Musculoskeletal strain or overuse: This is often the most common cause of bilateral upper extremity heaviness due to repetitive activities, poor posture, or overexertion, leading to muscle fatigue and strain.

Cervical spondylosis or cervical radiculopathy: Conditions affecting the cervical spine can cause radiating pain or heaviness into the upper extremities due to nerve root compression.
Thoracic outlet syndrome: Compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib) can cause pain, numbness, and heaviness in the arms.
Peripheral neuropathy: Conditions such as diabetes, vitamin deficiencies, or autoimmune diseases can cause damage to the peripheral nerves, leading to symptoms like heaviness, numbness, or tingling in the arms.

Acute coronary syndrome or cardiac conditions: Although less common, cardiac issues can sometimes present with arm heaviness, especially if there is associated chest pain or shortness of breath. Missing a cardiac cause could be fatal.
Stroke or transient ischemic attack (TIA): Sudden onset of arm heaviness, especially if accompanied by other neurological symptoms like facial drooping, speech difficulties, or leg weakness, could indicate a stroke or TIA, which requires immediate medical attention.
Spinal cord compression or injury: Conditions like spinal stenosis, herniated discs, or spinal fractures can compress the spinal cord, leading to bilateral upper extremity symptoms, including heaviness, and require prompt diagnosis and treatment to prevent permanent damage.

Amyotrophic lateral sclerosis (ALS): A progressive neurological disease that affects nerve cells in the brain and spinal cord, causing loss of muscle control. Early symptoms can include muscle weakness, atrophy, or heaviness, but it is much less common than other causes.
Multiple sclerosis: An autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including numbness, weakness, or heaviness in the limbs, though bilateral upper extremity heaviness as a sole presenting symptom would be unusual.
Plexopathies: Conditions affecting the brachial plexus, such as neuritis or traumatic injuries, can cause arm pain, weakness, and heaviness but are less common causes of bilateral symptoms.