Knuckle Cracking happens when a character cracks his or her knuckles to show that he or she is ready to deal out a whuppin' or tackle a challenge. Frequently used as an intimidation tactic in fights, this is a particularly effective way of showing that The Stoic is bothered enough by someone to get serious. Cracking knuckles may also be sign that a character is ready to get to work on a particularly arduous task, especially one that requires a lot of muscle.
Sometimes the knuckle cracking sound is removed, maybe for the sake of realism, maybe because some people find it too squicky and then it will be replaced by the hero crushing something in his hand. Bonus points if its made out of metal, points removed if its only a paper note.
Sometimes other joints are cracked as well, most often the neck, but sometimes the shoulders or back.
It might be useful to know that the sound released has nothing to do with the bones themselves, but rather with the joints between the bonesnote The popping sound is thought to be trapped bubbles of gas being released, amongst other theories, and that you're not damaging the joints or bones significantly by doing this, and does not increase the chance of having arthritis - contrary to an old piece of "advice". What it does is provide increased looseness and flexibility of a joint temporarily following a crack, at the cost of a slight cumulative reduction in the joint's strength - crack your knuckles and you get a weaker grip.
Subtrope of Kinetic Clicking.
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The protagonists from Dragon Ball do this all the time before and during a fight.
Super Buu did a LOT of neck-cracking. Which is funny when you think about it, as he didn't have bones.
In his introduction story in the anime, Ryôga crushes several walnuts to dust with his fist, vowing to make Ranma pay for something or other. In the manga, he crushes a single walnut... between index and thumb. Effortlessly. Since Ryoga constantly does things like lifting immense boulders and smashing walls by banging his head against them in frustration, this isn't surprising
Ranma is pretty fond of the "cracking knuckles and popping joints to show he's getting serious" version, and much like Inu-Yasha he favors the "cracking fingerjoints with an open fist" method. Also, in at least the anime version of Hinako's arrival, he effortlessly bends a yen coin completely in half after taking it from her. Between thumb and forefinger.
While there's not that much obvious noise, when Blue or Toboe from Wolf's Rain start clenching shaking fists, somebody's in for an asskicking.
Dr. Stein from Soul Eater cracks the joints in his neck at one point. This is tension leaving his body as he stops holding back his insanity.
Keroro Gunso. This is practically Natsumi Hinata's go to move. When you hear cracking knuckles, a certain green house guest is fixing to get a beating.
Tengen Toppa Gurren Lagann: Arc-Gurren Lagann cracks its neck shortly after its first transformation, and immediately before wiping out a swathe of Mugann. That's right, a city-sized mecha cracks its neck.
Bleach. Kenpachi Zaraki does the neck crack version during his fight with Nnoitra Gilga in Hueco Mundo.
An old standard of Kung Fu movies is to have a group from a rival school (often Japanese) come in and start disrespecting the Hero's school and his recently deceased master, maybe making a pass at his girlfriend. Fist of Fury, which uses the Chin Woo school as a basis, has a scene where this occurs and we get Lee standing there, taking the insults but with a knuckle crack to show how angry he is getting and to step up the tension.
On the flip side, an example of the "cool" variety, before his fight with Chuck Norris at the Colosseum in Way of the Dragon, both fighters step up to each other take off their shirts and then turn around to warm up. Norris only does some knuckle cracking before starting on some karate moves but Lee goes on to crack his entire upper body and show off his muscular definition.
Kung Fu Hustle uses it as a parody of/homage to (it's hard to tell which with Stephen Chow) Bruce Lee, when the Landlady menaces Brother Sum with knuckle-cracking to warn him away from Pig Sty Alley.
My Demon Lover, a film where Nick from Family Ties turns into The Devil whenever he gets an erection, has the, er... "hero" going nuts over a misplaced pack of Fritos followed by some gut-wrenching knuckle cracking.
In the X-Men film series, it appears a side effect of mutant healing factors is mutant joint-cracking, with Wolverine and Lady Deathstrike really going at this trope for all it's worth. Deathstrike really crunches those fingers of hers. Of course, they don't have to worry about arthritis. Justified in Deathstrike's case, since her claws are inside her fingers. They need to be kept loose, and are probably sore quite often. Bonus points for adding a metal clanking noise when they crack their knuckles, hinting at their adamantium skeletons.
In Snatch, "One-Punch" Mickey stretches and cracks his knuckles to warm up for a fight while his opponent his pounding on him.
In The Dark Knight, when the Joker is captured, and taunting the cop assigned to watch him, he finally gets a rise out of the guard, who tells him "I know you're gonna enjoy this. I'm just gonna have to enjoy it even more." The Joker's only response is to stoically crack his neck.
Variation in Titus Groan, in which Flay is constantly followed around by a leitmotif of joint-cracking, and his Let's Get Dangerous scene involves him wrapping his knees in bandages to allow him to sneak up on his enemy without his skeleton betraying him.
Spoofed on an NCIS episode. McGee tries to act tough by doing this, while Tony (falsely) points out that doing so is an warning sign for the early stages of arthritis.
