Regarding stress, during the 1960's the stress hormones of US Army Special Forces A-Teams ("Green Berets") in combat in the highlands of Vietnam were being compared to the stress hormones of BUDS/SEAL (Basic Underwater Demolition School/SEAL) candidates in San Diego by a team of researchers. What they discovered was that hormone levels such as cortisol were higher in the wanna-be SEALS in training than they were in the Green Berets - even during combat! (How would you have liked to have been a phlebotomist in that study?
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Blood analysis and psychological studies of the participants noted that the stress levels of the Green Berets were greatest prior to an expected attack. The greatest concern for the combatants was the fear that they would somehow make mistakes that could result in the death of their teammates or the indigenous Montagnard troops they were advising. The fear of letting a buddy down was greater for most than the fear of the loss of their own lives. Once in combat, training, experience and instinct took over and the stress hormones were at lower levels than in the periods in which combat was anticipated.
For the BUDS/SEAL candidates wanting to be in naval special forces, the stress hormones were higher during the introduction of new skills, new training phases, and new equipment. Even the addition of a face mask being added to fins during ocean swims caused increased cortisol. From the psychological viewpoint, the greatest fear of the candidates was that they would fail the course. Performance anxiety created higher levels of stress hormones in training than men experienced under the stress of combat.
Ultimately, fear of failure to perform appropriately was more problematic for both groups.
This explains my GUE Tech 1 buddy's position. He believed that Tech 1 was more stressful than any training he had in the Marine Corps, including sniper school and recon training, that GUE Tech 1 was harder in one week, than his entire tour of duty in Force Recon, and why he was more stressed and scared in class than he was during combat in the Middle East.
For many many GUE-F students, lack of diving experience creates confusion over what is important, what can be sacrificed or modified and when, what is a drill to help you dive better vs. how you actually dive, why team position is where it is for class vs. where you really would be, feedback during class vs. feedback while diving for fun, when, where and why certain skills are used, and when it is okay and not okay to use the tools in the environment in class vs. for real. It's like a point value in the mind is identical for everything rather than certain things carrying more points than others at different phases of a dive. Even experienced divers sometimes have trouble. This adds to the anxiety.