Stock & Resources
Broadly defined, Action Stock is any photo-reference in which the subject is doing SOMETHING. That 'something' does not need to be extreme or particularly kinetic, it just has to show the effects of various forces on the bodies (or other objects) in the image. Perusing an artifact, sitting on someone's knee, even enjoying the aroma of a cup of coffee are great examples of actions that, when explored as stock images, can be of great value to artists.
Of course, there is nothing wrong with extreme, kinetic, over-the-top actions, either...
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It's worth stating the obvious: if you are interested in exploring the more "kinetic" end of the action spectrum, know your limits. Don't work beyond your skill level and observe all necessary safety precautions; no shot is worth getting hurt over.
BODIES IN MOTION
Consider the visible effects of forces on objects when designing the shot. Clothing, hair, and body position/tension are the things that most clearly show gravity and motion.
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Sometimes you may have to hack gravity a little but...
AN ADDED DIMENSION
As with any photo-reference image, action shots can benefit from the addition of interesting/unexpected angles and foreshortening.
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Likewise, subject orientation gives the artists who use action shots more options...
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The subjects of gear and technique are too broad for this forum, but it's probably worth mentioning that you should understand your gear well enough to get the result you want. If you want to freeze your subject in mid-flight you need enough light to allow for the appropriately fast shutter speed; generally this means the use of either strobes or daylight.
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If you've never played around with action-oriented stock images I hope you'll give it a try; the main thing is to have fun and be careful. The sky's the limit.
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