Problem description:
Once, when you were sitting and expanding your chest, you heard something like pectoralis major (probably Smith's voice). Is this normal?
Analysis:
Theoretically speaking, it is impossible for muscle tissue to make sound, but I have encountered this situation before. As an action of expanding chest and waving arms outward, some unnatural sounds may be made.
Because there is no reasonable explanation, I personally think that this is a kind of vibration inside the chest cavity: the sounding area is roughly below the clavicle, which is the starting position of the chest cavity and the upper margin of the lung, and the cavity in this area can sound due to the drastic volume change.
Pathologically, there is a saying that chest vocalization seems to be a manifestation of pneumonia. The principle is that the contraction and expansion of the chest are abnormal because the muscle elasticity is insufficient.
There is also an explanation about the singing method of * * * in vocal music. I am not familiar with it, so I won't say much.
But one thing is visible; Below the clavicle, there is a cavity in the deep layer of the pectoral muscle (large and small), which can make sound because of vibration or drastic volume change. The landlord only caused this sound because of a "bigger" chest expansion action. Obviously, stretching in the normal range will not produce this sound. Because of the pain caused by this action, you may mistake it for a muscle tear.
In addition, as far as I know, muscle tearing is silent, at least you can judge that your muscles are not torn because of this sound.
I wish the landlord a successful training.