1. A video device for realtime pedaling frequency estimation, mounted on a bike and comprising: an image capture unit capturing continuous dynamic images of an upper body of a biker; an image recognition unit electrically connected with said image capture unit and recognizing images of symmetric regions of said biker and images of swings of said symmetric regions from said continuous dynamic images; a microprocessor electrically connected with said image recognition unit, calculating a frequency of periodical swings of said biker from said images of said symmetric regions and said images of said swings of said symmetric regions, and then obtaining a pedaling frequency; and a display device electrically connected with said microprocessor and presenting information of said pedaling frequency; wherein said images of said symmetric regions are a left shoulder image and a right shoulder image of said biker, and wherein a first reference point and a second reference point are respectively established at said left shoulder image and said right shoulder image of said biker, and wherein images of continuous variation of a relative tilt angle of said first reference point and said second reference point are used as said images of said swings of said symmetric regions; and wherein said image recognition unit uses an edge detection method to detect edge pixels having dramatic gradient variation in said continuous dynamic images and uses a Hough Transform to connect said edge pixels to define a left shoulder line of said left shoulder image and a right shoulder line of said right shoulder image.