Which type of motor unit can contract for many hours, or even days if necessary?

The slow motor unit is characterized by its ability to sustain contractions for extended periods, which can last for many hours or even days. This endurance stems from its physiological adaptations, including a high number of mitochondria, an abundance of myoglobin, and a rich capillary supply. These features enhance its aerobic capacity, allowing it to efficiently utilize oxygen for energy during prolonged activities, such as distance running or cycling.

In contrast to slow motor units, fast fatigable motor units are designed for short bursts of high-intensity activity, relying primarily on anaerobic metabolism, which limits their duration of exertion and leads to quick fatigue. Fast intermediate motor units have a mix of characteristics and can perform at varying intensities, but they still do not match the endurance capabilities of slow motor units. Fast oxidative motor units, while having some endurance capabilities, are generally not designed for ultra-endurance activities. Thus, when considering the ability to contract for extended periods, slow motor units are the clear answer.