Why can't I lift the same heavy weight as last time for the first reps

Question

For example, I usually do 21kgs for my triceps, but if I have just started weightlifting that day, I can barely do a single rep or two and I have to rest. After 15 minutes, I can do the same 21kg with ease for several sets and sometimes even with higher weight.

Why can't I do the same 21kgs I've been doing for months now from the moment I start weightlifting?

I experience the same with primarily abdominals, forearms and traps but not with other muscles. Say, I can do the same weight as last time with my biceps as soon as I start weigthlifting.

Answer 1

After much time without training, your "other" muscles are back to their previous state as before.

Meanwhile your arms still have approximately the same power because:

1. you use them in any other action you do during the everyday life.
2. Arms, calves and abs are what is called "little muscles", this muscles tend to recover quickly from workouts and some people use to hit them 2 days a week with no signs of overtraining and with great results.

Conclusion : You should start training your back, chest and shoulders again like you've never done it before, start with mid weights and focus mainly on the workout and the technique you're using to lift the weights.

Good Form is better than heavy weights.

Have a nice day, hope you enjoy your workout