NEURAL CONTROL OF RAPID MOTOR OUTPUT
Date
2020-05
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Publisher
University of Delaware
Abstract
Understanding the neural control of rapid movements is important for efforts to reduce the physical slowing that is commonly seen in older adults and with the symptom of bradykinesia in people with Parkinson’s disease (PD). PURPOSE: Rapid isometric contractions and rapid dynamic contractions at a single joint are commonly studied in research literature, but are seldom examined simultaneously. This study examined the scaling of motor rate with motor amplitude in young, healthy adults in both isometric and dynamic conditions. It was first hypothesized that both isometric and dynamic conditions would exhibit robust scaling of rate with the amplitude of motor output, and the scaling factors (rate of force development scaling factor, RFD-SF, and velocity scaling factor, V-SF) would be correlated across the two conditions. Second, it is hypothesized that the rate of rise of the first EMG burst (RER) of the agonist muscle (triceps brachii) would scale with the rate of motor output in both conditions. METHODS: Data were collected from subjects (n=10) between the ages of 18 and 30, who were free from any known neurological conditions or injuries. A custom device was built that allowed for collection of data in both isometric and dynamic conditions during elbow extension, and electromyography was used to collect data from the triceps and biceps. For the isometric condition, 5 sets of 15 force pulses were performed at 30%, 45%, and 60% of the subject’s MVC in a balanced order. For the dynamic condition, 5 sets of 15 rapid elbow extensions were performed at 30°, 45°, and 60° in a balanced order. ANALYSIS: Spike2 software and custom MATLAB routines were used to calculate peak rates (RFD, velocity, and RER) and linear regression was used to obtain dependent measures (slope and R2) to describe the scaling of motor output. RESULTS: In isometric conditions, the median RFD-SF was
15.0 s-1 with a median R2 value of 0.894. In dynamic conditions, the median V-SF was 5.67 s-1 with a median R2 value of 0.990. For the isometric condition, the median slope of the relationship between RFD and RER was 0.696 %MVC/%EMG-Max., with a median R2 value of 0.245. For the dynamic condition, the median slope of the relationship between angular velocity and RER was 0.107 °/%EMG-Max., with a median R2 value of 0.262. The two scaling factors (RFD-SF and V-SF) were correlated at R2 = 0.27. DISCUSSION: Strong linear relationships between the peak rate and amplitude of motor output were observed in both isometric and dynamic conditions, in agreement with previous studies. That the scaling of rate with amplitude was moderately correlated across conditions provides support for similar neural control mechanisms across subjects. The data collected in this study from young, healthy adults will prove useful in future studies by enabling us to compare these scaling factors in older adults or people with PD to provide more insight into differences in their control of rapid motor outputs.
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Keywords
neural control, exercise science, motor output