There are numerous factors as explained above, but don't forget your own heartbeat !!!
I can handhold at speeds much slower than a pal of mine can, but after 1/30th at 100% there is generally some movement showing - though it doesn't often matter enough to worry about
However, if you shoot slow speeds on your camera's highest burst rate you'll find some are clearly sharper than others, and this is usually your own heartbeat adding to the problem. Hence, shoot for 1 sec and you should have images that are sharper than during your pulse beat
Dave