After my daughter was finished with her workout she was kind enough to let me practice. This is my attempt at a 3 light setup with black (actually blue blanket) background. Timer set to 10 sec so I could use spray bottle for some fx. processing done in LR.
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https://www.flickr.com/photos/ap2380/
Excellent lighting once again!
A few minor suggestions:
1) You framed your daughter very centered. Cropping or shooting her slightly to the left would make for a more interesting composition.
2) She is doing a one-arm dumbbell curl. Ideally, the biceps would show better medially on the arm, i.e. if she were curling with her left arm at this body angle. The dumbbell also wouldn't be blocking some of her torso and the width (her back to extended DB) would fill the frame better and look more interesting.
2) If you want to photograph her curling with her right arm as you have, keep some space between the DB and her halter top. As is the grey hex dumbbell is touching the grey halter top. Having some abdomen showing inbetween the two would make for a better photo.
You did all the hard stuff well. Positioning your model differently is easy and quick.
--
'When nothing seems to help, I go and look at a stonecutter hammering away at
his rock perhaps a hundred times without as much as a crack showing in it.
Yet at the hundred and first blow it will split in two,
and I know it was not that blow that did it,
but all that had gone before.'
-- Jacob Riis (1849 - 1914)
Stay Well,
Pete K.