rotoscoping moving limbs
To directly answer your question, where Rotoscoping is concerned, it is simply the task of masking out, or cutting a hole in a moving image, for every frame.

Technically in almost any software capable of Rotoscoping, It is achieved by using a spline connected by a set of points that conforms and deforms each frame in your footage, to match the shape/outline of the object you wish to cut a hole through. 

In the bigger view of what is required for your video, it seems like you wanted to "keep the background intact". That is a much tougher requirement than it sounds, both artistically and technically. 

Basically when we take a photo, shoot a video, of a car on a grass field, the camera does not take an x-ray imagery of what is behind the car. Therefore when we mask out and remove the car, we cut a hole through the video clip, and there is nothing behind to show through. 

If you are familiar with the idea of compositing layers of images on top of each other, one way to deal with this is to take a similar looking patch of the background (grass, ground, etc) that is correct in perspective, and place it on top of the gaping hole created by the rotoscoped mask. 

If your footage is shot by a camera on the move (panning, dollying), it adds to the challenge. 

In this case you will need to then animate that patch of missing background in position and maybe rotation, to pin it to the ground so it moves together with the rest of the shot. 

On top of all that, you will need to clean up and blend the edges of this patch of ground with the rest of the footage. Also you will need to do adjustments like colour, brightness correction all so that your patched up area does not stand out and call attention to itself. 

Another trick you can use is to take a part of the footage that hasn't been obscured by the running person (maybe 2 frames before he passes over that grass/ground), you can take a frame of that ground /grass , freeze that image, and place it and match it over the hole created by the Rotoscoping and then animate/track that patched up piece as the camera moves so it appears to stick solidly on the ground. 

These are some techniques of visual effects workflow. Sometimes the concept sounds easy but the real pain is figuring how to solve the little problems that crop out, pushing the shot to look better, if your work requires a more refined result. 

If you want to see someone actually doing stuff like that, Video Copilot has huge number of tutorials where you can see how visual effects tasks can be accomplished. 

Www.youtube.com/user/videocopilot
Www.videocopilot.net

Watch a couple of tutorials, and you will soon get a good idea of the processes that are involved for compositing, Rotoscoping, background replacement, background reconstruction.