Have you ever heard the expression "Your body is like a machine"? I regard this as the most inaccurate and unhelpful analogy in the fitness world. In fact, our bodies are NOT at all like machines. While machines should operate in the exact same way every time they are in use, your body will achieve a different fitness level every single day! Our bodies' performance levels are tied to many nuanced factors: rest, injury, mood, energy source, focus, recovery, warm-up, etc. On Wednesday you might approach your workout with 8 hours of sleep and muscles teaming with glycogen from 24 hours of good eating. Accordingly, you might lift heavier and run faster than you have in weeks. You might even achieve a personal best! But Friday might be a different story entirely. Maybe you now have tight hip flexors from a long run the day before and low blood sugar from skipping breakfast. So working your HARDEST on Friday does not look like working your hardest did on Wednesday.
This is not to say you should not push yourself during every workout! I am a huge proponent of intensity. Instead this means you will not be physically CAPABLE of the same performance every single day. For example, earlier this week I was feeling great. I dead lifted over 300lb and strapped on an extra 55lb for my pull-ups. But yesterday I was RUN DOWN! The most I could accomplish was a few sets of light squats and dips -- far from my personal best!
How do you put this information into practice? Be aware of your daily capability! Jogging a mile at the beginning of your workout is a good way to gauge your levels. Does it feel easy today or does it feel like Hell? If you feel good, pick up heavier weights. Don't be SHACKLED to the green weights, or the yellow weights, or the blues! Don't let a color define you! Remember a different day means a different fitness level!!!
There are even programs and apps that will help determine your readiness for a workout. Check out Bioforce HRV. This program uses heart rate variability to assess how well you responded to previous workouts and how recovered and ready you are for the next workout! If you have trouble assessing your own capabilities this technology could help you.
Remember, not enough intensity equals no gains! Too much intensity equals injury and setback! So adjust your performance accordingly!