What you can do in the gym, well-rested, lightly attired and unstressed, is a far cry from the potential conditions of exertion when you are tired, hungry, wet and cold, after sucking dust, diesel fumes and pollen for days in the field, with little sleep and under the pressure of assessment, or worse, having to consider the possibly lethal results of poor decision-making. Imagine trying to do it in the continued presence of every trigger for the condition you can imagine being present.
Mike