Rounded shoulder posture, also known as kyphosis or "hunchback," is a condition characterized by the forward rounding of the shoulders and an exaggerated curvature of the upper back. Several muscles are affected by this posture, primarily those that are responsible for shoulder movement and stabilization, as well as those that support the spine. Some of the key muscles affected include:

Pectoralis Major: This muscle, located in the chest, becomes tight and shortened in rounded shoulder posture, contributing to the inward rotation of the shoulders.
Pectoralis Minor: Another chest muscle, the pectoralis minor, can also become tight in rounded shoulder posture, further pulling the shoulders forward.
Serratus Anterior: This muscle is located on the sides of the chest and wraps around the ribcage. Weakness or tightness in the serratus anterior can contribute to rounded shoulder posture.
Rhomboids: The rhomboid muscles, which are located between the shoulder blades, become lengthened and weakened in rounded shoulder posture due to the forward positioning of the shoulders.
Trapezius: The trapezius muscle, which spans the upper back and neck, can become overstretched and weakened in individuals with rounded shoulder posture.
Levator Scapulae: This muscle, located at the side and back of the neck, can become tight and contribute to the elevation of the shoulders in rounded shoulder posture.
As for the effects of rounded shoulder posture on the spine, it can lead to increased thoracic kyphosis, which is the excessive curvature of the upper spine. This can potentially cause issues such as:

Increased Strain on Intervertebral Discs: The exaggerated curvature of the spine can increase pressure on the intervertebral discs, potentially leading to disc degeneration or herniation.
Muscle Imbalances and Weakness: Rounded shoulder posture can lead to imbalances in the muscles supporting the spine, contributing to weakness in some areas and tightness in others.
Reduced Range of Motion: The forward positioning of the shoulders can restrict shoulder mobility and contribute to stiffness and discomfort.