Complex sentence: It was the advent of Nautilus machines that made resistance training apealing to the general public, fueling the fitness boom of the 1970s and 80s and resulting in Nautilus gyms in strip malls across America.
Split into simpler sentences: It was the advent of Nautilus machines that made resistance training apealing to the general public fueling the fitness boom of the 1970s and 80s. Which resulted in Nautilus gyms in strip malls across America.