John B. Pierce Laboratory, New Haven, Connecticut 06519
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ABSTRACT |
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Fuglevand, Andrew J.,
Vaughan G. Macefield, and
Brenda Bigland-Ritchie.
Force-frequency and fatigue properties of motor units in muscles that
control digits of the human hand. Modulation of motor unit
activation rate is a fundamental process by which the mammalian nervous
system encodes muscle force. To identify how rate coding of force may
change as a consequence of fatigue, intraneural microstimulation of
motor axons was used to elicit twitch and force-frequency responses before and after 2 min of intermittent stimulation (40-Hz train for 330 ms, 1 train/s) in single motor units of human long finger flexor
muscles and intrinsic hand muscles. Before fatigue, two groups of units
could be distinguished based on the stimulus frequency needed to elicit
half-maximal force; group 1 (n = 8) required 9.1 ± 0.5 Hz (means ± SD), and group 2 (n = 5) required 15.5 ± 1.1 Hz. Twitch contraction times were
significantly different between these two groups (group 1 = 66. 5 ms; group 2 = 45.9 ms). Overall 18% of the units were fatigue
resistant [fatigue index (FI) > 0.75], 64% had intermediate fatigue
sensitivity (0.25 FI
0.75), and 18% were fatigable
(FI < 0.25). However, fatigability and tetanic force were not
significantly different among groups. Therefore unlike findings in some
other mammals, fast-contracting motor units were neither stronger nor
more susceptible to fatigue than slowly contracting units. Fatigue,
however, was found to be greatest in those units that initially exerted
the largest forces. Despite significant slowing of contractile
responses, fatigue caused the force-frequency relation to become
displaced toward higher frequencies (44 ± 41% increase in
frequency for half-maximal force). Moreover, the greatest shift in the
force-frequency relation occurred among those units exhibiting the
largest force loss. A selective deficit in force at low frequencies of
stimulation persisted for several minutes after the fatigue task.
Overall, these findings suggest that with fatigue higher activation
rates must be delivered to motor units to maintain the same relative level of force. Questions regarding classification of motor units and
possible mechanisms by which fatigue-related slowing might coexist with
a shift in the force-frequency curve toward higher frequencies are discussed.
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INTRODUCTION |
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Motor commands generated in the CNS of mammals
ultimately are translated into skeletal muscle force through two
interrelated processes, 1) by varying the number of motor
units that participate in a contraction (recruitment) and 2)
by modulating the rate at which action potentials drive active motor
units (rate coding). Of the two processes, rate coding appears to play
a more significant role in terms of the overall capacity to grade
muscle force (Botterman et al. 1986; Kernell
1992
). The transformation of discharge rate into force by motor
units therefore represents a fundamental feature by which the nervous
system controls skeletal muscle (Heckman and Binder
1991
).
Whereas the relationship between activation rate and isometric force is
known to have a sigmoid form (Bigland and Lippold 1954;
Cooper and Eccles 1930
), the specific shape
depends in a complex way on the contractile speed of a motor unit
(Botterman et al. 1986
; Kernell et al.
1983b
). Because of their protracted time course, slow twitch
units summate individual force impulses more readily than do fast
twitch units. Consequently, the activation rate needed for half-maximal
or maximal force is usually lower for slow than for fast twitch units
(Botterman et al. 1986
; Kernell et al.
1983b
). With fatigue, not only does force magnitude decline but
also contractile speed may decrease (e.g., Sahlin et al.
1981
). Such a fatigue-related change in contractile speed
should, in theory, reduce the motoneuron discharge rates required to
maintain maximal force (Bigland-Ritchie et al. 1983
).
Indeed, motor unit discharge rate decreases during sustained maximum
voluntary contractions (Bigland-Ritchie et al. 1983
;
Grimby et al. 1981
). Accordingly, it was hypothesized
that the reduction in motor unit discharge rates seen during prolonged
activity may help to optimize force output as the contractile
properties of the motor units change rather than directly contribute to
force decline, a phenomenon termed muscular wisdom
(Bigland-Ritchie et al. 1983
; Marsden et al.
1983
).
Few studies, however, actually examined how the force-frequency
property changes with fatigue in mammalian motor units. Initial studies
on motor units in the cat hindlimb (Powers and Binder 1991) and in thenar muscles of the human hand (Thomas et
al. 1991a
) suggest that fatigue-related adaptation in the
force-frequency relation may diverge for different classes of motor
units. Fatigable units tended to require higher rates, whereas
fatigue-resistant units either showed little change or required lower
rates to attain half-maximal force after fatigue from intermittent stimulation.
To understand better how rate coding of force is modified with fatigue,
intraneural microstimulation was used to elicit force-frequency responses in single motor units of extrinsic and intrinsic muscles of
the human hand before and after a standard fatigue protocol (Burke et al. 1973). For the majority of units tested,
higher stimulus frequencies were needed to maintain the same relative level of force after the fatigue protocol. Furthermore, the degree of
shift in the force-frequency relation toward higher frequencies was
directly related to the fatigability of the motor unit. Preliminary account of this work was presented previously as an abstract
(Fuglevand et al. 1995
).
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METHODS |
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Nineteen experiments were performed on 11 healthy human
volunteers (4 female, 7 male, ages 23-55 yr). The Institutional Human Investigation Committee approved the procedures, and all subjects gave
their informed consent to participate in the study. The method of
intraneural stimulation was described in detail elsewhere
(Macefield et al. 1996; Westling et al.
1990
). Subjects reclined in a dental chair with the upper arm
abducted to ~70°, the elbow was extended to ~150°, and the
forearm and hand were supinated and stabilized in a vacuum pillow. A
rubber sphere, mounted on an adustable metal frame, was pressed down in
the center of the palmar surface to secure the hand in place. Force and
electromyographic (EMG) responses to intraneural stimulation of single
axons in the median and ulnar nerves were recorded from long flexor
muscles of the digits [flexor digitorum superficialis (FDS), flexor
digitorum profundus (FDP), and flexor pollicis longus (FPL)] and from
intrinsic hand muscles controlling the thumb [adductor pollicis (AdP)
and abductor pollicis brevis (AbPB)].
