Which of the following explains how a person straightens his arm at the elbow?

• A. Both The Biceps And Triceps Muscles Relax, And The Arm Straightens To Its Default Set Point.
• B. The Biceps Muscle Is Used To Push The Forearm Away From The Upper Arm While The Triceps Muscle Is Relaxed.
• C. The Biceps And Triceps Muscles Work Together To Pull The Ulna Away From The Upper Arm.
• D. The Triceps Muscle Contracts, Pulling The Ulna Away From The Upper Arm While The Biceps Muscle Is Relaxed.

Answer: (D)

Straightening the elbow is produced by the triceps contracting. The triceps attaches to the olecranon on the ulna, so when it contracts it pulls the forearm away from the upper arm, extending the elbow. The biceps, which flexes the elbow, must be relaxed to allow this movement; if the biceps were active, it would resist extension and bend the elbow instead. That’s why the statement describing the triceps contracting while the biceps relaxes best explains how the arm straightens.