When I do repetitive motions like brushing teeth or even just holding an arm up while using a phone, I quickly get a burning pain in the activated muscle. The discomfort dissipates within a few seconds of stopping, but it limits many everyday activities. Climbing stairs does the same to my legs. This, combined with quick exhaustion and shortness of breath, was the reason I could never swim for more than short bursts. I thought this was normal and attributed it to the popular "feel the burn" exercise trope until I found a thread about it on another forum where it was a common complaint. Is it most likely due to poor circulation? Acid accumulation? Inflamed muscle tissue?