I think it is important to warm up a retriever and slowly cool him down,
just like it is with human athletes...especially us older athletes.
Warm up may reduce the risk of injury, especially with older retrievers.
Stretching by having the lab chase a bumper in a figure 8 pattern through the legs is great fun for most retrievers...that is my last routine to stretch muscles after warming up.
Cool down slowly to lessen lactic acid in muscles...
Here is a youtube video from an expert: