Phone accelerometers don't have enough range and sampling frequency to even begin with. Raw, on some rare phones you can sample 800 Hz (enough-ish), but on most 100 Hz max, Web API is capped at 60 Hz, this is all way too low for any quaternion math. They also have much higher noise density which is the silent killer of all kinds of IMU navigation.

I also wouldn't trust a strap to drop a loaded bar from snatch :D https://youtu.be/nrgnH9fTfGo?si=6LLeu3y02iFrwfis&t=65

As someone who has been very deep down this rabbit hole and hacked together multiple path and velocity trackers over the years (specifically for olympic weightlifting), there is no extra information that tracking bar path will give you that simply looking at the video won't, and often just adds more clutter. You don't need to graph bar path to see that the bar is looping too far forward after hip contact in the snatch.

Velocity on the other hand is a great metric to track and is used as a proxy for RPE. Mike Tuchscherer was the first one to systematize it for powerlifting a while back, if you've been lifting for 20 years you're probably aware of the name.

Thanks! I think for "canonical" lifts (squat, deadlift, row, to some extent military press) the vertical bar path is mathematically optimal, and for all kinds of lateral or sagittal movements you do more work with weak stabilizing muscles and load joints laterally too. Is it productive work that strengthens your core? Possibly, but it's hard to quantify. It it something that can lead to injury? Absolutely yes.

For more complicated lifts like bench press (J-shaped) or snatch (S-shaped), for example, I would rather set a "golden sample" path with a coach and compare to that.

It's unlikely to be the sole metric, especially given the inverse kinematics of different body types (long/short femur, etc), but together with bar speed, over time, it can provide a lot of good feedback.

Not OP but velocity is typically what these devices are used for. Its a great measure of between-set intensity.

For me it's a bit of an inverse problem. I go to a public gym (hard to sustain motivation at home) and I absolutely don't want to film myself there.

Thanks! Working on it, for now it's literally a taped breadboard. :D