Which muscle is a weak/phasic muscle in Upper Cross Syndrome?

• A. Middle trapezius
• B. Rectus abdominis
• C. Pectoralis minor
• D. Sternocleidomastoid

Answer: (A)

In Upper Cross Syndrome, there’s an imbalance where scapular retractors and deep neck flexors become weak, while chest and neck extensors become tight. The middle part of the trapezius functions as a key scapular retractor, pulling the shoulder blades toward the spine and helping stabilize the scapulothoracic joint during arm movements. In this posture pattern, that muscle is underactive or inhibited, so it’s a weak/phasic muscle. This weakness contributes to reduced scapular retraction and poorer shoulder stability, reinforcing the rounded shoulder posture. Strengthening the middle trapezius (and pairing it with the lower trapezius and serratus anterior) helps restore proper scapular rhythm and posture.

Pectoralis minor and sternocleidomastoid are typically tight and overactive in UCS, driving protraction and forward head posture, not weakness. Rectus abdominis is not a primary muscle described as weak in this syndrome, so it doesn’t fit the classic UCS pattern.