I don't know that a historian can address this because your premise is mistaken. Energy is the thing you're overlooking. Muzzle loaders fired large and heavy projectiles at low velocity.

The British "Brown Bess" .75 caliber muzzle loader (with .69 caliber ball) fired a 740 grain bullet at or near 1,000 feet per second. That comes out to 1,650 foot pounds of energy.

By comparison a WWII era M1 Garand fired a 152 grain bullet at 2,800 feet per second which is 2,650 foot pounds of energy.

What that means is that a .30 caliber rifle firing a cartridge with smokeless powder has much more recoil than a black powder musket had if both rifles weigh the same. And modern rifles generally are slightly lighter than a muzzle loader so the felt recoil of the modern rifle will be slightly higher.

I cannot answer this question directly--I do not know what methods they used to keep bruising down. I only speak from the experience of live and blank firing 18th century muskets for my job.

Does it recoil? Yes. Does it hurt? Eh? You have it braced properly, and in reg coat and layers of uniform you have a decent padding. You are more pained from having to touch an incredibly hot barrel to reload, and your equipment, or if you are firing without a flash guard, the flash being rather hot on the face.

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Some caveats:

• We use fewer grains of powder than they probably did in the American revolution, since we aren't particularly concerned about the ball hitting an enemy.
• We also don't rapid fire, but in many cases soldiers were not rapid firing either.
• We don't stack ranks (the line behind the front resting their barrel on the shoulder of the front).
• I'm a modern human who finds discomforts in 18th century uniform/clothing/experience that a regimental soldier would not have felt.

I want to quickly nitpick some of the other responses here. While it's true that smoothbore muskets had a "relatively" low muzzle velocity, the earliest measurements made by Benjamin Robbin's new ballistic pendulum in the mid 18th century showed the bullet fired by a .75 caliber musket to be moving at 1700 fps just several feet from the barrel. Later experiments by Charles Hutton in England and Mordecai in the United states, even as the quality of powder increased, continued to aim for about 1400-1700 fps with the standard "war charge" of a smoothbore musket. Still not as high as many modern rifles, but early muskets for the most part would definitely try to push muzzle velocity to the limit of what most soldiers could endure in order to increase their point blank range and penetration. Ironically, it was the introduction rifles firing the minie bullet that lead to their standard muzzle velocity being lowered to just 900-1000 fps, partly because the elongated bullets tended to be much heavier than round balls, and partly because higher velocities tended cause the bullet's lead "skirt" to simply tear off rather than engage the rifling properly.

As a quick aside, check out this previous post which gives a rough overview of how muskets changed over time and the difference between a musket and an arquebus.

Anyways, on the subject of recoil. Going back to the late 16th century when the term "musket" referred specifically to a very large, long-barreled weapon which could weigh up to 20 lbs or more, required a forked rest to shoot, and often fired a bullet weigh as much as 2 ounces with a similar weight of powder, i want to start with a passage written by Sir Roger Williams in 1590 where he's advocating switching most gunmen from the arquebus/caliver to the more powerful "musket" like the Spanish were doing:

Touching their often discharging, nimblenes & profit, I answered before. For recoyling there is no hurt, if they bee streight stocked after the Spanish manner. For their weight and sure shooting, the Muskets haue aduantage on all the other small shot, by reason they shoote in their rests: true it is, were they stocked crooked after the French manner to be discharged on the breast, fewe or none could abide their recoyling, by reason of their great charges of powder: but being discharged from the shoulder after the Spanish manner, with the thumbe be∣twixt the stocke and the face, there is neither danger nor hurt, if the shooter haue any discretion; especiallie not to ouerload their pecces, and take heede that the bullets ioyne close to the powder.

So to start with, we see that recoil suppression technology was involved, starting with the growing popularity of the "straight" stock designed to brace against the shooter's shoulder along with a more appropriate grip. In fact, you see many of these early firearm stocks start to include a small notch where the shooter can place his thumb and get a good grip on the weapon. Obviously you don't want to injure your thumb either, but it helps distribute some of the weapon's recoil so that you don't have the entire kicking into your shoulder.

This wasn't the first time that straight stocks were invented, but during the second half of the 16th century, the most popular design had ended up becoming the curved "French" stock generally meant to be braced against the shooter's cheek or chest and often shown being carried by even Spanish soldiers. Proponents apparently considered this design a bit more convenient to carry around, quicker to put on target, and a bit easier to aim with, with some writers even continuing to recommend that crooked stocks continue to be used by light skirmishers armed with the less powerful arquebuses or calivers, but by the end of the century, the design was quickly falling out use in favor of the improved recoil of the shoulder stock.

Anyways, there are a number of other properties that those designing early firearms could consider when it came to improving the recoil of the weapon.

There was the shear weight of the weapon, which Williams mentions briefly. If the weight of the weapon is increased while the force of the shot stays the same, then it will recoil backwards more slowly and consequently be easier to control. So long as your arms aren't already worn out from lugging the thing around anyways.

Then there's the length of the barrel and using the correct gunpowder. While we can say that the quality of gunpowder "improved" over time in the general sense, there was no one universal "best" gunpower. Rather powder ideally would have to be matched to a particular gun based on it's barrel length, caliber, and projectile weight. The burn rate would then be altered using either a different recipe or different sized grains so that it finishes burning right when the projectile leaves the barrel. This not only maximizes efficiency by giving you more energy out of less powder while putting the least amount of stress on the weapon, it also means that you can reduce recoil's force by using a longer barrel and a slower acceleration to achieve the same final velocity. In theory, it would have been possible to make a much smaller, short barreled carbine which fired the same sized bullet with the same velocity of a musket by using a much finer, quicker burning powder in a greater quantity to accelerate the bullet much more quickly, but the recoil would feel far more extreme and it would be much more likely for the barrel to burst unless heavily reinforced.

