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Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Potvin, J. R. Effects of muscle kinematics on surface
EMG amplitude and frequency during fatiguing dynamic contractions. J. Appl. Physiol. 82(1): 144-151, 1997.
Fifteen male subjects performed a repetitive elbow
flexion/extension task with a 7-kg mass until exhaustion. Average joint
angle, angular velocity, and biceps brachii surface electromyographic
(EMG) amplitude (aEMG) and mean power
frequency (MPF) were calculated with each consecutive 250-ms segment of
data during the entire trial. Data were separated into concentric or
eccentric phases and into seven 20°-ranges from 0 to 140° of
elbow flexion. A regression analysis was used to estimate the rested
and fatigued aEMG and MPF values. aEMG values were expressed as a
percentage of amplitudes from maximum voluntary contractions (MVC).
Under rested dynamic conditions, the average concentric aEMG amplitude
was 10% MVC higher than average eccentric values. Rested MPF values
were similar for concentric and eccentric phases, although values
increased ~20 Hz from the most extended to flexed joint angles.
Fatigue resulted in an average increase in concentric and eccentric
aEMG of 35 and 10% MVC, respectively. The largest concentric aEMG
increases (up to 58% MVC) were observed at higher joint velocities,
whereas eccentric increases appeared to be related to decreases in
velocity. Fatigue had a similar effect on MPF during both concentric
and eccentric phases. Larger MPF decreases were observed at shorter
muscle lengths such that values within each angle range were very
similar by the end of the trial. It was hypothesized that this finding
may reflect a biological minimum in conduction velocity before
propagation failure occurs.
biceps brachii; power spectrum; concentric; eccentric; electromyography
INTRODUCTION SURFACE ELECTROMYOGRAPHY (sEMG) has been studied
extensively to determine its relationship with muscle fatigue. However,
with relatively few exceptions, studies using sEMG for this purpose have done so under isometric conditions where muscle length, velocity, and force were strictly controlled. Fatigue is generally accompanied by
increases in sEMG amplitude (8, 31) and shifts in the sEMG spectrum to
lower frequencies (spectral compression) (17, 31, 36) during prolonged
submaximal contractions. It has been proposed that muscles must respond
to fatigue with increased spatial or temporal motor unit recruitment
(25, 38) and/or synchronization (28, 29) to compensate for
decreases in force capacity and that this response accounts for the
observed increases in sEMG amplitude. The spectral compression
associated with fatigue is influenced predominantly by decreases in
action potential conduction velocity (4, 32).
The interpretation of sEMG signals from dynamic contractions is much
more difficult because movement introduces additional factors that
affect their characteristics. Muscle force capacity is highly
dependent on fiber length and is also inversely related to shortening
(concentric) velocity (15, 39) and directly related to lengthening
(eccentric) velocity (16, 19). The sEMG frequency spectrum is also
related to muscle length because shifts to higher frequencies have been
demonstrated when length is decreased (2, 24). Few studies have
analyzed the effects of muscle kinematics on sEMG spectrum
characteristics during dynamic contractions. However, some efforts have
been made to quantify fatigue during prolonged dynamic movements. These
studies, like those using isometric contractions, have demonstrated
increases in sEMG amplitude (13, 30, 34) and spectral compression (6,
9, 13) during concentric contractions. However, Tesch et al. (34) found
no changes in the sEMG-to-torque ratio or spectral characteristics
during repetitive eccentric contractions. Doud and Walsh (9) monitored
biceps brachii sEMG and muscle length during slow repetitive concentric
contractions (elbow flexion). In subjects under rested conditions, they
observed that the relationship between muscle length and sEMG frequency
was consistent with isometric studies (2, 24). However, they also found
that fatigue-related shifts to lower frequencies were more pronounced
at shorter muscle lengths.
sEMG signals have proven to be useful for quantifying muscle fatigue
during prolonged isometric contractions. However, relatively little is
known about the effects of changing muscle activity, length, and
velocity during dynamic movements on the interpretation of sEMG
statistics. The purpose of this study was to evaluate the effects of
muscle kinematics on biceps brachii sEMG amplitude (aEMG) and frequency
characteristics during a repetitive elbow flexion/extension task.
Specifically, this study was designed to determine the feasibility and
limitations of using sEMG to monitor muscle fatigue during
unconstrained, dynamic contractions.