Spoofed on Family Matters. When Urkel uses a machine to turn himself into Bruce Lee, he starts out an fight with an extended series of cracking, the noises of which barely even match up with the motions that are supposed to be making them. Then it goes Up to Eleven and Overly-Long Gag when Urkel, Richie and 3J all become literal Bruce Lee Clone and begin cracking and whining noises.
Whenever Kotaro Minami transforms in Kamen Rider Black, his fists make cracking noises.
The henshin devices in Juken Sentai Gekiranger require the users to perform knuckle-cracking movements to activate.
Buffy the Vampire Slayer. In "Hush" when demons steal the voices of everyone in Sunnydale, Giles cracks his knuckles before undertaking the task of...changing the overhead transparencies in order to deliver his usual exposition.
One of the finest examples of the "crushing" variant comes in Phoenix Wright: Ace Attorney: Trials and Tribulations. As cool, collected, magnificently sardonic Diego Armando comforts Mia over the disastrous end of her first trial, his hand tightens on his omnipresent coffee mug. He doesn't even flinch as it shatters, holding up his bleeding fist as he delivers one of the most profound lines in the series.
Before him, on being driven to the wall, Dee Vasquez tightens her grip on her pipette so much that it snaps.
In Breath of Fire III, Bunyan cracks his knuckles after catching your team robbing his house red-handed.
Marshall and Forest Law, also, had some knuckle and wrist cracking and are complete Bruce Lee Clones.
In one skit of Tales of Destiny 2, Loni Dunamis starts mentioning on how no guy will be attracted with Nanaly Fletch, because... she's a tomboy. Then cue to Nanaly cracking her knuckles (with sound effects), which scared Loni shitless and the rest, she starts cracking his bones.
Jill and Wesker do this in Resident Evil 5 at the beginning of your fight with them assuming you don't shoot them first.
Not Jill. She stands like some soldier at the ready while Wesker cracks up, then hisses in some wildbeast pose in the second fight.
Guilty Gear's Sol Badguy does this in one of his fight intros.
Shepard invokes this trope in Mass Effect a lot, usually as warm up in order to extract information from a hesitant informant. Unsurprisingly, if you understand anything about Shepard, it works. One volus was savvy enough that all s\he had to do was crack knuckles to comply.
Joker does this also quite a few times, most often when he's about to make Normandy dance. Which is strange, seeing how he could break his fingers quite easily..
A deleted scene from Spider-Man: The Movie: The Game has Shocker doing this. With no sound, oddly.
The character select screen in Warhammer Online has the Chaos Chosen flex his fingers and clench his fist in a soundless example of the trope, but the Orc Choppa simply grabs his own head and noisily cracks his neck.
Tokugawa Ieyasu from Sengoku Basara dramatically cracks his knuckles before his Basara attack, appropriate since he's a fist fighter. Kuroda Kanbe also cracks his neck during his taunt, complaining that he "needs a break".
Van Von Hunter sort of plays with this trope; the signature move of the Flaming Prince is saying "I will CRUSH you like this [noun]!" while acting this out. Often said object is something he really does not want to crush, due to it being valuable or very painful.
In the BDSM webcomic Sunstone, Lisa does this at the very start of the series before beginning the memoirs and again later when to help wake herself up she cracks her neck. It's one of the few things about her that annoys the crap out of Ally.
Mini Fett's Team Fortress 2 weirdo, Spyper, normally speaks with a Spy's voice, but he'll randomly twitch and start talking like a Sniper, usually to spout something loud and tactless. Every time it happens his neck cracks audibly.
Of all people, Richie Rich's mother had this as a habit.
Occasionally played for comedy, with a piano player about to attempt a difficult piece. For example, Bugs Bunny in Rhapsody Rabbit.
In one Batman: The Animated Series episode there's a crook who is loudly declaring "I'm no squealer" that Batman needs to get information out of. Batman just cracks his knuckles and narrows his eyes threateningly. The guy talks.
As for neck cracking, Starfire on Teen Titans does one badass neck cracking in "Go", where you really see what an upbringing by a warrior race will do for you when you're scared and in a strange place having just escaped being a war prize and implied sex slave. Scary.
A more humorous, yet still serious example occurs near the end of "Only Human." This time, it's Cyborg's turn for a double neck crack, but since half of his head is robotic, the second crack is actually a mechanical whine.
Would you believe that Optimus mimes the knuckle-cracking move in episode 41 of Transformers Cybertron (or 42, if you follow Galaxy Force)?
Another example of "character who has no bones and is therefore invincible but shows being invincible by cracking his neck despite having no bones after being beaten up" is in Ultimate Avengers 2, when the alien Nazi shapeshifter (who is kind of a colony of slugs) does this after falling from a great height.
The first aired (but not "true" first) Regular Show episode, Mordecai and Rigby vigorously crack their knuckles, spines, and shoulders for a few seconds before engaging in..... Rock, Paper, Scissors
In The Cleveland Show, Roberta gets ready to put a beatdown on some college guys for harassing her and punching out her boyfriend (in "B.M.O.C."). She cracks her neck, then shoulders, then knuckles, then boob knuckles.