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Clinical Trial
. 2002 Jun 15;541(Pt 3):937-48.
doi: 10.1113/jphysiol.2001.013305.

Modulation of synchrony between single motor units during precision grip tasks in humans

J M Kilner  1 , M Alonso-Alonso, R Fisher, R N Lemon
Affiliations
Clinical Trial

Modulation of synchrony between single motor units during precision grip tasks in humans

J M Kilner et al. J Physiol. .

Abstract

During precision grip, coherence between motor cortex and hand muscle EMG oscillatory activity in the 15-30 Hz range covaries with the compliance of the manipulated object. The current study investigated whether short-term synchrony and coherence between discharges of single motor units (SMUs) in the first dorsal interosseous (1DI) muscle were similarly modulated by object compliance during precision grip. Eight subjects used index finger and thumb to grip two levers that were under robotic control. Guided by visual feedback of the lever force levels, subjects held the levers against a steady force of 1.3 N for 8 s; they then linearly increased the force to 1.6 N over a 2 s period and held for a further 8 s before linearly decreasing the force back to the 1.3 N level over another 2 s period. Subjects performed the task at two different levels of compliance, each with identical grip force levels. Both surface EMG and SMU activity were recorded from the 1DI muscle. Short-term synchrony between the discharges of pairs of SMUs was assessed in the time domain by cross-correlation and in the frequency domain by coherence analysis. Coherence was seen in two frequency ranges: 6-12 Hz and 15-30 Hz. The compliance of the gripped object had a significant effect on both short-term synchronisation and coherence in the 15-30 Hz range between SMUs; both were greater for the more compliant condition. There was no change in the 6-12 Hz coherence.

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Figures

Figure 2
Figure 2. Modulation of a single motor unit discharge by task conditions
A and B, the position of the finger (upper trace) and thumb levers during precision grip task performance under the most compliant (HI-COMP) and least compliant (LO-COMP) conditions, averaged across 25 trials. The scale of the lever displacement at the tip of the digit is shown to on the left. C and D, the firing of a single motor unit (SMU) under the two task conditions. Each point represents a discriminated discharge from the SMU. Data are aligned to the onset of the first hold period. Trial number increases from the bottom up. E and F, the average rectified EMGs of the 1DI muscle recorded during the task conditions. G and H, the interspike interval histogram (ISIH) calculated across the entire task for the single motor units shown in C and D. The ISIH were calculated between 0 and 400 ms and had a binwidth of 1 ms.
Figure 1
Figure 1. Synchronous bursting of 1DI SMUs during precision grip
A, top panel, a section of surface EMG activity from the 1DI muscle during the HOLD2 period of the task. The bottom panel shows the corresponding section of one of the raw needle electrode channels. The dashed vertical lines indicate the time of discrimination for one of the SMUs discriminated from this electrode. The time of discharge of this SMU is also indicated by the black arrows. The dark and light grey arrows show the times of discrimination of two other discriminated SMUs. The open arrows show the time that would have been expected for discrimination of SMU1 based on its average firing rate over the entire task. For this section of data SMU1 would have been expected to fire twice. B, the ISIH for the unit identified by the black arrows in A. Note that there is a clear peak around intervals of ≈80 ms, reflecting the firing rate of the unit at 11 Hz, but also a sub-peak around 180 ms (5.5 Hz) as a result of missed units in the discrimination process, as shown by the open arrows in A.
Figure 3
Figure 3. Properties of motor unit discharge under different task conditions
A, the mean of the mean firing rate of the analysed SMUs calculated from the ISIH for the first and second hold periods of the task for SMUs with more than 500 events per period. Error bars are s.e.m.s (n = 12 HI-COMP HOLD 1, n = 19 HI-COMP HOLD 2, n = 13 LO-COMP HOLD 1 and n = 20 LO-COMP HOLD 2). B, the same data using the mean of the modal firing rate calculated from the ISIH.
Figure 4
Figure 4. Motor unit synchronisation: single subject data
A-F, the synchrony measured in the time domain between three different units recorded from the right 1DI muscle during the HI-COMP task (subject JL). The data shown were pooled across both hold periods. A-C, the cross-correlations with a lag of 150 ms. D-F, the same data but only plotting the central 100 ms. A and D, the correlation between SMU1 and SMU2; B and E, the correlation between SMU1 and SMU3; and C and F, the correlation between SMU2 and SMU3. The black lines show the cross-correlograms calculated with a 1 ms resolution and the grey lines show the cusum. The ordinate on the cross-correlograms is the ratio of synchronous events to the total number of reference events. The scale of the cusum is the value ‘b’ (see Methods). Number of trials 25; number of events: n = 2038 for SMU1; 1314 for SMU2 and 1627 for SMU3.
Figure 5
Figure 5. Motor unit synchronisation: pooled subject data
A and B, the cross-correlation data and (C-E) the cusum data for motor unit pairs from all five subjects analysed. A, the percentage of significant ‘k’ values for the HI-COMP (open bars) and LO-COMP (black bars). The total number of motor unit pairs is shown under each column, with the number significant shown in brackets. C, the corresponding data for the percentage of clear positive peaks in the cusum. B, the mean ‘k’ value for the two different task conditions. D shows the mean cusum ‘b’ value from all correlations with a clear positive deflection whereas in E the mean cusum ‘b’ value from correlations with significant ‘k’ values only are shown. The n number is shown under each column. Standard error bars are shown where possible. *= significant difference, P < 0.05 Mann-Whitney, NS = non-significant difference.
Figure 6
Figure 6. Single subject data: power and coherence spectra during the HI-COMP task
A-C, the power spectra calculated using a 1024 pt FFT window for three SMUs recorded from right 1DI and pooled over the two hold periods (subject JL). These SMUs are the same as those shown in Fig. 3. D-F, the coherence spectra calculated between pairs of the power spectra shown in A-C. The dashed horizontal line indicates the 95 % confidence limit. Peaks in the coherence spectra were only considered significant if three consecutive bins were above this confidence limit. Superimposed on each plot is the power spectrum of the 1DI surface EMG recording (dashed line) calculated over the two HOLD periods. The scale of these power spectra does not correspond to the scale given on the y axis, which only applies to the SMU data.
Figure 7
Figure 7. Task-dependent changes in SMU-SMU coherence
A, the percentage of motor units pairs that showed significant coherence in the 6-12 Hz range for the HI-COMP (open bars) and LO-COMP (black bars) task conditions. Motor unit pairs were considered significant if three consecutive bins were above the 95 % confidence level. The number of total motor unit pairs analysed for coherence is shown at the bottom of each column, with the number significant shown in the brackets. B, the mean level of the coherence in the 6-12 Hz range for each task condition as in A. The total number of significant motor units pairs is shown at the bottom of each column. The error bars indicate the standard error. C and D, the corresponding analysis for the 15-30 Hz range. E, the linear regression of the ‘k’ value against the mean coherence in the 15-30 Hz range for each subject and for each task condition.
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