Type I motor units are recruited first during exercise. Hence the correct option is a.
During exercise, the recruitment of motor units is determined by the principle of the size principle, which states that motor units are recruited in an orderly manner based on their size and type. Type I motor units, also known as slow-twitch or oxidative fibers, are characterized by their endurance and resistance to fatigue. These motor units are recruited first during low-intensity activities or the initial phases of exercise. Their oxidative nature allows them to sustain contractions for prolonged periods without experiencing rapid fatigue.
As exercise intensity increases, the recruitment pattern progresses to involve larger motor units, such as Type IIA and Type IIB (or Type IIX) motor units. Type IIA motor units are fast-twitch oxidative fibers, while Type IIB (Type IIX) motor units are fast-twitch glycolytic fibers. These larger motor units contribute to higher force production but are more prone to fatigue. Hence the correct option is a.