I plan to use my Apple Watch Sport predominately for tracking fitness goals. I am curious if turning the watch to the inside of my wrist will affect its performance when measuring my heart rate. (I like to use kettlebells, so I will surely smash the screen if I keep the watch on the outside of my wrist.)
Today I wore my Apple Watch on the inside of my wrist while weight lifting. I tightened it one additional notch compared to how I normally wear it during daily activities. Otherwise I had noticed that it would often lose my heart rate, even on the top of my wrist. This usually happened when my arms were vertical and/or the particular exercise required me to twist my wrists.
I am happy to report my heart rate was detected without skipping a beat. The main downside is Activate On Wrist Raise is very finicky. I suppose that is to be expected since the Watch assumes it's being worn on the top of your wrist.
Possibly-relevant piece of information: I am Vietnamese with fairly light skin on the inside of my wrist. Your mileage may vary if you have darker skin.
Also note that I'm only verifying the proper function of the heart rate monitor. I haven't tested distance or step count estimates while wearing the device this way, although I don't see why that should affect those functions.
Edit: I tried again the following day and the Watch lost my pulse on occasion, on either side of my wrist, even when tightened as much as possible. I slid the Watch up my wrist a little and my pulse was detected correctly again. I think this is because when the Watch is too low, movements of your hand tug on the skin and make it harder to measure your blood flow.