back from a cold and a sprain

I decided I'll actually start studying how to train.
Today it landed me up on isometrics and tendon training.
Idk wth isometrics mean but tendon training is basically loading up on ur tendons until they start yelling.
I never heard my tendons yell , until today.
Sissy squats and squat pauses started making quiver harder than a white girl after bbc.
I also tried out training till failure and ho boy, it pumped me up like never before.
I felt done after just bicep curls and lateral raise.
I doubt if I'm doing lateral raises correctly. It isn't producing any results.