What your seeing is motion blur.
If your shooting at 24 FPS, set your shutter to at least 1/50 of a second. This still may not help so you’ll have increase the shutter speed to something higher. To capture a human in motion sharp as a tack, a shutter should be set to 1/250 of a second or higher. Be careful though, if you go too high, the motion (although sharp) will look unnatural.
Increasing the shutter speed will let less light into the camera so you’ll have to do one or more of the following to compensate: increase the light falling on the front of the subject, increase the camera ISO, open the aperture on the lens, drop your frame rate. Dropping your frame rate will help, but in your case I wouldn’t recommend it.
My recommendation would be to open the aperture as wide as you can. Set the frame rate to 24, set the shutter to 1/50 of a second. Then, set the ISO to something no higher than you need to avoid noise (visual grain and splotchiness).
At this point, if your overexposed, then close the aperture until the exposure is correct.
If needed, add light to the front of the subject. In your case, you’ve got a lot of light coming from the background windows that you can put to work. Just hang up a couple of white sheets on both sides of the camera just out of the frame. These will bounce that light right back on to the subject and help balance out the light coming from the windows.
Robert Olding
http://www.8streetstudio.com
Minneapolis, MN