It is no secret [among athletes ]that[ in order to improve performance ]you've go to work hard .

It is no serect that 已不是秘密

in order to do .

在运动员中,为了提高表现而不得不努力训练已经不再是秘密.

Improvement only occurs during the rest period following hard training.

提高只是出现在艰苦训练后的休息之中.

[During recovery periods] these systems build [to greater levels to compensate for the stress] (that you have applied.)

to greater levels 达到更高水平

compensate for 弥补

在恢复期, 这些系统被锻炼到更高的水平来弥补曾经忍受(付出)的压力