Technically speaking, there is no realistic way to "defy" gravity. Gravity is a universal and inevitable force that acts upon all objects at all times at an equal rate. However, there are ways to change the conditions under which you are influenced by gravity in order to minimize its effects. In the case of an athlete, it is a matter of man (or woman) vs. gravity, with no technological aide. The only controllable variables are speed, strength, and position. Common sense would tell you that the more momentum and force you have going into a jump, the farther or higher you are going to go. Also, the more area faced with resistance, the longer it will take to touch back down. Of course, in a human jump, we're speaking in terms of miliseconds... Basically, the answer is no, no living or nonliving thing withing the confines of the Earth can truly defy the great and inevitable force of gravity.