Which concept best describes the effects of prolonged bed-rest?

Overload refers to applying a training stimulus greater than the body is accustomed to in order to drive positive adaptation - the opposite of what bed rest represents.

Specificity is the principle that training adaptations are specific to the mode, intensity, and muscles involved in training - it does not describe the consequences of inactivity.

Detraining refers to the partial or complete reversal of training-induced adaptations when physical activity is substantially reduced or eliminated, as occurs during prolonged bed rest. During bed rest, cardiovascular capacity declines, skeletal muscle atrophies, and metabolic efficiency deteriorates as the body loses previously gained adaptations. This principle directly and accurately describes the physiological consequences of extended inactivity.

Overreaching describes a short-term state of excessive training load without sufficient recovery, which is conceptually opposite to the inactivity of prolonged bed rest.