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1 Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309-0354; and 2 Institut National de la Santé et de la Recherche Médicale/Equipe de Recherche et d'Innovation Technologique 0207, Unité de Formation et de Recherche Sciences et Techniques des Activités Physiques et Sportive, Université de Bourgogne, 21078 Dijon, France
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Twenty-four men (n = 11) and women (n = 13) supported an inertial load equivalent to 20% of the maximum voluntary contraction force with the elbow flexor muscles for as long as possible while maintaining a constant elbow angle at 90°. Endurance time did not differ on the three occasions that the task was performed (320 ± 149 s; P > 0.05), and there was no difference between women (360 ± 168 s) and men (273 ± 108 s; P = 0.11). The rate of increase in average electromyogram (EMG) for the elbow flexor muscles was similar across sessions (P > 0.05). However, average EMG during the fatiguing task increased for the long head of biceps brachii, brachioradialis, and brachialis (P < 0.05) but not for the short head of biceps brachii. Furthermore, the average EMG for the brachialis was greater at the start and end of the contraction compared with the other elbow flexor muscles. The rate of bursts in EMG activity increased during the fatiguing contraction and was greater in brachialis (1.0 ± 0.2 bursts/min) compared with the other elbow flexor muscles (0.5 ± 0.1 bursts/min). The changes in the standard deviation of acceleration, mean arterial pressure, and heart rate during the fatiguing contractions were similar across sessions. These findings indicate that the EMG activity, which reflects the net excitatory and inhibitory input received by the motoneurons in the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, these results contrast those from a previous study (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003) when the goal of the isometric contraction was to maintain a constant force. These results, from a series of studies on the elbow flexor muscles, indicate that the type of load supported during the fatiguing contraction influences the extent to which endurance time can change with repeat performances of the task.
fatigue; electromyography; task; sex; fluctuations in acceleration
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE MECHANISMS THAT LIMIT the endurance time of contractions sustained at low-to-moderate forces appear to vary with the requirements of the task. For example, the endurance time for a contraction maintained at 15-20% of maximal voluntary contraction (MVC) force of the elbow flexor muscles 1) was longer for women compared with men (14), 2) improved with repeat performances of the task when maintaining a force and pushing against a force transducer (maintain-force task) (15), and 3) was longer for a maintain-force task compared with the requirement to maintain a constant elbow angle while exerting an equivalent net muscle torque (maintain-position task) (16).
The longer endurance time of women for the maintain-force task was associated with a lower target force and a reduced pressor response compared with the performance of men (14). However, the average electromyogram (EMG) at exhaustion was similar for both sexes, which suggested that the difference in endurance time between men and women involved a mechanism distal to the site of muscle activation. In contrast, an increase in the endurance time of some men and women across repeat performances of the maintain-force task were associated with a reduced rate of increase in motor unit activity (15). The alterations in motor unit activity were indicated by a decrease in EMG burst activity, a decline in the rate of increase in the average EMG, and an alteration in the distribution of average EMG among the elbow flexor muscles. Because prolongation of the endurance time was independent of the sex of the individual, the target force exerted, and the amplitude of the pressor response, the limiting mechanism likely involved alterations within the nervous system.
In another study, the endurance time for a submaximal maintain-force task was twice as long as that for a maintain-position task (16). The EMG of the elbow flexor muscles increased at the same rate for the two tasks but was less at exhaustion for the maintain-position task compared with the maintain-force task. Accordingly, the rates of increase in mean arterial pressure (MAP) and heart rate (pressor response), ratings of perceived exertion (RPE), and the fluctuations in force and acceleration were greater for the maintain-position task. Because the pressor response is mediated by central command and feedback from group III and IV afferents (1, 11, 23, 24), these results suggest that the amplitude of the excitatory inputs from descending commands and inhibitory inputs from the muscle onto the motoneuron pools (10) were greater for the maintain-position task. However, the identical rates of change in EMG activity indicated that the net output of the pool was similar for the two tasks (16). Consequently, despite a similar net muscle torque for the two tasks, the briefer endurance time for the maintain-position task was attributable to a greater impairment of neural mechanisms.
