The following guide lists the results of my experimentation into the current mechanics of arm strength and stamina. Since the game is in early access, these are likely to change.

Low stamina means your weapon sways in smoother, up/down movements from heavy breathing. Low arm strength means your weapon sways in more jagged movements. When both are low the effects are combined.

Neither arm strength nor stamina seem to affect how long you can hold your breath to stabilize the view, which seems to be 5 seconds regardless.

Note that arm strength has a noticeable impact on your aiming when you've only lost 20-30 % of it. Even though it takes one to two minutes to lose it all when aiming, you'll see noticeable effects after just 10-20 seconds, less with a heavy weapon. More arm fatigue increases the frequency and magnitude of the sway movements.

• Regain times from zero to max arm strength are 30 seconds when stationary, 60 seconds when walking and 90 seconds when running.
  
• Only movement speed affects arm strength regain rate. (Specifically, weapon weight, high vs. low ready and standing vs. crouching does not have an impact.)
  
• Low and high ready always regains arm strength, regardless of movement speed (except sprinting).
  
• Sprinting draws both stamina and arm strength rapidly.

• Arm strength is drained based on movement/aiming and weapon weight.
  
• Standing and crouching are the same in all situations, except that crouching 'run' counts as walking/standing still.
  
• Only total loaded weapon weight of currently held weapon affects drain rate. (Specifically, total encumbrance does not. Attachment placement has no effect, only their weight.)
  
• You only drain arm strength when pointing your weapon forwards (ready or aiming down sights) or sprinting.
  
• Aiming down sights drains at a fixed rate regardless of movement.
  
• With a pistol you only drain arm strength when aiming down sights or sprinting. "Close ready" (pointing weapon forwards/middle position) also regains arm strength.

Drain time (these are affected by weapon weight):

Ready/walk: 3 minutes
All crouched speeds (incl. run): 3 minutes
Aiming down sights: 2 minutes
Running upright: 40 seconds
Sprinting (standing/crouched): 30 seconds

Regain time (these are always the same):

Still: 30 seconds
Walk: 60 seconds
Run: 90 seconds

Weapon weight factors. Divide the times above by these.

Pistols: 1.0-1.1
SMG: 1.2-1.4
Shotgun: ~1.3
Assault rifle: 1.4-1.6
Sniper/battle rifle: 1.5-1.75
LMG: 2.1-2.3

(For example, with a pistol you get nearly the full 2 minute ADS time until you're completely out. With an LMG you get under half.)

• Is drained faster based on total encumbrance.
  
• Does not affect arm strength in any way.
  
• Does not affect breathing stabilization.
  
• Only running, sprinting and jumping costs stamina.
  
• Climbing objects does not cost stamina, neither do ladders.
  
• Standing/crouching does not affect stamina use.
  
• Jumping costs 20 % of max stamina, multiplied by weight factor.
  
• Magazines affect your weight, but weigh the same regardless of if they're empty or full.
  
• Shotgun shells have individual weight (subtracted when loading).

• Stand still to regain arm strength the fastest.
  
• Never run with your weapon pointed forwards.
  
• A laser drains arm strength less than aiming down sights for CQB.
  
• You want arm strength near maximum since sway increases quickly.
  
• Don't sprint if you expect to shoot soon, unless you have to.
  
• Crouching has no effect on arm strength. Very unrealistically, it's the same as standing up. The only exception is 'running' while crouched.
  
• Stamina mostly isn't an issue, except when sprinting.
  
• Neil Armstrong is your hero.