METHODS
,
Common Mode Rejection Ratio = 130 dB at 60 Hz, and
gain = 1,000). A potentiometer was positioned over the elbow
and fastened to the forearm and upper arm to monitor the elbow joint
angle. The potentiometer was calibrated at 0 and 90° before its
application. For each task, the sEMG and potentiometer signals were
analog-digital converted (1,024 Hz) on an IBM personal computer with
LabVIEW software (National Instruments, Austin, TX). Consecutive EMG
and potentiometer data segments of 250 ms (256 samples) were collected,
processed, and saved continuously with a double-buffered acquisition
system throughout the MVC and fatiguing dynamic contractions.
Data analysis.
The average EMG amplitude (aEMG) was calculated with the rectified
signal from each 250-ms MVC and dynamic segment. The maximum sEMG
amplitude was determined from the MVC contractions. The aEMG was
normalized as a percentage of the maximum value (%MVC). For each
dynamic segment of EMG data, a fast Fourier transform was performed,
and the mean power frequency (MPF) was calculated as the frequency
centroid of the spectrum. The potentiometer data were used to calculate
the average elbow joint angle (°), and a linear regression was used
to determine the average joint angular velocity (AV) as the slope of
the joint angles.
The number of dynamic contractions and the total duration of the
dynamic trial were recorded for each subject. All trial durations were
normalized to the endurance limit. Data segments with average AV
magnitudes between 
5 and 5°/s were removed so that
contractions close to isometric were not included in the analysis. The
dependent variable data (MPF, aEMG, and AV) were sorted into two
velocity directions [concentric
(AVcon) and eccentric
(AVecc)] with seven angle
ranges within each direction (from 0-20° to 120-140°
in increments of 20°) for a total of 14 groups. Second-order
polynomial regression analyses, with normalized time as the independent
variable, were used to characterize the time history of each dependent
variable throughout the dynamic trial for each combination of velocity direction and angle range. For example, a second-order equation was
calculated for each subject to represent the MPF time history for all
observations when there was a concentric contraction and the elbow
joint angle was between 80 and 100°. For the MPF data, segments
with average aEMG levels <5% MVC were removed from the analysis
because they were found to be unreliable for characterizing the
frequency content of the EMG signals. The intercepts from these
regression models were used as an estimate of the rested dependent-variable magnitudes for each velocity-joint angle
combination. For each regression line, the intercept was subtracted
from the value at the fatigue limit, and this dependent variable was
used to represent the fatigue-induced change in MPF, aEMG, and AV over the course of each subject's dynamic trial. The second-order nature of
the regression equations allowed for a determination of the curvilinear
time history of changes throughout the trials. The models for each
individual subject were used to estimate the MPF at the start and at
every subsequent 1% interval of the dynamic trial duration. Group
averages were calculated at each interval to determine the average time
history of changes.
Statistical analysis.
Two-way repeated-measures analyses of variance were used to determine
the effects of velocity direction (concentric and eccentric) and joint
angle range (from 0-20° to 120-140°) on the
intercepts and changes of the MPF, aEMG, and AV dependent variables.
Orthogonal means comparisons were used to determine the significance of
differences among individual means when significant main or interaction
effects were observed. Comparisons were made between concentric and
eccentric values at each joint angle range, and the values from the
0-20, 60-80, and 120-140° angle ranges were compared
with each angle range, within both the concentric and eccentric
velocity directions. Significance was set at
P < 0.05.
RESULTS
The average number (±SD) of elbow flexion/extension cycles was 47.9 ± 18.7, the average duration of the dynamic trials was 155.3 ± 64.0 s, and the average cycle frequency was 19.1 ± 4.1 cycles/min. Typical raw EMG time histories and spectra are presented in Fig. 1 from rested and fatigued phases, respectively. An example of the reduced kinematic and EMG data from three consecutive flexion/extension cycles is presented in Fig. 2.
) and elbow joint angular
velocity (
). B: EMG data with MPF
() and aEMG (). MVC, maximal voluntary contraction.
Rested conditions. The average rested concentric amplitudes (22.8% MVC) were significantly higher (P < 0.01) than the rested eccentric values (12.8% MVC) (Fig. 3, A and B). Angle range was found to have a different effect on concentric and eccentric aEMG (aEMGcon, aEMGecc, respectively; P < 0.01). There was a progressive increase in aEMGcon with increased angle range. The 0-20° values were lower than all ranges >80° (P < 0.05), and the 120-140° aEMGcon were higher than those in all other ranges (P < 0.05) (Fig. 3A). The aEMGecc was unaffected by joint angle (Fig. 3B) (P > 0.5).
