Just thought I'd share some info from a Strength and conditioning coach's perspective
This is not a simple question but infact at the very core of why I have a job lol. That being said, I'll try and help you guys out in your understanding of the problem and from there you can make some "educated" choices or read into specific areas. (I apologize for any grammer and punctuation mistakes as I'm writing this at 3:45 am with a fever, but it beats lying in bed)
Plateus occur due to a number of factors.
1)Overload
2)Improper Periodization
3)Not creating Mesocycles and Microcycles
4) Variation
5)Not taking advantage of Supercompensation.
I'll define and try and explain each of these so that you can apply them, however things like periodization and cycles, as well as supercompensation take years to fully grasp and apply correctly. That is why I get paid to program for athletes.
Overload: As an athletes body adapts to the greater physiological challenges to the neuromuscular system you must increase the training stress or loads placed on the muscles in order for gains in strength or performance to continue. A few ways of doing this are increasing weight, increasing over all volume, increasing the amounts of workouts in a week (total volume), or increasing sets of excercises that specifically overload muscle groups.
An explanation on volue can be seen as follows. I could do 2x4 reps @ 250lbs (2000lb total volume) or I could do 2x10@ 125lbs. These will give me completely different metabolic reactions and adaptaions while maintaining the same volume. Many people will change reps and set limits without addressing the total volume, as such, when they change cycles they are not correctly overloading or creating the right types of muscles fibers.
Cycles.
Hypertrophy: Increasing muscular cross sectional area and metabolic base for more intense future training (amount of muscle fibers able to be recruited and glycogen/use/storage. This preps for power or high volume low intensity micro cycles.)
Endurance: Increases the metabolic efficiency and endurance of the body but does not develop large amounts of cross sectional area.It does this by creating and recuiting more oxidative than glycolytic muscle fibers. This is a great reboot after active rest or used to train the neuromuscular circuits and increase the metabolic base of new athlets in order to allow the body to handle the larger volume loads of a strength/power or hypertrophy cycle a couple of micro cycles later.
Strength: The good stuff. Increase maximal muscle FORCE. Moderate volume and intensity. Depending on the client's needs for either sports, general population, or strength training/Olympic Lifting, maximal volume can be adjusted as well as rep sets and rest period. During a strength phase where you are using between 85-95% of your 1RM it is imperative that you allow proper rest between sets to allow for your glycogen stores to completely restore.
It is also important to consume enough carbohdrate and protein BEFORE your workout to maintain glycogen stores via carbohydrate breakdown.
Overall volume can be achieved here by not only focusing on increasing sets while maintaining low rep limits but also by overloading muscle groups on multiple days in the week or multiple sessions per day. It is important however to note that you should not be going over 6 reps as a rule of thumb (with some exceptions) on strength lifts, as the maximal strength will deplete your glycogen stores within 2-3 lifts. The body needs roughly 3minutes to fully come back after both strength and power lifts.
Remeber if you can do 10 reps at a weight it means you could be more effective at generating the correct metabolic response by doing 5 reps at a higher weight.
Strength/Power: If strength is lifting more, Power is accellerating that same weight from point a-b FASTER. This is done by not only focusing on power lifts (squat, bench, Deadlift) but with the mentality that it will also help your olympic lifts or sport specific goals. Here, you focus on LOW volume and high intensity. This is often where I will also mix plyometric and eccentric loading into a clients program on top of using explosive sets of 3x3s for power lifts. Olympic lifts will also take more of the training volume in order to allow for the appropriate metabolic reactions and neuromuscular developments to sequence movement. (For example on a hang clean: Hinge, iso hinge brings the bar to 1/4 quad, second hinge extends through hips, knees, ankles to create speed, Shrug, drop catch, stand) All of these motions must be perfectly sequenced to create maximal power throughout the range of motion. (Many times athletes will sacrifice performance due to poor sequencing where reciprocal inhibition will restrict maximal power throughout a range of motion).
(Competition or peak) - self explanatory, if you have a competition you try and line your cycles up to peak at this point before doing a mesocycle taper. we can spike athletes with large gains during the preseason but once the season begins the combined stress and timing of games and trainings means that limited performance gains can be achieved and it is more of a slow step by step buildup along with maintenance and rest management.
