reduce frames to 15 per sec:
# ffmpeg -i stepup.mp4 -r 15 stepup15.mp4

make new directory for frames and put to stepup/ a video to split
#mkdir stepup/; mkdir yolo_stepup/

split to frames:
# ffmpeg -i stepup15.mp4 stepup_%04d.png

lets begin the games!
run this script, make sure are you not in stepup/, you shold be in darknet/

for file in stepup/*.png
  ./darknet detect cfg/yolo.cfg cfg/yolo.weights "$file"
  mv predictions.png yolo_"$file"

images to video (with 15 fps):
#ffmpeg -r 15 -f image2 -s 1920x800 -i stepup_%04d.png -vcodec libx264 -crf 25 -pix_fmt yuv420p stepup15.mp4

get audio from video:
#ffmpeg -i stepup.mp4 stepup15.aac

copy original audio to new video:
#ffmpeg -i stepup15.mp4 -i stepup15.aac -codec copy -shortest stepup15_audio.mp4