These observations raised the question of whether the endurance time for a submaximal contraction with the elbow flexor muscles would increase with repeat performances of the maintain-position task, as it had for some subjects with the maintain-force task (15). The purpose of the study was to compare the endurance time and muscle activation of the elbow flexor muscles across three performances of a maintain-position task. Contrary to expectations, there were no alterations in either endurance time or the rate of increase in EMG activity across the three sessions for the maintain-position task. However, muscle activation differed among the elbow flexor muscles during the fatiguing contraction; there was no increase in the average EMG of the short head of biceps brachii, whereas there was an increase in the average EMG of the long head of biceps brachii, brachialis, and brachioradialis muscles. Furthermore, the brachialis muscle exhibited twice the rate of EMG bursts compared with the other elbow flexor muscles. A preliminary account of these results has been published in abstract form (19).
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Twenty-four healthy adults [11 men and 13 women; 26 ± 6 (SD) yr; range, 19-34 yr] volunteered to participate in the study. None of the subjects had any known neurological disorder or cardiovascular disease, and all were naive to the experimental protocol and procedures. Before participation in the study, all subjects gave informed consent, and the Human Subjects Committee at the University of Colorado approved the protocol.
Subjects performed isometric contractions with the elbow flexor muscles of the nondominant arm on 3 separate days (7 days apart). The fatiguing contractions required the subject to maintain the elbow joint angle at a right angle (1.57 rad) while supporting an inertial load that was equal to 20% of the maximum voluntary contraction (MVC) force. The subject was provided with visual feedback of the elbow angle during the fatiguing contractions and was required to sustain the contraction until exhaustion.
Mechanical Recording
Subjects were seated upright in an adjustable chair with the nondominant arm abducted slightly and the elbow resting on a padded support. The elbow joint was flexed to 1.57 rad so that the forearm was horizontal to the ground and the force at the wrist was directed upward when the elbow flexor muscles were activated voluntarily. Two nylon straps were placed vertically over each shoulder to restrain the subject and minimize shoulder movement. The hand and forearm were placed in a modified wrist-hand-thumb orthosis (Orthomerica, Newport Beach, CA) for the MVCs and constant-force tasks. The forearm was placed midway between pronation and supination.The force exerted at the wrist in the upward vertical direction was measured with a Siebe-Lebow load cell (300-lb. range; Eaton, Troy, MI). The load cell was mounted on a steel frame and connected to a wrist strap by a steel cable. The force exerted by the wrist in the downward vertical direction due to contraction of the elbow extensor muscles was measured with a force transducer (JR-3 Force-Moment Sensor, JR-3, Woodland, CA) that was mounted on a custom-designed, adjustable support. The orthosis was rigidly attached to the force transducer. The forces detected by the transducers were recorded on digital tape (DAT Sony PC 116, Montvale, NJ; bandwidth direct current to 5 kHz). The force exerted in the vertical direction was displayed on a 14-in. monitor that was located at eye level ~1.6 m in front of the subject.
Elbow angle during the position task was measured with an electrogoniometer (XM110 and K100, Penny and Giles, Cwmfelinfach, Gwent, UK) that was taped to the lateral side of the elbow joint. The output was recorded on digital tape and displayed on the 14-in. monitor. An inertial load equivalent to 20% of MVC force was suspended from the wrist, at the same location that the wrist was attached to the force transducer for measurement of MVC force of the elbow flexor muscles. Two uniaxial accelerometers (model 7265A-HS, Endevco, San Juan Capistrano, CA) were mounted on a right-angled aluminum platform that was secured to the orthosis near the thumb. One accelerometer was aligned to record acceleration in the vertical direction and the other accelerometer was oriented to record side-to-side acceleration. The accelerations were recorded on digital tape.
