Assuming that you and your injury would otherwise be covered by your employer's workers' compensation insurance, I'm confident that you were, in fact, acting withing the scope of your employment when you were injured.  In 2009, the Supreme Judicial Court of Massachusetts issued an interesting opinion.  In the case of Karen Sikorski, 455 Mass. 477, the Court considered a case where a teacher was injured while serving as a volunteer chaperon on a school club skiing trip.  The school system argued that the teacher was not covered under its workers' compensation policy, because the activity was recreational and voluntary.  But the court found that the "employee's injury arose out of and in the course of her employment, in that, even though her participation as a chaperon was voluntary, the activities involved constituted work connected to her employment; and where the injury did not fall within the statutory exclusion for injuries resulting from voluntary participation in a recreational activity, in that the employee's responsibilities as a chaperon, though voluntarily undertaken, were an extension of her employment duties as a teacher."

Your case is not exactly the same, but it seems you were injured during an activity that, although voluntary, was an "extension of your employment duties."  Of course, if I were you, I would talk to a Massachusetts personal injury attorney, someone who, unlike me, has actual experience in this area of the law.  Good luck.