Possible Clues:

• 6-Down rooms
• Treadmill settings
• Rooms full of sweaters?
• Where people get pumped up
• Dreaded areas for grade-school nerds
• They're often full of dumbbells
• Reps' places?
• Spots to spot six-packs?
• 'Physical training facilities, in short (4)'
• Rooms to sweat in