Skyrim:Heavy Armor

Heavy Armor is the skill governing your ability to move and fight in heavy armor. Each skill point increases your armor rating from heavy armor by 0.4% (1.5% for NPCs). The Heavy Armor skill tree has a total of 8 perks, requiring a total of 12 perk points to fill. 

In-game description: Those trained to use Heavy Armor make more efficient use of Iron, Steel, Dwarven, Orcish, Ebony, and Daedric armors.

Additional types of heavy armor that are forgeable: ancient Nord armor, bonemold armor , chitin heavy armor , dragonplate, Imperial armor, Nordic carved armor , and stalhrim armor . There are also a few types of generic, unforgeable heavy armor: Dawnguard heavy armor , Blades armor, Falmer armor, and wolf armor.

Skill Perks
  
 

 

Usefulness of Reflect Blows
While the description of Reflect Blows seems potent, it does not entirely work as described. Indeed, opponents who have the perk (such as the Ebony Warrior) can one-hit-kill you in certain circumstances if the perk triggers, the damage is calculated a little differently for players. 

The perk will not reduce damage taken. When it triggers, a melee attacker will take some of the damage dealt to you, but you will also take that damage. If your character that has reached the armor cap of 567 (which grants an 80% damage reduction) it means in the damage reflected back at attacker is also reduced by that same 80%. The damage reflected back is also affected by the Block skill, which caps at an 85% damage reduction. The usefulness of the perk becomes questionable as the most damage you can mitigate with heavy armor and block is 97%, meaning you will reflect only 3% of the damage an enemy does to you. On Master difficulty the perk becomes almost worthless because while you take double damage, the damage you reflect is also subject to the half damage you deal.

Comparison to Light Armor
Advantages:
By default, heavy armor is more protective than light armor - at least, until the armor cap is reached.  However, the higher base stats of heavy armor makes it easier to achieve higher armor ratings early in the game. Due to the way Armor Rating functions, each additional point of armor is more valuable than the last, making even small differences more potent than they might otherwise appear.
Heavy Armor incentivizes players to take damage, thereby leveling their Heavy Armor skill up faster. A player will reach high levels of Heavy Armor significantly faster than they would with equivalent Light Armor, where dodging is more encouraged, which does not reward XP.
The Fists of Steel perk provides an extra source of damage for unarmed combat, which is unique to heavy armor. This can be combined with the Fortify Unarmed enchantment and any racial bonuses to provide a viable alternative to weapons and magic. This is particularly useful in situations where a player's weapon may be Disarmed.
Heavy armor has perks for significantly reducing falling damage.
The Tower of Strength perk renders players heavily resistant to stagger effects, most notably Unrelenting Force. This substantially reduces the frequency of the player being ragdolled and needing to climb to their feet, mitigating what can be a significant vulnerability even for otherwise powerful characters.
The Reflect Blows perk essentially grants every set of heavy armor a 10% chance to punish a melee attacker.  Though the usefulness of this perk is questionable, as long as the player does not mind taking a lot of hits in melee, the bonus damage helps.

Disadvantages:
Heavy armor weighs more than light armor, which causes the player to move more slowly and takes up more of their available carry weight.  For most players, this disadvantage can be negated with the Conditioning perk or the Steed Stone effect.
Due to the extra weight of heavy armor, stamina is consumed at a greater rate while sprinting in heavy armor without the Conditioning perk or the Steed Stone effect.
Due to the heavy nature of heavy armor, specializing in heavy armor is a bad choice to players who prefer to be agile and/or wish to most conveniently carry multiple sets of armor/clothing with a variety of enchantment combinations.
Once armor cap is reached, light armor and heavy armor become equal, as they all provide the same 80% physical damage reduction.
Heavy armor is not stealth friendly.  Wearing heavy armor creates more noise while moving, which makes it harder to sneak.  This can be negated with the Muffle enchantment or spell as well as the Silence perk in the Sneak tree.
Heavy armor does not boost stamina regeneration, whereas the Wind Walker perk turns every set of light armor into an extra source of stamina regeneration, making light armor a great help whenever stamina is needed for attacking, blocking, and/or sprinting.
Heavy armor does not (have a perk to) completely negate incoming damage, whereas the Deft Movement perk allows every set of light armor to randomly completely negate the damage from 10% of melee attacks.  Since no armor can provide more than 80% physical damage reduction, the Deft Movement perk makes light armor more protective than heavy armor against melee attacks once the 80% cap is reached.  Unlike the Reflect Blows perk, Deft Movement is always useful, especially for surviving on Master/Legendary difficulty.

