Why does energy go down as workout progresses?

Question

Ok, I started working out 2 months ago and lost 20 lbs. I am now 31 years, 6'1, and 235. My goal has been to gain muscles while losing fat and this is working. I watch my diet (about 2,000 cals a day) and mix up cardio ( 3 days) with strength training (3 days). When lifting weights I generally get a good workout in an hour or so doing every muscle group superset-style in 3 sets of 12 and keeping the heat rate in Zones 1 and 2 (140-165 bpm). Keeping all this constant, I cannot figure out why by the time I get to the smaller muscle group exercises (example biceps and triceps) near the end of workouts, I feel fatigued and have difficulty doing even 2 sets of them at comparatively lower weights to the larger muscle groups... My heartbeat is the same and I am not winded... Any ideas on why I get fatigued and how to get over it? Thanks!

Answer 1

If your carbs are too low, your body will try to preserve them because they are the primary energy source for the brain, red blood cells, and kidneys. The best way for your body to preserve them is to discourage you from wasting energy by making you low on energy.

Keep your caloric intake the same but make the carbohydrates a slightly higher portion of your calories.

Here is some supporting information.
http://spartafit.com/articles/requirement%20for%20calories.html

Answer 2

your cals are simply too low for your bodyweight and daily level of activity. you didn't say what your body fat percentage is but your BMR alone is probably close to 2,100 cals

check your cals against the website below :

http://www.protraineronline.com/past/july2/nutrient.cfm

Answer 3

save bulking. i'm bulking to 180lbs till now slicing too. yet after the decrease, i will bulk lower back besides. basically pick to adventure the slicing technique for the 1st time. save working stressful, looks you're doing good.

Answer 4

my friend you have muscle fatigue. i know what you are describing but i couldn't describe the reason properly so here's the link...

http://en.wikipedia.org/wiki/Muscle_fatigue