Exercise for Earth-bound people is a choice, but for astronauts up there in the International Space Station (ISS), it is anything but. Without exercise in an environment with extremely low gravity, their bone density and muscle mass will rapidly decrease, so NASA mandates that their astronauts spend at least two hours per day working out. According to the Huffington Post, there are multiple ways in which they can do this in what is effectively a space gym.

The first contraption of note is a second-generation U.S. treadmill. The Combined Operational Load Bearing External Resistance Treadmill, or COLBERT, is named after the comic late-night talk show host. Being a space gym variant, this is of course no ordinary treadmill: It features data collection nodes that track how much bone density and muscle mass is gained or lost during the astronaut’s time away from Earth.

^{The COLBERT in action. NASA}

^{The logo for the device, featuring the eponymous talk show host. NASA}

The Cycle Ergometer with Vibration Isolation System, or CEVIS, is another initially indecipherable-sounding device. Essentially a mechanical bicycle nailed to the “floor,” it’s the equivalent of cycling in mid-air.

^{The mid-air cycling equipment CEVIS onboard the ISS. NASA}

Next up is the Advanced Resistive Exercise Device, or ARED, a slightly more visually odd instrument for physical exercise. Piston-driven vacuum cylinders act as grips that simulate the lifting of weights in “normal” gravity.

^{The more unusual-looking ARED. NASA}

The Russian segments of the ISS also contain exercise instruments, including the treadmill BD-2 (similar to the COLBERT) and the VB-3, an ergometer bike (similar to the CEVIS).

^{The BD-2. NASA}

All the exercise equipment on the ISS comes with a harness to keep the users strapped in the microgravity environment.

^{The VB-3. NASA}

So if you’re having a particularly lazy day, you might want to think about the astronauts above your head, running up to 20 kilometers per hour (12.4 miles per hour) on a treadmill, orbiting around the Earth at 7.7 kilometers per second (4.8 miles per second).