A few episodes back in the HBO version of A Game of Thrones, there was a brief scene of Daenerys getting off her first long ride on a horse. Her three servants had to help her down and all but carry her to her tent. Scott turned to me and smirked. "You know that feeling, huh?"
Strangely enough, not many new writers do.
The truth: Riding a horse is hard work!
I ride 1-2 times a week and I've worked my stamina up to a little over an hour in the saddle. Still, if I'm doing a lot of trotting or climbing hills, I'm reaching for ibuprofen the next day. And, usually, I'm wobbly for a good 10-15 minutes after dismounting.
Because, you don't just sit on a horse. If you did that, you'd fall off. Or hurt yourself, or hurt your horse… which brings us back to you eventually leaving your saddle unintentionally.
With a saddle
Your feet go in the stirrups with the ball of your foot on the stirrup, but your heels actually hold your weight and balance. That means your calves are getting a good stretch. You stabilize yourself with your knees, especially if you're leaning forward while going up a hill or holding yourself up for a trot. Every muscle around your thigh should also be helping you keep your balance. Your glutes and abs adjust to maintain your center of balance. If you've got a particularly spirited or stubborn horse, your arms and wrists are getting one helluva workout while steering and turning. If you're stressed, your neck and shoulders are going to tense and ache. (While that's not good riding, it's a common occurrence.) Now, if you're female and not wearing proper support… which, per most fantasy cover art seems to be pretty common, your whole chest is just burning in pain, and that affects your pecs, shoulders, neck, and back. If you're a male and you're not sitting properly, using your thigh muscles and glutes to protect your manhood, well, you're in a whole lot of other hurt.
Anyone learning to ride should expect a lot of pain in the legs and abs from working them. Also, anyone learning to ride ought to expect a lot of pain from strains and poor posture (that we hope, eventually, gets fixed.)
A saddle better distributes weight through the lower body and, if well made, helps ease muscle strain. It can also offer a certain amount of psychological support since new riders are often scared to actually grip a horse's mane no matter how many times they're told it doesn't hurt the horse.
Saddles also offer some extra potential injuries. If you get thrown or fall and can't get your foot out of the stirrup, you get dragged and/or kicked. Saddles can slide - forwards, backwards, and side to side. This will scare a horse and totally mess with your own balance. Imagine your horse breaking into a canter and - oops! - you didn't tighten your girth enough (another newbie mistake) and the saddle, with you in it, floops right to your poor horses belly!
Without a Saddle
So, your character hops on a horse without a saddle. He or she better have especially strong thighs or it's take-off and hard landing time. All the grip has to be in the thighs and core muscles to stay on, even at a walk. You feel every move of the horse, every turn, and there's nothing for you to brace against, like stirrups, when the horse changes gears - either faster or slower. Balance is key.
On top of that, it can be more painful for the horse because there's nothing distributing the character's weight over a larger area. The pressure is right on the spine, so it's more work for the horse to keep her balance and move. A horse laden with armor and a heavy saddle can be better balanced with a heavy rider than a somewhat heavy rider - especially a newbie who may not have his or her balance, so is bouncing on the spine - going bareback.
The Inevitable Stomp and Fall
Starting on the ground: It is an Indisputable Truth that if you spend copious amounts of time around horses, you WILL get stepped on. On purpose, accidentally… but likely both.
I've lost count of times I've been stepped on. In those times, I can honestly say the best foot protection is modern hiking boots with a reinforced - but NOT metal - toe. Or, of course, proper riding boots with reinforced toes.
A note on that metal bit… if you've got a knight in full mail with metal boots of some sort… a stomp on that metal will actually do more damage. Why? The metal will dent, if not bend or split, to the hoof, and it can cut off circulation to the toes. Unless someone actually cuts away that metal bit very quickly, your character is in deep manure. I've heard of many horror stories of steel-toe boots leading to lost toes or substantial foot and nerve injuries.
Well-made leather boots are your character's best safety measure on the stomping, but it will happen, and your character will likely limp for a few hours if it was an accident, a day or so if it was on purpose.
The second Indisputable Truth about horses is that you WILL fall off. Master horsemen and horsewomen fall off. In fact, they've lost track of when they lost count of how many falls they've taken.
A fall off a horse is close to the equivalent of a low-to-mid-speed car accident. Actually being thrown from a royally pissed off horse can be as much damage as a car accident at 50-70 miles per hour.
And if you're writing high fantasy, unless you're in armor of some sort, remove all those fun safety measures that cars have.
Assume there is a certain degree of whiplash and at least a bruised rib or two, if not a cracked rib or two. If your character's not wearing a helmet, there's potential for a concussion - or at least a good hard goose-egg and some dizzy vision. There's a high possibility of bruised hip-bones, sprained or strained ankles and/or wrists. We're not even getting into broken bones, or worse, broken backs/necks or immediate death from head injuries
The funny thing about most horse fall injuries, though? Unless they are serious, you don't feel the pain of them right away. That's why most people can get right back on a horse after a fall… you miss the part where they can hardly get out of bed the very next day. As soon as the body realizes it's falling or is getting thrown, adrenaline rushes in and masks the pain. Also, once you realize you can stand and aren't actually dead or have a major broken bone (if you're lucky), there's also a huge feeling of relief and euphoria - I just survived that!
When you're planning to have characters ride horses, be prepared for the pain if you want to be realistic. While a fall or an injury - or even general wear and tear - might be a bit inconvenient to your Save-the-World plot, it makes for good character development and can be used as a good plot device when you need a character to be a bit weaker.
Happy Riding & Writing!