I still remember setting up my bike in the transition area of my first sprint triathlon. I was really nervous. I recall watching people around me to get some queues from the more experienced racers. I noticed several of them riding and/or running out of transition and running. I wondered, "Is a warm-up really necessary before a sprint triathlon? I barely have the endurance to do the event, let alone waste my energy before the race begins."
Beginner Sprint-Distance Triathletes
Most beginner triathletes with early developing fitness, like the fitness I had prior to my first race, don't need to warm up before the race. Developing fitness means you have recently become an endurance athlete and your endurance is minimal. The sprint triathlon will be your longest race experience, though you may have completed a bike workout as long in time as the total time you expect to be racing.
If you have the opportunity to do a short warm-up before the swim, go ahead and swim 100 to 200 yards. This is especially preferable if the swim is in open water. Many triathletes, even those that have solid swimming experience, have some shortness of breath when putting their faces into cold water. A short warm-up helps you get past that feeling. Rather than thinking about the event ahead, focus on what you need to do to be successful in the water; that builds confidence.
Once past the very short warm-up, or no warm-up at all, use the swim as your first opportunity to warm up. Begin the swim at a comfortably easy pace, similar to the pace you've used to warm up in training. Plan to gently increase the pace about halfway through the swim.
After swimming, your body will have some warm-up time. When heading out on the bike and run, plan to race both these segments in a negative-split manner. That is, begin each sport at a slower pace than you anticipate doing at the finish. These paces are the same as you've practiced in training. You're using the first part of each leg of your sprint triathlon as a warm-up.
Since your primary focus is on comfortable event completion, more than likely your race intensity will be mostly aerobic (Zone 1 to 2) and perhaps include some intensive endurance pacing (Zone 3).