Assumptions on Shields. If two fighters are equal, and they have 50% chance of hitting each other in a given amount of time. This is assuming this is due to footwork, no parrying, and while they are within reach of each other. Then a shield the relative size of Heavy Infantry Shields to the wielder will bring down the odds of hitting by 2/3rds. That gives shields a 33.34% or rounded off to the "benefit of the doubt" it reduces the odds of hiting each other to around 15%. This means in combat, with trained fighters who manage their effort and wait for the highest utility over time opportunity to hit there is a 15% chance they hit each other.
Where did I get the 2/3rds rule on shields? At 5'9" or 175cm the average person's movement range occupies an circular area of 2.4sq.m. or 2.8sq.yrds (height/2 radius). Then factor in stances in combat where it is typically profiles of about 2/3rds which grant me 1.6sq.m or 1.9sq.yrds. area. A scutum is roughly 1sq.m. or 1.2sq.yrds. which is roughly a 2/3rds comparative ratio.
Feel free to replace these variable assumptions.
Taking Damage. Even if they do hit each other, these fighters improve their ability to judge how to best manage their attention regarding his opponents and judgement regarding attacks. This reflects how finely they can keep maximum secondary defense. Relatively a fighter gets better in part by being able to take more punishment: when it comes to pure kinetic energy fighters learn to tense up their muscles in the "combat form". In combat with edged weapons, this is by keeping an optimum form of easy last minute changes for evasion and rolling with the blow.
In reflection the first thing I learned sparing with my brother is that despite how much I want to go for cinematic blows, small whittling attacks against the fundamental skills is best against an equally skilled fighter. All-out-Attack or Reckless attack is relies heavily in context. In a situation where I leave my defense has momentary lapsed to "kill" my opponent technically is not reckless but calculated.
Mobility is the first defense. A shield or the toughest armor is only as good until the opponent is in a position or opportunity to ignore it. AC10, GURPS dodge 8, To be hit 10 or what ever defense typically assumes mobility. Many systems assumed this this because it is regardless of type of attack: projectiles vs melee or if the character is carrying a weapon or tool for parrying. Parrying is not a factor in basic defense when the defense assumes no difference with ranged or melee attacks except for distance and size. At 40m/s (fastest baseball pitch) or 73yrd/second is pretty hard to parry. If I saw the wind up or where person aiming the bow the easiest thing to do is to move at any perpendicular direction at the moment of release.
The role of Armor in all of this. Ancient up to Modern armor is centered around the torso for a good reason. When i was a kid out of pure aesthetics I cannot imagine armor without extending to the extremities. The idealized imagery overcoming the rationality was quite natural being a kid, growing up and getting some self confidence I see aesthetics just a circumstantial bonus and not the requirement.
Torso armor typically covers the chest and abdomen. Many close-quarter weapon using martial arts rely on belly slitting or reaching the vitals from the belly. One of the unusual things I learned about "killing blows" is when using knives I have to train to make as many multiple strikes on all vitals to bring up the certainty of bringing down a threat. When I consider this and double tap, burst fire, and sword techniques that very quick rapid succession attacks: the single blows don't kill so easily. The torso is a lot of person and the body can operate even when internal systems are junk for a shock fails to kick for a few critical moments.
Armor Familiarity - I own a sparring suit for Arnis/kali and really appreciate the amount of training one needs to be good at fighting in it. Although my guro was telling me to practice more with the armor, It was much easier not to. The the lack of movement range, visibility (helmet), center of gravity, and heat was some effort to get over. Armor familiarity is assumed, i guess if the person gets 40 hours of practice one can consider it covered. In terms of performance in DnD terms -1 or 5% to -2 or 10% physical actions without familiarity.
it goes without saying that this kind of familiarity can only be inforced it players role-play or account for their character's professional conditioning.
Armor Cumbersome Myth. Dan Howard in Myarmory and many others had done much to dispel this myth. There are certain key weights all armors share. It follows this logic: if a human can easily move around with 10-14lbs on his torso, he will use material that has the best hardness to weight ratio comparative to the cost: in this case layers of cloth or leather up to metal.
Helms. The shield covers half the face behind it and the rest of the head is behind the helm. In drawing, the basic facial proportions puts eyes dead center of the head. peeking behind a shield without a helmet means half one's head is exposed. I would like to point out that the area presented by the head is just as big as those presented by the hand or feet behind a heavy infantry-man shield. Given the option to hit limbs and the head, risk times utility when it comes to head shots will always be greater if you want the opponent to cease being a threat.
As a modern people, we know that the head is the center of our nervous system and its destruction can kill us before the body even realizes. Ancient people thought the liver, heart, stomach etc. was where life exists and must be protected in order to survive. Unusually, despite their beliefs warriors wore helmets at the expense of a Cuirsass If a good shield was available.
Here are armors, locations and their typical weights:
Torso: The Chest-rib cage, shoulders, and abdomen. It ends with the waist. armors can range from 10 to 15lbs.
Groin: The waist to the edge of the crotch is the Groin area. This is the point where flexible or articulated armor is used. This typically weighs 3-5lbs.
Skirt/Fauld: From the waist to the knees. this weighs 5 to 7lbs.
Arm: the arm refers to the shoulder up to the elbow. This is around 2-3lbs
Fore-arm: this refers to the elbow to the wrist. This is around 2-4lbs.
Leg: This refers to the part of the legs from knee to the ankle.
Feet: These are called Sabattons or Sollerates. This usually protects the ankle, the toes, and the metatarsus. This weighs around 1.5 to 2lbs
Half-helm or Cap: Just the skull and sometimes along the nose bridge. 2lbs
Helm: The skull and sides of the face on the joint of the jaw. Sometimes along the nose bridge.2.5-3lbs
Faced Helm: Armor that extends to the cheek bones and much of the jaw. Sometimes along the nose bridge.
Mask: A effigy of a face or a ornate face mask. usually 1lb.
Coif: a hood of flexible armor covering most of the face and the whole head. Around 2lbs.
7150sq,cm. is the area of a long sleeved shirt for medium sized men.