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A lot of the confusion on this issue comes from people trying to apply principles that work for steroid users to natural lifters. Natural and steroid training isn't the same game, and many don't know this, so chaos and confusion ensues.
You really don't "need" to change routines. I would look at training as more of an evolution. Change your training based on needs, instead of jumping from one program to another out of fear that gains will stop.
The idea that wholesale changes need to be made or gains will stall is a myth (at least for natural lifters). As long as you have a core group of great muscle building exercises, and focus on progression of reps and weight using good form, gains won't stall. At least not in the sense that it is painted by many.
This is not to say you can't change programs...you can. Just make sure to keep the core muscle building lifts in the program. If the core lifts are in the mix, you can "shuffle the cards" every several months if you'd like. But do you need to change to make gains? no.
I used a variation of the same routine for 20 years and evolved my training.
Natural lifters experience slower gains the longer they train. This is a natural occurrence, and doesn't mean you are doing anything wrong. In general, the "average" amount of muscle naturals gain looks close to this:
Year 1 - 16 pounds
Year 2 - 8 pounds
Year 3 - 4 pounds
Year 4 - 2 pounds
Year 5 - 1 pound
When gains start to slow, lifters assume the body has adapted and often do radical things, which aren't necessary. Natural lifters are not able to "re-ignite" super spurts of muscle gains. Their best bet is a religious pursuit of simple progression of weight. You might be able to jump on a new program and shock
a couple pounds of muscle out, but naturals have finite muscle gain limits. Simply stated - the more you gain now, the less you will gain in the future. So while a shock might work once, it is not a viable long term approach because you can't keep shocking new gains unless you're on steroids.
Simply put, try to add one more rep to each lift each workout session. If you can add more, great! But don't fret if you can't. Even adding two reps to a lift per month will lead to great muscle and strength gains over a 5 year period.
Strength does stall, and over time it will be harder to add reps, but this "stall" does not indicate that a program is ineffective. Like I said, even if you are adding "only" a couple reps per month after you reach the intermediate lifting stages, you are still making what I consider to be good progress - and I don't consider it a stall at all.
A pure stall - or wall - is when you have not made progress at all on a lift in about 1-2 months. When this happens it might be technique related, or you might have a glaring weakness, say tricep strength. When these stalls happen it's time to "evolve" your training a bit, and make a few changes to address these specific needs.
I recommend taking a look at the John Christy thread, and some of the old school workouts on the site. These lifters didn't make wholesale changes every 6-8 weeks and built incredible natural physiques.
John Christy advocates the same program week in and week out for naturals, and I believe it to be a very smart way to train.
Again, you CAN change to stave off boredom and routine, but do you need to change? No. Absolutely not!
Steroid users can "re-ignite" gains after they have stalled by shocking and awaking their receptors with a volume of intense contractions. This is where this myth developed. It is not applicable, nor needed for natural lifters.