Electrical Recordings
EMG signals were recorded with bipolar surface electrodes (Ag-AgCl, 8-mm diameter; 20-mm distance between electrodes) that were placed over the long head of biceps brachii, the short head of biceps brachii, brachioradialis, and the triceps brachii muscle. Reference electrodes were placed on a bony prominence at the elbow or shoulder. The EMG of the brachialis muscle was measured with an intramuscular bipolar electrode inserted into the muscle 3-4 cm proximal to the antecubital fold. The electrode comprised two stainless-steel wires (100-µm diameter) that were insulated with Formvar (California Fine Wire, Grover Beach, CA). One wire in each pair had the insulation removed for ~2 mm to increase the recording volume of the electrode. A surface electrode (8-mm diameter) placed on a bony prominence served as the reference electrode. The EMG signal was amplified (500-2,000×) and band-pass filtered (20-800 Hz for the surface EMG and 20-1,000 Hz for the intramuscular EMG) with Coulbourn modules (Coulbourn Instruments, Allentown, PA) before being recorded on digital tape and displayed on an oscilloscope.Cardiovascular Measurements
Heart rate and blood pressure were monitored throughout the fatiguing contraction with an automated beat-by-beat, blood pressure monitor (Finapres 2300: Ohmeda, Madison, WI). The blood pressure cuff was placed around the middle finger of the dominant hand, and the arm was placed in a sling so that it was relaxed with the hand at heart level. The blood pressure signal was recorded on digital tape.Experimental Protocol
Subjects were required to perform the protocol on three occasions, with 1 wk separating each session. Before the first experimental session, each subject visited the laboratory for an introductory session to become familiar with the equipment and the procedures, and each performed several trials of the MVC task. All subjects were naive to the experimental protocol and procedures before the familiarization session. The protocol comprised an assessment of the MVC force for the elbow extensor and the elbow flexor muscles, determination of the EMG-force relations for the elbow flexor muscles, and performance of a fatiguing contraction that was immediately followed by assessment of MVC force of the elbow flexor muscles. The experiments for each subject were performed at the same time of the day on each occasion.MVC force. Each subject performed three MVC trials with the elbow extensor muscles and three trials with the elbow flexor muscles in each experimental session. The MVC task consisted of a gradual increase in force from zero to maximum over 3 s, with the maximal force held for 2-3 s. The force exerted by the wrist was displayed on a 14-in. monitor, and each subject was verbally encouraged to achieve maximal force. There was a 60- to 90-s rest between trials. If the peak forces from two of the three trials were not within 5% of each other, additional trials were performed until this was accomplished. The greatest force achieved by the subject was taken as the MVC force and used as the reference to calculate the target force for the fatiguing contraction.
EMG activity. The EMG activity of the involved muscles was recorded in standardized tasks so that the amount of muscle activation could be compared across sessions. For the elbow flexor muscles, the subject held constant-force contractions for 6 s at target forces of 20, 40, and 60% MVC force. The subject was given a 60-s rest between each contraction. The order of the contractions was randomized across subjects but remained constant for each subject on the 3 experimental days. These data were used to evaluate the reliability of the EMG measurements across sessions for each subject.
Fatiguing contraction. The fatiguing contraction of the elbow flexor muscles was performed with a load hung from the wrist. The load was equivalent to the 20% MVC force as determined from the MVC performed on that day. The fatiguing contraction was terminated when the elbow angle declined by 10° from the right angle for greater than ~5 s or when the subject lifted the elbow off the support for greater than ~5 s, despite strong verbal encouragement.
The RPE was assessed with the modified Borg 10-point scale (3). The subjects were instructed to focus the assessment of effort on the arm muscles performing the task. The scale was anchored so that 0 represented the resting state and 10 corresponded to the strongest contraction that the arm muscles could perform. The RPE was measured at 30-s intervals during the fatiguing contraction.Data Analysis
All data collected during the experiments were recorded on digital tape and subsequently digitized (analog-to-digital converter, 12-bit resolution) and analyzed off-line by using the Spike2 data-analysis system (Cambridge Electronic Design, Cambridge, UK). The force, position, acceleration, and blood pressure signals were digitized at 200 samples/s, whereas the EMG signals were digitized at 2,000 samples/s.The MVC force was quantified as the average value over a 0.5-s interval that was centered about the peak force. Similarly, the maximal EMG for each muscle was determined as the average value over a 0.5-s interval that was centered about the peak rectified EMG. The rectified EMG of the constant-force contractions for the elbow flexor muscles performed at 20, 40, and 60% of MVC force was averaged over the middle 4 s of the 6-s contraction.
The fluctuations in acceleration during the fatiguing contraction were quantified in the vertical and horizontal (side-to-side) directions. The acceleration signals were high-pass filtered at 4 Hz with a fourth-order Butterworth filter (Coulbourn Instruments, Allentown, PA). The fluctuations in acceleration were characterized as the standard deviation of acceleration for the first 30 s; 15 s on both sides of 25, 50, and 75% of endurance time; and the last 30 s of the endurance time for the fatiguing contraction.
The EMG activity of the elbow flexor muscles and elbow extensor muscles during the fatiguing contraction was quantified in two ways: 1) for statistical purposes, as averages of the rectified EMG (AEMG) over the first 30 s; 15 s on both sides of 25, 50, and 75% of endurance time; and the last 30 s of the endurance time for the fatiguing contraction; and 2) for graphic presentation, as the AEMG for every 1% of the endurance time. The EMG was normalized to the peak EMG obtained during the MVC.
