Your healthy athletes should be able to hold their breath more than just a few seconds during the heaviest part of the lift, commonly referred to as the sticking point. Instruct them to take a larger than normal breath, not excessive but a little bit bigger than normal, and then hold it through the sticking point.
Not only does maintaining control of your breathing contribute to a stronger physical effort, it can relax your body and mind. Dr. Yessis states that inhaling and exhaling before a physical effort helps the body to relax. However, this does not mean a total relaxation of the muscles.
Prior to beginning these movements there has to be some muscular tension throughout the body. For example, when doing the dead lift, this tension is brought about by taking the slack out of the bar before the lift begins. This places enough tension on the muscles to produce sufficient strength to lift the weight off the floor once the pull begins.
Yessis, Michael, Dr. Build a Better Athlete, Equilibrium Books
Zatsiorsky, V.M. and Kraemer, W.J. Science and Practice of Strength Training
Verkhoshansky, Y. and Siff, M., Supertraining 6th edition, published by Verkhoshansky