Which muscle or muscles are the most important used in inspiration and inhalation?

Inspiration

* Primary muscles: Diaphragm, intercostal muscles (external intercostals and internal intercostals)

* Accessory muscles: Scalenes, sternocleidomastoid, pectoralis major, serratus anterior, latissimus dorsi

Expiration

* Primary muscles: Internal intercostals, abdominal muscles (rectus abdominis and external obliques)

* Accessory muscles: Diaphragm, scalenes, sternocleidomastoid