A plateau in the gym — when the weight stops moving
A stretch of several weeks when your strength or weight on a lift doesn't grow while you train the same way.
A plateau isn't one bad workout. Everyone has a couple of bad days. A plateau is when your estimated max sits still for several weeks while you keep training the same way.
I tell two cases apart. One — the weight hasn't moved for three workouts in a row and you're doing 12 or more reps: that's not a stall, it's time to add weight, and I'll remind you. The other — over the last eight weeks your estimated max has grown slower than half a percent a week: that's a real plateau, and the coach breaks it down.
The causes are usually boring: poor sleep, too little food, too little or too much volume, the same scheme for months. What to do about it step by step is in the guide on breaking a plateau.
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In the diary I work out all of these numbers myself — you just log the set.