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macpatgh-Sheldon
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A resting muscle has myosin and actin fibrils overlapping only slightly lengthwise so the muscle is in its longer state.
When a muscle contracts, the myosin and actin fibres slide along each other due to twisting back and forth of "myosin heads" after activation by ATP, which follows the movement of Ca++ out of the sarcoplasmic reticulum (ER of a myocyte - Greek myo = muscle; cytos = compartment here cell), so MUSCLE SHORTENS.

As it shortens the muscle pulls the bone on which it is inserted towards the bone of attachment e.g. the biceps muscle (most well-known one) is attached to the humeral head (bone of upper arm) and the outer (lateral) part of the clavicle and its insertion is on the upper radius (one bone of forearm), so when it contracts it flexes the arm at the elbow as it shortens and pulls the forearm to reduce the angle between the upper arm and forearm.

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(Original post by macpatgh-Sheldon)
A resting muscle has myosin and actin fibrils overlapping only slightly lengthwise so the muscle is in its longer state.
When a muscle contracts, the myosin and actin fibres slide along each other due to twisting back and forth of "myosin heads" after activation by ATP, which follows the movement of Ca++ out of the sarcoplasmic reticulum (ER of a myocyte - Greek myo = muscle; cytos = compartment here cell), so MUSCLE SHORTENS.

As it shortens the muscle pulls the bone on which it is inserted towards the bone of attachment e.g. the biceps muscle (most well-known one) is attached to the humeral head (bone of upper arm) and the outer (lateral) part of the clavicle and its insertion is on the upper radius (one bone of forearm), so when it contracts it flexes the arm at the elbow as it shortens and pulls the forearm to reduce the angle between the upper arm and forearm.

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