ACTIVE REST - This is perhaps the most important and overlooked aspect of periodization from an amateur perspective. This is the idea that you MUST allow physiological and mental recovery through limited low-volume and low-intensity resistance training or performance of activities unrelated to ones sport. These can be incorporated by including tapers between microcycles (3 weeks hypertrophy 1 week taper) or between mesocycles (3 weeks endurance 3 weeks hypertrophy 3 weeks strength 3 weeks power PEAK followed by active rest before beginning the next meso or macrocycle.
Variation:
Some people have touched on it but I'll tell you a way you can achieve it.
There are a couple different views on variation, one is to vary the muscles recuited to assist in a movement, either by changing position or overloading one group and then doing a complex multijoint movement to subsequently overload the other assisting muscles. Second is to vary the intensity at which you use them (therefore recruiting different types of muscle fibers and creating more of that type of fiber).
ex.
Changing position: Doing a Sumo Deadlift or Snatch grip deadlift will change which muscles are recuited at the heaviest point in the range of motion or "hole" to initiate the pull.
Doing a wide arm push upwill allow for more outer pectoral recruitment and activation while doing a narrow grip one will increase the amount of tricep extension and inner pectoral recruitment.
Ex. Burn out triceps before bench pressing to force the pectorals to overcompensate rather than recruiting the triceps in the middle range of motion. This can be done in a drop set as maximal volume after the tricep fatigue sets in will limit volume in subsequent sets.
ex.
Varrying intensity doing a plyo push up or explosive benchpress in which you do a pause followed by a maximal acceleration of the bar off the chest. Or using chains to normalize the resistance at every point in the range of motion (effectively eliminating the sticking point or point of least mechanical advantage) and doing a slow steady rep.
ex.
Intensity can also be viewed as the amount of proprioceptive stability required to do a lift, for example changing a bilateral movement (Squat) into a unilateral movement (Split Squat) Or using a Trx to do a 1 legged lunge as opposed to having both feet on the ground
Varrying how you do these excercices as well as how you cycle your focus will yield a net gain in development of fast glycolytic muscles needed for heavier lifting.
Type1 Slow oxidative have high oxidative capacity and are fatigue resistant but contract and relax slowly (endurance/active rest)
Type II a Fast oxidative Glycolytic have some oxidative capacity but are larger and more powerfule with moderate to high anaerobic metabolic capability (Some Hypertrophy/strength/plyometric)
Type IIx Fast glycolytic Fast/Powerful but easily fatigued (Strength/power/explosive plyometrics)
All types will adapt regardless of resistance training however, by creating microscycles you can piggy back off of focusing on the growth of specific types and cycling through them. Think increases in the types of fibers as follows
+ 3 + 1 -1 Endurance
+1 +3 -1 Hypertrophy
-1 +1 +3, Stength
+1 -1 +3 Power/plyo
you will see a net gain of +5 +5 +4 from that cycle
Lastly and perhaps the most hard to actually take advantage of for normal people with busy schedules is the Idea of
Super compensation.
Basically supercompensation is as follows.
Training - Drop in perforamance/recovery - followed by the body jacking up glycogen and muscles stores in preperation for the next harder workout known as the supercompensation phase
If your next training happens in the window of your supercompensation, performance will increase, if the next training happens outside of that window, then performance will remain the same. Glycogen stores will often replenish faster which allows for two workouts in a day (which elite athletes and bodybuilders will take advantage of) After ~a 10 hour rest period your body enters its metabolic supercompensation of glycogen production and usage. However, if you workout too quickly, symptoms of overtraining may occur, resulting in a loss in performance or damage to the mitochondrial process.
If you look at the graph below you will notice that while glycogen stores will have been restored your complex parameter of fitness level will not have reached its supercompensation arc yet. It is important to balance the glycogen window with the neuromuscular stress and overall fatigue of a second training in a day. Often it is used as an auxilary overload of low intensity and low volume simply to increase maximal volume before the next supercompensation for your overall fitness which will allow for your next lifting session.
Sorry for the lengthy response but hopefully there are some helpful ideas once you read through.
If you guys have any questions on anything in this ask away and ill check back on this thread, or go over to my programming thread and post your questions up there. I can go as scientific or as lay as you want in my response.