Biceps Tendinitis: This condition is characterized by inflammation of the biceps tendon, which can cause pain, especially with activities that involve contraction of the biceps muscle. Sustained contraction would exacerbate the condition, making it the most likely diagnosis.

Biceps Strain: A strain or pull in the biceps muscle can cause pain, particularly with contraction. This is a common injury in sports or activities that involve heavy lifting or bending.
Repetitive Strain Injury (RSI): RSI can affect the biceps among other muscles and tendons, leading to pain with sustained or repetitive contraction.
Radial Tunnel Syndrome: This condition involves compression of the radial nerve and can cause pain in the back of the hand or the forearm, including the area where the biceps is located, especially with activities that involve wrist extension or forearm rotation.

Brachial Plexus Injury: Although less common, an injury to the brachial plexus can cause pain, weakness, and numbness in the arm, including the area served by the biceps. Missing this diagnosis could lead to permanent nerve damage.
Cervical Radiculopathy: Pain radiating from the neck down to the arm, including the biceps area, could be indicative of a cervical disc problem or other neck issues. This condition can lead to significant morbidity if not addressed.

Bicipital Apophysitis: Inflammation of the bicipital apophysis, the growth plate at the origin of the biceps tendon, can cause pain, especially in adolescents.
Infectious Tendinitis: Although rare, bacterial or fungal infections can cause tendinitis, which would present with pain, swelling, and possibly systemic symptoms like fever.
Tumor or Cyst: Rarely, a tumor or cyst affecting the biceps tendon or surrounding tissues could cause pain with contraction. Examples include a ganglion cyst or a soft tissue sarcoma.