Related to the previous two points, the weapons caliber could also greatly change the amount of recoil produced. This might seem obvious, but the quirk here is that the overall damage/penetration that a bullet is capable of tends to be more closely related to its overall kinetic energy (which has the formula 0.5mv^2 ) while the amount of recoil transferred by the bullet to the weapon and the shooter has more to do with the bullet's momentum (expressed simply as m*v), in one formula velocity is squared while in the other one, it isn't. If we have two guns, one which fires a 20 gram bullet at 200 meters per second and one which fires a 10 gram bullet at 400 meters per second in the same amount of time, then both weapons are going to have a very similar recoil while the muzzle energy actually calculates out to 400 Joules for the first weapon's bullet and 800 Joules for the second weapon's bullet. This is likely why handheld arquebuses in the early 1500s were often being made with such small bores, sometimes as small as half an inch, compared to many later weapons. Accelerating a small bullet to very high velocities would tend to produce less recoil in addition to being much easier and safer given the trial and error nature of the technology at the time. Smaller caliber weapons may have also been a bit more efficient at penetrating armor at close range. A small projectile generally requires less energy overall to penetrate a metal plate since it makes a smaller hole, though against soft targets a small bullet would be more likely to over-penetrate and pass clean through the target without doing much damage in addition to the fact that a small, light, round bullet will tend to lose velocity due to air resistance much more quickly over time than a larger one. At the start of the 17th century, Maurice of Nassau instructed that cavalry's pistols should be made with barrels 26 inches long and have a bore made to fire bullets weighing 36 per pound of lead (36 gauge). This was when the pistol was supposed to serve as a horseman's dedicated anti-armor weapon instead of a mace and meant to be fired only at a very short range, sometimes described as the barrel of the gun literally touching the opponent's armor. By the English Civil war when complete armor had become much less common, the specifications for cavalry pistols had reduced to just an 18 inch barrel bored 20 to the pound. Then in the 18th-early 19th century you start to see much more pistols with even shorter barrels sometimes bored large enough to fit a standard musket bullet, but would fire it at a much lower velocity.

Lastly there's the fact that muskets just didn't fire as often as most modern weapons and would have likely given the shooter's shoulder much more time to rest in between shots. At the end of the 18th century there were supposedly prussian musketeers who figured out how to fire up to 5-6 shots per minute, but the average rate of fire on the battlefield likely rarely ever exceeded more than 1-2 shots per minute. And perhaps an even bigger concern than a sore shoulder might would probably be the the fact that repeated firing could rather quickly cause the weapon's barrel to heat up to the point where the soldier could no longer touch it. Especially in the 16th-17th century at this point a musketeer would ideally be marched back to the rear of the army to let his weapon cool down, rest, and resupply himself with ammunition while another squadron took his place to continue the skirmish. If it was necessary to keep firing for a long period of time it was sometimes reccomended to switch to using only half as much gunpowder per shot once the weapon started to overheat so that the barrel wouldn't explode. This would have likely helped quite a bit to reduce the recoil as well.

First of all, the premise of your question is a bit flawed as other users have already pointed out. Due to the slower burn of black powder and overall lower muzzle energy of the typical black powder firearm, the recoil would be lower than that of a modern gun.

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However, you do have a point in that, up until the adoption of lower-powered intermediate cartridges, the recoil from service rifles was generally something that'd be too much for many shooters today. However, it wasn't as much of an issue as one may think for a variety of reasons. A lot of it is just basic physics. Most of these service rifles were designed for traditional line warfare, where units would be firing volleys in formations several ranks deep. Although for the first generation of smokeless powder rifles (think the typical service rifle in WW1), this obviously didn't end up reflecting the reality of warfare, the thinking did play a part in the design of many of these rifles. The most obvious design element we get from this thinking was the length of those first-generation service rifles - something intended to give the rifleman a gun long enough to reach past several ranks when firing, as well as a makeshift spear when a bayonet is mounted. These factors had the side effect of adding weight and generally having shooters firing from positions where recoil is easier to manage. The weight is straightforward - basic principles of physics mean that a heavier gun is going to have less felt recoil than a lighter one. That's a factor we can see both historically with some of the more common complaints about cavalry carbines and in more recent times with the significantly lighter weight of sporting shotguns and rifles. We can also see it in other classes of weaponry, as automatic rifles and light machineguns tended to be significantly more heavy than service rifles to make it more controllable and bearable for the shooter in sustained fire.

As for positioning, it's most easily understood in practice by just shooting a rifle standing versus on a bench, but it's essentially that shooting a rifle offhand - standing and unsupported - tends to cause recoil to be better spread through the body than when the rifle is supported by something other than the shooter. Along a similar vein, holding the rifle properly goes a long way to reducing felt recoil as well. Properly holding the butt tight to the correct part of your shoulder significantly reduces recoil, and, on the other side of things, recoil can be incredibly painful even in lighter cartridges if the gun is held improperly. Obviously, recruits were going to have this drilled fairly heavily to ensure that they've got the correct stance to properly aim and fire their rifles without injury.