Disclaimer: Make sure the treadmill is unplugged and turned off. It is a safety hazard to perform maintenance tasks while the treadmill is turned on.
For a bike some common issues are looseness at either the crank or the pedals. For looseness at the crank, this will feel like your foot is sliding side to side or you may have a thump noise associated with it if it’s severe. The way that you would fix that would be to get yourself a flathead screwdriver to take off the hub protector. This is just a plastic cap (be cautious when removing as to not break the protector). Sometimes they only have a hole in them, in which case you’ll have to pry them off from the side. This one (the Horizon Comfort U) has a slot so that you can unscrew it.
Getting to the internal issue
Then, you would find a hex nut underneath there (underneath the hub protector). Normally 13 mm, but your bike may vary, it may be 14 or 15 mm on the internal (nut). If you turn your ratchet to tighten and you can actually tighten it (which this bike has some play in it, that would explain why I was having that feeling), problem solved. You can tighten it just about as tight as you can get it. You don’t need to put 1000′ lbs of torque or anything on it, but now you can see this crank no longer moves side to side. We will no longer feel the issue while we are using it (the exercise bike).
If the crank arm won’t tighten further
If you found that you had no play, or room to tighten here at the crank hub, and that has not alleviated your problem, you may have (an issue) where your pedal has backed off from the crank arm. You can see (in our example) that we’ve got a fairly good distance between the crank and the shoulder of the pedal. This is very easy to fix. I have a pedal wrench here, you can use a standard (open ended) wrench, they are generally 15 mm, or 9/16, depending on the age of your bike. You would just tighten until the shoulder runs right up firmly against the crank. You want to give this a decent amount of pressure because that will lock it (the pedal) into place so that it (the pedal) doesn’t back back off.
The important thing to remember is on the right pedal, everything tightens normally (i.e. clockwise). On the left pedal everything is counter threaded, so to tighten you actually go counter clockwise (on the left pedal).