Hey Bob...
I'll try and answer your question, however though it is very wide...
A properly fitted kydex/synthetic sheath is for sure enough to hold a neck knife, not only for running, but even for hardcore activities...
Now the determining factor is ,,is the knife fitted properly....A lot of factory (OEM) sheaths are notorious for poor fit.. Several reasons for this..Mass production being the number one factor...
Also some of the OEM sheaths found are not kydex, but what I believe to be injection molded ABS or Zytel,,something to that effect...
Most everything injection molded will have a funny feeling fit,like pushing a knife into clay, to falling out under it's own weight... There are exceptions though, but most are poor fitting.
If it's a custom knife, then more than likely it will come with a better fitting sheath, as the maker will have wanted to pay special attention to the sheath.. There are also exceptions to this as well...
If you are in doubt, the best way to relieve your mind is to order an aftermarket sheath, if the quality and fit isn't up to your standards....
There are several very good Sheath makers here on the forum..Ask around,,ask questions,, and look at some websites,, starting with mine..
I'd be happy to answer any questions you may have..
Hope that helped..
ttyle
Eric....