First let's get our semantics straight. This list is about trends, not fads. The difference being that a fad is a phenomenon that blows up quickly, lasts only for a short time, then, fades away. A trend is something that gains momentum and can create long-term impact in the way people think or behave. In other words, a trend is a "movement."
The truth is it doesn't take long for the body to start losing strength, endurance and aerobic power and anaerobic capacity. In just days or weeks some of these qualities can start to decline. The adage "use it or lose it" has more scientific relevance than you may think. You work out hard and the body adapts by getting fitter, you stop and that hard work starts to deteriorate.