A final problem is what’s being measured. Athletes want to know what will maximizes their performance, strength, power, speed, throwing, etc. Researchers invariably measure stuff of less relevance to athletes and coaches. Nitrogen balance, amino acid uptake, sometimes actual muscle growth is measured over the length of the study. Is the amount of protein needed to optimize performance different than what’s needed to maximize some aspect of muscular physiology?
An added issue is that solely looking at skeletal muscle may be missing pathways of importance to athletes. Immune system, connective tissue synthesis and a host of other pathways use amino acids; presumably athletes will upregulate those pathways. Meaning that true protein requirements, if you only look at what’s going on in the muscle, may be under-estimating what athletes truly need to maximize every aspect of performance.