You can use electrical stimuli, but that only tends to bulk them up by about 10 percent, which isn't sufficient and it's also expensive. (scientificamerican.com)
Don't forget that here lies the spinal cord, which communicates with the brain and your muscles, controlling their work through electrical stimuli. (fitnessandpower.com)
Increasing intensities of electrical stimuli were delivered to each participant's hand until they reached a painful but tolerable level of shocks. (sciencedaily.com)