With their sleek aesthetics, club-inspired music, and cult-like followings, cycling studios have been on top of the trendy boutique fitness totem pole for the better part of the last decade, led by grapefruit-smelling, feel-good-mantra-spouting SoulCycle (which people seem to either love or hate). Competitors such as Flywheel and Peloton have tried to knock Soul off its throne, but there's a whole new game-changer beginning to take over the world of sweaty, sexy group workouts: indoor rowing. That's right, the machine that everyone ignores at the gym, the same old one collecting dust in your grandpa's basement, is poised to be...