This study investigates the orderly recruitment of human motor units during voluntary isometric contractions. Motor units from the first dorsal interosseus muscle were analyzed using new techniques. Twitch tensions varied widely, from 0.1 to 10 g, and were nearly linearly related to the level of voluntary force. The number of motor units recruited increased sharply at lower force levels, but declined at higher levels, suggesting that recruitment contributes less to force increases at higher levels. Contraction times ranged from 30 to 100 msec, with over 80% of units having contraction times less than 70 msec, indicating they are fast twitch units. Larger motor units, recruited at higher thresholds, had shorter contraction times than smaller units. The study confirms the "size principle," where larger motor units are recruited at higher thresholds and produce more force. The relationship between twitch tension and threshold force was nearly linear, with high correlation coefficients. The distribution of motor units showed a wide range of twitch tensions, with small units more common than large ones. The number of motor units recruited decreased exponentially with increasing twitch tension or threshold, indicating that recruitment becomes less effective at higher force levels. The study also found that contraction time was related to threshold force, though the correlation was weaker than that between twitch tension and threshold. The results support the idea that motor units are recruited in an orderly fashion during voluntary contractions, consistent with findings in animal studies. However, the study notes that the results from a single muscle should not be extrapolated too far, and that the findings are consistent with general principles observed in other studies. The study also highlights the importance of recruitment and firing rate changes in adjusting voluntary force levels.This study investigates the orderly recruitment of human motor units during voluntary isometric contractions. Motor units from the first dorsal interosseus muscle were analyzed using new techniques. Twitch tensions varied widely, from 0.1 to 10 g, and were nearly linearly related to the level of voluntary force. The number of motor units recruited increased sharply at lower force levels, but declined at higher levels, suggesting that recruitment contributes less to force increases at higher levels. Contraction times ranged from 30 to 100 msec, with over 80% of units having contraction times less than 70 msec, indicating they are fast twitch units. Larger motor units, recruited at higher thresholds, had shorter contraction times than smaller units. The study confirms the "size principle," where larger motor units are recruited at higher thresholds and produce more force. The relationship between twitch tension and threshold force was nearly linear, with high correlation coefficients. The distribution of motor units showed a wide range of twitch tensions, with small units more common than large ones. The number of motor units recruited decreased exponentially with increasing twitch tension or threshold, indicating that recruitment becomes less effective at higher force levels. The study also found that contraction time was related to threshold force, though the correlation was weaker than that between twitch tension and threshold. The results support the idea that motor units are recruited in an orderly fashion during voluntary contractions, consistent with findings in animal studies. However, the study notes that the results from a single muscle should not be extrapolated too far, and that the findings are consistent with general principles observed in other studies. The study also highlights the importance of recruitment and firing rate changes in adjusting voluntary force levels.