Balance, posture and locomotion (wk 9)

Question 1

What is balance?

Answer 1

Quiet standing involves keeping the centre of mass (COM) within the base of support (BOS). If the COM moves outside the BOS, a fall or compensatory step will happen. Need to keep COM inside the base of support.

Question 2

What is the inverted pendulum model of human standing?

Answer 2

During normal quiet stance, most motion occurs at ankle joint. Torque provided by continuously active calf muscles. Can be represented as a point mass, tending to fall forwards, with active torque stabilisation.

Question 3

What is centre of pressure and centre of mass?

Answer 3

-Centre of pressure has a larger range than the centre of gravity (or mass) for balance
-Centre of Pressure (COP) actively oscillates forward/ back to maintain the centre of mass (COM) within the limits of stability. Sway can be quantified by COM motion

Question 4

Why do we sway and what are the 3 processes?

Answer 4

-Why do we sway? -> Standing is a sensorimotor control process. Imperfections in this process arise from:
1. Sensory estimation – an imperfect process with sensor noise
2. Motor output – also an imperfect process
3. External/ internal perturbations – e.g. breathing, wind, being pushed etc

Question 5

What information keeps us upright?

Answer 5

Vision (eye muscles have no proprioception muscles), efference copy of motor command and prediction, vestibular apparatus, touch and proprioception

Question 6

Comparisons of sensory inputs:

Answer 6

Visual control for slow/low frequency control of sway. Vestibular input for fast/ high frequency control.

Question 7

How does removing visual information increase sway?

Answer 7

1. Visual acuity -> Reducing visual acuity increases sway
2. Blocking the visual field -> Blocking parts of the visual field increases sway. Peripheral vision just as important as foveal.

Question 8

Why is the quality of visual information important?

Answer 8

Closer objects are more useful for controlling sway. Greater image displacement on retina. Distant visual information effectively useless for balance – no retinal motion. May as well be blind when standing on a cliff edge.

Question 9

What is motion parallax?

Answer 9

Motion parallax is when objects appear to move relative to each other, depending upon focus point. This is also true during the small motion caused by sway. Objects appear to move relative to each other when we sway – provides more information. Largest sway reduction observed when foreground + background scenery are present together and furthest apart.

Question 10

What is perturbing visual inputs?
-Visual vection

Answer 10

: A false sense of motion induced by a moving scene. E.g. watching an adjacent train depart the station, you may get the impression that your train has started to move. This has consequences for control of standing posture. Subjects sway in the direction of visual scene movement. The brain interprets forward scene motion as backward body motion. Therefore produces a compensatory forward response.

Question 11

Describe the habituation of visual response and the effect of predictability on visual response?

Answer 11

-Habituation of visual response -> Reduced sway upon repeated exposure to moving room. Visual responses easily ignored.
-Effect of predictability on visual response -> Visual response readily modified by expectation/ prior experience

Question 12

What are the 5 characteristics of visual perturbation response?

Answer 12

1. Initial sway in the direction of visual field movement, begins ~1s after onset
2. Corrective sway after 2-3s by sway in opposite direction
3. Responds best to slow, low frequency movement (0.1Hz, <5 deg/s)
4. Responses habituate quickly i.e. modifiable by expectation and cognitive influence
5. Visual field motion can be ambiguous. Therefore the brain must interpret each visual stimulus and distinguish between object- and self-motion. This involves expectation and prior knowledge

Question 13

How do you test the balance system?

Answer 13

-> Standing body can be considered as a simple inverted pendulum. The body tends to topple forwards. Calf muscle activity generates torque around the ankle to resist this tendency. Very small imperceptible torque perturbations uses to test the balance system. Resistance to waist pull measured to determine stiffness. Changes in stiffness represent changes in muscle reflexes.

Question 14

What is the effect of sensory removal on postural responses?

Answer 14

-> Standing is possible with only proprioceptive input. However, balance reflexes are more effective with all senses available. Balance reflexes can also be ‘tuned’ by intention – i.e. trying to stand still.

Question 15

Describe the motor output for standing and the stiffness of the ankle with passive standing :

Answer 15

-Continuous calf muscle contractions prevent us falling forwards. Can we set a fixed level of muscle activity, and then ‘forget’ about the task/ just rely on stretch reflexes. This depends upon passive ankle stiffness.
-The stiffness of the ankle is too low to permit passive standing – i.e. standing without active modulation of EMG. The limiting factor in stiffness is the tendon and soft tissues of the foot. Stiffness will not be changed by intention of sensation

Question 16

How should an ankle be held to maintain balance?

