I find that especially for my heavy days, there are so many factors that impact my workouts. I think I'm older than you (35), so maybe my body is more sensitive to these things than you, so keep that in mind. 1. Sleep. Crappy sleep (less than 8 hours) 1 or even 2 days before a workout and I see a negative impact. 2. Food. Crappy nutrition within the 24 hours prior to a heavy workout and I see a negative impact. 3. Stress. Too much stress (for me, mostly job related) and my workout suffers. 4. Time of day. I've noticed that my "sweet spot" is right around 3-4pm. Too early or too late and I see a negative impact. So, I guess what i'm saying is, try not to be too disapointed with a single "bad" workout because there are so many things that could have caused it that are not purely muscle strength. Hope this helps!