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. 2010 Mar 15;588(Pt 6):967-79.
doi: 10.1113/jphysiol.2009.185520. Epub 2010 Feb 1.

Corticospinal contribution to arm muscle activity during human walking

Affiliations

Corticospinal contribution to arm muscle activity during human walking

Dorothy Barthelemy et al. J Physiol. .

Abstract

When we walk, our arm muscles show rhythmic activity suggesting that the central nervous system contributes to the swing of the arms. The purpose of the present study was to investigate whether corticospinal drive plays a role in the control of arm muscle activity during human walking. Motor evoked potentials (MEPs) elicited in the posterior deltoid muscle (PD) by transcranial magnetic stimulation (TMS) were modulated during the gait cycle in parallel with changes in the background EMG activity. There was no significant difference in the size of the MEPs at a comparable level of background EMG during walking and during static PD contraction. Short latency intracortical inhibition (SICI; 2 ms interval) studied by paired-pulse TMS was diminished during bursts of PD EMG activity. This could not be explained only by changes in background EMG activity and/or control MEP size, since SICI showed no correlation to the level of background EMG activity during static PD contraction. Finally, TMS at intensity below the threshold for activation of corticospinal tract fibres elicited a suppression of the PD EMG activity during walking. Since TMS at this intensity is likely to only activate intracortical inhibitory interneurones, the suppression is in all likelihood caused by removal of a corticospinal contribution to the ongoing EMG activity. The data thus suggest that the motor cortex makes an active contribution, through the corticospinal tract, to the ongoing EMG activity in arm muscles during walking.

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Figures

Figure 1
Figure 1. Methods
A, transcranial magnetic stimulation (TMS) was applied over the arm area of the motor cortex and motor evoked responses (MEP) were recorded in the EMG signal of the arm muscles. B, modulation of the amplitude of the MEP (marked by arrows) in posterior deltoid was assessed by applying TMS at 1.2 × MEP threshold (T) at different times during gait cycle. Each trace starts at the ipsilateral heel contact, and the delay is calculated from that heel contact. C, short interval intracortical inhibition (SICI) was tested by using a conditioning-test design, where the test stimulus (1.2 T) was given alone (control; 0 ms) or with a conditioning stimuli below threshold (0.7 T) through the same coil at a short interstimulus interval (ISI) of 2, 3, 4, 5 or 6 ms (see lower panel). The largest inhibition was obtained with ISI of 2 ms. Value at each data point is presented as mean ±s.e.m. D, stimulation of the motor cortex by subthreshold TMS. At this intensity which is below threshold to induce an MEP, an inhibition is demonstrated (here in triceps) when comparing the stimulated trace (dark) with the EMG activity without stimulation (control, light grey). The start and end of the inhibition is marked by vertical lines. The time of stimulation is represented by the arrow.
Figure 2
Figure 2. EMG and kinematics
A, stick figures representing the right arm and forearm of a subject during locomotion at 3.4 km h−1. B, shoulder (black), elbow (dark grey) and wrist (light grey) marker excursion of the same subject walking on the treadmill in the transversal plane, movement along the anteroposterior axis. The origin of the y-axis (0) was set at the front of the treadmill belt. The x-axis represents the time of the gait cycle and corresponds to the EMG averaging of the same subject walking on the treadmill, shown in C. Heel trigger corresponds to the time of the heel contact of the ipsilateral leg, which represents the beginning of the gait cycle. The stance phase (pale grey) and swing phase (dark grey) are also displayed.
Figure 3
Figure 3. Modulation of MEP in PD
A, timecourse of MEP amplitude at different times during gait cycle of a single subject. Lower panel is the rectified EMG averaging of the PD muscle. X-axis represents the time between heel strike (0) and the application of the stimulation. Upper panel displays the amplitude of MEP as % of maximal voluntary contraction (MVC) at each stimulation point. The dashed lines mark the onset of EMG bursts. Arrows point to the three time-points assessed during gait to analyse the modulation of PD MEP amplitude. B, averaged PD MEP amplitude and background EMG during walking for all subjects at 3 different time-points during gait: when the PD EMG activity was low, i.e. while the arm was mid-way during the forward movement (200–300 ms after heel strike, varying between subjects; black column, middle forward movement). The grey column represents the amount of MEP during the bursts of EMG activity at the maximal forward movement (400–700 ms after heel strike; maximal forward movement). The white column is at the time of the maximal backward excursion of the arm (900 to 1200 ms after heel strike; maximal backward movement. One subject did not exhibit activity in the PD during that period). C, comparison of MEP amplitude during locomotion (taken at maximal forward movement; grey) and tonic contraction (black) for all subjects. The comparison was made at similar shoulder angle and similar background EMG (right axis). MEP_N: normalized MEP; B-EMG_N: normalized background EMG. Amplitude is presented as mean ±s.e.m.
Figure 4
Figure 4. Modulation of SICI in PD
A, timecourse of SICI at different time-points during the gait cycle of a single subject. Amplitude of SICI activity is plotted as a percentage of control MEP. X-axis represents the time between heel strike (0) and the application of the stimulation. B, group comparison of SICI, test MEP and background EMG at 3 different times in gait for all subjects: middle point of the forward movement (black column), maximum forward excursion of the arm (grey column), and maximal backward excursion of the arm (white) On the left axis is the amplitude of SICI and on the right axis, the amplitude of test MEP (MEP normalized; MEP_N) and background EMG (normalized; B-EMG_N). C, comparison of SICI during locomotion (grey) and tonic contraction (black) was made at the same amplitude of background EMG, MEP and shoulder angle. D, amplitude of SICI during tonic contraction was plotted against different levels of background EMG. In B and C, amplitude is presented as mean ±s.e.m.
Figure 5
Figure 5. Subthreshold inhibition in PD
A, B and C, traces of PD EMG during locomotion when subthreshold TMS was applied 600 ms after heel strike (dark trace) or in the control state (light grey) at different intensities. Vertical lines show the onset and offset of inhibition. The white arrows point to the facilitation and the black arrows point to the inhibited portion of the EMG.
Figure 6
Figure 6. Subthreshold inhibition in other arm muscles
Inhibition of ongoing EMG evoked by subthreshold TMS in triceps (A), biceps (B) and ECR (C).
Figure 7
Figure 7. Subthreshold inhibition – group results
The inhibited area of EMG for individuals (grey) as well as the group (mean ±s.e.m.; black) is plotted as a percentage of background EMG during control trials, in both locomotion and tonic contraction. The y-axis depicts the area of the EMG during the inhibitory period as a percentage of the control EMG in steps without stimulation. n= 7.

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References

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