Your mind and the body often have a nasty habit of not always being in sync with one another.
This will lead to what you think you are going to do, and what you actually do, being at odds with one another.
As a result of this truth any movement must first be thought of correctly in the mind before the desired physical result can occur. If the mind wanders ever so slightly, so does everything else.
Keep your mind sharp. Focus. Then move. But, never move before your mind is also committed to your physical actions. Since a thoughtless movement is always a movement made alone, and the result as you may have already experienced can be quite disheartening indeed.
Remember: "Mental quality before physical quality, or no quality at all".