Which motor units contract optimally during very light work?

The correct answer indicates that red slow twitch motor units contract optimally during very light work. These motor units are characterized by their aerobic capacity and endurance, allowing them to sustain activity over long periods without fatigue. They contain a higher number of mitochondria and myoglobin, which contribute to their ability to efficiently use oxygen for energy production.

During light activities such as walking or light jogging, the demand for energy is relatively low, and slow twitch muscle fibers are capable of meeting this demand without significant fatigue. They are well-suited for activities that require endurance and prolonged use, making them the primary type of motor unit recruited during light work.

Other types of motor units, particularly those associated with fast twitch fibers, are better suited for high-intensity activities that require quick bursts of power rather than sustained, low-intensity efforts.