Force and EMG recording
The distal interphalangeal joint of the test digit was enclosed
in a cylindrical fitting 2 cm in length. Fittings of different internal
diameters were used to accommodate fingers of various sizes. A Velcro
strap spanned a small gap in the cylinder to secure the digit in the
fitting. An orthogonal, biaxial force transducer was then attached to
the fitting. One axis of the isometric transducer was aligned to detect
flexion forces, and the other axis was aligned to detect abduction and
adduction forces. Digits other than the test digit were immobilized
with straps. Force was reset 10 ms before the delivery of stimuli to
the nerve by means of an analogue circuit to minimize the effect of
baseline fluctuations caused by respiration and arterial pulse pressure
waves. Stimuli were triggered and delayed from the R-wave of the
electrocardiogram to evoke twitches during the plateau phase of the
pulse pressure cycle (Westling et al. 1990). In some
cases subjects were also encouraged to breath-hold during brief
stimulation sequences to lessen baseline oscillations caused by respiration.
EMG signals were detected by five sets of surface bipolar electrodes (Ag-AgCl, 4-mm diam, ~2-cm interelectrode spacing). These were located 1) on the proximal forearm ~3 cm medial to the midline to record FDS, 2) on the distal forearm ~2 cm medial to midline to record FDP, 3) on the distal forearm ~ 2 cm lateral to midline to record FPL, 4) over the thenar eminence to record AbPB, and 5) on the palmar surface of the hand just distal to the thenar eminence, to record AdP.
Stimulation procedures
A tungsten microelectrode (1- to 5-µm tip diam, 5- to 10-µm
uninsulated length, 250-µm shaft diam, 100-300 k impedance after insertion) was inserted through the skin on the medial surface of the
upper arm to penetrate either the median or ulnar nerve. Low-intensity
negative pulses (1-5 V, 0.2 ms, ~1 Hz) were delivered through the
electrode via an isolated constant voltage stimulator. An adjacent
uninsulated electrode inserted subdermally served as the anode. The
intraneural electrode position was adjusted manually until a site was
found that elicited motor responses in one of the target muscles.
Activation of flexor digitorum profundus was distinguished from that of
flexor digitorum superficialis by the presence of evoked movements in
the distal phalanx. The force transducer was then fixed to the digit
exhibiting the largest responses. The microelectrode site was then
tested to determine whether a single motor axon could be activiated in
isolation. This procedure involved a gradual increase in stimulus pulse
intensity until all-or-nothing responses were seen simultaneously in
EMG and force. If these responses were stable over a range of
intensities above threshold (termed the safety margin), it was assumed
that a single motor axon was stimulated (cf. Macefield et al.
1996
). If a clear safety margin could not be demonstrated, the
electrode position was readjusted, and the process was repeated.
Once a site was found that yielded unitary responses, the axon was
stimulated in the following order: 1) individual pulses triggered and delayed from the R-wave of the electrocardiogram at ~1
Hz to obtain twitch responses, 2) tetanic stimulation
involving a 3.1-s train of stimuli to potentiate the force responses in which stimulus frequency increased smoothly from 5 to 80 Hz and then
returned to 5 Hz, 3) pulses triggered on the R-wave to
obtain posttetanic twitch responses, 4) a series of
constant-frequency trains, 5 s at 2 Hz, 2 s at 5 Hz, and then
1 s at 8, 10, 15, 20, 30, 50, 80, and 100 Hz to obtain
force-frequency responses, 5) a standard fatigue protocol
involving 330-ms trains at 40 Hz, 1 train/s for 2 min (Burke et
al. 1973), 6) immediately after the fatigue
protocol, the force-frequency sequence was reapplied, 7) the
stimulus safety margin was reevaluated with ~1-Hz stimuli, and
8) after 10 min of rest, stimulus sequences 1-4 were
repeated. Individual trains in the force-frequency sequence were
triggered and delayed from the R-wave such that there was a 1- to 3-s
delay between trains.
Data acquisition and analysis
Force, EMG, and stimulus pulse signals were digitally sampled at
0.8, 3.2, and 12.8 kHz, respectively, by a computerized data acquisition and analysis system (SC-Zoom, University of Umeå, Sweden).
Resultant forces were computed from the vector addition of the two
orthogonal force signals. For FDS and FDP muscles the resultant force
was practically equivalent to the flexion force vector. Twitch
parameters were measured from the ensemble average of five twitch
responses obtained before and after tetanic stimulation. These
parameters included peak force, contraction time, half-relaxation time,
time constant of relaxation after half-relaxation time, total duration,
area, and the maximum rates of rise and relaxation of force (normalized
to peak twitch force). For the force-frequency sequence, mean force was
computed over the final 250 ms for each train for stimulus frequencies
10 Hz. For trains of lower-stimulus frequency, mean force was
computed over an epoch equivalent to twice the interstimulus interval.
From the fatigue protocol, peak force, half-relaxation time, time
constant of force decay after half-relaxation time, and the maximum
rates of rise and relaxation of force (normalized to peak force) were
measured from the first five and last five trains. Changes in tetanic
response during the fatigue protocol were then computed as the ratio of
the average values obtained from the first five trains to that of the
last five trains. Paired t-tests and linear regression
analysis were used to evaluate whether the fatigue protocol caused
significant changes in the mechanical properties of motor units. Values
are expressed as means ± SD, and differences are considered
significant at P < 0.05.
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RESULTS |
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Experiments typically lasted 2.5-4.0 h. Most subjects reported experiencing light tingling sensations in small regions of skin in one or two fingers or in the palm when the electrode was moved or during electrical stimulation. On occasion, while manipulating the electrode, some subjects reported a local dull ache in the vicinity of the electrode. In those cases, the electrode was withdrawn slightly, and the angle of penetration was adjusted to alleviate the discomfort.
Reliable unitary responses to intraneural stimulation were obtained for 13 motor units. Eleven of these were supplied by axons in the median nerve, and two were supplied by axons in the ulnar nerve. The larger proportion of median nerve-supplied units reflects the preponderance of experiments involving the median nerve (15/19). Ten units belonged to extrinsic, long-flexor muscles (7 FDS, 2 FPL, and 1 FDP), whereas three units were from intrinsic muscles controlling the thumb (2 AbPB and 1 AdP). Because of the relatively small sample of units from individual muscle or muscle groups, intermuscle comparisons were not performed.
