You should probably look into how the commercial or Indie engines handle these things.
In most cases, the render thread just runs as fast as possible to prevent any dropped frames (vsync off). Even if that is pointless because you are getting 140 FPS on a 60 Hz screen.
Most modern games give you the possibility to synchronize your render thread with the screen refresh (vsync on), which will prevent the tearing seen when the buffer changes while the screen refresh is still in progress, on the other hand lead to dropped frames when the buffer cannot be filled fast enough while the screen refreshes twice, thus players see a "microfreeze"
In the second case, you would see the render thread slow down to 60 Hz, 30 Hz or 20 Hz (in the worst case :) ), thus no longer using 100% of resources (though these are GPU resources hopefully, unless the game is Draw Call constrained)
On the CPU side, the Physics would be one engine subsystem that could consume a core 100%. Which is fine in case of multicore systems, but would mean you have no chance of handling higher physics load without the physics engine starting to "throttle" (like calculating less iterations for physical effects), which may or may not decrease the quality of the simulation.
One reason why many physics engine run at a different, FIXED rate, completly separate from the render thread frames per second.
Main reason is to make the physics simulation more stable... but it can decrease the load on the CPU if physics is only calculated at 30 or 20 Hz instead of 60 Hz.
As has been mentioned, you can simulate these behaviours with sleeps. Basically you calculate first how long a single iteration should take (1/'fixed FPS' s), then you make sure a new iteration is only started every 1/'fixed FPS' seconds (or multiples thereof, should an iteration ever run longer). This would give you the option to test how your game behaves at differen fixed FPS settings.
Of course, question is if the high CPU load is a problem after all. Depends on your game, and what part of the game loop we are talking about. In some cases, it is more important you can keep a constant FPS than getting the highest FPS (physics, renderer to some extent for example), there it would make sense to look into such a mechanism (vsync in case of the renderer). In some cases, it might not matter at all, as long as your game runs on PC.