The treadmills at the gym are so popular that I often have to wait in line to use one.

treadmills 是 treadmill 的复数形式。treadmill 由 tread（踩踏）和 mill（磨坊）组成，原指古代用人力或畜力踩踏驱动的磨坊设备，现在主要指健身用的"跑步机"。