It often recommended to brine salmon with salt before sous vide, so it's firmer. On the other hand, salt tenderizes meat. What is the explanation of this opposite behavior.
Excellent question. The salt, through osmosis, takes humidity out of the meat. Less moisture leads to a denser product.
The main structural component of the muscle fibers in meat is myofibril, which is itself composed of thick and thin filaments. Higher-than-normal levels of salt cause these filaments to swell and separate from one another and depolymerize, or break down. This process makes meat more tender.
For tenderization, salt is usually added to meat via a brine solution comprised of salt, water, and sometimes other flavoring compounds. Harold McGee describes the effect of brining in his book "On Food and Cooking"
I hope that answers your question.
Edit I only answered half the question.
Link to a more detailed explanation about osmosis.