) and fatigued ()
conditions for each of 7 angle ranges.
A: aEMG during Con.
B: aEMG during Ecc.
C: MPF values during Con.
D: MPF values during Ecc. Values are
means ± SE for each angle range (n = 15 subjects).
There was a progressive increase in concentric MPF (MPFcon) with increased angle (decreased muscle length) (Fig. 3C). The 0-20° MPFcon values were lower than those of all other ranges (P < 0.05), and the 120-140° MPFcon values were higher than those for all other ranges (P < 0.05) (Fig. 3C). Joint angle had a somewhat different effect on the eccentric MPF (MPFecc; P < 0.01). There were no differences between MPFecc values for the 0-20°, 20-40°, and 40-60° ranges and then a progressive increase with further elbow flexion. The 120-140° MPFecc values were higher than all other ranges (P < 0.05) (Fig. 3D). The mean rested AVcon was observed to increase from low levels at full extension to a peak of 130.3 ± 11.2 (SE) °/s in the 60-80° range and then decrease again to low levels at full flexion (Fig. 4). A similar pattern was followed during the eccentric phases, although the peak AVecc magnitude of 123.0 ± 9.3°/s occurred in the 80-100° range (Fig. 4B). The AV in the 0-20° and 120-140° ranges were overestimations of the real values because segments with AV magnitudes <5°/s were not analyzed, and most of these segments occurred in the end ranges of motion.
) and fatigued ()
conditions for each of 7 angle ranges during Con
(A) and Ecc
(B). Values are means ± SE for
each angle range (n = 15 subjects).
Fatigue-related changes. Fatigue resulted in significant increases in aEMG (P < 0.01), although the nature of these changes was different across joint angles (P < 0.01) and between the concentric and eccentric phases (P < 0.01). The average increases in aEMGcon and aEMGecc at the endurance limit were 34.6% MVC (158% of the rested value) and 10.5% MVC (84%), respectively. The largest average increase in aEMGcon was 53.8% MVC in the 40-60° range, and the smallest increase was only 5.9% MVC in the most flexed range (Fig. 3C). Changes in aEMGecc were <6% MVC for angles <60°, and the largest increase of 20.7% MVC occurred near full flexion (Fig. 3D). No significant differences were observed between the MPF changes during the concentric and eccentric phases. When data were pooled, fatigue resulted in MPF decreases in each angle range (P < 0.01), and the magnitude of this decrease became progressively larger as flexion angle increased (P < 0.01). The decreases at the endurance limit ranged from 10.6 Hz (17.3% of the rested value) to 28.5 Hz (35.0%) as elbow flexion angle increased from the 0-20° to the 120-140° range. The MPF changes in the 0-20° range were significantly lower than in all ranges >60°, and those in the 120-140° range were higher than those in all other ranges (both P < 0.05) (Fig. 3, C and D). Figure 5 illustrates the average polynomial regression curves, pooled across subjects, for the MPFcon within each angle range. The joint angle effect on MPFcon, observed under rested conditions, was greatly diminished as fatigue progresses during the trial.
The average decrease in AVcon (39.5°/s) was significantly larger than the decrease in AVecc (13.5°/s; P < 0.01). The AVcon decreases were at least 19.5°/s for all angles <120°, with the largest decrease being 66.9°/s in the 80-100° range (Fig. 4). The AVecc did not change for angles <60°, and the largest decrease of 32.7°/s was observed in the 80-100° range (Fig. 4B).
DISCUSSION
The main finding of this study was that muscle length and velocity interact to affect the magnitude of fatigue-related changes in biceps brachii sEMG during repetitive flexion/extension movements. Fatigue was observed to have a larger effect on force-generating capacity at high concentric velocities while having only a small effect during all eccentric muscle actions. The effect of fatigue on MPF was observed to be similar for concentric and eccentric phases of the task. The effect of muscle length on MPF was prominent under rested conditions but diminished as fatigue progressed.
