Intrinsic muscles of the back that promote extension of the spine (or head) include all but:
a. Splenius muscles
b. Semispinalis muscles
c. Scalene muscles
d. Erector spinae

Verified step by step guidance

1. Step 1: Understand the function of intrinsic back muscles. These muscles are primarily responsible for movements such as extension, rotation, and lateral flexion of the spine and head.
2. Step 2: Identify the muscles listed in the options and classify them as intrinsic or extrinsic muscles of the back. The splenius muscles, semispinalis muscles, and erector spinae are intrinsic muscles involved in extension of the spine or head.
3. Step 3: Recognize that the scalene muscles are located in the lateral neck region and primarily function in flexion and lateral bending of the neck, as well as assisting in respiration. They are not intrinsic back muscles and do not promote extension of the spine or head.
4. Step 4: Conclude that the scalene muscles (option c) are the exception among the listed muscles because they do not promote extension of the spine or head.
5. Step 5: Summarize that the correct answer is the muscle group that does not belong to the intrinsic back muscles promoting extension, which is the scalene muscles.

Key Concepts

Intrinsic Muscles of the Back

Intrinsic back muscles are those located deep within the back and primarily responsible for movements and stabilization of the vertebral column. They include groups like the erector spinae, splenius, and semispinalis muscles, which help extend, rotate, and laterally flex the spine and head.

Muscle Actions: Extension of the Spine

Extension of the spine involves straightening or bending the spine backward. Muscles that promote extension contract to increase the angle between vertebral segments, helping maintain posture and allowing movements such as looking upward or standing upright.

Scalene Muscles and Their Function

Scalene muscles are located in the lateral neck region and primarily function to elevate the first two ribs during respiration and assist in lateral flexion of the neck. They are not intrinsic back muscles and do not contribute to spinal extension.