What must skeletal muscles do to move?
A. engage their fibers slowly
B. twitch involuntarily
C. separate from the bone
D. contract and relax
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Answer:
D
Explanation:
Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint.
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