During the flexion of the elbow joint, the .......... muscle contracts while the muscle relaxes.

Explanation

**Step-by-Step Solution:**

1. **Identify the Joint Movement**: We are discussing the movement of the elbow joint, specifically during flexion.

2. **Understand Muscle Roles**: In muscle movements, there are typically two types of muscles involved: the agonist (the muscle that contracts) and the antagonist (the muscle that relaxes).

3. **Identify the Agonist Muscle**: In the case of elbow flexion, the primary muscle that contracts is the biceps brachii. This muscle is located in the upper arm and is responsible for bending the elbow.

...