The mean (±SD) stimulus intensity for eliciting a threshold response
in the 13 motor units was 701 ± 558 mV. The increase in stimulus
intensity above threshold within which unitary responses were obtained
was 225 ± 200 mV. F waves (sporadic delayed responses to
stimulation caused by antidromic activation of the motoneuron) were
seen in two units (1 FDS and 1 AbPB). The EMG waveforms for these
F-wave responses were identical to those of the direct responses, which
provided additional supportive evidence for unitary stimulation (Westling et al. 1990).
Twitch properties
Twitch responses potentiated on average by 29.5 ± 30.7%
after 3.1 s of tetanic stimulation. Posttetanic twitch amplitude
varied over a wide range of values, from 2.6 to 135.3 mN with a mean value of 70.7 ± 41.3 mN. Twitch contraction times were unimodally distributed and ranged from 43 to 91 ms (mean 57 ± 14 ms). Twitch half-relaxation times ranged from 50 to 92 ms (mean 71 ± 12 ms), the time constant of force decay varied from 21 to 56 ms (mean 40 ± 10 ms), and total twitch duration ranged from 156 to 301 ms (mean
224 ± 43 ms). The maximum rates of rise and relaxation of force
during the twitch, normalized to twitch amplitude, varied from 19 to 53 s1 (mean 28 ± 9 s
1) and from
7 to
33 s
1 (mean
12 ± 7 s
1),
respectively. Twitch area values extended from 0.3 to 14.3 mN/s (mean
7.7 ± 4.6 mN/s).
Contraction time correlated significantly with half-relaxation time
(r = 0.64), twitch duration (r = 0.80),
and normalized maximum contraction rate (r = 0.60)
but not with normalized relaxation rate or the time constant of force
decay. As in previous reports on human motor units (Macefield et
al. 1996
; Sica and McComas 1971
; Thomas
et al. 1990
), no significant associations were found between
twitch amplitude and temporal features of the twitch nor was twitch
amplitude correlated with force-rate parameters. As would be expected,
however, twitch amplitude was positively correlated with twitch area
(r = 0.98).
Prefatigue force-frequency relation
Force and EMG responses to different stimulus frequencies from 2 to 100 Hz are shown for an FPL unit in Fig.
1. Figure 1, inset, depicts
five superimposed twitch responses. Maximum force was attained in this
unit at a stimulus rate of 80 Hz, whereas the greatest increases in
force occurred for stimulus frequencies between 10 and 30 Hz. The ratio
of twitch to maximum tetanic force for this unit was 0.17. Fusion
(absence of stimulus-related force fluctuations) was nearly complete in
this unit at a stimulus frequency of 15 Hz, well before the stimulus
frequency required for maximum force. This was typical of other units
tested in this study and is similar to that reported previously for
human toe extensor motor units (Macefield et al. 1996).
Occasionally, a stimulus pulse failed to evoke a response. This can be
seen in the 20-Hz train of Fig. 1 (sixth train from the left) as a
small gap in EMG record and a transient fall in force. At high stimulus
frequencies (80 and 100 Hz), the decay in EMG amplitude was likely
caused by signal cancellation associated with temporal overlap of
long-duration, biphasic action potentials (~16 ms) detected with
surface electrodes (Fuglevand 1995
).
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Maximum force within the range of stimulus frequencies tested was attained for all units between 50 and 100 Hz (mean 85 ± 21; median 100 Hz). It is possible that even greater forces might have been exerted had we stimulated at frequencies greater than 100 Hz; however, above 30 Hz force increased very gradually with increased frequency. Force was normalized to the maximum force generated by each unit. Stimulation at 50 Hz elicited on average 91% of maximum force. The relation between force (relative to maximum force) and stimulus frequency is shown in Fig. 2 over the frequency range from 2 to 50 Hz to highlight the range of frequencies within which most force was developed. Two groups of units could be clearly distinguished based on the stimulus frequency needed for half-maximum force (estimated by linear interpolation; Fig. 2, dashed arrows); group 1 (n = 8) required 9.1 ± 0.5 Hz, and group 2 (n = 5) required 15.5 ± 1.1 Hz. Twitch contraction times were significantly different between these two groups as were the normalized maximum contraction rates (Table 1). Indeed, there was no overlap in the range of values between the two groups for contraction time and minimal overlap for normalized contraction rate.
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Parameters related to the relaxation phase of the twitch, although
tending to be slower in group 1 units (those units requiring lower
stimulus rates to attain half-maximum force), were not significantly different between the two groups. For both groups of units combined, the stimulus frequency required to generate half-maximal force was
significantly correlated to twitch contraction time (r = 0.69) but not to other twitch properties. This finding is similar
to that shown previously for human toe extensor motor units
(Macefield et al. 1996
) and suggests that the time
course of the rising phase of the twitch appears to be the most
important determinant of the position (along the frequency axis) of the
force-frequency relation (cf. Kernell et al. 1983b
).
The peak slope (% maximum force/Hz) of the force-frequency curve was
twice as steep on average for group 1 units compared with group 2 units
and occurred at significantly lower stimulus frequencies (Table 1).
There was no overlap in the range of values between group 1 and group 2 units for the stimulus frequency of half-maximum force, peak slope, or
for the stimulus frequency of peak slope. Maximal tetanic force and the
frequency of maximal force, however, were not different between the two
groups. The ratio of twitch amplitude to maximal tetanic force was
significantly larger for group 1 (0.36 ± 0.08) units compared
with group 2 units (0.25 ± 0.06). These values for
twitch-tetanus ratio are similar to that previously reported for human
toe extensor motor units (0.28) (Macefield et al. 1996).
The larger value of the twitch-tetanus ratio for group 1 units
suggests that these units may have less capacity to vary force by rate
modulation. The difference between the two groups of units in this
regard is even greater when the rate of discharge at recruitment threshold is considered (Fuglevand et al. 1993). The
minimal rate of discharge during human voluntary contraction appears to
be similar for most motor units in a muscle and is normally ~8 Hz (De Luca 1982
; Milner-Brown et al. 1973b
;
Monster and Chan 1977
). The average force produced with
8-Hz stimulation was ~40% of maximal force for group 1 units and
17% of maximal force for group 2 units (Table 1). Therefore the
capacity to grade force by rate coding is substantially narrower for
group 1 than for group 2 motor units.
