The context you've given is somewhat confusing to me (albeit, I'm coming at it from wondering why would an unconsious person be in a cell? Especially if not under arrest. They should be in hospital). In these circumstances, I would revert to the fact a cop's most important tool is his mouth.
But, taking your scene in isolation without consideration to wider context and just operating under the parameters you've given - You seem to suggest the subject is standing, arms crossed, facing torwards, compus mente. Officer must exert presence or dominance to show they can restrain. No suggestion of excitablity giving a resistance to pain.
On the arm which is on the outside of the folded arms (normally right over left) I'd have a light hold on the person's right wrist with my right hand. My left hand would be just above their right elbow (Present as reassuring, "Are you feeling okay?" those kinds of verbals). To look at, it may seem a very benign posture. Even a friendly rub on the elbow may be warranted.
If the subject starts to struggle, you then have two options.
They're already quite tied up as their arms are folded. I might be inclined to just push them back firmly (especially if a bed or matress behind) leave the cell, close it up and let them chill out before trying again, or gather a larger team to safely restrain.
Option two, which is more than likely what you are looking for is what's called a "Straight arm bar takedown".
My reassuring left hand on the left elbow will then pincer into the flesh between the tricep and elbow. A very sensitive spot (perfect if you get the elbow funny bone right). This will give a distraction and also gain control of the arm at the elbow.
My right hand will grip the wrist, this will be firm, but lighter as I need the wrist to be able to rotate beneath my hand.
I will pull the wrist towards me, (hopefully out of the weak side of the arm folded position) push the arm straight with my left hand while twisting. My grip on elbow will evolve to a sawing motion with my forearm. If they need any extra persuasion, or their strength is higher than expected, I would put in a knee strike, aiming for a dead leg. I will drive them down towards the ground. This seems quite long winded when written, but it will be quite a snappy motion.
I will be shouting as loud as I can "Get Down". Two purposes - a distraction in itself, and because whether consiously or unconsiously, people sometimes respond to commands.
From there, I would go into a ground pin. The officer would now be expending zero effort, while the subject will be fatiguing fast through struggling or unable to move full stop. I'd either call for others, so we can get him all tied up, or be able to work at a slower, more considered pace to get him into cuffs while doing him the minimum harm. (from that position you can use pain compliance).
"Put your other hand in the small of your back"
"No, officer. I do not wish too. Go forth and multiply."
(officer tweaks arm by pushing it towards centre of back) "Do it now."
"Ouch... Ok"
I'd be very mindful of what we call positional axphyxia in that position. The subject would struggle to breathe and may suffocate. But assuming they're doing okay like that, the cop can take as long as he needs to take to get the guy in cuffs safely, or calm them down.
Videos:
This shows a basic straight arm bar takedown. (for your scenario modified as above to get control of arm out of arms folded)
This shows a take down to ground pin. (personally, I prefer to be stood up with the arm locked between the knees. It reduces risk of positional asphyxia as you have no weight on them, I feel more stable and in the city centre environment, you normally have a load of the subject's mates around baying for your blood, so its easier to react to them). Note the twist of the wrist. That's so pain compliance can be applied and also puts the wrist in the correct position for handcuffing.
I hope that's clear enough. Sometimes difficult to explain, easier to show. Send me the part on here for critique if you wish.