The key is controlling just how much stress reaches the joint.
The device is known as the Alter-G. It is a treadmill that allows patients to defy gravity using an inflatable bladder that surrounds the pelvis and lifts the runner, controlling how much body weight is transferred to the legs.
Computer controls allow therapists to adjust how much body weight is transferred to the legs, allowing post-op patients to slowly strengthen muscle without putting excess strain on the healing joint.
Physical therapists at UCSF's rehabilitation center at Mission Bay are using the device to rehab injuries ranging from knee and hip surgeries, to heart patients. Joe Mendez lost strength in his left side after suffering two small strokes during a medical procedure, but he is now rebuilding his balance and endurance. He is training to run again.
"It's a huge benefit because it gives me an opportunity to move in ways I can't with my full weight on me. And, so there's a lot of ease that comes with it and it helps with the progressing," he told ABC7.
The trainer was originally conceived for use by NASA for astronauts returning from space flight. But, two years ago the FDA approved it for medical applications.