Fatigue
Eleven units were followed for the duration of the fatigue protocol. Figure 3A shows force and EMG responses of one of those units (for comparison, the same unit as depicted in Fig. 1) during 2 min of intermittent stimulation at 40 Hz. Force declined in this unit by ~50% during this period [fatigue index (FI) = 0.52] with little change in the magnitude of the EMG responses.
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In this study, the distribution of motor units with respect to
fatigability was continuous and without distinct groupings of units.
Nonetheless, when conventional criteria were applied (Burke et
al. 1973), 2 of 11 units (18%) were categorized as fatigue resistant (FI > 0.75), 7 of 11 (64%) units were of intermediate fatigability (0.25
FI
0.75), and 2 of 11 (18%) were
considered fatigable (FI < 0.25). The two units identified as
fatigable were from extrinsic flexors muscles (FPL and FDS).
In addition to changes in force magnitude, the time courses of the
tetanic responses were slower at the end of the fatigue protocol
compared with the beginning (Fig. 3B). For the 11 motor units, the mean value of the first five tetani differed significantly from that of the last five for normalized maximum contraction rate
(11.1 ± 2.2 vs. 8.6 ± 2.2 s1), normalized
maximum relaxation rate (
6.7 ± 3.0 vs.
4.9 ± 1.9 s
1), half-relaxation time (142.2 ± 62.2 vs.
208.6 ± 62.1 ms), and time constant of force decay after
half-relaxation (72.4 ± 36.5 vs. 117.9 ± 44.2 ms). There
was no significant correlation, however, between any of these
parameters or change in these parameters and FI. For the current
population of units, therefore, the degree of slowing in the tetanic
response was unrelated to change in force magnitude (cf. Thomas
et al. 1991b
).
Relationship between fatigability and prefatigue contractile properties
FI was significantly correlated with both twitch amplitude
(r = 0.80) and maximal tetanic force
(r =
0.71) (Fig.
4A) but not with contraction
time (Fig. 4B) or any other contractile speed property
measured before fatigue. This finding is consistent with that
previously reported by Thomas et al. (1991b)
for human thenar motor
units, namely, fatigue was greater in those units that initially generated the largest forces but was unrelated to contractile-rate parameters. In other mammals, fatigability also is consistently correlated with motor unit force (Kernell et al. 1975
;
Lev-Tov et al. 1988
; Olson and Swett
1966
; Reinking et al. 1975
;
Tötösy de Zepetnek et al. 1992
) and in some
studies correlated with contractile speed (Bakels and Kernell
1993
; Gardiner 1993
; Lev-Tov et al. 1988
; Reinking et al. 1975
), although this is
not always the case (Bigland-Ritchie et al. 1998
). There
was no difference between group 1 and group 2 units with regard to
fatigability (mean FI: group 1 = 0.51 ± 0.30; group 2 = 0.46 ± 0.22).
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Fatigue-related change in the force-frequency relation
Immediately after the fatigue protocol, motor units were activated
with the same sequence as before fatigue with brief trains from 2 to
100 Hz (Fig. 5). The most striking
feature of the postfatigue responses depicted in Fig. 5 (same unit as
in Figs. 1 and 3) was the absence of force at low frequencies of
stimulation (i.e., low-frequency fatigue) (Edwards et al.
1977; Jami et al. 1983
). This deficit occurred
despite the presence of fully developed motor unit action potentials.
Only for stimulus frequencies greater than or equal to 15 Hz were
detectable force responses elicited. Maximal tetanic force after
fatigue, however, was only moderately diminished from that obtained in
this unit before fatigue (215 vs. 252 mN, or 85% of prefatigue level).
For the 11 motor units, the average maximal tetanic force immediately
after fatigue was 81 ± 31% of that before fatigue. This change
with fatigue, however, did not attain statistical significance
(P = 0.06).
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The relationship between stimulus frequency and normalized force,
derived from the recordings of the unit depicted in Figs. 1 (before
fatigue) and 5 (immediately after fatigue), are shown in Fig.
6A. The force-frequency curve
shifted to the right (i.e., toward higher frequencies) with fatigue in
this unit. The frequency needed for half-maximal force was 16.9 Hz
before fatigue and increased to 24.5 Hz after fatigue (Fig.
6A, arrows), a 45% increase in this parameter. The
fatigue-related change in the frequency for half-maximal force,
expressed as a percentage of the prefatigue value, is plotted for the
11 units as a function of FI in Fig. 6B. For all units but
one, the frequency for half-maximal force increased (positive change),
indicating a rightward shift in the force-frequency relation with
fatigue. The average percentage change in the frequency for
half-maximal force was 44% (± 41%). In addition, there was a marked
correlation (r = 0.80) between the FI of the unit and
the degree of shift in the force-frequency curve. Those units that were
most fatigable also exhibited the greatest rightward shift in the
force-frequency relation.
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One parameter that has been used extensively to characterize
low-frequency fatigue is the ratio of force elicited at a low-stimulus frequency to that elicited by a high frequency (e.g., Edwards et
al. 1977). The 15-Hz-maximal tetanic force ratio was
significantly smaller after the fatigue task (0.44 ± 0.22)
compared with before (0.62 ± 0.12). This change was consistent
with the observed shift in the force-frequency curves toward higher
frequencies. Furthermore, the 15-Hz-maximal tetanic force ratio was
strongly correlated with the frequency for half-maximal force both
before (r =
0.98) and after the fatigue protocol
(r =
0.87). In addition, the regression equations
fitting these data were practically identical in fresh (Y =
0.036X + 1.04) and fatigued motor
units (Y =
0.031X + 0.96). These findings,
therefore, indicate that the 15-Hz-maximal tetanic force ratio, which
can be determined with relatively few measurements, could be used as a
robust predictor of the position of the force-frequency curve along the
frequency axis.
