Flirting with slower shutter speeds

 
One of the things I've been working on for a while is motion blur. It has such an aesthetic effect when done correctly, but it's so challenging to do well. If your shutter speed is too slow, you risk having everything blurred; if your shutter speed is too fast, you may have a nice sharp picture, but no sense of motion. This isn't necessarily a bad thing, but sometimes you get bizarre shots like this one where athletes seem eerily suspended in space:
 
So far I've found that the most important thing in getting a decent motion blur shot (assuming the exposure is ok) is keeping your main focal point in focus. There are a couple of ways to achieve this, including panning or using a fancy mode on your camera called second curtain flash mode which will "freeze" the subject in space while preserving a trail of motion blur behind it.

In the picture of the guys doing squats, I chose a much less sophisticated option: timing. I simply waited until my subject was momentarily stationary to take the shot. Prior to this, I focused on Yong's face (in black) while he was standing still. Once they started doing squats, I timed my shot so that it clicked just as he reached the top of his extension. I used a 1/15 sec shutter speed to give a slight blur to everything that wasn't stationary; in hindsight, I probably could have gone to 1/30 sec to get some more detail in the other squatters. But the result is that his face is (relatively) in focus while everything else has a slight blur to it.

How to decide what shutter speed to use? I'm sure there are probably some rules of thumb but I don't know what they are. I just use trial and error. The challenge is that different movements have different rates of speed so one exercise (like squats) may use a relatively slow speed while running might need a faster shutter speed. Since I don't have an innate ability to know what blend of ISO, aperture, and shutter speed will result in the best image, I basically try them all. It results in a lot of throw-away shots, but every now and then I get a little closer and closer to the effect I'm seeking. More on this later as I continue to experiment.