Cripple

Fallout and Fallout 2
Targets can be crippled by aiming for the body parts. Having a crippled leg prevents running and requires more action points to move. A crippled arm disables using two-handed weapons. If the arm crippled is the arm wielding a gun, the accuracy of the weapon will decrease significantly. If both arms are crippled, only unarmed attacks can be used.

In Fallout 1 & 2, crippling of the eyes results in blindness.

Fallout 3 and Fallout: New Vegas
Although body part damage can be achieved through manual aiming, V.A.T.S. allows more effective targeting of particular body parts. The effects of crippling specific body parts are listed below.
 * Crippling the head will reduce overall accuracy and Perception (-4) of the target. If inflicted upon the player character, the screen will intermittently get fuzzy and a ring will be heard through the speakers, loosely simulating the effects of a concussion.
 * Crippling the torso will result in the target flinching far more often from shots than normal. This can also affect the player character, throwing off aim and interrupting attacks (even in V.A.T.S.).
 * Crippling an arm will drastically reduce accuracy and may disarm the target if a weapon is held in the attached hand. This is particularly evident when using a scoped weapon, such as a sniper rifle. The weapon itself is not destroyed, and the target can pick it up again or use another weapon from their inventory.
 * A crippled right arm also reduces base melee damage done by 10% if wielding a one-handed weapon. If wielding a two-handed weapon (or unarmed), one crippled arm reduces base damage done by 5%, two crippled arms reduce base damage done by 10%.
 * Crippling a leg will result in impaired mobility. Creatures who rely on leaping attacks (e.g. deathclaws) will no longer be able to use them.
 * Crippling both of the player character's legs disables special melee and unarmed moves that are executed outside of V.A.T.S., such as Mauler or Ranger Takedown.
 * Crippling the held weapon will ruin it and make it unusable, and/or send it flying out of the target's hand. Although the condition is zero and it cannot be used, it can still be collected and repaired, or used for repairs. The player character's weapon may be damaged by enemy fire as well. If the weapon's Condition depletes, it will be unequipped as opposed to flying out of the hands. If it is an explosive ordnance (like a frag grenade), it will explode, dealing its damage to the target holding it (and any others nearby). Grenades can be targeted by V.A.T.S. while in the air after being thrown as well. If successfully hit on the fly, they will explode prematurely.
 * Some enemies have alternate targets. Robots, insects, turrets, and certain others have special targetable parts that when crippled will push them into a frenzied state where they stop discerning between friend and foe and attack indiscriminately.

Crippling any body part will result in the target momentarily pausing to clutch the now crippled body part or reel from the injury.

When using explosive and melee weapons specific body parts cannot be targeted while in V.A.T.S. The enemy is simply targeted as a whole.

Healing a crippled body part
In combat, the player character may also sustain crippling damage. This was originally to be a mini-game. In order to repair this damage, it requires:
 * Pay (50-100 caps) for a doctor to heal it.
 * Sleep in any bed.
 * Use a stimpak (either on the crippled body part directly or generally through normal stimpak-healing).
 * Develop advanced radiation sickness (400 rads) if one has the Rad Regeneration perk.
 * Use My First Infirmary.
 * Use a doctor's bag or hydra.
 * Use a healing poultice or weapon binding ritual.

Hardcore Mode (Fallout: New Vegas only)

 * Paying a doctor works the same.
 * Sleeping only heals if the bed is owned.
 * Stimpaks cannot be used to heal limbs.
 * Doctor's bags function like stimpaks on Casual.

Fallout 4
Unlike in previous Fallout games, crippled limbs in Fallout 4 will heal on their own after a short while. In Survival mode, legs can become crippled if the Sole Survivor is over-encumbered.

Fallout 76

 * Most enemy limbs can be crippled, preventing enemies from moving, attacking or flying. Crippling a scorchbeast's wing forces it to land. Some enemies will regenerate their crippled limbs while others such as ghouls remain crippled until killed. A legendary enemy that mutates has all of its limbs instantly restored.
 * Robots that are completely crippled will heat up and explode.
 * The Enforcer (perk) gives each shotgun pellet up to 30% chance to cripple a limb.
 * The Tormentor, Modern Renegade and One Gun Army perks also give a chance to cripple with each shot.
 * When activated, the crippling effect multiplies limb damage by 999.
 * Weapon upgrades exist that provide a 20% chance to cripple a limb.