Activation under rested conditions. The observation that rested aEMGcon values were significantly higher than eccentric values (Fig. 3, A and B, respectively) was consistent with previous dynamic studies (19, 24). These differences were due to the increased capacity to generate force during muscle lengthening (16, 19) and decreased capacity as shortening velocity increases (15, 39). Joint angle was shown to have no effect on aEMG during the eccentric actions while causing some increase in aEMGcon in the more flexed ranges. Given the nature of the task, the required activation at any time would have been dependent on a complex combination of the load moment, muscle length, velocity magnitude, and velocity direction. It appears that this combination resulted in relatively constant activation levels throughout the full range of eccentric actions and for most of the concentric ranges. The higher aEMGcon levels observed in the most flexed postures were likely due to the decrease in the contractile component strength (12) and elimination of the passive elastic contributions (22) that would have been associated with the shorter muscle length. MPF under rested conditions. MPF values were observed to increase significantly with decreased biceps brachii muscle length (increased flexion) for both the rested concentric and eccentric phases (Fig. 3, C and D, respectively). These results were consistent with previous studies of concentric biceps brachii contractions (9) and isometric contractions of the biceps brachii (2, 24). The frequency characteristics of sEMG are highly dependent on the conduction velocity of action potentials along the muscle fibers (5, 32). Muscle diameter increases at shorter lengths, assuming constant volume (1, 10), and muscle diameter have been shown to be directly related to conduction velocity (14, 18). Therefore, some authors have proposed that the higher MPF values associated with increased elbow flexion result from shorter muscle lengths having higher conduction velocities (1, 33). The present results are consistent with the proposed relationship among muscle length, conduction velocity, and MPF through the entire range of elbow flexion. The observation that rested MPFecc were slightly lower than MPFcon for three of the seven angle ranges may have been due, in part, to the lower eccentric activation levels (Fig. 3, C and D). However, the average rested MPFcon were only 1.8 Hz (3%) higher than MPFecc, and the highest difference within any angle range was only 5.1 Hz (8%). Previously, no difference was found between biceps brachii MPFcon and MPFecc values when the elbow flexion was near 90° (6, 24), although MPFcon has been observed to be 11 Hz higher than MPFecc in more flexed postures (24). In the present study, the largest concentric/eccentric differences were observed to occur in the more extended positions. The lower activation levels observed during the eccentric phases would be expected to result in relatively lower MPFs. However, it has been suggested that eccentric actions are associated with a preferential recruitment of fast-twitch fibers (26, 27) that results in increased MPF (20). This may account for the similar length effects on MPFcon and MPFecc, even though aEMGecc levels were significantly lower than aEMGcon at all angles >20°. Fatigue-related changes in activation. The fatigue-related increases in aEMGcon and aEMGecc were consistent with results from previous repetitive dynamic studies of the biceps brachii (13) and other muscles (30, 34). Similar findings have also been reported in numerous studies using prolonged isometric contractions (8, 31). These aEMG increases have generally been attributed to increased motor unit recruitment (25, 38) and/or synchronization (28, 29). Increased recruitment was considered to be a likely explanation for the present results because the initial activation levels were low and the biceps brachii has been shown to recruit new units up to 85% of maximum force (21). The increases in aEMGecc were more pronounced in the higher flexion ranges (Fig. 3B). Tesch et al. (34) observed no changes in the sEMG/torque ratios during repetitive isokinetic eccentric actions of the biceps brachii. This may mean that fatigue does not result in direct increases in aEMGecc. However, in the present study, fatigue was associated with decreased extension velocity magnitude for the same angle ranges that demonstrated increases in aEMGecc (Figs. 3B and 4B). With angles between 60 and 140°, the decreased lengthening velocity of the biceps brachii would have been associated with decreased eccentric strength (16, 19) and a subsequent need for increased activation levels. This velocity effect was not observed by Tesch et al. (34) because AV was set to be constant at 180°/s. The increases in aEMGcon from rest were as high as 54% MVC and significantly larger than eccentric changes, despite substantial decreases in concentric velocity that should have contributed to enhanced force capacity (15, 39). The strength-reducing effects of fatigue may have dominated the strength-enhancing velocity effect. However, it is not fully understood why the concentric aEMG increases were so much larger than eccentric increases. It may be that the effects of fatigue on the excitation/contraction-coupling process are more pronounced when the muscle shortens. The observation that concentric aEMG increases were highest in the ranges where joint angle was changing most rapidly (Figs. 3A, 4A) may also indicate that the peripheral effects of fatigue on force capacity are more prominent at higher shortening