Interestingly, despite the shift in the force-frequency curve to higher frequencies with fatigue (as indicated by the frequency for half-maximal force and the 15-Hz-maximal tetanic force ratio), the frequency that elicited maximal force decreased with fatigue (from 85.4 ± 21.5 Hz, median 100 Hz, before fatigue to 64.5 ± 23.4 Hz, median 80 Hz, with fatigue; P < 0.05). This change was mainly due to a slight decline in force at the highest stimulus frequencies for 9 of the 11 units (e.g., Fig. 5). Nevertheless, the combined effects of a decrease in frequency for maximal force and an increase in the frequency for half-maximal force meant that the slope of the force-frequency curve was steeper in the upper-frequency range with fatigue. Indeed, the average slope of the curve between 15 Hz and the frequency for maximal force was significantly steeper with fatigue (1.3 ± 0.7% maximum force/Hz) compared with before fatigue (0.6 ± 0.3% maximum force/Hz).
Persistence of low-frequency fatigue
The contractile responses of four motor units were reexamined 10 min after the fatigue protocol. All four units exhibited low-frequency fatigue (i.e., marked depression in force responses to low-stimulus frequencies) at this time. For two of these units, however, the specific deficit in force at low-stimulus frequencies was largely eliminated by a brief period of tetanic stimulation. Force and EMG responses to low-frequency stimulation before and after a 3.1-s train of stimuli in which stimulus frequency increased from 5 to 80 Hz and then returned to 5 Hz are shown for these two units in Fig. 7, A and B. The upper force trace (light line) in each record depicts the response of the unit obtained before fatigue. For both of these units, at 10 min after the fatigue protocol (dark lines), the twitch responses associated with 1-Hz stimulation were markedly diminished before the tetanus but were restored to approximately one-half the prefatigue level after the tetanus. The transition from depressed to restored force was abrupt and occurred as stimulus frequency increased above ~15 Hz (Fig. 7, A and B, dashed vertical lines). Before fatigue (light tracing), the force developed at this same frequency was ~75% of the peak force attained during the tetanus.
|
In a third unit, force was also abruptly restored as stimulus frequency increased above 15 Hz (Fig. 7C, dashed vertical line). However, force responses at low-stimulus frequencies were practically absent both before and after the tetanic stimulation in this unit. Noticeable force was developed during the subsequent constant-frequency trains only for stimulus frequencies greater than or equal to 15 Hz. Immediately after the last train in the series (100 Hz, Fig. 7C), twitches were momentarily visible, but these responses decayed rapidly and were difficult to resolve from baseline fluctuations within a few seconds. In a fourth unit (not shown), the constant-frequency trains were not preceded by tetanic stimulation. The responses in that unit to constant-frequency trains were similar to those shown in Fig. 7C, namely, detectable force was developed for stimulus frequencies greater than or equal to 15 Hz, and twitch responses were restored for only a few seconds after the 100 Hz train.
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DISCUSSION |
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Intraneural microstimulation of motor axons, mainly supplying long
flexor muscles of the fingers, was used to examine changes in
contractile properties of human motor units after a fatigue protocol
that has been used extensively to study motor units in other mammals
(Burke et al. 1973). The only comparable previous investigation in humans was carried out for motor units of the thenar
muscles (Thomas et al. 1991a
). Two groups of motor units could be distinguished in the present study based on the stimulus frequency required to attain half-maximal force. Those units with brief
contraction times needed higher-stimulus frequencies to achieve the
same relative force compared with units with longer-duration contraction times. Unlike findings in other mammalian muscles, fast-contracting motor units were neither stronger nor more susceptible to fatigue than slowly contracting units. Fatigue, however, was found
to be greatest in those units that initially exerted the largest
forces. After the fatigue protocol, the force-frequency relation was
displaced toward higher frequencies with the greatest shift occurring
among those units exhibiting the largest force loss. The significance
of these observations is considered after a brief discussion of some
technical limitations of the current investigation.
Limitations
The most significant limitation of the current study was the small sample of motor units from which a complete set of contractile properties could be measured. This problem should not be undervalued given the diversity of properties of motor units that normally exists within a single muscle, across different muscles, and across individuals. In this investigation, data from 13 motor units in 5 muscles from 11 subjects were pooled together for analysis. Clearly, this limits the generality of the conclusions that can be drawn about the organization of motor unit properties in these muscles.
The main factor that prevented successful isolation of more units was
the difficulty in establishing and maintaining unitary stimulation of
single motor axons. Obviously, it is not possible in human subjects to
physically isolate motor axons by successive division of the motor
nerve as is done in acute animal experiments. Consequently, successful
stimulation with microelectrodes depends on the fortuitous placement of
the electrode within a nerve fascicle such that a weak stimulus
activates only one motor axon supplying a target muscle. Because of the
relatively large size of the microelectrode in relation to axon
diameter (Wall and McMahon 1985), such a site likely
requires that all axons in the immediate vicinity of the target axon be
either sensory fibers or motor axons supplying muscles whose mechanical
action have minimal effect on the detected force. The probability of
finding such a configuration improves with distance between the target
muscle and electrode site because of the greater intermingling of
sensory and motor axons supplying diverse targets within more proximal
fascicles (Sunderland 1978
). Also, with greater distance
between electrode and muscle, it is less likely that contraction within
the target muscle will displace the electrode.
In a previous study, we examined twitch and force-frequency properties
of motor units in human toe extensor muscles in response to intraneural
stimulation in the common peroneal nerve (Macefield et al.
1996). We were unable, however, to measure fatigue properties in those units because safety margins invariably deteriorated during
the fatigue protocol. This was likely due to the short distance between
the electrode site and the muscles innervated by the peroneal nerve and
associated greater density of motor axons within more distal fascicles.
In the current study, the microelectrode was inserted into the median
or ulnar nerve many centimeters proximal to the muscles under
investigation. Both the mean threshold (701 mV) and mean safety margin
(225 mV) for motor axons in the medial-ulnar nerves were nearly double
those found in the peroneal nerve (411 and 116 mV, respectively)
(Macefield et al. 1996
). These differences probably
reflect a lower density of motor axons in fascicles of the
median-ulnar nerves. Consequently, minute alterations in electrode
position or axon excitability in these nerves were less likely to
activate neighboring motor axons or to interrupt activation of the
target axon. Hence it was possible to maintain unitary stimulation of
some motor axons for several minutes, which allowed us to record the
contractile properties of their muscle units before and after fatigue.
