Introduction: The aim of the study was to compare maximal force, force steadiness, and the discharge characteristics of motor units in the tibialis anterior (TA) muscle during submaximal isometric contractions for ankle dorsiflexion and adduction of the foot.
Methods: Nineteen active young adults performed maximal and submaximal isometric dorsiflexion and adduction contractions at five target forces (5, 10, 20, 40, and 60% MVC). The activity of motor units in TA was recorded by high-density electromyography (EMG).
Results: The maximal force was similar between dorsiflexion and adduction, despite EMG amplitude for TA being greater (p < 0.05) during dorsiflexion than adduction. Τhe coefficient of variation (CoV) for force (force steadiness) during dorsiflexion was always less (p < 0.05) than during adduction, except of 5% MVC force. No differences were observed for mean discharge rate; however, the regression between the changes in discharge rate relative to the change of force was significant for dorsiflexion (R 2 = 0.25, p < 0.05), but not for adduction. Discharge variability, however, was usually less during dorsiflexion. The CoV for interspike interval was less (p < 0.05) at 10, 20, and 40% MVC, but greater at 60 % MVC, during dorsiflexion than adduction. Similarly, the standard deviation of filtered cumulative spike train of the motor units in TA were less (p < 0.05) at 5, 10, 20, and 40% MVC during dorsiflexion than adduction.
Conclusions: Although mean discharge rate of motor units in TA was similar during foot adduction and ankle dorsiflexion, discharge variability was less during dorsiflexion resulting in less accurate performance of the steady adduction contractions. The neural drive to bifunctional muscles differs during their accessory function, which must be considered for training and rehabilitation interventions.
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