsoas  flexes hip due to lateral
rotation
b. Knee flexes 50-60 degrees.
c. Ankle dorsiflexes and falls in neutral
position.
7. Mid swing 
a. Hip further flexes by contraction of
adductors
b. Tibialis anterior  dorsiflexion of
ankle
c. Knee flexes to 60 degrees but extends
back
d. Sartorius  extends knee back to 30 degrees.
8. Late swing 
a. Hip flexion of 25-30 degrees
b. Locked extension of knee
c. Neutral position of ankle.




Proprioception
The gait cycle is controlled by Epidural Electrical
Stimulation (EES) in the spinal cord. EES
activates motor neurons by recruiting
proprioceptive circuits within the posterior
roots of the spinal cord. Spinal Inter-neuronal
Networks (SINs) are thought to represent the
functional circuit translating the higher center
commands and integrating peripheral feedback
to finetune the motor neurons. SINs consist of:

- Input neurons from various sources
including afferent and interneurons.
- Numerous types of interneurons which process and integrate the inputs.
- Target output neurons, being motor or interneurons, but also another subset of SINs.


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, During movement muscle spindles notice stretch and a signal is sent to the spinal cord. In the long-
latency response, the signal is passed on to the motor cortex in the cerebellum, a signal is sent back
to the spine to the muscle upon which it contracts.

Cortex-to-cortex or spinal cord interconnection often are based on a combination of convergence,
divergence and horizontal connectivity.

- Convergence  a muscle can be under the control of multiple brain regions/neuronal
networks at once.
- Divergence  single network can affect multiple different muscles at once.

Therefore there are multiple possible actions which can result into the same outcome, to reduce
complexity and number of neurons required to activate all these muscles, a low dimensional control
strategy is used during which divergence and synergies are very important.

In humans primarily four or five muscle synergies coordinate the gait cycle and are implemented by
CPGs, which call for rhythmicity and patterning to generate outputs, without being dependent on
sensory input.

- Synergy I: provides body support in early stance: Hip and Knee extensors and Hip Abductors
(Vastus Lateralis, Rectus Femoris, Gluteus Maximus and Gluteus Medius) are activated.
- Synergy II: primary contributor to forward propulsion in late stance and contributes to body
support: Ankle Plantar Flexors (gastrocnemius and soleus) are activated.




- Synergy III: generates dorsiflexion in early stance at heel strike and during early swing
contributing to foot clearance: Tibialis Anterior and Rectus Femoris are activated.




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