How do your muscles continue to get enough energy during high levels of activity when there is not enough oxygen?

Your muscle cell break down energy from food turning it into the process of respiration.

Respiration is when the body turn carbon dioxide into oxygen

Edit-

Oxygen is used to create ATP in the muscle cell site, however it is still possible to create ATP without oxygen. this is called Anaerobic glycolysis and is when energy from glucose (sugar) is broken down without the presence of oxygen. the byproduct of this reaction is lactic acid and causes the muscles to retard and tighten up.