velocities. This proposed mechanism is supported by the data of Beelan and Sargeant (3), who observed larger relative decreases in cycling power output as velocity was increased. They hypothesize that this effect may be due to a selective fatigue of the fast-twitch fibers that are more susceptible to fatigue. Fatigue-related changes in MPF. The decrease in sEMG MPF observed throughout the dynamic trials (Figs. 3, C and D, and 5) was consistent with previous dynamic studies of the biceps brachii (6, 9, 13). The MPF decrease associated with fatigue may be due to a temporal elongation of the motor unit action-potential waveform due to increased lactate levels (35) and/or impairment of the ionic pump (23) and a subsequent decrease in muscle fiber conduction velocities (4, 32). However, it appears that MPF decreases may not be entirely explained by conduction velocity decreases (4, 32) and that subsequent MPF recoveries are more rapid than lactate removal (30, 37). Tesch et al. (34) observed MPF decreases during concentric trials, but no changes were observed during repeated eccentric muscle actions. They stated that this difference may have resulted from an absence of fatigue during the eccentric trial. In contrast, the present concentric and eccentric phases demonstrated similar fatigue-induced decreases within each angle range. Both concentric and eccentric actions were performed within the same session, and the level of fatigue would have been very similar during each cycle. These results demonstrate that MPF will decrease during eccentric muscle actions if the muscle is sufficiently fatigued. One of the most interesting findings of this study was that the effect of length on MPF was progressively diminished over the course of the fatiguing trials. This finding was consistent with those of Doud and Walsh (9), who also used a regression-based method to monitor MPF changes in the biceps brachii during repeated slow (30-35°/s) controlled concentric contractions. They observed that the rates of decrease in MPF were similar at each length for the first 75% of the trials and that the shorter lengths had proportionately larger decreases only in the last 25% of the trial duration. In contrast, Fig. 5 demonstrates that the trends in the present study were essentially linear within each angle range and that the slope of these trends became more negative as joint angle decreased. These differences may have been due to the substantially larger AV used in the present study. Under rested conditions, increases in elbow joint angle from full extension to maximum flexion caused average MPFcon and MPFecc increases of 19 and 22 Hz, respectively. However, fatigue caused larger MPF decreases in the higher flexion ranges (Fig. 3, C and D), and the MPFs were observed to be relatively independent of joint angle at the end of the trials (Figs. 3, C and D, and 5). Under fatigued conditions, the average MPFcon and MPFecc differences between the 0-20° and 120-140° angle ranges were only 3.5 and 0.1 Hz, respectively. This convergence of MPF values with progressive muscle fatigue may indicate that there is some biological minimum for the action potential conduction velocities that dominate the sEMG spectrum. Recent data from Cupido et al. (7) support this hypothesis. They stimulated the biceps brachii at 20 Hz and concluded that propagation failure would occur beyond the point where conduction velocity had decreased ~50%. This decrease in conduction velocity appears to result from a slowing or complete impairment of fast-twitch motor unit action potential transmission (11). With long fiber lengths, the conduction velocity would already be relatively slow such that fatigue would only cause small decreases in velocity before transmission failure. However, in shorter fibers the initial conduction velocities would be much higher and could experience larger fatigue-induced decreases before conduction would no longer be possible. Summary. Under rested conditions, the concentric sEMG amplitudes were observed to be higher than eccentric values due to the influence of velocity on force-generating capacity. MPF values were similar for concentric and eccentric phases of the movement, although values increased with decreased muscle length. This finding was attributed to the increase in action potential conduction velocity that results when a muscle becomes shorter and its diameter increases. Fatigue resulted in an increase in the concentric sEMG amplitudes, although this increase was not uniform across elbow joint angles. Larger increases were observed in the midrange of concentric movement, and it was hypothesized that this was related to the higher velocity magnitudes that occurred in this range. There appears to be a disproportionate decrease in concentric force capacity with increased velocity and a subsequent need for higher activation. No concentric/eccentric differences were observed in the fatigue-induced changes in MPF. Larger decreases were observed at shorter muscle lengths such that final values fell within a small range. It was hypothesized that this finding may reflect a biological minimum in conduction velocity before propagation failure occurs.ACKNOWLEDGEMENTS
The author thanks Leah Bent for contributions to the data collection for this study.
FOOTNOTES
Address for reprint requests: J. R. Potvin, Department of Human Biology and Nutritional Sciences, Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1 (E-mail: jpotvin.ns{at}aps.uoguelph.ca).
Received 29 April 1996; accepted in final form 30 August 1996.
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