Therefore despite the limited sample size, these records represent some of the only data of this kind obtained in human subjects. Furthermore, the similarity of features of this small population of motor units with
that of larger samples of human units obtained with intramuscular stimulation, e.g., absence of relation between twitch force and contraction time (Elek at al. 1992
; Garnett et
al. 1978
; Mateika et al. 1998
), and with that of
nonhuman motor units, e.g., significant correlation between
fatigability and tetanic force (Botterman et al. 1985
;
Fritz and Schmidt 1992
; Goslow et al.
1977
; Reinking et al. 1975
;
Tötösy de Zepetnek et al. 1992
; Zajac
and Faden 1985
), suggests that the current collection of motor
units did provide a coarse-grain view of motor unit organization in
muscles that control the digits of the human hand.
Classification of motor units
This investigation was consistent with previous studies in both
human and nonhuman muscle in that the distribution of motor unit
contraction times was relatively continuous and unimodal (Elek
et al. 1992; Goslow et al. 1977
; Kernell
et al. 1983a
; Thomas et al. 1990
). Consequently,
motor unit populations typically cannot be separated readily into slow
and fast twitch groups on the basis of twitch contraction speed
(Reinking et al. 1975
; Tötösy de Zepetnek et al. 1992
). Indeed, one conventional criteria
distinguishes fast from slow units not on the basis of twitch responses
but on whether units exhibit a small loss in force during a short train
of stimuli ("sag") (Burke et al. 1973
). Although sag
represents an important property for categorizing motor units in some
muscles, e.g., cat gastrocnemius (Burke et al. 1973
),
cat tibialis posterior (McDonagh et al. 1980
), cat
tenuissimus (Lev-Tov et al. 1988
), and rat gastrocnemius
(Kanda and Hashizume 1992
), it has also been shown to be
poorly correlated with contractile speed in other muscles, e.g., rat
tibialis anterior ( Bakels and Kernell 1993
; Tötösy de Zepetnek et al. 1992
) and cat
extensor carpi ulnaris (Fritz and Schmidt 1992
), and to
be absent altogether in human motor units (Macefield et al.
1996
; Thomas et al. 1990
). When twitch
contraction time itself was used to distinguish fast from slow units,
the boundary delineating the two groups is often chosen arbitrarily
(Olson and Swett 1966
) or may be selected based on a
property unrelated to contractile speed, such as fatigue resistance (Kernell et al. 1983a
). In addition, the contraction
time used to separate fast from slow units can vary over a wide range
of values from as little as 14 ms in rat tibialis anterior
(Bakels and Kernell 1993
) to as much as 99 ms in human
gastrocnemius (Garnett et al. 1978
).
Therefore rather than impose an arbitrary criterion to segregate motor
units into fast and slow categories based on twitch properties, we
instead chose to classify motor units into two groups based on the
configuration of the force-frequency relationship. Motor units recorded
in this study clustered into two distinct groups based on the stimulus
frequency required for half-maximal force (Fig. 2). Similar criteria
were also used previously to classify motor units in nonhuman muscle
(Botterman et al. 1985; Fritz and Schmidt
1992
; Gardiner 1993
). The force-frequency
relation represents the fundamental input-output property of motor
units; it indicates how the neural code, in terms of motor neuron
discharge rate, is transformed into a mechanical response that can be
applied to the external environment. The force-frequency property,
therefore, would seem to possess greater functional significance than
would twitch speed per se. Although features of the force-frequency relation can be predicted from the time course of the twitch
(Botterman et al. 1986
; Kernell et al.
1983b
), the relationship between these two properties is not
simple. For example, the stimulus frequency needed to attain
half-maximal force can be markedly different for motor units with
identical twitch contraction times but residing in different muscles
(Kernell et al. 1983b
). Consequently, we used a
parameter that more directly denoted the configuration of the
force-frequency relationship for different motor units, namely, the
stimulus frequency for half-maximal force. On the basis of this
parameter, we discerned two groups of units that differed in how they
encode isometric force through rate modulation.
Distinct clustering of motor units based on fatigability was not
observed. The majority of units had intermediate sensitivity to fatigue
similar to what was described for some muscles in various species,
e.g., rat tibialis anterior (Bakels and Kernell 1993; Tötösy de Zepetnek et al. 1992
), cat
tibialis anterior and extensor digitorum longus (Goslow et al.
1977
), cat tenuissimus (Lev-Tov et al. 1988
),
and human first dorsal interosseous (Stephens and Usherwood
1977
). This result, however, is the converse of that described
for other muscles in which a clear bimodal distribution is found such
that most units are either fatigue resistant or fatigable, and few
units are categorized as having intermediate fatigue properties, e.g.,
cat medial gastrocnemius (Burke et al. 1973
), cat
peroneus longus (Kernell et al. 1983a
), and cat tibialis posterior (McDonagh et al. 1980
). Also, the current
distribution of fatigue indices was different from that observed in
human thenar muscle (Thomas et al. 1991b
) and ankle
extensors of the skunk (Van de Graaff et al. 1977
) in
which the largest proportion of motor units was fatigue resistant, a
smaller proportion was classified as intermediate fatigable, and no
units were categorized as fatigable. Therefore there seems to be
several designs by which motor unit populations can be organized
according to fatigability. These differences probably relate to
variation in the repertoire of motor behaviors across muscles and
species (Van de Graaff et al. 1977
).
It is also possible that species differences in fatigability might
arise because the same stimulation protocol has been applied to induce
fatigue despite large differences in contractile speed (Bigland-Ritchie et al. 1998). For the relatively slow
human motor units, 40-Hz stimulation produces near maximal force,
whereas it elicits only partially fused, submaximal force in cat and
rat motor units. Accordingly, it may be more appropriate to induce fatigue with a stimulus frequency selected for each motor unit that
evokes a particular percentage of maximal force rather than use the
same frequency for all units.
