M&M 3E allows an Athletics check as a Free action to add one rank to your speed that round, effectively doubling your speed.
You can make a DC 15 Athletics check as a free action to run faster: one or more degree of success increases your ground speed rank by +1 for one round.
It's a nice way to let players sprint when they need to pour on the speed, but my players have come to realize that, much like the Acrobatics check to rise from Prone as a Free action, there is no drawback built into the system to try it every time in case you get lucky.
Is there a good way to balance it out so that there's some risk, but not enough that the players never take advantage of it?
I considered lifting the 2E setup for All-Out-Movement and only allowing it for a total number of rounds equal to one's Constitution (Stamina in 3E) before you start having to make checks for Fatigue, but that still allows a lot of running, and given it's often an intermittent thing, I suspect they'd probably try to argue that they got some rest while jogging in between.
Another option would be providing for a consequence if they miss the check by more than one degree of failure, but some of them have bonuses (or Skill Mastery) high enough that that will never happen.
Or is this something where I'm probably just best off handling it in-character with it being hard to be stealthy while running, and/or people looking at them weird for occasionally breaking into a sprint?