Scaling of Rate of Force Development with Force Amplitude is Impaired by Motor Segmentation in Parkinson’s Disease

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Abstract Healthy nervous systems scale peak rates of force development (RFD) to peak force in rapid isometric contractions, with low variability. In people with Parkinson’s disease (PwPD), scaling of RFD is reduced and more variable compared to healthy adults. Some PwPD also have transient reductions in RFD (motor segmentation) within individual force pulses, but how this affects scaling is unclear. Thus, we aimed to determine how the magnitude and variability of rate-scaling differs between PwPD with (PD Seg ) and without (PD NoSeg ) motor segmentation and compared to healthy young (YA) and older adults (OA). Twenty YA, 22 OA, and 58 PwPD performed 60 rapid isometric finger abduction force pulses to 20–80% maximal voluntary contraction force (MVC). The slope (RFD-SF) and variability (R 2 ) of the relationship between peak force and peak RFD were computed per individual. PwPD with a median of ≥ two force segments were classified as PD Seg . The Kinesia ONE system quantified bradykinesia and total motor symptom scores. In PwPD, the 42 PD Seg had reduced RFD-SF and R 2 compared to PD NoSeg (p ≤ 0.004), OA (p Seg , peak RFD was lower in segmented force pulses compared to pulses without motor segmentation (p 2 but not RFD-SF (p ≥ 0.154) in PwPD. Motor segmentation reduces overall RFD and impairs rate-scaling in PwPD, but with remarkable preservation of rate scaling in the first segment. Rate-scaling and motor segmentation measures provide specific information about Parkinson’s disease impairments that is not visible in other assessments.

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