
Nervous system
What causes it
Heavy lifting above about 85% of your max. Top speed sprints. Hard jumping and landing. Low rep power work. Any session where the bar moves slowly because it is heavy.
What it feels like
Flat. No pop. Weights feel heavier than they are. Reaction time is off. You can still grind, but nothing is snappy.

Muscles and joints
What causes it
Muscle building work like 6 sets of 6 with two minutes rest. Long lowering phases. Downhill running. Lots of stopping, starting and changing direction. A long flight or a day on your feet.
What it feels like
Sore and stiff. Heavy legs. Stairs are a thought. You move like you are 20 years older for a day or two.

Engine
What causes it
High volume sessions. 3 sets of 12 with a minute of rest. Tempo runs, intervals, long swims, anything repetitive for a long time. A big week of training volume.
What it feels like
Drained and hungry. Breathing hard on things that are normally easy. Sleep is heavy but you still wake tired.

Head
What causes it
A big game, a pitch, a presentation. A new team or a new job. Conflict at home or at work. Travel. Anything you read as a threat, even a small one. Or things are just not right and you cannot say why.
What it feels like
Wired, not tired. Short fuse. Busy mind at 2am. You skip the warm up and rush the session, or you cannot make yourself start.
Section 05 gives you the matching list for each one: cold water, stretching, easy movement, compression, breathing, light and sleep, with times and temperatures.