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1 Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
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(Received 23 September 2004;
accepted after revision 30 November 2004;
first published online 2 December 2004)
Corresponding author R. H. Westgaard: Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Email: rolf.westgaard{at}iot.ntnu.no
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Introduction |
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Trapezius motor units can be continuously active for up to one hour (Thorn et al. 2002). Whole-day SEMG recordings have shown that trapezius has a more sustained activity pattern than upper and lower limb muscles (Mork and Westgaard, unpublished observations; Kern et al. 2001). Trapezius motor unit firing pattern deviates from motor units of a distal hand muscle (first dorsal interosseus; FDI) in slow ramp contractions, by sudden onset and higher firing rates of new relative to previously recruited motor units (Westgaard & De Luca, 2001). Abrupt changes in recruitment threshold during sustained contractions have been observed, causing ‘silent’ periods without motor unit firing, and ‘substitution’ by other motor units with initially higher threshold (Westgaard & De Luca, 1999; Westad et al. 2003). Altogether, this shows that the control of low-threshold trapezius motor units has characteristics that are different from those of limb muscles.
As it is not possible to determine motor unit force in a postural muscle like trapezius with the available technology, we wanted to examine whether motor unit size could be deduced from SEMG recordings. The representation of single motor units in the SEMG signal was determined by spike triggered averaging (STA). This was anticipated to generate relatively consistent motor unit potentials, as trapezius has a sheet-like appearance with mean thickness 5 mm at the preferred SEMG electrode location (range 3–9 mm; Jensen et al. 1994). A muscle thickness of 5 mm corresponds well to the typical extent of motor unit territory (Buchthal et al. 1957; Roeleveld et al. 1997).
The first aim of this study was to examine the hypothesis that low-threshold trapezius motor units show high consistency in their contribution to the SEMG signal, but with an increase in amplitude (i.e. size) of motor unit potentials with higher recruitment threshold as formulated in the Henneman size principle. However, it appeared the STA-derived potentials of the same motor unit were highly dependent on SEMG amplitude. A second aim was therefore to examine the cause of the increase in the area of STA-derived motor unit potentials with contraction amplitude. A possible explanation is short-term synchrony in motor unit firing, which causes an increase in the apparent SEMG amplitude of STA-derived motor unit potentials (Kirkwood, 1979; Yao et al. 2000; Taylor et al. 2002). Motor unit synchrony was examined by constructing peristimulus time histograms (PSTHs) of firing events for pairs of motor units.
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Experimental procedures
Simultaneous electromyographic (EMG) recordings by surface and intramuscular electrodes were performed. The indwelling EMG signal was used to detect firing events of identified trapezius motor units while STA-derived potentials were extracted from the surface EMG (SEMG) signal. The trapezius SEMG signal was also root-mean-square (RMS) detected and used to indicate and, in some procedures, to control trapezius contraction amplitude by displaying signal amplitude on a visual display screen in front of the subject. The SEMG signal was calibrated in technical units (µV) and as a percentage of the RMS-detected EMG activity at maximal voluntary contraction (%EMGmax).
The feedback-controlled contractions consisted of slow ramp contractions (nominally from 0 to 10% EMGmax in 3 min, rise time 0.055% EMGmax s–1; some contractions reached lower amplitudes) and constant amplitude contractions of one to 30 min duration, amplitudes ranging from 1 to 8% EMGmax. All feedback-controlled contractions were carried out with the subject seated and straps placed over the shoulders to provide resistance to the attempted movement of elevating the shoulders. Shoulder elevation was performed bilaterally, with EMG data collected from the left trapezius. Three 30-min and four 10-min contractions with imposed brief periods of transient increase in SEMG activity (Westad et al. 2003) and 10 contractions of 10 min duration without transient increase in amplitude (Westad et al. 2004) provided long-duration, constant-amplitude recordings. These contractions were used to examine the stability of STA-derived potentials over time.
Motor tasks mimicking muscle activation in daily living, such as motor activity in typing and trapezius motor response to mental stress were carried out (Westad et al. 2004). These tasks were of 10 min duration and were performed without shoulder straps or any other device (except EMG electrodes) that could influence shoulder geometry or shoulder movement. Contraction amplitude varied within a range of 2–4% EMGmax during these contractions, which had mean amplitudes from <1–9% EMGmax. The mental strain task consisted of a complex, attention-demanding two-choice reaction test presented on the computer screen. An open (‘frame’) and a solid (‘brick’) quadrangle were placed in a square pattern, and an alphanumeric suggestion on how to move the brick to superimpose on the frame was given (Westgaard & Bjørklund, 1987). The subject responded by pressing one of two keys, ‘correct’ or ‘wrong’, with the right or left index finger. A new position of the ‘brick’ and ‘frame’ in the square pattern and a new suggestion then appeared. The execution of the test was self-paced, but the subject first carried out the test for 2 min at a steady pace while attempting to maintain a low failure rate. On the basis of this performance the subject was offered a small monetary reward to perform 10% faster with the same or lower failure rate for 10 min. Feedback provided on the computer screen informed the subjects about the response speed (very slow, slow, OK, fast, or very fast) and whether the answer was correct (Wærsted et al. 1994).
