Gymnastics is the most exhausting both, physically and psychologically. First of all physically because of all the preparation and strength you need. I am not saying other sports are hard but gymnastics is over all of them because of how you need to lift your body and then staying on uncomfortable positions that prove your strength. Now it comes the psychological training, everyday you go to train and you only got 2 or 3 meets in the year, to stay motivated it should be by your own because even your coach seems to be against you: he always shout and you and tell you how bad you are. "Gymnastics tells you no. All day long. It makes you fall you over and over again. Telling you you're out of your mind. That you're crazy. If you like running full-speed towards a stationary object, vault's for you. If you like pealing pieces of skin the size of quarters of your hands... bars is for you. Because the only thing more fun then rips, is when your rips get rips. It's super sexy. And floor, are you serious, I mean who doesn't want to parade around in a leotard getting wedgies and doing dorky choreography? It's delicious. If you like falling, then gymnastics is the sport for you! You get to fall on your face, your butt, your back, your knees, and your pride! It's a good thing I didn't like falling... I LOVED IT!" (Stick it, 2006)
Gymnastics is not the gardest and more exhausting psicollogycally and physically, the one that is truly the hardest is cycling, because you have to train hundreds of kilometers a day, just to race some few, also this is very exhausting on the psicological side because you have to spend hours on the bike and also, it is very exhausting phisically because you are riding for several hours on a race, and at the end you have to push yourself to your limits so you can win the other persons, it"s so hard that your legs even get cramps. Also the injuries you suffer are more painful than on gymnastics because if you fell of your bike you fall at a speed higher than 30 kilometers per hour and you fall on rocks or if you are lucky on hard concrete. ;D
If we are talking about injuries you got no idea about how painful the injures from gymnastics are, the cyclist injuries are maybe some rips, but gymnasts get rips everyday in each training because of the friction with the bar. The real injuries are broken bones, muscles rip and bruises that are of the size of a tennis ball, and all this are common injuries, I am not even talking about what happens when you seriously fall, you can even die. In gymnastics you do double back flips, triple twists, and all this exercises can kill, like it happened to a young girl who was training a double twisting double back dismount that fell on her head and lost conscious, minutes later she passed out. We are talking about serious stuff in gymnastics, that is why we use safety equipment, but even with it you are in danger.
If you are talking about diyng, sure the cicling is worse because you fall at high speeds and on races there are people that are behind you so if you fall they pass over you so you can die or be severly injuried, on the case of broken bones, you can breac a bone even without falling because on some races there are ramps so if you land wrong you can break your arms.On the training part, you train indoors, but we need to train outside on the hardest conditions, in every moment we can be run over by a car or even get sick from the cold or rain, your training is more explosive for short moments so this means that you just do some effort for a little time, but here you have to train many hours to race more hours and you always have to give your 100 and think wisely because you have to save some energy for the end