When you start lifting (or re-start after a long hiatus), you experience what are called "newbie gains" or "beginner gains."
Every workout, you show improvement. Often fast improvement - slapping another pair of 5s or 10s on the bar each workout, rapidly adding to your maximum pushups, getting stronger and faster than you'd thought possible. Gains are essentially linear - each time, it's more. You are generally stronger the next time than you were this time (subject to proper rest and recovery, of course).
Then it suddenly slows down, and then stops. You hit a wall. You can't add any more weight to your squat, to your bench press, to your deadlift. Or if you can, you can't add as much as you did. You run down into the 2.5 pound plates and then 1.25 pound plates, and then even those won't improve your maximum. You might even backslide.
At this point, it's tempting to quit. Or tempting to go back to whatever you started with (assuming you've changed) and try it again. After all, Starting Strength got your squat from 3 x 5 wobbly reps x 85 pounds to 3 x 5 solid reps at 225. Then you stalled and couldn't move, right? But this time, it'll add another 140 pounds to your squat! But it doesn't...because you've already gone past your capacity to gain that quickly on that kind of training.
I outlined this in this fashion, because I've done it. Ran up a good run on a beginner program (albiet not a very good one), then stalled out. Tried many different things, but no real progress. Went back to the first program again, but it stalled out almost immediately. I did that for years.
You have to learn to use periodization. Simply put, it's learning to vary your training in a systematic fashion to make progress. You learn to go light to recover, to go heavy to improve, to alternate rep and set counts to zig-zag towards your goal. As you get stronger, and get closer to your genetic potential, you must use more complex training to achieve progress.
When simple, linear progression stops working, you need to change to a different form of periodization. Probably the easiest way to learn about periodization is to start with Mark Rippetoe and Lon Kilgore's excellent Practical Programming for Strength Training.
If you don't want to read that, you're going to need to select a program that will do it for you. Madcow's Advanced 5 x 5, 5/3/1, the Texas Method. You can't simply bash yourself against the weights you've stalled at; your body has adapted beyond that. You have to back off, and then use a more complex training system to build up to and past your plateaus.
It's that simple, and that complicated.
If you've been working on a basic strength program and then stalled out, periodization is your answer.
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