Many conditions, including injuries, metabolic diseases, neuromuscular diseases and toxins in the body, can cause pain and weakness in the arms, according to MedicineNet. Weakness describes a lack of strength or loss of muscular function and should not be confused with fatigue, which refers to tiredness.
Weakness is determined by an individual's ability to exert the arms, legs and other muscles with normal effort, according to WebMD. Overexertion is often the cause of sore muscles, and muscle weakness typically subsides within a day or two. In some cases, a mineral deficiency, such as that of iron or potassium, may bring on the condition. Hypothyroidism, which is caused by low thyroid levels, and hyperthyroidism, which results from high thyroid levels, may disrupt the body's normal use of energy, causing weakness, fatigue, weight changes, moods and sensitivity to temperature.
Muscle loss that comes on suddenly may indicate a stroke or serious nerve problem, explains WebMD. In rare cases, Guillain-Barré syndrome, a nerve disorder that can lead to paralysis, and Myasthenia gravis, which causes rapid muscular fatigue, causes muscle weakness.
Additional causes of arm pain that may be associated with muscular weakness include fibromyalgia, which is characterized by musculoskeletal pain, fatigue, mood and memory loss, according to Mayo Clinic. Heart attack, tendonitis, carpal tunnel syndrome and a dislocated elbow are also common causes of arm pain.