What does it mean that movement generates energy?

Physical activity increases available energy rather than depleting it.

Example: You have been sitting at your desk for six hours. You skipped your morning walk because you had too much to do. By 2 PM you are foggy, irritable, and rereading the same paragraph for the fourth time. You tell yourself you cannot afford to take a break — the deadline is tomorrow. So you stay seated, drink another coffee, and force yourself to keep staring at the screen. By 4 PM you have produced almost nothing of value. The next day, facing an identical workload, you force yourself out the door for a ten-minute walk at noon despite the same deadline pressure. When you sit back down, the paragraph that defeated you yesterday resolves in minutes. You write more in the next ninety minutes than you wrote in the entire previous afternoon. The walk did not cost you time. The sitting cost you output.

Try this: Run a three-day movement experiment. On day one, work as you normally do — track your energy at 9 AM, 12 PM, and 4 PM on a 1-to-10 scale and note your total productive output. On day two, insert a ten-minute brisk walk before your most important work block. Rate energy at the same three points. On day three, add a second ten-minute walk at the point where your energy typically crashes. Compare the three days. You are not measuring whether exercise is healthy — you already know that. You are measuring whether movement changes the energy available for the specific cognitive work you care about.