KIN Mechanics of posture by Stephen Robinovitch, Ph.D.

Transcription

1 KIN Mechanics of posture 2006 by Stephen Robinovitch, Ph.D.

2 Outline Base of support Effect of strength and body size on base of support Centre of pressure and centre of gravity Inverted pendulum model of balance maintenance Muscle synergies for balance control Anticipatory postural responses Postural set

3 Postural stability requires that the COG is within the base of support A stable static posture exists if the vertical line of action of the force of gravity passing through the whole-body centre-of-gravity (COG) is within the base of support provided by the feet = COG mg stable mg unstable (pending stable?) mg unstable

4 The base of support is the region on the ground where the centre-of-pressure (COP) can be feasibly located to support body weight. A non-zero velocity of the COG changes the effective size of the BOS (e.g., see Pai et al., 1996) poor stability cane good ML stability crutch good AP stability

5 Stability is decreased if the size of the base of support is reduced If the size of the base of support is reduced, a smaller destabilizing force is required to initiate instability. (Does strength influence the size of the base of support?) mg mg d 1 d 2 > d 1

6 A smaller angle initiates instability if the height of the COG is increased Since instability occurs when sinθ > (r/h), an increase in the height h of the COG above the ground reduces the angular displacement θ required to initiate instability. θ 2 r 2 = r 1 h 2 > h 1 θ 2 < θ 1 mg θ 1 mg mg mg h 2 h 1 r 1 r 2

7 A greater recovery torque is required if the height of the COG is increased Once instability has occurred, the moment required to recover stability increases with the height of the COG. In the figure at right, the moment M 2 required to stabilize system 2 is greater than the moment M 1 required to stabilize system 1 mg θ m d 1 M 1 system 1 h 1 m θ mg h 2 > h 1 M 2 d 2 > d 1 system 2

8 Balance Challenges 1. Stand with your heels against a wall, and bend forward (flexing your hips, not your knees) to pick up an object located at your feet. 2. Stand in a doorway with your nose and abdomen touching the door jamb. Then rise up on your toes. Why are these tasks difficult?

9 Sway during quiet stance: cat and mouse between COP and COG Even during quiet stance, the body s COG is never stationary. Postural stability requires that we constantly move the foot COP (distance p in the figure at right) to catch the COG (distance g in the figure at right) ω ω ω ω α α α α α R R R R R g g g g g W W W W W p p p p p ω

10 Sway during quiet stance (cont) if p < g, then α will be cw if p > g, then α will be ccw if α is cw and ω is cw, then p must be moved to be > g if α is ccw and ω is ccw, then p must be moved to be < g ω ω ω ω α α α α α R R R R R g g g g g W W W W W p p p p p ω

11 Sway during quiet stance (cont) the COP continuously moves anterior and posterior to the COG to maintain balance therefore, COP excursions are of similar frequency but greater amplitude, when compared to COG excursions

12 Inverted pendulum model of balance maintenance via the ankle strategy

13 Minimum ankle stiffness for stability. Equation of motion (EOM) : I 0 " # mg l 2 sin" = #T A. If " is small, sin" $ " : I 0 " # mg l 2 " = #T A. Modeling the ankle as a linear spring : T A = k", the pendulum's EOM becomes : I 0 " + % k # mg l ( ' & 2 *" = 0. ) This system will only be stable if % k # mg l ( ' * > 0 OR k > mg l & 2) 2.

14 Muscle synergies for balance maintenance and recovery ankle strategy involves distal-toproximal sequence of muscle activation hip strategy involves proximalto-distal activation the two strategies activate opposite muscles typically observe a mixed (ankle + hip) response

15 Anticipatory postural responses proceed voluntary movements anticipatory postural responses (APAs): muscle activations and corresponding movements which precede the primary movement, and ensure the COG is maintained within the base of support programmed based on an internal model of body dynamics (feedforward control) improved by learning, which increases the accuracy of the the internal model and knowledge of the expected disturbance

16 Postural set: postural response selected in advance of predicted stimulus