Physiological recordings and analyses
The SEMG signal was detected by an active differential electrode with two circular recording surfaces (6 mm in diameter, 20 mm interelectrode distance). The electrode was positioned with the medial recording surface 20 mm lateral to the midpoint of a line between the C7 spinous process and the acromion (Jensen et al. 1993). The SEMG signal was band-pass filtered at 10–1000 Hz. The RMS-detected SEMG signal was averaged at a time resolution of 0.2 s. The intramuscular EMG signal was recorded with quadrifilar wire electrodes, constructed by bonding together four 50-µm nylon-coated nickel–chrome alloy wires (‘Stablohm 800A’, California Fine Wire Co, Grover Beach, CA, USA). The wire bundle was cut transversely, exposing only the cross-section of the wires. The wire bundle was placed in a 27-gauge needle and a hook was formed at approximately one millimetre from the exposed end of the wire. The needle was inserted to a depth of approximately 10 mm at a location approximately 10 mm medial to the midpoint of a line between the C7 spinous process and the acromion, along the direction of the muscle fibres recorded by the SEMG electrode. The insertion aimed for the midpoint of the transverse extent of the SEMG electrode pick-up area. The needle was removed and the wire bundle remained lodged in the muscle. Three pairs were chosen as the differential input to the amplifiers. The signals were band-pass filtered from 1 to 10 kHz. All EMG signals were stored on a digital recorder (DATaRec-A160, Racal-Heim Systems GmbH, Bergisch Gladbach, Germany). The signals were subsequently reconverted to an analog form and digitized at a sampling rate of 50 kHz on a PC.
The intramuscular EMG signals were resolved into individual motor unit firing trains using the Precision Decomposition technique (LeFever & De Luca, 1982; De Luca & Adam, 1999). This technique uses template matching, template updating, firing probabilities and superposition resolution to identify the individual firing times of the motor units (Mambrito & De Luca, 1984). The firing rates of the motor units were obtained by inverting the time series of the interpulse intervals. Firing rates were low-pass filtered at 0.5 Hz for display purposes, but STA motor unit potentials and estimates of motor unit synchrony were derived from the unfiltered firing rates.
Data analysis
Firing events of identified motor units were used as triggers in the STA analyses, which were based on recording intervals of 20 s, corresponding to 180–250 firings, unless otherwise stated. The intramuscular electrode was located medial and the SEMG electrode lateral to the endplate region (Jensen et al. 1993), making the STA potential sometimes appear to start before the firing event. The size of the STA potential was quantified as the area underneath the curve traced by the rectified STA potential. All derivations of STA area for a continuous recorded motor unit used the same start and end points. In long-duration, constant-amplitude contractions, STA potentials were extracted at the start of the experiment and 3, 10 and 30 min into the recording. Additional epochs were used to examine STA potentials at different SEMG amplitudes or at marked changes in firing patterns, e.g. STA potential before and after motor unit silent periods.
Motor unit synchronization was determined by constructing peristimulus time histograms between simultaneously firing motor units (PSTHs; e.g. Kirkwood et al. 1982). The CUSUM method (Ellaway, 1978) was used to determine the number of extra triggering events synchronous with the reference motor unit and the width of the central peak in the PSTH. CUSUM values were normalized by number of firings of the motor unit with the lowest firing rate.
Synthesized SEMG responses from STA-derived motor unit potentials
Ten STA-derived motor unit potentials were extracted at low SEMG amplitude (range 0.6–1.6% EMGmax). The rectified area of the motor unit potentials differed by a factor of 2 (from 0.017 to 0.037% EMGmax s). Trains of firings at 10 and 12.5 pulses per second (pps), corresponding to mean firing rates in constant-amplitude contractions of approximately 1 and 5% EMGmax (Westad et al. 2004), were constructed. The start of the individual trains of firings was decided by random draw within the intervals 0–100 and 0–80 ms for motor unit trains at 10 and 12.5 pps, respectively. The SEMG response for the set of motor unit firings was determined by algebraic summation of the motor unit trains (Day & Hulliger, 2001) and average rectified value (ARV) over 0.2 s intervals determined. The simulation of the SEMG signal was repeated, using a STA-extracted motor unit potential from the first dorsal interosseus (FDI), recorded by an electrode with bar-shaped recording surfaces that had a separation of 10 mm (Westgaard & De Luca, 2001). Ten trains of firing were constructed, with the motor unit potentials scaled to mimic the variation in area of the trapezius motor unit potentials.
