Hi Xue,
Great images.
What brand is that machine?
The lateral part of the biceps muscle bellies is the long head component.
There are a few options.
I know how good you are so it is not going to be anisotropy in your hands. The long and short head bellies can be really anisotropic with one going white while the other goes black really dramatically and then with a small adjustment of transducer angle it reverses. This is especially apparent in younger fit people. But this is not the case in your example.
The answer I think is an old long head of biceps tendon origin rupture. This may have happened ages ago. The vinculum will likely be intact and hence there is not a “popeye” muscle deformity. If you look on the bicipital groove you may see a tendon, however when you look over the humeral head in the rotator cuff interval it will be absent.
The atrophy in your case is typical of this type of old injury.
The patient demographic also supports this diagnosis. So does the lack of current symptoms. The patient will have some relative supination strength compared to the other side, but this might not be apparent to the patient. Elbow flexion strength will be preserved as this is provided by the healthy short head of biceps component.
Other options for this appearance on ultrasound which I think are not the case in this example are:
DOMS overuse injury to the long head muscle belly
Isolated denervation from a Parsonage Turner (Neuralgic Amyotrophy) event.
But I think in your case it is a chronic long head of biceps rupture from the superior labrum / glenoid with preservation of the vinculum.
I always love your work Xue,
Steve