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. 2023 Oct 12;18(10):e0292807.
doi: 10.1371/journal.pone.0292807. eCollection 2023.

Evaluation of stretch reflex synergies in the upper limb using principal component analysis (PCA)

Frida Torell  1
Affiliations

Evaluation of stretch reflex synergies in the upper limb using principal component analysis (PCA)

Frida Torell. PLoS One. .

Abstract

The dynamic nature of movement and muscle activation emphasizes the importance of a sound experimental design. To ensure that an experiment determines what we intend, the design must be carefully evaluated. Before analyzing data, it is imperative to limit the number of outliers, biases, and skewness. In the present study, a simple center-out experiment was performed by 16 healthy volunteers. The experiment included three load conditions, two preparatory delays, two perturbations, and four targets placed along a diagonal path on a 2D plane. While the participants performed the tasks, the activity of seven arm muscles were monitored using surface electromyography (EMG). Principal component analysis (PCA) was used to evaluate the study design, identify muscle synergies, and assess the effects of individual quirks. With PCA, we can identify the trials that trigger stretch reflexes and pinpoint muscle synergies. The posterior deltoid, triceps long head, and brachioradialis were engaged when targets were in the direction of muscle shortening and the perturbation was applied in the opposite direction. Similarly, the pectoralis and anterior deltoid were engaged when the targets were in the direction of muscle shortening and the perturbation was applied in the opposite direction. The stretch reflexes were not triggered when the perturbation brought the hand in the direction of, or into the target, except if the muscle was pre-loaded. The use of PCA was also proven valuable when evaluating participant performance. While individual quirks are to be expected, failure to perform trials as expected can adversely affect the study results.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. The robotic platform and experimental setup.
(A) The participant manipulated the position of a robotic handle while playing a simple computer game with four potential targets situated along the Y-dimension. Throughout the experiment, the participants sat in an adjustable chair and had their right arm secured to the handle. All visual stimuli were projected onto a one-way mirror. The participants could not view their arm or the robotic handle, instead, the position of their hand was represented by a cursor (see Materials and Methods for more details). (B) Each new trial was initiated when the cursor was brought to the origin and stayed there for 1–1.5 seconds. This action initiated the load condition of the particular trial (no load or a slow-raising 4N load in either the +Y or -Y direction). Regardless of load condition, the participant must remain immobile inside the origin. A target was cued by turning red and remained cued either for 250 ms or 750 ms. This preparatory delay was followed by a rapid position-controlled perturbation of the hand (3.5 cm in 150 ms, during which the cursor position was frozen). At the end of the perturbation, the cued target turned green (the ’Go’ signal), and the participant had to move the cursor to the target as straight and swiftly as possible. (C) The continuously displayed potential targets. Origin diameter 1.3 cm. Near targets were spaced 3.5 cm from the center of the origin and had a diameter of 5.1 cm. The far targets were placed 9 cm from the center of the origin and had a diameter of 2.4 cm. Abbreviations: T3 = target 3, T1 = target 1, T2 = target 2, T4 = target 4, cc = center-to-center distance.
Fig 2
Fig 2. First component of the PCA.
(A) Score plot. The normalized EMG data was centered. The first component explained 20% of the variation. Light gray indicates a +Y perturbation, Dark gray indicates -Y perturbations. Colored by perturbation. Sorted by trial (numbers indicate trial number). (B) Loading plot. The colors represent the different muscle functions, muscles that were triggered by a +Y perturbation were colored purple, while muscles that were triggered by a -Y perturbation were colored blue, and the muscles that responded to both were colored gray. EMG1: brachioradialis, EMG2: biceps, EMG3: triceps lateral head, EMG4: triceps long head, EMG5: anterior deltoid, EMG6: posterior deltoid, and EMG7: pectoralis.
Fig 3
Fig 3. Second component of the PCA.
(A) Score plot. The normalized EMG data was centered. The second component explained 13% of the variation. Colored by perturbation and sorted by trial (numbers indicate trial number). (B) Loading plot. The colors here represent the different epochs, very light gray = pre-perturbation, light gray = post-perturbation, gray = SLR epoch, dark gray = early LLR, and black = late LLR. The placement of the peaks is still the same–EMG1: brachioradialis, EMG2: biceps, EMG3: triceps lateral head, EMG4: triceps long head, EMG5: anterior deltoid, EMG6: posterior deltoid, and EMG7: pectoralis.
Fig 4
Fig 4. Contribution to flexion/extension overview.
For clarity the data is displayed in the same timeframe instead of a long sequency. (A) The three different columns represent the three different loads that were applied to the left the load was applied in the -Y direction, in the middle no load was applied and to the right a +Y load was applied. For each of these trials the targets were in the -Y direction. The haptic perturbation was applied in the +Y direction. ‘Contribution to extension’ refers to contribution to shoulder extension, with an internal rotation and an elbow extension. (B) Loads and perturbation same as in (A), but the cued targets were in the +Y direction. ‘Pushed into target’ refers to the trials when the perturbation brought the handle into or in the direction of the target. (C) Loads same as (A), targets were in the +Y direction and the perturbation was applied in the -Y direction. ‘Contribution to flexion’ refers to contribution to shoulder flexion, with an internal rotation and a simultaneous shoulder extension that is required to reach the cued target. (D) Same loads as in (A), targets in the -Y direction and perturbation in the -Y direction, meaning that the perturbation brought the hand into or towards the cued target. The colors represent the different muscle functions, muscles with stretch reflex that were triggered by a +Y perturbation were colored purple (EMG1, EMG4 and EMG6), while muscles that were the stretch reflex was triggered by a -Y perturbation were colored blue (EMG5 and EMG7), and the muscles’ that responded to both were colored gray (EMG2 and EMG3). EMG1: brachioradialis, EMG2: biceps, EMG3: triceps lateral head, EMG4: triceps long head, EMG5: anterior deltoid, EMG6: posterior deltoid, and EMG7: pectoralis.
Fig 5
Fig 5. Participant overview using PCA score plot.
The UV-scaled PCA had two significant components where the first and second components explained 12% and 10% of the variation respectively. The ellipse resents the 95% confidence limit for Hotelling’s T2. Fourteen of the participants end up in close proximity while two participants deviate from the group, these were named participant x and participant y.
Fig 6
Fig 6. Contribution plot Participant x and y.
(A) Participant x. The participant called x mainly deviated in the first component, hence the contribution plot only depicts the first component. (B) Participant y. The participant called y deviated in both the first and mainly the second component, hence the contribution plot only depicts both components. Colored by epoch. Abbreviations: Py = participant y, Px = participant x.
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Grants and funding

The author received no specific funding for this work.
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