The biceps muscle is the proximal biceps tendon that attaches to the shoulder. The proximal biceps tendon has two parts: the long head and the short head. Almost all injuries to the proximal biceps are to the long head of the tendon.
The long head of the biceps tendon attaches inside the shoulder joint, just above the socket of the shoulder joint. The tendon actually attaches to the rim of cartilage that surrounds the shoulder socket called the labrum.
Source: Verywell Health