It is impossible to argue with someone who knows more than you do. During the intermission of a show where I was the pianist, a young man came up to me saying that he came to the show just to watch me play. He went on to disclose that he knew I had healed myself of tendonitis, an affliction which he was currently suffering from and he wanted to see me play for himself and talk to me about how I healed. The conductor was listening in and immediately chimed in with his opinion on the matter. He began by saying that he had a doctorate in piano pedagogy and trained with some names of people whom I never heard of. He opined that our injured inquirer needed to work through his pain, build endurance and do strengthening exercises to overcome his malady. He couldn't be more wrong and since the injured pianist was giving him ear, I quietly slipped away.
The truth is that it doesn't take very much strength nor endurance to play the piano. The fallacy here is that a lot of pianist feel that the keys are heavy and they need more muscle to dig into the keys. Everything we need we are born with and our everyday movement is enough to equip us with all the muscle we actually need. To play properly it is more a matter of what not to do. For instance, it doesn't require strength to depress a key, only a small amount of arm weight. When a pianist feels they need to play harder to get the keys down, that is actually a symptom of a dual muscular pull - they are using two diverse muscles to make the hand or fingers to go in two directions at the same time which makes them feel weak. The muscles are fighting one another to move the bones. A dual muscular pull will cause tension, pain and fatigue which is not an issue of endurance or strength but, poor technique and lack of knowledge.
Go to a piano and press down a key, notice that it doesn't take very much strength to make the key go down, nor a lot of weight. Notice also that after you reach the point of sound, when the note plays, the key rests on the key bed. A mistake a lot of pianists make is to play into that key bed. No matter how hard they play, once the key reaches that key bed, no amount of pressure is going to make more sound nor make playing easier. The sound has already been made. Go ahead and play a note, then press into the key bed as hard as you can. You will probably feel fatigue and pain. The solution to the fatigue and pain you are now feeling is to allow only enough weight to play the key to it's point of sound, then no more. Many educated pianists will say that it is impossible to play to the point of sound but that is because they can't do it. In that case, they are correct. It can't be done, by them.
Every motion requires an equal and opposite motion. As you sit at a keyboard, rest your hands on the keys. If your arms are totally relaxed, your hands should fall off the keys and dangle to your side. The body wasn't designed to sit in that static position but it can overcome it by forward shifting, shaping and playing with rounded motions which are all equal and opposite to playing down. Many pianists attack the instrument with brute force because they don't know what effortless playing feels like, so instead they force themselves to feel effortless with strength and endurance. What they are really doing is training the body to accept fatigue and as my doctorate friend said, build strength and endurance to fight through it. Fighting tension with tension is a no win situation. If you play using natural arm weight, you won't be using muscles to the point of fatigue by pressing into the key bed in the first place.
Let's look at body building. Many people who go to gyms and work out on machines which are only isolating certain muscles. Stand barefoot on one leg (if you can) and look down at your ankle. You should see dozens of tendons and muscles come into play in an effort to maintain balance. Chances are that you've never isolated and trained each of those individual muscles but, your everyday normal motion is enough exercise and maintain those muscles. Normal and beneficial activity incorporates many muscles at once. To exercise your ankles or legs on a machine, the machine will exercise one specific muscle and both legs at the same time because that is how these machines are designed. Exercising each body part separately but whole would be better.
If someone were to bench press with a single bar with weights on both sides, both arms will assist in the balance and pressing of the bar. If that person were to use two separate dumbbells, each arm will have to engage all the ancillary muscles to adjust and maintain balance, just like your ankle did. Going to gyms and working on those machines can be a waste of time. It would be better to work with free weights. Free weights are also more demanding of the core so it is like exercising more body parts at one time. You can't get that whole body workout on a machine designed to isolate a muscle.
Strength and endurance have little functional value in playing the piano just as weight lifting doesn't in our everyday lives. If I can bench 350 pounds but work at a desk five days a week, all that training is of no value to real life. The performance demands in our every day life consists mainly of manipulating our own body around desks and computers and pushing pencils. When is the last time you exerted yourself while writing a memo or reading a report?
Okay, working out makes you look good. That's another issue best discussed with your mental health provider.
