I am relatively new to swimming but I know a warm-down is essential after a practice.One thing I do not know though is WHY exactly it is needed.I was just curious what you miss out on if you don't warm-down.
I'm just an average swimmer, but from what I've heard from a variety of coaches, active recovery (cooling down) allows heart rate, body temperature, and blood pressure to decrease gradually, promotes the circulation of hormones which counteract the effects of stress hormones like adrenaline and endorphins, and promotes the elimination of metabolic waste products like lactic acid. Suddenly halting exercise has been shown to increase the probability of dizziness, muscle cramping and soreness. Warmdown is also an ideal time to stretch warm muscles and increased flexibility will help you avoid injury in sports and in life generally.
Personally, my current masters coaches often include all-out sprints near the end of the workout, so cooling down makes me feel much more comfortable and calm. After a 4000 yd workout, we typically do about 200 yds of easy swimming to cool down.
The warm down keeps the blood flowing while not straining the muscles as much. It allows your body to rid the muscles of lactic acid - which is the stuff that causes soreness.
It probably is good to allow your heart to slowly ramp down too, but I'm not sure about that.
In addition to the reasons already mentioned, a good warmdown can also provide an opportunity to engrain efficient stroke technique. For more information on this, see:
www.totalimmersion.net/.../packlearning.html
