SAN FRANCISCO — Close to what should be one of the biggest events of Fitbit’s life — its initial public offering of stock — the maker of wearable health trackers is facing an unexpected challenge: a lawsuit from one of its biggest competitors.

Jawbone sued Fitbit in California State Court here on Wednesday, accusing its rival of “systematically plundering” confidential information by hiring Jawbone employees who improperly downloaded sensitive materials shortly before leaving.

“This case arises out of the clandestine efforts of Fitbit to steal talent, trade secrets and intellectual property from its chief competitor,” lawyers for Jawbone wrote in the complaint.

The legal action is an unusual twist for Fitbit, which this month filed for an initial public offering, a move meant to take advantage of huge demand for wearable devices. Fitbit’s trackers, as well as a range of offerings like Jawbone’s Up bands and the Apple Watch, have drawn users eager to document and quantify ever more aspects of their health.