Explanation:
T-Tubules are extensions of the sarcoplasmic reticulum. They extend down into the muscle fibers to activate deeper muscle tissue. Initiation of deeper muscle tissue ties into the motor unit recruitment, as a higher voltage of motor neuron activation generates summation of muscle fibers. A higher load yields a higher voltage of stimulation, requiring more muscles to move an object. In effect, T-tubules are necessary for motor unit recruitment as a heavier load activates more motor neurons, deeper muscle tissue, faster twitch, and glycolytic fiber types (Type IIa & Type IIb/x)