Answer 16

-Passive ankle stiffness insufficient to maintain balance – active modulation required.

Question 17

Describe the swing stance pattern?

Answer 17

Alternating pattern of swing-stance. Flexors active during swing (e.g. TA, hamstrings, hip flexors). Extensors active during stance (e.g. triceps surae, quadriceps, gluteus)

Question 18

Describe ground reaction forces while walking:

Answer 18

-> Large vertical force upon heel-strike. Accompanied by a decelerating (backward) shear force (dotted line). Push-off includes an accelerating shear force, accompanied by a secondary vertical force.

Question 19

Describe ground reaction forces while running:

Answer 19

-> Forefoot runner, the impact transient is absent. With heel runners, there is a prevalent transient impact

Question 20

Describe the muscles and muscle contractions which control walking:

Answer 20

• Alternating pattern of eccentric and concentric muscle contraction
• At beginning of stance – eccentric contraction of extensors provides a ‘braking’ action
• At end of stance – concentric action of extensors provides ‘push-off’ power
• During swing, hip, knee and ankle flexors carry leg through the air

Question 21

Describe the tibialis anterior action while walking:

Answer 21

-> Foot clearance during swing is remarkably well controlled – the toes only clear the floor by a couple of cm (with foot drop/ dorsiflexor paralysis). Various disorders can cause paralysis of this muscle (polio, stroke, damage to peroneal nerve).

Question 22

Describe the strut and pendulum action and passive dynamic walking:

Answer 22

-Strut and pendulum action -> Bones of the leg/hip act similar to rigid strut that pivots about the ankle joint. Causes characteristic vertical motion of the COM. During running, the leg ‘compresses’ during stance. Hence, vertical COM is the opposite to that during walking.
-Passive dynamic walker -> Gravity used to supply energy to walker. Human-like gait is achieved with entirely passive mechanisms. Legs work like pendulums – just like human walking. Nervous system exploits the passive dynamic properties of the limbs to produce walking action

Question 23

What is ‘decerebrate’? and spinal when studying locomotion in animals?

Answer 23

• Decerebrate -> Locomotion initiated either spontaneously or by stimulation of brainstem region called: Mesencephalic locomotor region (MLR). Midbrain transection. Brainstem and cerebellum left intact. Electrical stimulation of MLR induces locomotion. MLR does not produce locomotor pattern, MLR triggers locomotion and determines speed.
• Spinal -> Spinal cord transected at lower thoracic level. Isolates spinal segments controlling hind limbs. Sensory feedback intact.

Question 24

What is a central pattern generator (CPG) and what are CPG half-centres?

Answer 24

-Central Pattern Generator: basic generator of locomotor pattern -> A relatively complex (spinal) neural network capable to producing functional locomotor muscle activation patterns without any contribution from afferent feedback (although in normal circumstances, feedback does contribute to the locomotor pattern).
-CPG: half-centres -> Graham-Brown proposes the concept of the ‘half-centre’ for generating CPG pattern

Question 25

Describe CPG half patterns:

Answer 25

1. Two populations of excitatory motor neurons – flexors and extensors
2. Inhibitory interneurons cause mutually inhibition
   i.e. when flexors are active, extensors inhibited (and vice-versa)
   -Stimulation of sensory neurons can induce an alternating pattern of flexion-extension. These neurons are flexor reflex afferents.
   -CPG can be modulated by sensory feedback (and reflex rehersal) -> Proprioceptive information from hip flexors initiates swing phase.

Question 26

What are the descending influences on CPG in the supraspinal brain areas:

Answer 26

1. Motor cortex -> alters locomotion output based on visual signals e.g. obstacle avoidance
2. MLR -> initiates locomotion
3. Cerebellum/ brainstem -> adjusts pattern based upon ongoing sensory feedback

Question 27

Describe greyhound running in locomotion to humans:

Answer 27

-Greyhound running -> Human running speed around curves – tighter curve leads to slower speeds. Unlike human athletes, these dogs do not need to slow down when racing round a tight bend. In humans, the propulsion muscles also perform an anti-gravity function (i.e. calf muscles, quads, glutes). In dogs, the propulsion muscles are largely uncoupled from gravity. This supports the idea that greyhounds power locomotion by torque about the hips, so -just as in cycling humans- the muscles that provide the power are mechanically divorced from the structure that supports weight.