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How do the human biceps and triceps move the lower arm in opposite directions?
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Locomotion
These birds largely reside in coastal areas of North America, although some populations live in the continent's interior along large rivers.
The human biceps and triceps move the lower arm in opposite directions through a process known as reciprocal innervation. This principle involves the coordinated contraction of one muscle group while its antagonistâthe muscle with the opposite functionârelaxes.1
- Forearm Flexion: The biceps brachii acts as a primary flexor. When it contracts, it decreases the angle at the elbow, pulling the hand closer to the shoulder.23
- Forearm Extension: The triceps brachii acts as the primary extensor. When it contracts, it increases the angle at the elbow, moving the hand away from the shoulder.23
Under reciprocal innervation, when the nervous system signals the biceps to contract for flexion, it simultaneously inhibits the motor neurons of the triceps, causing it to relax. Conversely, when the triceps is signaled to contract for extension, the biceps relaxes. This sequential ordering of contraction and relaxation allows for smooth, controlled movement of the forearm.1