Frictional resistance modifies splitting of a sinusoidal central command to wrist antagonists

We recently observed that when a limb is voluntarily oscillated across its passive equilibrium position, the movers’ EMG assumes a cyclic half-wave pattern that switches between antagonists as equilibrium is crossed. Instead, when a limb oscillates all above or below equilibrium, only one muscle is recruited over the whole cycle and its EMG starts increasing at the movement onset. On this basis, a simple neural network was proposed that splits one sinusoidal central command between antagonists when equilibrium is crossed. It may be asked, however, whether this neural model also holds in those cases in which the equilibrium is not confined to a single position. For example, an equilibrium range of ~25° is present in the flexion-extension excursion of the hand, when it is held vertical, between prone and supine, and oscillated in the horizontal plane. In this condition, 5 subjects rhythmically flexed-extended their hand all lateral to, all medial to or across the equilibrium range. Wrist angular position and EMG from wrist flexors and extensors were averaged over 10 cycles. In the across set-up, muscles were activated in a half-wave pattern: EMG started in extensors when hand crossed the medial limit of the equilibrium range and in flexors when it crossed the lateral limit. Only extensors were recruited in the all lateral oscillations, EMG starting at the onset of extension. The reciprocal was true in the all medial case. Adding a frictional load expanded the equilibrium range to more than 80°, allowing the hand to oscillate all within the equilibrium range. Antagonists were still activated in a cyclic half-wave pattern but now the EMGs switched at the onsets of flexion and extension, i.e. about 90° in advance with respect to the previous condition. This command splitting, that occurs in the absence of equilibrium crossing, can still be described by the proposed model if a position feedback controller, which matches the central command with the actual position, is supplemented.