A consistent although unexpected feature of human motor units is the
absence of a relationship between twitch speed and force magnitude.
This was reported for a variety of human muscles, including those
involved in posture and locomotion (Garnett et al. 1978; Macefield et al. 1996
), intrinsic muscles of the hand
(Elek et al. 1992
; Thomas et al. 1990
),
intrinsic muscles of the foot (Sica and McComas 1971
),
respiratory muscles (Matieka et al. 1998
), muscles involved in mastication (Nordstrom and Miles
1990
), and extrinsic muscles that control the digits of the
hand as found in the current study. In most nonhuman muscles, weak
motor units tend to be slow (Bakels and Kernell 1993
;
Burke et al. 1973
; Dum et al. 1982
;
Gordon et al. 1986
, 1990
; Kanda and Hashizume
1992
; Kernell et al. 1983a
). In contrast there
is little evidence to suggest that weak motor units are inevitably slow
or that strong units are necessarily fast in human muscle
(Bigland-Ritchie et al. 1998
).
Data from this study as well as from others (Nordstrom and Miles
1990; Thomas et al. 1991b
) also indicate
that contractile speed and fatigue sensitivity are poorly correlated
among pools of human motor units. Interestingly, this is also the case
for a wide array of nonhuman muscles, including rat tibialis anterior (Tötösy de Zepetnek et al. 1992
), rabbit
masseter (Kwa et al. 1995
), cat flexor digitorum longus
(Olson and Swett 1966
), cat extensor digitorum
(Fritz and Schmidt 1992
), and cat tibialis anterior and
extensor digitorum longus (Goslow et al. 1977
). For some
of these muscles, the absence of a correlation between contractile speed and fatigue was probably due in part to the preponderance of fast
twitch units that are known to have a broad range of fatigabilities and
a narrow range of contractile speeds (Fritz and Schmidt
1992
; Goslow et al. 1977
; Kwa et al.
1995
). Along these lines, it is also conceivable that
intraneural stimulation might have biased our sample toward those units
with larger diameter (more excitable) axons that presumably supply
fast, fatigable, and strong muscle units. However, the majority of
units recorded in the present study could be considered slow based on
the relatively long-duration twitches and the low-stimulus rates
required to attain half-maximal force. Therefore it seems in some
mammalian muscle and in particular human muscle that contractile speed
is in no predictable way related to fatigability among a pool of motor units.
One aspect of motor unit organization that seems consistent across
muscles and species is that strong motor units invariably fatigue more
rapidly than do weak motor units. A significant association between
motor unit force (twitch or tetanic) and fatigability was found in
practically every muscle in which the relation was examined in both
nonhuman (e.g., Fritz and Schmidt 1992; Gardiner 1993
; Goslow et al. 1977
; Kernell et al.
1975
; Kugelberg and Lindegren 1979
;
Lev-Tov et al. 1988
; McDonagh et al.
1980
; Olson and Swett 1966
; Reinking et
al. 1975
; Tötösy de Zepetnek et al.
1992
) and human muscle (Garnett et al. 1978
;
Stephens and Usherwood 1977
; Thomas et al.
1991b
), including this study. This finding probably
relates to systematic differences in the extent of daily activity
across a pool of motor units. It is well established that motor units
typically are recruited in an orderly sequence from those units that
exert the smallest forces toward those that generate the largest forces
(Henneman and Mendell 1981
; Milner-Brown et al.
1973a
; Zajac and Faden 1985
). Most forms of
muscular activity involve only a fraction of the motor unit population
(Enoka 1995
). Consequently, low-threshold, weak motor
units will tend to be active for a much greater duration of a day
compared with high-threshold, strong units. Because sensitivity to
fatigue can be dramatically reduced by increasing the amount of chronic
activity (Kernell et al. 1987
), it follows that
low-threshold motor units will tend to be more fatigue resistant than
high-threshold motor units. In some respects then, categorization of
motor units according to force capacity and fatigue sensitivity could
be considered more physiologically relevant and more harmonious across
species than prevailing classification schemes. In other words, it may be more meaningful to characterize motor units based on the conjunction of fatigability and contractile strength rather than that of
fatigability and contractile speed.
These two schemes were compared for the current set of data in Fig. 4. When contraction time was plotted against FI, as is ordinarily done to classify motor units, no relation between the two variables was seen nor was there distinct clustering of data into subgroups (Fig. 4B). On the other hand, when tetanic force was plotted against FI (Fig. 4A), not only was the relationship between these two variables significant but three clusters of motor units also could be distinguished. One group was weak and fatigue resistant, a second (and most abundant) group was of intermediate strength and intermediate fatigue sensitivity, and a third group (one unit) was strong and fatigable. Because of the limited sample size, however, it is important that these findings not be overgeneralized until comparable criteria were applied to larger motor unit populations, including those from other muscles and species.
Fatigue-related change in the force-frequency relationship
Prolonged activation of muscle not only leads to loss in force but
may also cause the contractile responses to slow. In the absence of
other changes, slowing in contractile speed should cause the
force-frequency curve to shift toward lower frequencies (Bigland-Ritchie et al. 1983). In this investigation,
despite marked slowing in the contractile responses with fatigue (e.g., Fig. 3B), the force-frequency curve for most motor units was
displaced toward higher, rather than lower frequencies. Similar
findings were described previously for single muscle fibers of the
mouse (Westerblad et al. 1993
), motor units in the cat
hindlimb (Powers and Binder 1991
), motor units in human
thenar muscles (Thomas et al. 1991a
), and whole human
muscle (Bergström and Hultman 1990
;
Binder-McLeod and McDermond 1992
; Edwards et al.
1977
).
How can fatigue-related slowing in contractile speed be reconciled with
a shift in the force-frequency curve toward higher frequencies? One
possible explanation is that other modifications in neuromuscular
function override the effect that contractile slowing alone would have
on the force-frequency relation. One candidate for such a modification
is a frequency-dependent impairment of excitation-contraction
coupling. Virtually no force was developed in motor units at
low-stimulus frequencies immediately after (Fig. 5) and for several
minutes after the fatigue protocol (Fig. 7). This absence of force
occurred, even though EMG responses were fully developed, suggesting
that the site of impairment was distal to the sarcolemma. Because the
deficit in force at high-stimulus frequencies was small, the intrinsic
capacity of cross-bridges to generate force appears to have been
minimally affected by the fatigue protocol. By exclusion then, it
appears that impairment of intervening processes associated with
excitation-contraction coupling were the principal contributors to
fatigue. These arguments follow those previously articulated to explain
similar findings, generally referred to as low-frequency fatigue, in a
variety of preparations (Edwards et al. 1977;
Grabowski et al. 1972
; Jami et al. 1983
;
Powers and Binder 1991
; Westerblad et al.
