Which of the following is NOT a muscle involved in breathing or respiration?

The quadriceps is primarily a group of muscles located in the front of the thigh that are responsible for extending the knee and playing a crucial role in activities like walking, running, and jumping. In contrast, the diaphragm, intercostals, and sternocleidomastoid muscles are directly involved in the mechanics of breathing.

The diaphragm is the primary muscle of respiration, contracting to allow air to enter the lungs during inhalation. The intercostal muscles, located between the ribs, assist in expanding and contracting the chest cavity, further facilitating breathing. The sternocleidomastoid muscle, although primarily associated with head movement, assists in inhalation when the neck is fixed, contributing to deeper breaths.

Thus, the quadriceps is the only muscle listed that does not participate in the respiratory process, making it the correct answer to identify as NOT being involved in breathing.