Marc Greenwood, a Ramsey Ambulance Corps member and Mahwah police dispatcher, was at Retro Fitness off Route 17 when the victim collapsed, Police Chief Bryan Gurney said.
Greenwood, who used to work at the same gym, began CPR.
Officer Bryan Stevens arrived soon after and brought the victim back to full consciousness with one shock from a defibrillator, the chief said.
Ramsey EMS took him to the Valley Hospital in Ridgewood.
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