People can easily think they are lacking in energy, when in fact they are lacking in focus.
The two do overlap as well as being different.If you lack energy, you will no doubt lack focus, but you can very easily lack focus while having adequate energy. Here's an example - roughly 2 months ago I started using caffeine in my pre-workout shake, and boy have I noticed the difference! I have also used beta alanine for quite a while, but if I'm bluntly honest, I don't feel an increased focus, but I do feel an increase in energy. Since using caffeine, I can honestly feel the difference in my focus levels. I've had workouts where I've had so much on my mind that I couldn't focus properly on my my session, and that leads to a feeling of a lack of energy. But the caffeine turbocharges my mind and I'm 100% in the zone.
When you're mind isn't focused, you're not fully concentrating on your reps, so you feel a bit lethargic - hence the feeling of a lack of energy. But your body isn't struggling for energy, it's simply struggling to focus. If you have ever seen a bodybuilder workout, you'll notice that they are 100% focused on lifting the weights every single rep. Then you see the 'posers' in the gym who lift moderate weights, checking themselves out in the mirror, looking around to see if anyone is looking at them, they are not concentrating on the exercise at hand. If they tried to lift heavy and do the same 'routine', they wouldn't be able to get anymore than about 3 reps out, because they aren't focused.
I have had workouts where someone has interrupted me, told a funny joke, or something that made me lose focus, and at the same time I lost strength and had to quit that set. If you got in a press up position, and then listened to your favorite comedian, try to do a set of 20 press ups while laughing your head off, you'll notice a drop in strength, because you have lost your focus.
I remember watching a video from Nick Mitchell on YouTube, and he said his 'pre-workout' is a cup of coffee, which helps him focus during his workouts. I personally don't like coffee, so I didn't try it, but instead I got myself some caffeine, and I'm glad I did.
Unless you walk into your gym feeling absolutely shattered, you will have energy in your system, so don't think that you're struggling for energy if your workout is poor, because it's more likely you're simply struggling to focus. Some people can instantly get focused when they put on certain music, whereas others need a stimulant of some kind (i.e caffeine).
So the next time you're working out and thinking "I've got nothing left in tank", it's probably that you're struggling to focus on your workout. I do recommend trying caffeine - coffee or powder form, it really works for me.
Lee Gregory Fitness