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Expiration: The process in expiration won't involve muscles at all, it's going to be a passive process where it doesn't require extra muscle for air to exit, to be pushed out, for rib cage to go back to normal.

In expiration passive/ tidal expiration we have a grouping called elastic recoil forces, this combination of three forces coming together will push air out of the lungs by decreasing the chest wall space, decreasing the rib cage volume, pushing air out but there's no muscle active contraction involved.

The first piece that contributes to passive exhalation is the pull of gravity. That in combination of muscles are relaxed, diaphragm will relax to its normal domed position, that will push out and decrease rib cage volume. The diaphragm relaxes, the external intercostals will relax contraction. That will decrease and push down the rib cage to resting point.

Respiratory will collapse back to its original position. Inside of that space, pressure is starting to increase and that will push air out and you'll have an exhalation. Air will flow out, lungs will return to resting volume.

You go from an extended volume, with combination of gravity, muscle relaxation and the last piece in elastic recoil is tissue. Tissue has an elastic component that wants to get back to its original shape. Tissues will return to normal, the muscles will relax and air will be pushed out by pressure changes. Pressure will increase, wants to get the same as the same atmospheric pressure and push air out of the lung.

In tidal breathing there is no muscle involvement in expiration. The combo of these forces will help push air out of the lungs.

Active expiration: where there is speaking involved, you need an increase volume of air, you breathe faster and need more control over expiration then certain muscles come into play.

Active expiration will involve more muscle control.

One situation is when you want to push out more air beyond that resting level. To do so we need more...