M & M 3E allows a track and field check as a free action to add a rank to your speed this round, doubling your speed.
You can perform a DC 15 Athletics check as a free action to run faster: one or more degrees of success increase your ground speed rank by +1 for a turn.
This is a good way to let players sprint when they need more speed, but my players have come to realize that just like the Acrobatics test that allows you to switch from Prone to a free action, the system does not present any inconvenience. every time in case you are lucky.
Is there a good way to balance the game so that there is a risk, but not enough for players to never benefit from it?
I've planned to lift the 2E configuration for All-Out-Movement and only allow it for a total number of laps equal to its Constitution (Endurance in 3E) before you start having to check fatigue, but that still allows a lot of running and since it's often an intermittent thing, I guess they'd probably try to argue that they were resting while jogging between both.
Another option would be to foresee a consequence if they miss the test of more than one degree of failure, but some of them have sufficiently high bonuses (or mastery of skills) for this to never happen.
Or is it something for which I'm probably better off with the character, with the fact that it's hard to be stealthy running, and / or that people are looking at them oddly to start occasionally in a sprint?