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(1) When an athlete falls, the human body does work by gravity. After falling on the trampoline, the direction of the elastic force on the athletes is upward. When the athlete falls to the lowest point and just rebounds, the elasticity of the athlete is greater than gravity. When an athlete bounces, the elasticity is upward.

(2) When the athlete just leaves the trampoline, the elasticity is 0n. After rebounding, he couldn't recover to his original height because part of the mechanical energy was converted into internal energy when he contacted the trampoline. In the process of athletes ascending in the air, mechanical energy is conserved because air resistance is not considered.

So the answer is: (1) heavy; Upward; Greater than; Upward; (2)0; Protect.