Hi Nick,
Many of my personal training clients WANT to stretch between sets. I am not a fan of this personally. When do you encourage your clients to stretch?

Answer: This is a question that a strong personal trainer should have a great handle on. Obviously, I don't need to say that static stretching is totally out before any workout. To those of you who don't know the reason for this it is because static stretching can shut down the Golgi tendon organ reflex (stretch reflex) that makes you stronger and protects the muscle when it is in a vulnerable elongated position (think about how you stretch your arm back to throw a ball and you'll follow what I am writing about). The BEST way to stretch is to use dynamic/ballistic/PNF (if you have a good personal trainer) stretching PRIOR to beginning any serious work – be it resistance or high impact such as sprinting.

There is no reason to do this between sets though – once warmed up save your energy for the stuff that counts.

However, I do recommend stretching the OPPOSING muscle group to the one being trained BETWEEN sets (apologies for the excessive capitals…I am in an exclamatory mood today!). For example, stretching the hamstrings in between sets of quadriceps work will make the quads stronger through increased neural drive (think of it as your brain firing the muscle fibres of the quad more effectively) as a direct result of the stretch.