As far as research tells us, muscle growth is caused by the combined effects of high levels of mechanical tension and increased metabolic stress, with muscle
contractions during resistance training as the third most important contributor to anabolism.
The presence of different levels of muscle activity has been observed both using surface electrodes and fine wire (intramuscular electrodes) and
during maximal voluntary isometric
contraction (MVIC),
resistance training exercises, and normal humans movements such as gait.