The freak accident Wednesday prompted temporary suspension of construction of the controversial Marine Transfer Station on the Upper East Side.
Robin Cocking says she was exercising at Asphalt Green when the jack hammer chisel came crashing through the window around 9 a.m.
Cocking, who works at the gym, escaped with minor cuts.
Rep. Carolyn Maloney called for a halt to the construction until safety at the work site could be improved.
The city's Department of Design and Construction, which is overseeing the work, says it's conducting an investigation.