Excellent question, especially considering Steve quite easily kills people using the shield as a blunt weapon. How come he doesn’t do the accidentally-slammed-my-fingers-in-the-desk-drawer dance every time he catches his shield on the rebound?
1. Bone conditioning.
Martial artists use a technique called bone conditioning to strengthen high-impact areas (hands, elbows, knees, shins, feet) and prevent injuries during fights. All bones consist of a light-weight mesh of spongy bone tissue surrounded by a shell of compact bone tissue:
By applying various stresses to the bones (doing knuckle or fingertip push-ups, hitting hard surfaces repeatedly, etc.), martial artists cause microfractures in the spongy bone tissue which heal up as compact bone, making their bones stronger and less prone to compression fractures. Similarly, catching the shield probably delivers an impact hard enough to cause microfractures in Steve’s hand and finger bones (regular people would break their hand trying to catch it!); over time, he would build up additional compact bone tissue in his hands to aid in handling the shield and avoid more serious fractures.
The first time we see Steve using the shield in Captain America: The First Avenger, he’s not throwing it but holding it in front of him defensively like a regular shield and using a pistol as his primary weapon:
The next time, he’s switched to using the shield offensively as a blunt weapon but still doesn’t throw/catch it:
It’s not until the middle part of the Howling Commandos montage that we see Steve use his signature shield-throw-and-catch – he probably spent the intervening time practising how to wield the shield, and while doing so also conditioned his hand bones.
(Incidentally, I’ve read a number of fics mentioning Bucky’s gun calluses – mostly on his right index finger from the trigger, occasionally also on his shoulder from the rifle stock – but I’ve never read a fic mentioning Steve’s calluses, which is a crying shame. Even with his serum-fuelled healing factor, I’m sure he must have them in the palms of his hands and across the inside of his fingers from catching the shield, and maybe across his forearms from the straps or across his back from carrying the shield. I’m a sucker for details like that.)
2. Technique (*).
Secondly, it seems Steve also uses different techniques for catching his shield to avoid taking the brunt of hard impacts, depending on the shield’s forward momentum as it rebounds towards him (**):
2.1 Soft rebound (hitting flesh):
When Steve catches the shield after a soft rebound, such as when it rebounds directly off of someone’s body, he plucks it out of the air by letting it hit his palm straight on:
The low velocity (evidenced by the wobbling) indicates the shield has a reduced momentum on the return, allowing Steve to comfortably catch it even though the flight vector means Steve’s hand is taking the full force of the impact. Notice that his bent arm and bent knees (crouching slightly) at the moment of impact further serve as shock absorbers, reducing the stress on the rest of his body.
2.2 Semi-soft rebound (hitting bone/body armour):
In Captain America: The Winter Soldier’s opening sequence, we see Steve throwing the shield at an opponent in such a way that it rebounds off the man’s body (likely hitting either bone or body armour, as we saw in the previous example that the shield loses too much of its kinetic energy to rebound a second time when hitting flesh) and ricochets off a steel bulkhead before returning to Steve at a much higher velocity than in the example above:
Since the shield is travelling at a higher speed than in the previous example, it has an increased momentum (momentum = speed x mass), meaning it’ll hit Steve’s hand much harder (pressure = momentum/area/time).
However, Steve doesn’t catch the shield straight on this time, but grabs it sideways by the rim rather than letting it slam into the palm of his hand. This 1) reduces the shield’s force by applying a secondary force pulling to the side and back (since force vectors are accumulative), and 2) extends the amount of time it takes for Steve’s hand to absorb the momentum, making the shield exert a smaller pressure on his hand on impact.
2.3 Hard rebound (hitting steel/concrete):
Finally, when rebounding the shield directly off of hard, unyielding surfaces such as steel or concrete which don’t absorb much of the initial kinetic energy–
– it looks as if Steve avoids touching the rim of the shield at all, and instead reaches up underneath the shield to catch it by the leather straps:
The straps probably have enough give in them to reduce the shield’s momentum as they’re pulled, again extending the time it takes for Steve’s hand to absorb it, with the additional benefit that Steve avoids taking a direct hit from the thin vibranium edge (a small area = higher pressure); instead, the shield exerts its force through the thick straps, thereby reducing the potentially bone-breaking compressive strain on Steve’s hand bones.
In conclusion, depending on how much momentum the shield has, Steve has different ways of catching it to avoid breaking his hands.
Of course, that said, it’s fun to note how the Winter Soldier has absolutely zero shield-catching technique and just catches it straight on, stiff-bodied, outstretched arm and all (because he’s just that badass):
Considering how Steve is pushed backwards across the roof from the force of the return throw, I assume the Winter Soldier is bracing his foot against the brick parapet; otherwise, he’d probably have been catapulted clear off the roof by the shield’s momentum. 😉
(*) Please note that I don’t know if Steve’s shield-catching techniques were intended by the directors or (more likely) coincidental and I’m just reading too much into the film (I like to, though, it’s fun coming up with fan theories).
(**) I’m a physician, not a physicist, so please forgive (and preferably correct) any glaring errors regarding force, momentum, velocity, etc. 🙂
TWS is BAMF, but I suspect his spine n shoulder rang like a bell when he caught the shield as it doesn’t look like Steve pulled that throw at all.