“Ring rust” is a term that has varying strength. On one side there are recent cases like Yoshihiro Akiyama’s long-awaited return at UFC 116. Akiyama was never known for great cardio, but a year out of the cage did a number on his gas tank, leaving him blatantly exhausted midway through the second round. Conversely, former champ and genetic-jackpot winner Randy Couture spent 13 months playing retired before dominating Tim Sylvia for the heavyweight title in 2007.

Inconsistency aside, there is enough evidence to consider ring/cage rust a real problem. Missing is a clear explanation. There are three main factors that fighters must maintain during a break: physical conditioning, evolution of technique, and mental fortitude. While most cases of ring rust are probably a combination of the three, each affects performance in unique ways.

There are dozens of strength and conditioning programs available to athletes. But none of them can match real combat-sport competition. Firstly, it is impossible to train effectively with anyone you genuinely dislike–things get heated too quickly. There needs to be a level of trust; so fighters train with friends, which inadvertently attaches emergency brakes to the intensity level–a friend does not rush in for the finish when his sparring partner gets rocked, or crank on a submission like a title is on the line.

Training focuses on technical aspects, which are essential, but less draining than hunting for a knockout. Nothing can replicate the cardio test of a real fight. Fighters will run, jog or swim at the highest pace that they can confidently maintain, while paying close attention to their breathing. Conversely, fighting has an fluctuating pace that puts the heart and lungs through bouts of spiking intensity.

The obvious solution is focusing on sparring sessions to build cardio, but that’s a catch-22. Even if fighter is willing spar with full-blown heavy-contact, a good coach would not allow it because the chances of injury would skyrocket. Getting back into the cage is the only solution.

All fighters have room to grow and evolve, and time off means lost opportunities to learn from mistakes. Watching fight tapes is a huge part of top-tier competition, not only to learn about opponents, but to catch personal slip-ups. Dropping hands after throwing kicks, predictable combinations, and telegraphing strikes and takedowns are some common mistakes that creep into fighting styles.

Such problems can be absent throughout training and emerge under pressure. Aggressive coaching can be iron them out, but that takes weeks. Furthermore, not all training camps are truly well-rounded. For many, the only way to find subtle holes in their game is to fight a variety of opponents.

Finally, mental fortitude is the wild card because it can lapse without warning. Humans can adapt and even become comfortable under extreme stress. However, time away from a roaring crowd and a dangerous opponent makes for a visceral return experience. Fear can come out of nowhere and cut to the bone, leading to something even worse than diminished cardio: hesitation.

Repetition and “muscle memory” are what fighters rely on in the heat of the moment. Mental conditioning allows fighters to react to a strike, takedown, or submission. But with time off, even a few weeks, the conditioning begins to fade, and fighters have to think about actions that should be automatic.

When Rampage Jackson returned to face Rashad Evans at UFC 114, his wild swings were tell-tale signs of a long layoff. But the most embarrassing moment came in the third round when Jackson landed a hard left, and inexplicably stood back and allowed a bobble-headed Evans to recover. That match was Jackson’s 38th professional fight, but those few seconds were the crystal clear difference between occasional training and consistently competing.

Athletes from all sports seem out of sync after a long layoff, but it’s far more severe in combat sports because of the specific factors that contribute to ring rust. Still, rust does not hit all fighters equally, and is far from a guaranteed loss.

The next year will see some particularly high-profile fighters returning after extended layoffs. First, Thiago Alves, on the sidelines since UFC 100, will finally be returning for a rematch with Jon Fitch at UFC 117. And Vitor Belfort, out since September, is expected to return in the fall, probably against the winner of Anderson Silva vs Chael Sonnen.

It’s a dangerous situation for both fighters, but potentially extremely rewarding. Alves is being given a chance to jump ahead of Jon Fitch in the race for a second crack at Georges St. Pierre. Due to Fitch’s conservative (see: boring) style and reluctance to fight teammate Josh Koscheck, Alves can easily become the more popular choice.

Similarly, Belfort was originally promised a middleweight title shot due to his unique speed and striking power, despite never fighting in the UFC at 185lbs. The intriguing style matchup against Anderson Silva, and lack of real threats (including Sonnen) in the division, mean that Belfort may still walk into a title shot. It remains to be seen whether either man can overcome ring rust, as a loss for either fighter now would be mean a huge blown opportunity.