I'm pretty arhythmic too, was about 14 before I learnt to skip - spent many humilating lessons at primary school when the teacher told us to skip (without a rope) and they didn't understand how I couldn't do it. I can't clap in time at all, nor dance (another point of childhood humilation).

As it happens I took classical piano as a teenager, but always had trouble with the rhythm.

When getting a medical exam for scuba there was a suggestion that I had an arhythmic heartbeat and I've always wondered if that could relate - perhaps my brain just can't do rhythm in any way at all ...