Hi Beverly. In my opinion, the answer to your question depends more on the training goal for the particular client than it does on a specific ‘guideline.’ Each client is an individual, and will respond to a training stimulus in their own individual way. The beauty of our profession and the ‘art’ of what we do is that we need to design programming that will get the desired results for the client that we have in front of us. Sometimes, what we read in the books is the answer, and sometimes we have to use what is in the books to design our own program for the particular client’s needs.
I’ve used single sets and multiple sets in working with my clients. A lot of the time it comes down to something as simple as “am I working on raw strength, or strength endurance?” for a particular training session.
I hope that this helps.