Skill Increases
Character Creation
The following races have an initial skill bonus to Heavy Armor:
+10 bonus: Orc
+5 bonus: Imperial

Trainers
Gharol in Dushnikh Yal (Expert)
Farkas of the Companions in Whiterun (Master)
Isran  in Fort Dawnguard (Master)
Kuvar  in Thirsk Mead Hall (Master)

Note: Trainers that double as followers can train you for "free". They will still charge you, but you can get the gold you spent back by checking their inventory. 
:This exploit is no longer possible as of version 1.2.6 of the Unofficial Skyrim Patch.

Free Skill Boosts
+1 Heavy Armor from Brunwulf Free-Winter in Windhelm for clearing a bandit hideout.
+1 Heavy Armor (and other combat skills) from Giraud Gemane (Bards College) for completing the quest Rjorn's Drum.
+1 Heavy Armor (and Block, Archery, and One-handed) from Wulf Wild-Blood  for finding his brother during Filial Bonds .
+5 Heavy Armor (and other combat skills) by selecting "The Path of Might" from the Oghma Infinium after completing the quest Discerning the Transmundane.

Skill Books
2920, MidYear, v6
Chimarvamidium
''Hallgerd's Tale
The Knights of the Nine
Orsinium and the Orcs''

Gaining Skill XP
Armor skill increases are based on the raw attack rating (plus any power attack bonuses) of an enemy strike. 
Methods that improve armor rating, such as Smithing, skill enchantments, spells, potions or shouts, have no adverse effect on the XP gained. For example, a character with an armor rating of 400 will receive the same XP as a character with an armor rating of 100 for the same enemy strike.
The number of heavy armor items simultaneously worn by the player does increase XP gained. For example, receiving damage while wearing four pieces of heavy armor will result in more XP than wearing three pieces.
Regenerate Health potions and enchantments can help with staying alive while training, potentially allowing you to leave the game unattended. This can also be accomplished with natural health regeneration during brawling sessions with NPCs or when fighting very weak enemies like skeevers, though skill gains will be very slow.

Notes
Once your Heavy Armor skill is level 30 or higher, guards will begin to say, "Best offense is a good defense, am I right?"
Heavy Armor is the only skill that can directly modify unarmed attacks by way of the Fists of Steel perk. The perk modifies the damage done by unarmed attacks by the worn gauntlets' base armor rating, unmodified by perks, enchantments, or skill levels. This means Daedric gauntlets only add 18 damage to your unarmed attacks no matter what. The Khajiit Claws racial ability should be on par with the Orcish gauntlets (15 base armor rating). The Claws ability stacks with this perk (so Claws + 100% base armor rating of gauntlets + base damage = damage). Making use of the Fortify Unarmed Damage enchantment will further raise the damage done with fists.
Note that when leveling up Heavy Armor, you have to take damage from an outside source—Equilibrium, for example, will not level up the skill. The amount of damage a blow would cause will dictate the amount of experience you gain, not the number of hits you take. One way of leveling up Heavy Armor is by fighting giants. From a starting level of 15, by training against a giant you can reach a level of over 50 within 5 minutes of getting bludgeoned, provided you don't die. A healing spell would be most helpful while training. Note that at early levels, the giant's ground shatter attack with its club is often fatal, unless you have a lot of health to spare.

Bugs
 
If this happens, entering your inventory will let the game readjust your carry weight to its proper value.