Statistics
Pearson correlation coefficients were determined for regression of STA-derived motor unit area versus SEMG amplitude. A Student t test was used to compare STA potentials obtained with and without the use of shoulder straps. Wilcoxon signed ranks test was used to compare STA areas extracted at different times during sustained, constant-amplitude contractions to initial STA area. A P-value of 0.05, two-tailed, was considered to indicate significant differences.
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STA potentials of motor units with high and low recruitment threshold were compared by quantifying STA area of higher threshold motor units in percentage of the simultaneously determined STA area of the lowest threshold motor unit. Table 1 shows the results for lowest-threshold motor units recruited in the intervals 0–2 and 2–4% EMGmax. There was no systematic change in STA area with recruitment threshold for motor units with thresholds <10% EMGmax.
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STA potentials at start and after 3, 10 and 30 min of constant-amplitude contraction are shown in Fig. 4A for a continuously firing motor unit. The stability of these potentials is contrasted by the 2.5-fold increase in STA area during brief periods of elevated SEMG amplitude (Fig. 4B). STA area was also examined immediately before and after silent periods of more than 1 min duration. In the example of Fig. 4C there was a 10% reduction in STA area following the silent period; however, both a slight increase and decrease of the STA potential was observed following re-recruitment.
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CUSUM is 0.045 for the wide peak, i.e. an excess of 4.5% firings in synchrony with the reference unit. A narrow central peak within the wide peak, 5 ms wide with CUSUM = 0.025, is also defined. Some PSTHs presented a narrow peak only; in the example of Fig. 5B, peak width is 5 ms and CUSUM = 0.044. Histograms show CUSUM values and width of intervals with elevated probability of firing (Fig. 5C and E) and the corresponding values for the narrow central peak (Fig. 5D and F). Strength of synchronization was examined with respect to experimental condition (constant-amplitude, mental stress, typing), but no clear differences emerged. A comparison of the first versus the last 10 min of 30 min contractions did not show any indication of time-dependent change in strength of synchronization.
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Discussion |
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The uniform size of motor unit potentials at low SEMG amplitude is probably due to the sheet-like appearance of the trapezius muscle, located on average between 7 and 12 mm below the surface (Jensen et al. 1994). The motor units presented muscle fibres at the midpoint of the transverse extent of the surface electrodes, ensuring a location within the transverse pick-up area of the SEMG electrode. Distance from the recorded motor units to the SEMG electrode is thereby largely eliminated as a source of variation for STA-derived potentials in this study.
Motor unit synchrony clearly contributes to the increase in motor unit potentials with contraction amplitude. As an example, the motor units in Fig. 5B fired 223 times in synchrony (above the number expected by chance) during trains of 5100 and 5400 firings. The 4.4% (4.1% for second motor unit) added number of firings within ±2 ms of the trigger unit increases the STA-derived motor unit potential accordingly. The contribution from firing synchrony to the increase in STA area is for the most part defined by the narrow-peak PSTHs, as there will be increasing cancellation from opposite-phase motor unit potentials at wider dispersion of the firing events. This is consistent with the observed increase in width of approximately 4 ms for STA-derived potentials at increasing SEMG amplitude.
Although the increase in motor unit potentials with increasing SEMG amplitude is, to a major extent, due to motor unit synchrony, changes to the signal source may contribute (McComas et al. 1994). The mechanism is increased Na+–K+ pump activity due to elevated K+ concentration in the interstitial fluid, causing membrane hyperpolarization and larger muscle fibre action potentials both in active and inactive muscle fibres (Hicks & McComas, 1989; Kuiack & McComas, 1992). Several studies indicate an increase of 10–30% following tetanic stimulation, assessed as transient increase in M-wave (West et al. 1996; Harrison & Flatman, 1999) and in motor unit potentials elicited by nerve stimulation (Thomas et al. 2004). A third, less likely explanation of the increase in motor unit potentials with SEMG amplitude is more efficient signal transmission from the muscle fibres to the recording electrode. Contact pressure to the shoulder had the effect of reducing motor unit potentials at relatively high SEMG amplitude.
Contrasting the immediate effect of contraction amplitude on the size of STA-derived motor unit potentials, there was little or no time-dependent change in size and duration for constant-amplitude contractions within the time period studied. This indicates that the active motor units are little affected by fatigue. However, the sensitivity to detecting changes in motor unit potential is low, as the triggering motor unit may contribute as little as 25–30% to the STA-derived potentials, if the potential derived at low contraction amplitude is the true representation of the motor unit.
