Sprinting and Tiredness

Hey guys, I have a small issue, I would like my sprintSpeed variable to be proportional to the amount of energy my player has left, basically the more he sprints the slower he will sprint. I tried something like this, but it is not the desired effect:

 var energy = 100.0;
 var sprintSpeed = 12.0;
 
 function Update ()
 {    
 
     if(energy <= 100 && energy > 75)
     {
         sprintSpeed = 12.0;
     }
     if(energy <= 75 && energy > 50)
     {
         sprintSpeed = 10.0;
     }
     if(energy <= 50 && energy > 25)
     {
         sprintSpeed = 8.0;
     }
     if(energy <= 25 && energy >= 0)
     {
         sprintSpeed = 6.0;
     }
 }

Any help is appreciated