But to make voluntary actions such as lifting a soldering iron or kicking a soccer ball, you use your skeletal muscles! Your skeletal muscles allow you to do all the wonderful movements with which you pass your days. Your muscles contract and enable movement by sliding microscopic actin and myosin protein filaments across each other, with a full support cast of other players including proteins (troponin and tropomyosin), ions (Na+, K+, Ca2+), energy carriers (ATP), and blood circulation to deliver O2 and remove CO2.
Each of your muscles is subdivided into functional groups of muscle fibers called motor units (Again, see our Introduction to EMG's experiment). A motor unit is a motor neuron and all of the muscle fibers it innervates. To achieve great things, like lifting a heavy weight, motor units join together in a systematic way to supply the force required to achieve strength. This teamwork among motor units is called "Orderly Recruitment" by scientists, and as stated before, motor units with the smallest number of muscle fibers begin contracting first during a movement, followed by the motor units with the largest number of fibers afterward, to allow for a smooth, strong muscle contraction
As long as calcium and ATP are available, the actin and myosin will continue pulling on each other and the twitching will continue. Note that the calcium is rapidly