This paper simplified the human action recognition work into key frames matching. It works quite well when there is just one target model for recognition. Suppose we are given a database of several different actions, like walking, jogging or going upstairs/downstairs, will this method still output the best recognized action? We could image that some different models may share the same key frames or have similar patterns in these frames, e.g. jogging and running. One significant difference between these two actions is the pace. Obviously, the rate of action should also be taken into consideration when there are multiple action models. In this paper, they mentioned that they use simple Markov chain to prevent inappropriate key frames being chosen, does this imply that they give a specific order of key frames, if so will it allow deletion or insertion of these key frames when doing the matching?