In breathing the chest muscles expand the rib cage and allow air to be sucked into the lungs.
呼吸时胸部肌肉使胸廓扩大让空气吸入肺部。
《牛津词典》
2
When you have filled and emptied your lower lungs 8 to 10 times, add the second step to your breathing: Inhale first into your lower lungs as before, and then continue inhaling into your upper chest.
Instead of taking your usual big inhalation, take a smaller inhalation and then immediately relax all muscles, especially upper chest and all other breathing muscles.