Which muscle is weak (inhibited) in upper cross syndrome?

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In upper cross syndrome, the pattern is an imbalance where deep neck stabilizers are inhibited while certain superficial or postural muscles are overactive. The deep cervical flexors—those small, deep muscles that pull the head into a neutral position and stabilize the upper spine—become lengthened and neurologically inhibited when the head sits forward. This loss of activation reduces their ability to control neck movement and maintain proper alignment, which is a defining feature of the syndrome.

Meanwhile, muscles like the pectoralis major and the upper back/neck muscles (such as levator scapulae and the suboccipitals) tend to be tight and overactive, contributing to the forward head posture and rounded shoulders. That combination of overactivity in some muscles and underactivity in the deep cervical flexors is what produces the characteristic imbalance.

So the muscle that is weak (inhibited) in this pattern is the cervical flexors.