Working out makes you feel good. Actually, getting adequate sleep, eating a proper diet and drinking plenty of water makes you feel good. I've worked out before and to be honest, I did not feel good the next day.
There are two kinds of pain; there is the kind with lactic acid build up where swelling occurs when muscles tissue is torn and the body rushes oxygenated blood to the site in order to repair it (good) and pain from strain and stress on bone, ligaments and tendons (bad). Pressing into a key bed strains the bone, ligaments and tendons.
Ligaments hold our bones together. If you bend a bone or hyper-extend a joint beyond what is normal, you can tear or stretch a ligament. This is bad since they don't grow back. This happens often to football players, basketball players and skiers because their foot and knee alignment don't line up. One goes one way, the other goes another and the ligament in between bears the brunt of the misaligned torque. A third degree tear of a ligament can only be repaired with surgery. A first and second degree tear can either be tolerated or it may "scar down" but you will lose flexibility. For a professional athlete it might be better to have a third degree tear so that a surgeon can graft a new one in its place. That happened to me and my repaired knee is now stronger than my good knee.
Tendons move our bones around. When you stress a joint to the point of stretching or tearing a tendon, this too is bad since they can take years to heal. Tendons do not have strong blood supplies going to them so they scar before they heal. A pianist with scarred tendons will feel sharp pain as they move. That is the scar tissue tearing. The good news is that this can easily be healed with massage therapy and proper technique. Proper movement promotes healing.
If you tear muscle, muscle tissue can heal overnight or in a few days as muscles have an ample supply of oxygenated blood flowing to them.
People think the more they work out the more endurance they are building. Actually that isn't true. They are actually improving their economy of motion. Movement doesn't become easier because of endurance, the body is just becoming more efficient at that particular movement. Your body is requiring less strength and oxygen than you did prior.
Have you ever watched "Dancing With The Stars" and witness professional athletes and body builders who have no endurance? They not only come off the dance floor exhausted but all that muscle robs them of flexibility and true endurance as the muscle mass is starved for oxygenated blood. The body has to move all that weight and the large muscles get in the way of joint flexibility. I've even witnessed long distance runners get winded riding a bike. Why is that? They trained and isolated specific muscles rather than full body training. The muscles used to run are different than those used to ride a bike. That is why ballet dancers can do most everything with ease. Some astute football coaches even encourage their players to take ballet. People who have trained in a certain way generally have equal and opposite weaknesses equivalent to their strengths. Pianists are no expectation. Train for strength and you will be weak. Train to the laws of physics and you will play effortless - which is not the same as "strong."
Sharp pain and fatigue are not good symptoms to have. They indicate that you are doing something wrong. Any time you feel those two symptoms you should stop and not continue until you figure out what is wrong with the movement. Otherwise permanent damage may occur. If your car is giving you problems, continuing to drive won't make the problem go away, the problem will only get worse and become more expensive to repair. Remember the old adage that an ounce of prevention is worth a pound of cure. Live it or pay later.
What about people who exercise for weight management? The simple truth is, in order to lose weight (or burn fat - which is not the same thing since exercising to build muscle can put on weight in numbers even if you lose fat. A mirror is a better judge than the scale), you need to burn off more calories than you consume. If you eat more calories than you burn off, no matter how much you exercise, you won't lose anything.
The good news is that muscle by its mere existence burns fat without you having to do anything. One pound of fat can fuel the body for up to 10 hours of continuous activity. But most people can't and shouldn't go 10 hours without eating. Beside the amount you eat, what you eat is very important. Complex carbs, protein, vegetables and lots of water will build muscle and burn calories. Sugar and simple carbs that turn to sugar will not burn fat. You're burning the sugar and storing the leftover as fat.
I'm not a doctor but I have been injured. I've also been lucky to know people without PhD's who taught me to heal myself. Some of them didn't even have high school diplomas and they could do what no doctor could. Because of them, getting injured was the best thing to ever happen to me. If you are lucky, you'll never get injured but, getting injured might save you from technical and professional mediocrity if you have the capacity to heal. Consider Gandhi, failing the bar exam saved him from a life of professional oblivion. Failing isn't so bad, it is what you do with it that makes all the difference.