Explain muscle firing for stimulated contraction?

• A. Type I before Type II; asynchronous firing; intermittent lower firing
• B. Type II before Type I; asynchronous firing; varying firing rate
• C. Type II before Type I; synchronous firing; constant higher firing depending upon stimulator rate
• D. Type I before Type II; synchronous firing; constant low firing

Answer: (C)

Electrical stimulation activates motor units in a reverse pattern compared to voluntary effort: fast-twitch Type II fibers, which have larger-diameter axons and lower impedance to the external current, are recruited first. This means Type II fibers come online before Type I fibers when a stimulus is applied. The electrical pulse acts across the muscle area, causing motor units to be activated nearly at the same time, so firing is synchronous rather than staggered across units as in natural, voluntary contraction. The rate at which these fibers fire is determined by how fast the stimulator delivers pulses—the higher the stimulation rate, the higher the firing rate of the recruited units, leading from individual twitches toward a smooth, tetanic contraction. So, the pattern described—Type II first, synchronous firing, and higher firing rate that rises with the stimulator rate—best fits electrically stimulated contraction.