This type of weakness is pathognomonic for an abnormality of the neuromuscular junction such as myasthenia gravis. A characteristic form of muscular weakness that worsens after use of affected muscles. Myasthenia is caused by failure of neuromuscular transmission, which results from the binding of autoantibodies to proteins involved in signaling at the neuromuscular junction, including especially the nicotinic acetyl choline receptor (AChR) or, less frequently, a muscle-specific tyrosine kinase involved in AChR clustering.