Why do people with McArdle disease produce too little lactic acid during exercise?

People with McArdle disease produce little to no lactic acid during exercise because they lack the enzyme phosphorylase (specifically myophosphorylase) in their muscle cells.123

Normally, during physical exertion, muscles break down stored glycogen into glucose to meet energy demands, a process that leads to the production of lactic acid. In McArdle disease, the absence of phosphorylase prevents this initial step of glycogenolysis—the breakdown of glycogen into glucose-1-phosphate. Because the muscles cannot access their internal glycogen stores to fuel anaerobic glycolysis, they cannot produce the lactate that typically accumulates during intense exercise. Consequently, individuals with this condition do not experience the usual exercise-induced acidosis, though they frequently suffer from muscle pain, cramping, and weakness due to the inability to meet energy requirements during activity.24531