"I can barely walk today, what a great workout I had yesterday!" is commonly heard. Delayed onset muscle-soreness is often viewed as the barometer for the effectiveness of a training session. On the contrary, no muscle soreness is frequently interpreted as an ineffective workout. This translation is erroneous and misleading. Muscle soreness is simply a by-product of performing a level of muscle work to which your body is unacquainted, not the indicator of a single workout's effectiveness of improving your strength or fitness.
High-level strength and fitness is not the result of one workout. Quite the opposite, high-level strength and fitness are the precipitate of consistent training, time, and physiological adaptation. You see, physical adaptation is what you are striving to achieve. Physical adaptation is increased: strength, muscle mass, endurance, etc.
Muscle soreness goes away as adaption occurs. This is evident if you follow a training program for a few weeks. You are no longer sore. People view this as a negative, when rationally it is positive. You can now do the program without developing muscle soreness. You have taken a step closer to where you want to be without the negative side-effects. When you do progress the program, your body can respond favorably to it because of the adaptation that has occurred.
"The journey of a thousand miles starts with ones step."
Everyone should follow a training program. As I tell my clients, variety is not a bad thing, but when it interferes with adaption and progression it is counter-productive. You can't progress beyond a beginner-level if you keep changing things up. Additionally, you place yourself at greater risk of injury when your body is not adapted. Plus, it just feels good and is motivating when you can see your strength and fitness improve from week to week.
There are times when you can or should change your training program. But, that time frame is in weeks and months. This is an adequate amount of time to give your body for adaptation to that specific program. When you decide to change programs, it should be a subtle changes in variables that are still in-line with your long-term goals! The complete abandonment of your past training is misguided. If you are a runner, you need to still run similar distances/paces. You may make a few slight changes to the variables in your program. But, as a runner, you wouldn't abandon running, use the elliptical trainer, and expect yourself to improve or even maintain the level of running at which you had been. That is cross-training and is counter-productive! Remember specificity of movement.
The SAID principle or specific adaptation to implied demands is the primary principle that exercise training is based upon. It is ignorant to ignore the SAID principle. Then again, nearly all people buying those books, dvds, or attending an exercise class that promote "muscle confusion" are and will always be beginners. The muscular soreness they experience from those workouts gives them false sense of accomplishment. Muscular soreness (or even vomiting) can be induced with about five minutes of heavy exercise with a beginner. But, in reality they have only added one small brick in the training wall, or perhaps maybe even set themselves back if they can't train for a while because they are so sore. Adaptation to an appropriate program, followed consistently over time, with subtle changes made to it over time has and always will reap tremendous results. However, since the training adaptation process does take a time, it is easier to induce muscular soreness and declare it progress to the impatient trainee.