I look after my great nephew's chocolate lab when they are on holiday.  And he is a nightmare walking on the lead.  I walk him off it and he is a lot better.  Stops when I tell him and waits. He is great with other dogs off the lead but totally different on it.  If I have to put the lead on him I have one of those that extends.  When he was at the vet hospital for an op the Vet said that labs have very strong necks and it is true.  My other nephew cannot walk Jay as he is far too strong for him.