Question: Is there a single word that sums up "taking one for the team"?

In this instance specifically doing work alone that would normally be done by a small group, so that others can rest.

* Loyalty
* Dedication
* Integrity
* Sacrifice

all seem to (me to) be sub-par.

Answer: 
There are a few that would qualify: selflessness, self-denial, self-sacrifice, self-abnegation, etc. They all mean something like this [from NOAD]:

> 
> **selflessness**  
> 
> *adjective*  
> 
>  concerned more with the needs and wishes of others than with one's own; unselfish: *an act of selfless devotion*.
> 
> 
>
* *[workhorse](http://en.wiktionary.org/wiki/workhorse)*, "Anyone or thing that does a lot of work; who works consistently or regularly."
* *[spearhead](http://en.wiktionary.org/wiki/spearhead)*, "One who leads or initiates an activity" [Also see *point* or *point-man*, *quarterback*, *matador*, etc.]
* *[martyr](http://en.wikipedia.org/wiki/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](http://en.wiktionary.org/wiki/lead_dog)*, "one of the dogs at the front of the team, who set the pace"
* *[mainspring](http://en.wiktionary.org/wiki/mainspring)*, in sense "The most important reason for something", might refer to one who has been a main driver for a project
* *[yeoman](http://dictionary.reference.com/browse/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](http://en.wikipedia.org/wiki/Helot)* may suggest working like a slave in spite of personal risk, but to some extent suggests a cog, lackey, flunkey, slave, dogsbody.