Slowing down in PNP: relationship of joint velocities in an arm-movement test and the fast TUG test

Polyneuropathy (PNP) is a prevalent neurological disorder that affects both upper and lower limbs leading to a decline in motor and sensory nerve function and consequently, to movement impairments. In our earlier work, we were able to demonstrate that PNP patients' slower gait speed as compared to healthy subjects goes along with a relative speed reduction in all major joints of the body during gait, including the arms. It is not known yet, whether this speed reduction is confined to gait or whether it is a general phenomenon of cyclic movements in PNP, which may also show up in isolated goal directed arm movements. We aim here to assess joint speed traces across the body in PNP patients during gait and evaluate the relationship to joint velocities in a different task, i.e. goal directed repetitive arm movements. We compared performances of 20 PNP patients and 20 matched healthy individuals (CG) during (i) the walking sequence of the fast executed TUG test, and (ii) a fast repetitive goal-directed arm-movement test. We were able to reproduce the reduction in joint velocities across all relevant joints during walking. Moreover this reduction of joint velocities was almost evenly distributed across the velocity traces as a function of time. In the goal-directed arm-movement test, PNP patients showed again significantly lower joint velocities across all joints involved (upper body), compared to the CG, with mean and maximum velocities both significantly reduced. The mean velocities of arm-movement test and the degree of slowing observed in the arm-movement task was strongly correlated with TUG performance. Specifically, slower arm movements were associated with longer TUG times (ρ = -0.76, p < 0.001) and reduced gait speed (ρ = 0.79, p < 0.001). Arm movement slowing significantly correlated with clinical scales, including the Falls Efficacy Scale-International (FES-I, ρ = -0.55, p = 0.012) and the Performance-Oriented Mobility Assessment (POMA, ρ = 0.69, p < 0.001). We conclude that the evenly distributed reduction of joint speed as a function of time in PNP is not confined to gait, but also seems to appear in other repetitive motor tasks, here goal-directed arm movements. We assume that joint speed reduction is a general impairment in movement planning and execution of PNP patients. We speculate that this slowing is a consequence of less reliable/ less accurate sensory feedback in the sensorimotor control loop for movement execution. As a clinical application, this finding might help to quantify the amount of the impairment of the sensorimotor control loop in PNP when gait testing is not feasible. Moreover, it might lead to better targeted training strategies to enhance motor performance in PNP. Trial registration: German Clinical Trials Register (DRKS), Identifier: DRKS00016999.