Because low-force contractions of long duration are characterized by
bursts of EMG activity, we quantified the number and duration of these
bursts throughout the fatiguing contraction for each muscle (Fig.
1). The rectified EMG signal was
1) smoothed with a low-pass filter at 2 Hz for surface EMG
signals and at 3.8 Hz for the intramuscular EMG (brachialis),
2) differentiated to identify rapid changes in the EMG
signal, and 3) divided by the average of the rectified EMG
so that muscles with different EMG amplitudes could be compared. A
burst was identified when the smoothed, differentiated EMG signal
increased by more than 0.31 s
1 for the surface EMG and
0.28 s1 for the intramuscular EMG. These values
represented three standard deviations above the mean of the smoothed,
differentiated EMG signal. The 3-standard-deviation criterion was based
on sample EMG records of fatiguing contractions from the present data
set when the EMG signal displayed minimal bursting during the
contraction. The end of a burst was identified as the time when the
smoothed EMG signal decreased to the same amplitude as at the start of the burst. When the EMG signal did not decline to the same EMG amplitude at the start of the burst, however, the end of the burst was
then identified as the time that the differentiated EMG signal became
most negative before the start of the next burst. This criterion
represented the time at which the signal decreased most rapidly before
the beginning of the next burst. The start of two bursts was
constrained to be >2 s apart and the minimum burst duration was
0.5 s.
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Heart rate and MAP during the fatiguing contraction were analyzed from recordings at 25% intervals throughout the endurance time.
Statistical Analysis
Data are reported as means ± SD within the text and displayed as means ± SE in the figures. Separate two-factor (session × sex) ANOVA with repeated measures on session (StatView, SAS Institute) were used to compare the dependent variables for endurance time and change in MVC force. Separate three-factor ANOVAs (session × sex × time) with repeated measures on task and time were used to compare the dependent variables of heart rate, MAP, RPE, and standard deviation of acceleration in the vertical and horizontal directions. A four-factor ANOVA (session × intensity × sex × muscle) with repeated measures on session and intensity was used to compare the EMG-force relation for the constant-force contraction. Separate four-factor ANOVAs (session × time × sex × muscle) with repeated measures on session and time were used to compare the burst rate and AEMG during the fatiguing contraction. Because bursts were sometimes absent during a one-third interval of endurance time for some subjects, averages of the burst duration are reported and the results of independent t-tests are indicated where these analyses were possible. Post hoc analyses (Tukey-Kramer) were used to test for differences among pairs of means when appropriate. A significance level of P < 0.05 was used to identify statistical significance.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The purpose of the study was to compare muscle activation and
endurance time across repeat performances of a fatiguing contraction performed with the elbow flexor muscles when the subject supported an
inertial load at the wrist. The endurance time for the fatiguing contractions was similar across sessions (session 1:
315 ± 114 s; session 2: 337 ± 194 s;
session 3: 309 ± 132 s; P > 0.05). Furthermore, the endurance times were not statistically
different between the men (273 ± 108 s) and women (360 ± 168 s; P = 0.11) and there was no interaction
for sex and session (P > 0.05), indicating the
consistent absence of a sex difference across the three sessions (Fig.
2A).
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MVC force before the fatiguing contraction did not change across sessions (session 1: 251 ± 93 N; session 2: 248 ± 94 N; session 3: 254 ± 92 N; P > 0.05) (Fig. 2B). Men (337 ± 50 N) were nearly twice as strong as women (179 ± 44 N; P < 0.05), which meant that the inertial load supported by the men (6.9 ± 1.0 kg) during the fatiguing contraction was almost two times greater than that for the women (3.6 ± 0.9 kg). MVC force at exhaustion declined by 28 ± 13% (P < 0.05) for all subjects, and there was no difference either across sessions (P > 0.05) or between the sexes (P > 0.05).
EMG-Force Relation
The AEMG for the elbow flexor muscles was determined during the three sessions with isometric contractions held at 20, 40, and 60% MVC force. AEMG increased with contraction intensity for all the elbow flexor muscles (P < 0.05; Fig. 3) and was similar across sessions (P > 0.05). However, the brachialis muscle had greater AEMG at all target forces compared with the biceps brachii (short and long heads) and the brachioradialis muscles (P < 0.05). There was an interaction for sex and target force (P < 0.05) such that the women had greater AEMG at 20% MVC force compared with the men but not at 40 and 60% MVC force, and this was consistent across sessions. These results indicate that the relation between AEMG and force for the elbow flexor muscles was consistent across the sessions for men and women.
