When it comes to physical fitness, the phrase "stays on treadmill" can be replaced with a variety of synonyms. For example, one could say "remains on the treadmill" or "persists with the treadmill routine." Similarly, one could use "sticks to the treadmill" or "adheres to the treadmill regimen." Other possible synonyms include "maintains treadmill activity," "continues treadmill training," or "stays engaged with the treadmill workout." Ultimately, the choice of synonym depends on the context and intended meaning, but each of these options effectively conveys the idea of someone staying on a treadmill for the purposes of fitness.