It’s likely that during (and after) a SHTF scenario, the physical demand on the human body will drastically increase. Our bodies would suddenly become responsible for filling the productivity gap left from the wake of downed mechanical systems that significantly reduces our workload in providing for our every little need, with little effort. Life has become so easy most of us pay little notice the highly productive and efficient grid network built up around us and is working around the clock.
We are all guilty of doing less physical work in the modern era than our ancestors, to the point that sedentary lifestyles are not only becoming the norm, but are on the rise and have been classified as modern day health epidemic. If many people were abruptly forced with no choice into a mode of high physical stress for survival reasons, many may not survive simply because their bodies are not accustomed to the level of physical demands required in a hypothetical emergency survival situations.