noun
inner muscles; deep muscles
Anatomy and fitness term for muscles located deep inside the body, such as the transversus abdominis or multifidus, often contrasted with superficial muscles.
インナーマッスルを鍛えると姿勢が良くなる。
Training your inner muscles improves your posture.
ピラティスはインナーマッスルに効く
Pilates is said to work the deep muscles.