What words can be unscrambled from the letters e, i, r, i, l, l, p, v, r, v, r, e, g, i, g, i,

The answer would be fracture.

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)

Iwould put 'think' in the blank

Habit 1: be proactive  habit 2: begin with the end in mind  habit 3: put first things first  habit 4: think win-win  habit 5: seek first to understand, then to be understood  habit 6: synergize  habit 7: sharpen the saw