Background: Arm cycling on an ergometer is common in sports training and rehabilitation protocols, but has not been widely studied from an aspect of neural control. The hand movement is constrained along a circular path, and the user is working against a resistance, maintaining a cadence. Even if the desired hand trajectory is given, there is the flexibility to choose patterns of joint coordination and muscle activation, given the kinematic redundancy of the upper limb. With changing external load, motor noise and changing joint stiffness may affect the pose of the arm even though the endpoint trajectory is unchanged, unless a control mechanism maintains the same arm configuration in corresponding time points of the cycles. However, the effect of crank resistance on the variances of arm configuration and muscle activation has not been investigated, yet.
Methods: Fifteen healthy participants performed arm cranking on an arm-cycle ergometer both unimanually and bimanually with a cadence of 60 rpm against three crank resistances. We investigated arm configuration variances and muscle activation variances. Arm configuration was given by inter-segmental joint angles, while muscle activation by surface EMGs of arm muscles. Applying multifactorial ANOVA we evaluated the effects of resistance conditions.
Results: Arm configuration variance in the course of arm cranking was not affected by crank resistance, while muscle activation variance was proportional to the square of electromyographic muscle activity. Furthermore, the shape of the variance time profiles for both arm configuration and muscle activation was not affected by crank resistance independently on cranking being performed unimanually or bimanually.
Conclusions: Contrary to the prevailing assumption that an increased motor noise would affect the variance of auxiliary movements, the influence of noise doesn’t appear at the arm configuration level even when the system is redundant. Our results suggest that neural control stabilizes arm configurations against altered external force in arm cranking. This may reflect the separation of kinematic- and force-control, via mechanisms that are compensating for dynamic non-linearities. Arm cranking may be suitable when the aim is to perform training under different load conditions, preserving stable and secure control of joint movements and muscle activations.