Hi Xue,
Nice images,
I think they are normal.
The hypo echoic and hyper echoic area is from anisotropy and if you rock the probe a little I think the hypo echoic area will become more echogenic and the echogenic area will simultaneously become hypoechoc.
This looks like a strong, healthy muscle and they are very anisotropic when they are like this.
You say she is not an athlete but that muscle looks stronger than mine and yours!
I bet she would beat you easily at badminton.
Sometimes you can get pain with overuse as part of a delayed onset muscle soreness scenario (DOMS) and if it is bad enough it can progress to rhabdomyolysis. But I don’t see any changes suggestive of this. I would expect the muscle to be more echogenic if this was the case.
These muscles are innervated by the posterior interosseous nerve (PIN). You have shown the PIN beautiful in your video and you can see it entering the supinator at the arcade of Frohse. The PIN is fairly big in size but this can be normal.
I would so a resisted supination test and look for compression of the PIN at the arcade of Frohse.
It is possible this is the cause of the pain (radial tunnel syndrome). It is not a common diagnosis in my experience but worth considering.
What was the comparison with the other side like?
It may be the muscles are bigger as it is the dominant arm and badminton uses such a lot of forearm strength.
Did the muscle feel firm clinically compared to the other side?
Steve