I had that when walking quickly, turned out to be copper deficiency caused by taking zinc without copper. Being a former motorcyclist I was not scared. I was (censored) terrified.
Best source of copper is raw organic natural liver from grass-finished animals. Once per week. I don't know of any safe copper supplement, never found the right one.
I fixed problems with raw paleolithic diet, and now added the Iodine protocol. Raw works.