Czenglish: I went to fitness today.
English: Um, do you mean you went to the gym?
Czenglish: Yes! I am trying to get stronger.
English: How was it?
Czenglish: It was OK. I didn’t sleep well last night so I didn’t have much power.
English: Ah, that’s a common mistake – you didn’t have much strength, or you didn’t feel strong, or you can just say you didn’t have a lot of energy.
* If you really want to say ‘fitness’ you can say you went to a fitness centre but it is much more common to say you went to the gym.
** When talking about power in the sense of physical capacity it only refers to a combination of speed and strength. For example:
– Lebron James has a lot of power (he is strong and fast).
– I don’t have the power to be a 100m sprinter (I am not strong enough or fast enough to be a 100m sprinter).
If you are talking about how much energy you have at a particular time you should say energy or strength instead of power. For example:
– I have had a busy day but don’t worry, I still have enough energy for our lesson!
– They didn’t have any strength left after running the marathon.