No, I don't have the answers for why this happens, but what I have learned is when to hold em and when to fold em. Damn I feel gay for quoting a Kenny Rogers song.
In all seriousness, most of your training sessions aren't going to be the great ones. Let's get that right out there. You can't train balls out every session with the same intensity all the time and maintain it. Something in the machine will break down. Whether it becomes mental (dreading workouts) or physical (injuries and lethargy/fatigue), cycling your intensity is required for steady progress. To misquote Dan John, 80% of your workouts are just going to be getting it done, 10% are the great/out of this world ones, and 10% suck ass.
What you do need to try to achieve is for THAT particular session to be as good as it can be. Maybe you aren't going to feel as diesel as the last time you did this lift, but you need to have some things in place to help make each training session be as good as it can be (now I'm quoting Army commercials).
The Warm-up and pre-workout workout -
For a couple of years I did DoggCrapp training. The one thing that it took a while for me to get used to with that program was doing things like curls and calf work before squats. I grew up with that mantra of doing all of your "big muscles" first, then the small stuff last. This is not a bad principle to live by, but there are some benefits to doing some small muscle work before you get to your big stuff.
The one thing I noticed on the days that I trained biceps and calves before legs was that I rarely had a bad leg session. On the days I felt like crap I would have no idea how I was going to squat and do stiff legs when I could barely curl. But by the time I finished a curl and an exercise for forearms, I would feel much better, and almost always went on to have a great leg workout.
After I quit doing DoggCrapp I got away from this and didn't pay a lot of attention to it until I decided to start doing abdominal and neck work at the beginning of my back/bicep and pressing days. Usually movements for these areas are done at the end of a training session. If the training session had sucked and I was tired, I wouldn't do them at all. However when I started incorporating these first I noticed once again, even on what felt like was going to be a "bad day" usually ended up being ok. On a good day it was great. On squat and deadlift days I start them off with heavy calf work now. It doesn't effect my squat or dead in a negative way, and in fact my pumped calves make squats seem a little easier and more "greased" right out of the gate.
I go pretty all out on the top set of abs or calves and I can say that throwing these in first has helped because no longer do I feel like my abs aren't getting enough solid work, and my calves are back to 19" (yes this one is just about vanity as skinny calves look like shit).
How to incorporate your pre-workout workout -
The main thing you want to use the pre-workout workout for is to get your body primed for the big stuff, and to strengthen areas you know you need improvement on. Improvement can mean look or strength or both, that's an individual choice. The movement itself however, needs to be something that you can work hard on but not be thrashed when you are done. When you finish you should feel better than you did when you walked into the gym, and ready to attack the big shit.
So let's go over some movements that might work well in this regard. Remember not to get overzealous. Pick one or two exercises for 1-2 top sets. This is something in the mix of a warm up, and work stuff.
Abdominal - Pick One to be done on upperbody days
Decline Sit Ups - work up to a top set or two for 12-15 reps all out
Old-man Abs AKA Any Abdominal Machine - 1-2 sets of 15-20 all out
Hanging Leg Raises - shoot for a total number of reps and go after it
Neck - Pick one to be done before any session
Plate Raises - 2x15-20
Neck Harness - 2x15-20
Swiss Ball Pushes to the side (for the side of the neck, you don't need a ton of resistance here) - 3xmax reps
Forearms - Pick One for pressing or lower body days you don't pull on
Hammer Curls - 2x15-20
Reverse Machine Curls - 2x15-20
Behind the back barbell curls - 2x15-20
Calves - to be done on lower and/or upperbody days
Seated/Standing Calf Raises - 2x10-15
Calf Press - 2x15-20
Donkey Calf - 1xas many as your partner will let you do while they ride you
Traps - To be done on any day because traps are more important than anything on the planet
Hise Shrugs - 2x15-20
Upright Rows - 3x15-20 stay light
High Rep Shrugs - 100 reps This is to be done light. If you can pull over 600 go with 275, otherwise stick with 185 - 225. This should only take a couple of minutes so stay light.
Cuff Work - Any day
L-Flyes - 2x15-20
Internal and External Rotation - 2x15-20
When to flush it -
Sometimes you're just going to have a day when no matter what, things are going to suck. What I suggest doing is to look at a few things you have control over.
Sleep? Did you get enough the night before, and the night before that? Remember that if you have a night of sleep deficit, it usually takes a couple of nights of solid sleep to get back to "normal". If sleep was bad, flush it and go home and plan on getting good sleep that night and try the session again.
If your sleep is good, go to the next one...
Food? Did you eat well that day and the day before? You should be smart enough to know if you have eaten enough or not. One night at Krav we were fighting and I suddenly felt a drop in blood sugar and I almost collapsed. Lucky for me the head instructor is a smart guy and asked me "have you eaten enough today?" I could barely answer. He left the room and came back with an apple. If you haven't eaten enough, try a light session or flush it. Go home and load up for a meal or two with good carbs (Oatmeal, sweet potatoes). Take a concentrated look at your diet on training days and see if you are increasing caloric intake enough on those days. In my opinion, pre-workout eating is more important than what you do during the workout and post workout. You have to have enough calories to power through tough sessions. Pay close attention to what you are eating for the 3-4 hours before you train. If your eating was good go on to the next part...
Recovery? Did you just have some kind of other worldly good training session a day or two days before? If so, you could still just be suffering some systematic fatigue from that. You may need to examine your log and see if this happens more on some days than others. For example if you notice you always have bad training days two days after doing squats, move that workout to three days out. An extra day of recovery never hurt anyone. Flush it and go home.
Recovery? Did you just have some kind of other worldly good training session a day or two days before? If so, you could still just be suffering some systematic fatigue from that. You may need to examine your log and see if this happens more on some days than others. For example if you notice you always have bad training days two days after doing squats, move that workout to three days out. An extra day of recovery never hurt anyone. Flush it and go home.
If you have eaten well, and slept well, and haven't had a great workout in a few days or even weeks, maybe you need to quit being a puss and just man up.
Another option -
Another option -
Another option to think about is always having a backup workout to go to when things are in the toilet. What that is can be entirely up to you. If you feel good, but just don't feel like doing your normal thing that night go to something else. Just don't make this a habit. Consistency is the biggest key in making progress. Deviate from the path as little as possible.
I hate to use the term "instinctive training" because for a lot of guys that means driving to the gym and deciding what they want to do on that night. I think this is the intellectually lazy way to train. You can always fool yourself that you're making progress if you constantly change everything up. Have a measuring stick of some type that you always use to weigh progression.
But do be in tune with your body and know when you're having a night where you either need more rest, or just need to get the minimum reps and sets in. Progression is a balancing act of effort and recovery. Make sure both are weighed equally on the scale.