In a computer system, methods and computer-readable medium are disclosed for tracking motion of a subject in an activity captured by camera. The camera supplies pluralities of discrete images of the subject to a computing system environment. An event window, displayed on a monitor, has at least two cells for receiving a user input pertaining to an action of the subject in the activity. In a first of the two cells, the user indicates a specific image number corresponding to one of the plurality of discrete images and an estimate of a fractional percent thereof. In the second cell, the user indicates another specific image number and estimate of a fractional percentage thereof. Software calculates a delta between the specific image numbers and their corresponding estimated fractions. Scroll and position bars provide user-aid for navigating between discrete images. Example subjects include horses in a horse race activity while an action includes a horse stride length.