Carrier's constraint is the observation that air-breathing vertebrates which have two lungs and and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because: Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... The heart and lungs (from an older edition of Grays Anatomy) The lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. ... In a general sense, locomotion simply means active movement or travel, applying not just to biological individuals. ...

• the sideways flexing expands one lung and compresses the other.
• this shunts stale air from lung to lung instead of expelling it completely to make room for fresh air.

Consequences of Carrier's constraint

Lizards move in short bursts, with long pauses for breath.

Ways of avoiding Carrier's constraint

Sea snakes have only one lung.

Birds have erect limbs and rigid bodies, and therefore do not flex sideways when moving. In addition many of them have a mechanism which pumps both lungs simultaneously when the birds rock their hips.

Mammals have erect limbs and flexible bodies, which makes their bodies flex vertically when moving fast. This aids breathing as it expands and compresses both lungs simultaneously.

References

Carrier, DR (1987). The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. Paleobiology 13: 326-341