workhorse, "Anyone or thing that does a lot of work; who works consistently or regularly."
spearhead, "One who leads or initiates an activity" [Also see point or point-man, quarterback, matador, etc.]
martyr, "somebody who suffers persecution and death for refusing to renounce... a belief or cause" ["The hero continues, despite knowing the risk, out of commitment to the cause" is listed as characteristic of martyrs]
lead dog, "one of the dogs at the front of the team, who set the pace"
mainspring, in sense "The most important reason for something", might refer to one who has been a main driver for a project
yeoman, as in yeoman effort, refers to one who has "performed or rendered in a loyal, valiant, useful, or workmanlike manner, especially in situations that involve a great deal of effort or labor"
Of course none of the above precisely mean "doing work alone that would normally be done by a small group, so that others can rest", but some suggest hard, faithful, or important work being done, and calling a worker a martyr suggests sacrifice on his or her part. Helot may suggest working like a slave in spite of personal risk, but to some extent suggests a cog, lackey, flunkey, slave, dogsbody.