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Adjustments in AEMG
The amplitude of the AEMG (% peak MVC) for each elbow flexor muscle during the fatiguing contractions was similar across the three sessions (P > 0.05). However, there were differences between muscles in the amplitude and rate of increase in the AEMG during the fatiguing contraction. Although the AEMG of the long head of biceps brachii, brachioradialis, and brachialis muscles increased progressively until the endurance limit (P > 0.05), AEMG for the short head of biceps brachii was not different at the endurance limit compared with the first 30 s (P > 0.05; Fig. 4). At the start of the contraction (first 30 s), the normalized AEMG of the short head of biceps brachii (13 ± 4%) was similar to that of the long head of biceps brachii (12 ± 4%) and brachioradialis (12 ± 4%) muscles. At the end of the fatiguing contraction (last 30 s), however, the AEMG of the short head of biceps brachii (13 ± 5%) was similar to the start of the contraction and less than that for the long head of biceps brachii (20 ± 1%) and brachioradialis (21 ± 2%) muscles. Furthermore, the AEMG for the brachialis muscle was significantly greater at the start (20 ± 6%) and end (29 ± 9%) of the contraction compared with that of the other elbow flexor muscles (P < 0.05). Nonetheless, the rate of change in AEMG was similar for the brachialis, long head of biceps brachii, and brachioradialis muscles. These differences in AEMG between the elbow flexor muscles were consistent across sessions.
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The AEMG of triceps brachii during the fatiguing contractions was substantially less than that for the elbow flexor muscles, but the average values increased during the fatiguing contractions from the start of the contraction (2 ± 3%) to the endurance limit (5 ± 3%; P < 0.05). This increase was similar across sessions (P > 0.05). Although the triceps brachii AEMG was greater in the women (4 ± 3%) compared with the men (2 ± 2%, P < 0.05), this difference was similar across sessions.
Bursts of EMG Activity
The fatiguing contractions were characterized by a progressive increase in the number of bursts in EMG activity (Fig. 5). The burst rate for all the elbow flexor muscles increased from 0.2 ± 0.7 bursts/min in the first third of the contraction to 1.5 ± 2.5 bursts/min during the last third of the contraction (P < 0.05). The average burst rate was similar across sessions (session 1: 0.5 ± 0.7 bursts/min, session 2: 0.7 ± 1.1 bursts/min, session 3: 0.7 ± 0.8 bursts/min; P > 0.05). However, there was a main effect of muscle (P = 0.05) due to the greater burst rate of the brachialis muscle (1.0 ± 1.3 bursts/min) compared with the other elbow flexor muscles (0.5 ± 1.6 bursts/min; P < 0.05). Furthermore, women (0.5 ± 1.1 bursts/min) had a lesser rate of EMG bursts compared with men across all sessions (0.8 ± 2.1 bursts/min; P < 0.05), although the increase in burst rate across the fatiguing contractions was similar for men and women (P > 0.05).
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The mean burst duration of EMG activity for the elbow flexor muscles during the fatiguing contractions was 5.0 ± 5.8 s. The mean burst duration was similar for all three sessions (session 1: 5.0 ± 6.3 s; session 2: 5.4 ± 6.7 s, session 3: 4.6 ± 4.3 s; P > 0.05) and did not change across the fatiguing contractions (P > 0.05). There was no difference in burst duration between men and women (P > 0.05). Furthermore, the burst duration was similar for all muscles (P > 0.05): short head of biceps brachii, 6.3 ± 6.2 s; long head of biceps brachii, 4.1 ± 3.6 s; brachioradialis, 4.7 ± 7.3 s; and brachialis, 4.2 ± 3.7 s.
Fluctuations in Acceleration
The amplitude of the fluctuations in acceleration (standard deviation) in the vertical and horizontal directions increased progressively during the fatiguing contraction (Fig. 6). The relative increase in the fluctuations in the vertical acceleration (524 ± 443%) was greater than that for the side-to-side acceleration (184 ± 145%). There were no differences across sessions in either the amplitude or the rate of increase in the standard deviation of acceleration (P > 0.05) for the vertical and side-to-side directions. The standard deviation of acceleration for the men tended to be greater than for the women in both the vertical (P = 0.05) and side-to-side directions (P = 0.07). However, the relative increase in fluctuations during the contractions was similar for men and women in both directions (P > 0.05).
