This stuff about tight or imbalanced muscles sounds dubious to me. The body reacts very rapidly to the stresses placed upon it by building muscle and to prevent the limbs from curling up it builds opposing muscle. In the long term even the skeleton reacts to habitual stress by building bone where it's needed. How could a person be exercising so wrongly for so long that the body creates unbalanced muscles?
It's very common, I see it all the time.
Muscle builds as a consequence of the stress placed on it.
Hence pro cyclists have massive quadriceps and calf muscles.
But are not so strong elsewhere.
Inequality in muscle strength, and short but powerful muscles, combined with a lack of flexibility are a major cause of sports injurues.
In yoga we learn to work all the muscles equally , keep them close to the bones, and keep them long.
Things like fallen arches, and tight hamstrings can be made better by intelligent working to correct.
And extending the front edge of the spine to keep chest open for lung function is vital.
It takes time and knowledge to get right.
Just like adjusting a bike.
But given that your body is your primary piece of biking (and life) kit, it makes sense to learn, and practice this stuff.