Which energy source would you expect to be depleted first during continuous contractions caused by the tetanus toxin?

The answer is: ATP generated by oxidative or aerobic catabolism

Tetanus causes the continuous contraction of muscles, a condition known as rhabdomyolysis, that leads to depletion of oxygen in the body. Without that oxygen, the mitochondria in our cells can't make aerobic respiration, which produces a lot of ATP molecules. That's the reason why ATP is depleted.