Alter-G, a Menlo Park, Calif. company whose first round of funding we reported last year, has taken an equal follow-on amount of $2.5 million to help roll out its “anti-gravity” treadmills for physical therapy.

The device assumes a portion of the patient’s load by use of a pressurized that reduces effective weight. If that sounds odd, further description isn’t likely to help; just check out this video of the machine in action.

The company says it will use the additional fund for marketing, clinical trials and product development. The first prototype was created for Nike almost three years ago.

A lead investor was not named, but participants in the round include Frog & Peach Investments, Astrolabe Ventures and Funk Ventures. Alter-G previously took $2.5 million as the first half of this investment round, from NASA’s Red Planet Capital and Hellman & Friedman; its total to date is $5 million.