Changes in EMG coherence between long and short thumb abductor muscles during human development

Abstract

In adults, motoneurone pools of synergistic muscles that act around a common joint share a common presynaptic drive. Common drive can be revealed by both time domain and frequency domain analysis of EMG signals. Analysis in the frequency domain reveals significant coherence in the range 1-45 Hz, with maximal coherence in low (1-12 Hz) and high (16-32 Hz) ranges. The high-frequency range depends on cortical drive to motoneurones and is coherent with cortical oscillations at approximately 20 Hz frequencies. It is of interest to know whether oscillatory drive to human motoneurone pools changes with development. In the present study we examined age-related changes in coherence between rectified surface EMG signals recorded from the short and long thumb abductor muscles during steady isometric contraction obtained while subjects abducted the thumb against a manipulandum. We analysed EMG data from 36 subjects aged between 4 and 14 years, and 11 adult subjects aged between 22 and 59 years. Using the techniques of pooled coherence analysis and the chi(2) difference of coherence test we demonstrate that between the ages of 7 and 9 years, and 12 and 14 years, there are marked increases in the prevalence and magnitude of coherence at frequencies between 11 and 45 Hz. The data from subjects aged 12-14 years were similar to those obtained from adult controls. The most significant differences between younger children and the older age groups were detected at frequencies close to 20 Hz. We believe that these are the first reported results demonstrating significant late maturational changes in the approximately 20 Hz common oscillatory drive to human motoneurone pools.

Figure 2. Individual subject EMG–EMG cumulant and coherence

Cumulant density and coherence spectra calculated for 2 min of the EMG data sets shown in Fig. 1. A, cumulant density between APL and APB rectified EMGs in the subject aged 4 years. The horizontal dashed line shows the expected value of zero for uncorrelated EMG signals; the continuous horizontal lines indicate the upper and lower 95% confidence limits. The cumulant contains a central peak magnitude q(u) = 0.07, duration ±8 ms, on a asymmetric broad feature duration 200 ms. B, cumulant density between APL and APB rectified EMGs in the subject age 35 years; confidence limits as in A. The cumulant contains a central peak magnitude q(u) = 0.12, peak duration ±10 ms, troughs of decreased joint EMG firing probability are seen either side of the central feature, with small secondary peaks at ±50 and ±100 ms. C, coherence calculated between APL and APB EMGs of the 4-year-old subject associated with the cumulant in A. The dashed horizontal line denotes the upper 95% confidence limit based on the assumption of independence. Significant coherence is detected in the range 1–5 Hz (maximum at 1 Hz) with occasional crossings at higher frequencies. D, coherence calculated between APL and APB EMGs of the 35-year-old subject whose cumulant is shown in B, confidence limits as in C. Significant coherence is detected in the range 5–40 Hz, with maximal coherence of 0.19 at 19 Hz.

Figure 1. Rectified EMG and auto-spectra from two subjects

A, sections of rectified surface EMGs recorded from abductor pollices longus (APL) (top panel) and abductor pollices brevis (APB) (bottom panel) from the right hand of a healthy 4-year-old male subject during isometric extension/abduction of the thumb. B and C, autospectra (1–90 Hz) constructed from 2 min of continuous APL EMG data (B) and APB EMG data (C), same data as A. D, sections of rectified surface EMG recorded from APL (top panel) and abductor pollices brevis (APB) bottom panel from the right hand of a healthy 35-year-old male subject during isometric extension/abduction of the thumb. E and F, autospectra (1–90 Hz) constructed from 2 min of continuous APL EMG data (E) and APB EMG data (F), same data as D.

Figure 4. Pooled analysis, ages 7–9 years

Pooled autospectra, phase, coherence and cumulant calculated from records of APL and APB rectified EMGs recorded from the dominant hand of 13 subjects aged 7–9 years. A, pooled autospectra from APL. B, pooled autospectra from APB. C, plot of proportion of records (n= 13) that showed significant (P < 0.05) coherence at frequencies between 1 and 90 Hz. D, pooled phase for frequencies between 1 and 90 Hz. E, pooled coherence (n= 13) between 1 and 90 Hz. The coherence is significant between 1 and 14 Hz with additional significant coherence up to 40 Hz. F, pooled cumulant density plotted ±250 ms; the cumulant contains a central peak magnitude q(u) = 0.037. Confidence limits for coherence and cumulant density estimates as in Fig. 2.

Figure 3. Pooled analysis, ages 4–6 years

Pooled autospectra, phase, coherence and cumulant calculated from records of APL and APB rectified EMGs recorded from the dominant hand of 12 subjects aged 4–6 years. A, pooled autospectra from APL. B, pooled autospectra from APB. C, plot of proportion of records (n= 12) that showed significant (P < 0.05) coherence at frequencies between 1 and 90 Hz. D, pooled phase for frequencies between 1 and 90 Hz. E, pooled coherence (n= 12) between 1 and 90 Hz. The coherence is significant between 1 and 18 Hz, with some crossings of the 95% confidence limit up to 40 Hz. F, pooled cumulant density plotted ±250 ms; the cumulant contains a central peak of magnitude q(u) = 0.04. Confidence limits for coherence and cumulant density estimates as in Fig. 2.

Figure 6. Pooled analysis, ages 22–59 years

Pooled autospectra, phase, coherence and cumulant calculated from records of APL and APB rectified EMGs recorded from the dominant hand of 11 adult subjects aged 22–59 years. A, pooled autospectra from APL. B, pooled autospectra from APB. C, plot of proportion of records (n= 11) that showed significant (P < 0.05) coherence at frequencies between 1 and 90 Hz. D, pooled phase for frequencies between 1 and 90 Hz. E, pooled coherence (n= 11) between 1 and 90 Hz. The coherence is significant between 1 and 38 Hz, with maxima at 1, 12 and 22 Hz. F, pooled cumulant density plotted ±250 ms; the cumulant contains a central peak magnitude q(u) = 0.05. Confidence limits for coherence and cumulant density estimates as in Fig. 2.

Figure 5. Pooled analysis, ages 12–14 years

Pooled autospectra, phase, coherence and cumulant calculated from records of APL and APB rectified EMGs recorded from the dominant hand of 11 subjects aged 12–14 years. A, pooled autospectra from APL. B, pooled autospectra from APB. C, plot of proportion of records (n= 11) that showed significant (P < 0.05) coherence at frequencies between 1 and 90 Hz. D, pooled phase for frequencies between 1 and 90 Hz. E, pooled coherence (n= 11) between 1 and 90 Hz. The coherence is significant between 1 and 50 Hz. F, pooled cumulant density plotted ±250 ms; the cumulant contains a central peak magnitude q(u) = 0.062. Confidence limits for coherence and cumulant density estimates as in Fig. 2.

Figure 7. Statistical comparison of coherence at each frequency between different age groups

χ² extended difference of coherence test showing frequencies of maximum coherence difference. The dashed horizontal line in all plots denotes the 95% confidence limit (χ_(0.05,1)²= 3.84). A, adult subjects versus subjects aged 4–6 years. B, adult subjects versus subjects aged 7–9 years. C, adult subjects versus subjects aged 12–14 years. D, subjects aged 12–14 years versus subjects aged 7–9 years. E, subjects aged 12–14 years versus subjects aged 4–6 years. F, subjects aged 7–9 years versus subjects aged 4–6 years.