Skeletal Muscles have several essential roles-

1.movement (of limbs, torso etc.).

2. stabilization of tendons, ligaments and joints.

3. facilitation of partially controlled functions such as swallowing, breathing etc.


It is important to understand those physiological roles in order to understand what happens when there is muscle dysfunction, and to prevent secondary damage.

Impairment of movement is the easiest to understand.

But, muscle weakness also leads to strain on tendons and joints. This will take more time to be apparent and hence harder to connect with the weakness of the muscles. Sitting in a chair which is not well supported will lead to pain in the back and the neck due to strain on the vertebrae. walking when the muscles are weak can lead to strain on the knee and hip, even picking up something can lead to pain in the shoulder etc.
joints and ligaments have pain receptors because this is the way we know we have over-strained them. This kind of pain means-stop, improve your muscle strength or get better support before you go on.

Using a wheelchair (like you would use a car) to travel a longer distance, enables you to get there, while preserving your strength and avoiding putting too much strain on your body.
A healthy person (even if he/she is a trained athlete) who has a 30 minute drive to get to work, will quite likely use a car to get there and not run to work every morning, even if he/she can physically do it.
By the same token it makes no sense to struggle walking 100 meters and get there hardly able to move talk of breath, even if you can physically do it.

Because swallowing and breathing are only partially under our control and because both require co-ordination of multiple muscles, people do not always realize that they have such difficulties.

People may just feel they have had enough to eat, when their chewing and swallowing muscles get weak, or that they are tired, irritated, foggy, when their breathing muscles get weak.

The muscles in our limbs have sensors that tell us their position, their force etc. We do not have similar sensors in our swallowing and respiratory muscles.

Finally, pain in the muscles is usually the result of production of lactate and inflammation when the muscle has reached its threshold. It is not surprising that when you reach this threshold with "normal" everyday activities, you would have such pain in your muscles that a normal person would have after a full work-out in the gym.

All that being said, there are some muscle diseases in which pain is a prominent feature, and not the result of weakness. In those diseases you would usually (but not always) see elevated levels of muscle enzymes (such as CPK) in the serum, because they are accompanied by muscle damage and not only metabolic dysfunction.

I personally think there are many disease that can effect muscle function and endurance, which are not (yet) recognized. I believe many of those patients are blown off as suffering from " emotional problems" because those diseases are very hard to understand, and it is easy to think that such a patient is not putting enough effort and can easily do more.