An easy answer, and one that would not let Thomas fix the oncoming energy crisis in an afternoon, is just to have the magic be a way of forcing cells to use more energy and perform more, like a sort of magical adrenaline. It might cause tissue damage or other complications, but no matter what it will use more than the equivalent amount of un-boosted energy [due to increased inefficiency], and produce an equivalent amount of waste products [lactic acid, etc.].

It is still a massively MASSIVELY useful power, but at least then it is something explainable naturally and within the realm of possibility [and therefore game balance]

It is only marginally more useful then being able to move around small to medium amounts of kinetic or thermal energy, in fairness.