"Wouldn’t a better approach be to initially train the neural network on more than just one person? That is a great point! In fact Microsoft’s research arm published a paper last year doing exactly that. Although they achieved great results, their initial training cohort consisted of 94 participants! I, as a one man team, can’t possibly duplicate that. This is why I created a system that can adapt, eliminating the need of Microsoft level resources."

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Rather than using an accelerometer (OP attached an old iPhone to his person while doing the movements) - you could plausibly point a camera and try to use video as the input.

If that pans out the next step would be using Mechanical Turk to cheaply and quickly build the initial training set (no pun intended) using publicly available videos of people working out.