As a runner, when I’m in marathon training mode, my rest heart rate is very close to 40. Pro or very serious athletes can have a resting heart rate near 30. It’s actually a great way to tell your current fitness level (if you know your own baselines)

You would need to know a lot more before you worry about 50 being low.

And yes, I had to get an EKG and the doctor raised his eye brow at my heart rate, but I said I run and he immediately Moved on.