The consistent and largely invariant size and firing rate (Westad et al. 2004) of low-threshold motor units in sustained contractions, together with the simulation experiments allows an examination of motor unit estimates based on the presumed effects of firing synchrony on motor unit size. The mean STA-derived motor unit area was 0.06% EMGmax s at SEMG amplitude 5% EMGmax, while the area of singular firing motor units was ~ 0.02% EMGmax s. If the apparent increase in motor unit area is due to synchronization at the mean narrow-peak value (2.8%), about 70 motor units are required to achieve the observed effect. Mean firing rate of motor units in a sustained contraction at 5% EMGmax is 12.5 pps. The simulation indicates SEMG amplitude of 3.4–4.4% EMGmax. This is in reasonable agreement with the 5% EMGmax starting point, as several factors may perturb estimates both for the simulation trials and the synchronization effects. Such factors include changes to the motor unit source potentials and effects due to the distribution of narrow-peak and wide-peak synchronization. The simulation experiments are simplistic as they do not consider the effects of e.g. firing rate variability and firing rate synchronization. However, a recent simulation study (Keenan et al. 2004) concluded that both these variables may not exert much influence on the synthesized SEMG signal, while the width of the motor unit potential had a major influence, as is also shown by our simulations.
This study has practical implications for the interpretation of results using the STA technique. Shortcomings of the STA technique have been recognized for a long time (Kirkwood, 1979). Motor unit synchronization is suggested as a cause of deviant results in the comparison of motor unit force determined by STA versus nerve microstimulation (Keen & Fuglevand, 2004). Synchronization is usually considered less of a problem in spike-triggered averaging to determine motor unit potentials, due to the restricted width of these potentials, and the method has been used to quantify motor unit size (Conwit et al. 1997; Jensen et al. 2000), motor unit numbers (Boe et al. 2004), and muscle fibre conduction velocity by use of electrode arrays (Farina et al. 2002). However, it appears that motor unit synchrony has considerable influence on STA-extracted motor unit potentials, especially for muscles presenting wide potentials. This restricts the applicability of the STA technique in studies of large muscles located away from the skin surface and with large numbers of motor units active at low contraction levels. This restriction is clearly valid for the trapezius and is probably a general problem in STA-based studies of motor units in postural muscles. A reduction in the separation of the electrode contact areas from 20 to 10 mm does not appreciably reduce the width of the motor unit potential at the skin surface, as motor unit potentials extracted from trapezius recordings with electrode bars separated by 10 mm (Westgaard & De Luca, 1999) had similar width to those presented in this study.
Trapezius motor units are expected to increase in size with increasing recruitment threshold, according to the Henneman size principle (Henneman et al. 1965), which is established and later confirmed in studies of limb muscles (e.g. Milner-Brown et al. 1973). Less is known about the motor control of postural muscles, which do not lend themselves to this type of experimentation, due to the difficulty of performing reliable motor unit force recordings, but it is generally assumed that motor control principles valid for extremity muscles hold for (compartments of) postural muscles. The size of trapezius motor units is only indirectly determined by the area of STA-derived motor unit potentials. As the observed motor units probably contribute only a fraction of the apparent size of the motor unit potential, and the added contribution is the result of highly non-linear processes, an estimate of motor unit size by the STA technique cannot be performed. However, if the non-linear effects due to cancellation of opposite-phase motor unit potentials equally influence the size of simultaneous active early- and late-recruited units, the relative size of motor units derived in the same time interval should allow inferences concerning their proportional size. Table 1 shows there is no trend of increase in size of trapezius motor units with increasing recruitment threshold for contraction amplitudes <10% EMGmax. The proportional variation in size is similar to the range of variation for the 10 lowest-threshold motor units. On this basis, it may be speculated that trapezius motor units recruited below 10% EMGmax are of equal size. With due caution in consideration of the possible sources of error that would modify this conclusion, such a motor unit organization is quite different from that of FDI, which shows progressive increase in motor unit size with recruitment threshold also for low-threshold motor units (Milner-Brown et al. 1973). It may further be argued that such a difference in the organization of motor units makes aetiological sense in that there is little need for fine control of trapezius force, beyond controlling the number of motor units recruited. Instead, the habitual control requirement is sustained low-amplitude, long-duration force generation, which in part is maintained by load sharing through motor unit substitution.
In conclusion, STA-derived trapezius motor unit potentials show strong dependence on SEMG amplitude due to motor unit synchrony and, possibly, increased amplitude of muscle fibre action potentials. Motor unit potentials in trapezius and postural muscles with relatively long signal transmission distance are especially sensitive to the effects of motor unit synchrony. Finally, trapezius motor units active in contractions with amplitude <10% EMGmax may have a uniform size.
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References |
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, 2: 
).
), and contractions without restrictions on shoulder movement (
