Intensity is about focus, both energy and attention. Focused attention is simply concentrating on the muscle and the exercise being performed. Do I have good form? Am I getting a full extension out of that movement? Am I swinging the weight?

Focused energy involves a couple of considerations. First, is the energy expended on the exercise. Is there enough energy being spent to force the muscle to grow? Second, is the movement that the energy is being spent on going to force the muscle or muscle group to grow?

Stress is what makes muscles grow. More specifically, adapting to stress is what causes muscle growth. If sufficient energy to cause stress is not spent, muscle growth will not happen.

For example, I have found that if I increase the weight I’m handling by about 5% every 1-2 weeks, I will steadily get bigger and stronger. Comparatively, compound exercises allow you to lift heavier weights, creating greater stress, which causes more muscle growth.