In my experience there are two distinct types of muscular fatigue associated with intense progressive resistance training (only intense training is sufficient to trigger muscle hypertrophy) and these two types should be recognized and understood. The first type of fatigue is direct muscle soreness and is the result of a particular exercise targeting a specific muscle. Scientists are at odds as to the exact cause of muscle soreness but most believe that it is associated with some sort of cellular micro-trauma. Direct muscle soreness is usually the type of pain and discomfort that most folks experience when they begin serious progressive resistance training program.
There are varying degrees of muscle soreness and sometime the intensity of soreness can become so severe as to be debilitating. The muscles are actually sore to the touch. I have self-induced this type of soreness to every degree on every muscle ? once, as a 14-year old novice, I found a 10-pound solid dumbbell and proceeded to do 50-repetitions in the one-arm curl for each arm every hour on the hour for 10-straight hours. It seemed like a cool idea to my young and dumb mind but that went out the window the next day when both arms locked up to such a degree that I could not straighten my arms. Both biceps were so traumatized that they remained involuntarily contracted for the next 36-hours. My hands were held at my face and any attempt to straighten my arms resulted in excruciating pain. I had to ride it out until the biceps relaxed. This was an extreme example of muscle fatigue but extremely illustrative of this 1st type of muscle soreness/fatigue.
The second type of muscular fatigue is what I would describe as overall fatigue, I call it body shock. The body is a holistic unit and hard intense training done for long time periods has a cumulative effect. After a while a uniform sense of overall fatigue is experienced manifested by an overwhelming sensation of tiredness. This tiredness envelops the whole body. When in the throes of body shock it seems as if you are moving through water. In my experience this type of fatigue is a direct result of an accumulation of intense workouts. Fatigue and soreness come with the territory and if you never experience either version, likely you'll not make any significant physical progress.
In my experience, if I don't feel some degree of muscle soreness in the target muscle after a workout I become suspect that I didn't work hard enough or the exercise I selected was technically deficient and spread the muscular effect over too wide an area. In this respect I use controlled soreness (no too much, not too little) as a workout report card. When it comes to body-shock fatigue, to my way of thinking a much more serious type of fatigue, I will cut back on my training and kick up my calories, particularly my protein intake. When body-shock descends training through it is a bad idea: first, training poundage plummets (so what's the point?) and secondly there is a very real danger of fatigue-induced injury.
If you experience severe muscular soreness of the 1st type, avoid training that particular body part until the soreness reduces to tolerable levels. If body-shock envelops you cease and desist progressive resistance training and kick up the food intake. I have found that light to moderate cardio actually helps to dissipate muscle soreness. Accelerating circulation within a sore muscle stimulates recovery, assuming the resistance used is light, easy and not taxing. Use your common sense and be aware that even purposeful primitives paid heed to fatigue.
Marty Gallagher is a former fitness chat columnist for washingtonpost.com. He is also a former world champion powerlifting coach. Marty has written for publications such as Muscle Media, Muscle & Fitness, and Powerlifting USA. His website, http://www.martygallagher.com, assimilates years of accumulated knowledge from the athletic elite and makes them accessible to the common person. The "Purposeful Primitive" way has been proven effective time after time after time for weight loss, building muscle, increasing strength, and improving health.