This study investigates the orderly recruitment of human motor units during voluntary isometric contractions. The authors use a technique to measure the contractile properties of single motor units, including twitch tension, contraction time, half-relaxation time, and the threshold force for recruitment. The results show that larger motor units are recruited at higher thresholds, supporting the size principle. The data from three subjects are consistent, indicating an exponential decline in the number of recruited motor units with increasing twitch tensions or recruitment thresholds. The study also finds a nearly linear relationship between twitch tension and threshold force, suggesting that recruitment accounts for a decreasing proportion of force increases at high force levels. The findings confirm the orderly recruitment of motor units and highlight the importance of recruitment in generating force during voluntary contractions.This study investigates the orderly recruitment of human motor units during voluntary isometric contractions. The authors use a technique to measure the contractile properties of single motor units, including twitch tension, contraction time, half-relaxation time, and the threshold force for recruitment. The results show that larger motor units are recruited at higher thresholds, supporting the size principle. The data from three subjects are consistent, indicating an exponential decline in the number of recruited motor units with increasing twitch tensions or recruitment thresholds. The study also finds a nearly linear relationship between twitch tension and threshold force, suggesting that recruitment accounts for a decreasing proportion of force increases at high force levels. The findings confirm the orderly recruitment of motor units and highlight the importance of recruitment in generating force during voluntary contractions.