The fast-to-slow shift in myosin isoforms and its functional significance: which statement is true?

• A. Maximal Velocity Of Contraction
• B. Decreases Fatigue Resistance
• C. Increases Mechanical Efficiency
• D. Decreases Oxidative Capacity

Answer: (C)

A fast-to-slow shift in myosin isoforms means muscle fibers adopt slower, more oxidative myosin heavy chains, which slows the rate of cross-bridge cycling and shifts energy use toward aerobic metabolism. This change enhances the muscle’s endurance by increasing fatigue resistance because slow-twitch fibers have more mitochondria, greater capillary density, and higher oxidative enzyme activity. It also makes contractions more energy-efficient: for a given amount of work, slow-twitch fibers produce the same or greater force with less energy expenditure, improving mechanical efficiency.

So, the true statement is that this shift increases mechanical efficiency. By contrast, slowing the myosin isoforms reduces maximal velocity of contraction and actually raises oxidative capacity and fatigue resistance, not lowers them, so the other options don’t fit.