1993
).
An intriguing aspect of low-frequency fatigue as revealed in this study
was the abrupt transition from no force to near-normal force when
stimulus frequency exceeded a particular value, usually ~15 Hz (Figs.
5 and 7). One interpretation of these findings is that
excitation-contraction coupling was disrupted only at low-stimulus frequencies. Current theories, however, suggest that disturbances in
the calcium-release channel of the sarcoplasmic reticulum lead to a
decrease in the amount of calcium released per action potential, independent of frequency (Fitts 1996; Westerblad
and Allen 1991
;Westerblad et al. 1993
). This
implies that the concentration of myoplasmic calcium should be less for
all stimulus frequencies up to that needed to just attain calcium
saturation. As a consequence, the relation between relative force and
frequency should have a lower slope over the entire rising phase of the
curve, and the frequency needed for maximal force should be greater in
low-frequency fatigue. However, our findings as well as those of others
(Faulkner 1983
; Powers and Binder 1991
;
Thomas et al. 1991a
) indicated that the rate of rise of
force with frequency once initiated was steeper and that there was a
decrease in the frequency for maximal force with fatigue.
What then could account for the specific impairment of
excitation-contraction coupling at low but not at high stimulus rates and the prolonged time course of this effect? Efficient communication between voltage-sensitive dihydropyradine receptors in the t-tubule and
ryanodine-receptor calcium-release channels of the sarcoplasmic reticulum depends on the intimate juxtaposition of these molecules (Franzini-Armstrong and Protasi 1997). Recently, Gomez
et al. (1997)
demonstrated that reduced contractility in hypertrophied cardiac muscle cells was associated with an increased physical separation between adjacent dihydropyradine and ryanodine receptors. A
similar alteration might occur in skeletal muscle with fatigue. One
aftereffect of prolonged activity that could change the microgeometry between the t-tubule and sarcoplasmic reticulum and that follows a
relatively long time course is postcontraction swelling of muscle fibers (Eisenberg and Gilai 1979
; Lannergren et
al. 1996
). In swollen fibers, movement of the charged elements
of the voltage-sensitive receptor in response to a single action
potential, although normal (Gomez et al. 1997
;
Györke 1993
), might not be sufficient to provoke
calcium release from the sarcoplasmic reticulum. However, if subsequent
action potentials arrive before the charged elements fully relax to
their initial state, summation of displacements could occur. With
sufficiently brief time intervals between action potentials (i.e.,
higher stimulus rates), the overall displacement of the charged
elements may be adequate to induce calcium release from the
sarcoplasmic reticulum. Because the rate of calcium reuptake by the
sarcoplasmic reticulum is slowed with fatigue (Westerblad and
Allen 1993
), myofibrillar calcium could accumulate to
relatively high concentrations during trains of stimuli at activation
rates in excess of that required to initiate calcium release. In this way fatigue-related slowing of the contractile response (caused by
diminished rate of calcium uptake) feasibly could coexist with a
steeper but rightward-shifted, force-frequency relation.
Functional consequences
Finally, it is of interest to consider the functional implications
of fatigue-related alterations in the force-frequency relation. In this
study, the stimulus frequency needed to attain half-maximal force in
human motor units increased on average by >40% with fatigue. To
varying degrees, similar findings were reported by others in a variety
of preparations (Bergström and Hultman 1990;
Binder-McLeod and McDermond 1992
; Edwards et al.
1977
; Powers and Binder 1991
; Thomas et
al. 1991a
; Westerblad et al. 1993
). In
addition, there was a significant tendency for the most fatigable (and
strongest) units to exhibit the greatest rightward shift in the
force-frequency relation (Fig. 6B) (see also Thomas
et al. 1991a
). Therefore for the type of fatigue task
used in these studies, greater activation rates would be required to
maintain the same relative level of force with fatigue. Furthermore,
the demand for higher activation rates would be greatest for those
units least capable of maintaining force.
A crucial question then is whether the nervous system keeps pace with
fatigue-related changes in contractile function of motor units during
voluntary activity (Fuglevand 1996). The discharge rates
of motor units decrease by ~50% from an initial level of ~30 Hz
during sustained maximum voluntary contractions (Bigland-Ritchie et al. 1983
). This finding together with the results of this
study implies that fatigue-related adjustments in motor neuron
discharge may fail to accommodate for changes in the mechanical
properties of motor units and may directly contribute to force loss.
However, two additional and interrelated factors require consideration. First, whereas intermittent activity was used to induce fatigue in the
current investigation, little is known about how (if) motor neuron
discharge adapts during intermittent voluntary contractions. Second,
few studies have examined how the force-frequency relation changes as a
consequence of sustained rather than intermittent contraction.
Investigation into both these areas is needed to establish whether
adaptation in motor neuron discharge during prolonged activity helps
prevent or contribute to fatigue.
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ACKNOWLEDGMENTS |
---|
This work supported by National Institutes of Health Grants NS-14576 and HL-30062 to B. Bigland-Ritchie and AR-42893 to A. J. Fuglevand.
Present address: V. G. Macefield, Prince of Wales Medical Research Institute, Sydney, New South Wales 2031, Australia; B. Bigland-Ritchie, Dept. of Pediatrics, Yale University School of Medicine, 47 Deepwood Dr., Hamden, CT 06517.
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FOOTNOTES |
---|
Present address and address for reprint requests: A. J. Fuglevand,
Dept. of Physiology, University of Arizona, P. O. Box 210093, Tucson, AZ 85721-0093.
E-mail: fuglevan{at}u.arizona.edu
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 1 April 1998; accepted in final form 5 January 1999.
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REFERENCES |
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