Hypermobile and a weak back.
This was a client of mine who is a triathlete who needed to work on her swimming. She was very strong in her running and cycling portions but could not hold herself up (kept sinking) in the swim portion. You will see here in the video that she demonstrated hypermbility in the low back. She was having trouble stabilizing and engaging the low back so that it could be the connector for the arms and the legs.
That kick board underneath her was to help “float assisted” and feel supported. It was also used to get her to “feel” the stomach come into the spine, or what I call “belly to spine.”. The other issue you will notice is the high legs. The legs are supposed to be straight behind you in a streamline position. High legs can translate into bended knees or a bicycle kick. Just because someone can run many miles does not mean that they have the posture, strength or cardiovascular endurance to swim even a sprint traiathalon distance.