Abstract
Individuals with Parkinson's disease (PD) demonstrate deficits in muscle activation such as decreased amplitude and inappropriate bursting. There is evidence that some of these disturbances are more pronounced in extensor vs. flexor muscles. Surface EMG has been used widely to quantify muscle activation deficits in PD, but analysis of discharge of the underlying motor units may provide greater insight and be more sensitive to changes early in the disease. Of the few studies that have examined motor unit discharge in PD, the majority were conducted in the first dorsal interosseous, and no studies have measured motor units from extensor and flexor muscles within the same cohort. The objective of this study was to characterize the firing behavior of single motor units in the elbow flexor and extensor muscles during isometric contractions in people with mild-to-moderate PD. Ten individuals with PD (off-medication) and nine healthy controls were tested. Motor unit spike times were recorded via intramuscular EMG from the biceps and triceps brachii muscles during 30-s isometric contractions at 10% maximum voluntary elbow flexion and elbow extension torque, respectively. We selected variables of mean motor unit discharge rate, discharge variability, and torque variability to evaluate motor abnormalities in the PD group. The effects of group, muscle, and group-by-muscle on each variable were determined using separate linear mixed models. Discharge rate and torque variability were not different between groups, but discharge variability was significantly higher in the PD group for both muscles combined (p < 0.0001). We also evaluated the asymmetry in these motor variables between the triceps and biceps for each individual participant with PD to evaluate whether there was an association with disease severity. The difference in torque variability between elbow flexion and extension was significantly correlated with both the Hoehn and Yahr scale (rho = 0.71) and UPDRS (rho = 0.62). Our findings demonstrate that variability in motor output, rather than decreased discharge rates, may contribute to motor dysfunction in people with mild-to-moderate PD. Our findings provide insight into altered neural control of movement in PD and demonstrate the importance of measuring from multiple muscles within the same cohort.
Highlights
A hallmark of Parkinson’s disease (PD) is the presence of an abnormal pattern of muscle activity when performing voluntary movements [1,2,3,4]
A summary of individual participant and group mean discharge characteristics for biceps and triceps motor units for control and PD groups are shown in Table 2 and Figure 2
The present study investigated differences in motor unit discharge rate and discharge variability of an elbow flexor and an elbow extensor among individuals with mild-to-moderate PD and control participants without PD
Summary
A hallmark of Parkinson’s disease (PD) is the presence of an abnormal pattern of muscle activity when performing voluntary movements [1,2,3,4] This abnormal pattern is characterized by decreased EMG amplitude and the presence of multiple agonist bursts with highly variable duration rather than a single fused agonist burst. Greater deficits in extensor compared to flexor muscle function in PD have been demonstrated in the lower extremity [5,6,7,8,9,10] These reductions in strength and movement speed have been ascribed to impairment in the ability to activate the agonist muscle rather than to co-contraction of agonistantagonist muscles
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