1. The purpose of this study was 1) to characterize the decrease observed in mean firing rates of motor units in the first 8-15 s of isometric constant-force contractions and 2) to investigate possible mechanisms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mean firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the first dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 19 healthy subjects during a total of 85 constant-force isometric contractions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of computer algorithms to decompose the signal into the constituent motor unit action potentials. Time-varying mean firing rates and recruitment thresholds were also calculated. 3. Motor units detected from the TA muscle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at force values ranging from 30 to 80% of maximal effort and a duration of 8-15 s. Likewise, motor units detected in the FDI muscle displayed a decrease in firing rate in 32 of 41 trials performed during constant-force isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing rates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitment thresholds and lower firing rates tended to display the greater decreases in firing rate over the constant-force interval, whereas motor units with lower recruitment thresholds and higher firing rates had lesser rates of decrease. Furthermore, increasing contraction levels tended to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the maintaining of force output while motor units decreased their firing rates: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first because none was observed during the 8-15 s duration of any of the 85 contractions. Furthermore, contractions outside the physiological range of motor unit recruitment (at 80% of maximal effort) revealed the same decreasing trend in firing rates, ruling out recruitment as the means of sustaining force output. 6. The role of agonist or antagonist muscle interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar needle electrodes from the wrist extensor muscles while myoelectric activity in the wrist flexor muscles was concurrently monitored with surface electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extensor muscles simultaneously decreased while the flexor muscles were determined to be inactive. 7. All the findings of this study regarding the behavior of the firing rates could be well explained by the reported characteristics of twitch potentiation that have been previously documented in animals and humans. 8. The results of this study, combined with the results of other investigators, provide the following scenario to explain how a constant-force isometric contraction is sustained. As the contraction progresses, the twitch force of the muscle fibers undergoes a potentiation followed by a decrease. Simultaneously, the "late adaptation" property of the motoneuron decreases the firing rate of the motor unit. Findings of this study suggest that voluntary reduction in firing rates also cannot be ruled out as a means to augment the adaptation in motoneurons. (ABSTRACT TRUNCATED)