8 years ago I learned pace control of work assembling and disassembling pens

I remember the first day of job rehabilitation 8 years ago.

The first task I was given was assembling ball-point pens.
At first I didn't know what to do.
I was so panicked that I couldn't even say my name.
In the first half hours the only thing I could do was crying, sitting on the floor.
Other 5 participants seemed to know at least what to do.
An hour passed and they finish their task one after another.
It wasn't until then I took up the parts, screwing the shaft into the tail screw cap, put the ink refill into the capped shaft, then putting the front screw cap into them, and finally put the cap over the tip of the pen.
I was stuck as long as it took other participants to finish assembling all parts of pens, and it took me no longer time than them once I succeeded to began.
I continued assembling pens in the first week, and by the end of week, I became as fast as any other participants.
After we assembled all boxes of parts, we disassembled them until there was no complete pen.
Next week we had to assemble another type of things, electric plugs.
This time again we assembled all stock in the center and then disassembled.
Before long I became confident of the speed of assembling, but some days later, my elbow had so much pain that I couldn't lift my arms.
Through the pain of my elbow, I learned that I had a difficulty to set a pace to continue work long at a constant pace.