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Muscles generally develop forces sufficient to maintain:
a rotational equilibrium around a joint
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We assume that mm act to oppose:
gravity
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Rotational equlibrium=
the sum of all the moments that act around a joint's axis is equal to zero
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All the adductor moments are balanced by:
equal and opposite abductor moments
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moments:
products of forces
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When do we encounter disequilibrium?
we don't always activate mm to counter the effects of gravity. Sometimes, we move in such a way that gravity assists our movements, as when we throw a ball from a height, or slam a book downward on the table
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Muscles generally develop forces sufficient to maintain:
a rotational equilibrium around a joint
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What are the 3 assumptions of gravity and muscle wars?
- the opposing moments are exactly equal
- the mm produce a moment that exceeds the moment which gravity produces
- the mm produce moment that opposes the effect of gravity but is nevertheless smaller than moment that gravity produces
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Mm=Mg
- joint doesn't move so muscle's length remains constant
- isometric muscle action
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Mm>Mg
joint moves as muscle shortens in concentric action
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Mm<Mg
joint moves in direction dictated by gravity's moment. while muscle's activity exerts force on attachments, and controls gravity's effect on joint, muscle still elongates. Eccentric muscle action
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Factors affecting the amount of muscle force required:
- muscles must produce a moment to match the moment that gravity produces at a joint
- Mm=Mg
- Therefore:
- Fgsg=Fmsm
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Factors affecting the amount of muscle force required:
the muscle force sufficient to balance this equation may change depending on:
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1. Changes in externally applied moment of force:
while Fg doesn't change, its moment arm does change throughout a movement task
since the muscle must balance the moment which gravity produces, the force required of the muscle changes
- 2. Changes in the muscle's moment arm: if sm changes through a joint's range of motion, the muscle must produce different levels of force to maintain a constant moment
- if the muscle's moment arm increases anywhere in the range of motion, the muscle can produce less force and still produce the same moment around the joint
- if the muscle's moment arm decreases, it must develop additional force to maintain the same moment
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