With the G-trainer (alter-g.com) making its way into gyms this year, bum knees will no longer be an excuse for missed workouts. This freaky gravity-defying treadmill (which requires being fastened into rubber shorts) lifts you off your feet so you can run with minimal stress on your joints. Currently, the NASA-inspired machine is a rehab tool for college and pro teams, from Texas A&M to the New York Knicks. But through April, it'll be parked next to the conventional treadmills and ellipticals at the Sports Club/LA on NYC's Upper East Side. Permanent fixtures here and at other health clubs nationwide are in the works.

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