A Hypothesis to Account for the Mary Walker Phenomenon

Certain myasthenic patients show that their skeletal muscles produce a substance during ischemic exercise which, when released into the circulation, increases muscle weakness. This phenomenon has been named after Mary Walker, the physician who popularized its demonstration. It is suggested that ischemic exercise produces lactic acid that binds calcium, reducing ionized and total serum calcium. The decreased calcium has an adverse effect on skeletal muscle function especially in patients with myasthenia gravis, where neuromuscular function is often so precarious it may be interrupted by weak inhibitors.