Final answer:
Overtraining and depressed muscle protein synthesis have not been proposed as mechanisms for why concurrent training impedes strength development.
Step-by-step explanation:
The mechanisms that have not been proposed to explain why concurrent training impedes strength development are overtraining and depressed muscle protein synthesis. Overtraining refers to excessive training without sufficient recovery time, which can lead to decreased strength development. Depressed muscle protein synthesis is when the body is unable to effectively build new proteins in the muscles, which can hinder strength development. The other proposed mechanisms for decreased strength development during concurrent training include neural factors, such as interference between endurance and strength adaptations.