Final answer:
The biceps and triceps muscles work together to bend and straighten the arm at the elbow joint.
Step-by-step explanation:
The muscles in the upper arm, specifically the biceps and triceps, work together to bend and straighten your arm at the elbow joint.
When you want to bend your arm, the biceps muscle contracts, pulling on the forearm and bringing it closer to the upper arm. At the same time, the triceps muscle relaxes, allowing the biceps to exert its force.
On the other hand, when you want to straighten your arm, the triceps muscle contracts and the biceps muscle relaxes. This causes the triceps to pull on the forearm, extending and straightening the arm at the elbow joint.