MOTOR UNIT CONTROL PROPERTIES IN CONSTANT-FORCE ISOMETRIC CONTRACTIONS

1. The purpose of this study was 1) to characterize the decrease obser ved in mean firing rates of motor units in the first 8-15 s of isometr ic constant-force contractions and 2) to investigate possible mechanis ms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mea n firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the firs t dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 1 9 healthy subjects during a total of 85 constant-force isometric contr actions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of comp uter algorithms to decompose the signal into the constituent motor uni t action potentials. Time-varying mean firing rates and recruitment th resholds were also calculated. 3. Motor units detected from the TA mus cle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at f orce 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-f orce isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing r ates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitmen t thresholds and lower firing rates tended to display the greater decr eases in firing rate over the constant-force interval, whereas motor u nits with lower recruitment thresholds and higher firing rates had les ser rates of decrease. Furthermore, increasing contraction levels tend ed to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the mai ntaining of force output while motor units decreased their firing rate s: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first bec ause none was observed during the 8-15 s duration of any of the 85 con tractions. Furthermore, contractions outside the physiological range o f 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 muscl e interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar n eedle electrodes from the wrist extensor muscles while myoelectric act ivity in the wrist flexor muscles was concurrently monitored with surf ace electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extens or muscles simultaneously decreased while the flexor muscles were dete rmined 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 docum ented in animals and humans. 8. The results of this study, combined wi th 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 und ergoes a potentiation followed by a decrease. Simultaneously, the ''la te adaptation'' property of the motoneuron decreases the firing rate o f the motor unit. Findings of this study suggest that voluntary reduct ion in firing rates also cannot be ruled out as a means to augment the adaptation in motoneurons. As the duration of the contraction increas es and the twitch force decreases, the force output of the muscle woul d be reduced below the intended force level and a voluntary increase i n the excitation to the motoneuron pool would be required to sustain t he intended force. This excitation would manifest itself in an increas e in the firing rates of the active motor units and a recruitment of n ew motor units.