Protein synthesis and energy use
The energy supply for the synthesis of protein in the body fluctuates during, and after exercise. During exercise, the supply of energy used for the synthesis of protein decreases, thus leaving the door open for increased protein degradation. Furthermore, the uptake of amino acids from the blood which normally would go into the muscles is also depressed at this time.
This means that during heavy exercise the mass of proteins within the muscles are being catabolized (turned into waste products) faster than new proteins can be synthesized. The result is the protein availability in the muscle automatically decreases after a workout while the amount of catabolites (waste products) increases.
After a workout, the amino acid uptake is higher than at normal resting states with the resultant Supercompensation and above normal saturation of protein in the muscle tissues.
Science and practice of strength training, Vladimir M. Zatsiorsky, Human Kinetics 1995