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MAP and Heart Rate
MAP increased during the fatiguing contractions for all three sessions and for men and women (P < 0.05; Fig. 7A). There was no difference in the MAP across sessions at rest and during the fatiguing contractions. The resting MAP was similar for session 1 (83 ± 10 mmHg), session 2 (80 ± 8 mmHg), and session 3 (83 ± 7 mmHg; P > 0.05) and for men (83 ± 3 mmHg) and women (81 ± 8 mmHg; P > 0.05). The MAP increased similarly across sessions from 100 ± 8 mmHg at the start of the contraction to 134 ± 14 mmHg at the endurance limit. Furthermore, MAP was similar at the start of the contraction for men (99 ± 6 mmHg) and women (99 ± 10 mmHg; P > 0.05) and at the endurance limit for men (135 ± 12 mmHg) and women (134 ± 16 mmHg; P > 0.05). These results indicate that the rate of increase in MAP during the fatiguing contractions was similar for men and women (Fig. 7A).
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Heart rate was similar across all three sessions at rest (67 ± 10 beats/min; P > 0.05), at the start of the contraction (81 ± 11 beats/min; P > 0.05), and at the endurance limit (100 ± 11 beats/min; P > 0.05). Furthermore, men and women had a similar heart rate at rest, the start of the contraction and the endurance limit (Fig. 7B). For example, heart rate was similar at the start of the contraction for men (81 ± 13 beats/min) and women (82 ± 9 beats/min; P > 0.05) and was similar at the endurance limit for men (98 ± 11 beats/min) and women (103 ± 10 beats/min; P > 0.05).
RPE
RPE during the fatiguing contraction began and ended at similar scores for all three sessions (P > 0.05). The rate of increase in RPE was similar across sessions and for men and women, progressing from 2.2 ± 1.1 at the start to 4.2 ± 1.5 at 25% of endurance time, 6.9 ± 1.8 at 50% of endurance time, 8.6 ± 1.3 at 75% of endurance time, and 10 ± 0 at the endurance limit.| |
DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The purpose of the study was to compare the endurance time and muscle activation of the elbow flexor muscles across three performances of a maintain-position task. The endurance time and the pattern of muscle activation during the fatiguing contraction did not change across the three sessions. Accordingly, there were no alterations across sessions in the other neural indexes, including RPE, amplitude of fluctuations in acceleration, and MAP. However, the amplitude and rate of increase in activation during the fatiguing contraction varied among the elbow flexor muscles: 1) the AEMG of the long head of biceps brachii, brachioradialis, and brachialis increased during the fatiguing contraction, whereas that for the short head of biceps brachii did not, and 2) both the AEMG and burst rate for brachialis during the fatiguing contraction were greater than the other elbow flexor muscles. In contrast to a maintain-force task (15), there was no sex difference in endurance time or the rate of increase in MAP for the maintain-position task.
Endurance Time Across Sessions
The endurance time for the fatiguing contraction when supporting the inertial load was similar across sessions. This was accompanied by a similar reduction in MVC force at the endurance limit and similar rates of increase in fluctuations of acceleration and RPE during the fatiguing contraction across sessions. These results indicate that the performance during the contraction and the relative fatigue at the endurance limit were consistent across sessions when the task involved supporting an inertial load with a postural contraction.We expected the endurance time for a maintain-position task to increase across sessions in some subjects, as had been observed previously with the maintain-force task (15). The previously observed increase in endurance time across sessions for the maintain-force task at 20% of MVC force was associated with changes in the rate of rise in AEMG, AEMG at the endurance limit, bursting activity of the interference EMG, rate of rise in RPE, and fluctuations in force (15). Furthermore, because higher levels of EMG were attained at the endurance limit during the maintain-force task compared with the maintain-position task when performed at 15% of MVC force (16), and a deficit in the net motor unit activity at the endurance limit is commonly observed for a sustained low-force contraction when compared with maximum levels of EMG attained before the contraction (9, 34), it seemed reasonable to expect that practice might improve the ability to increase motor unit activity during the maintain-position task. However, this did not occur: endurance time, rate of increase in the AEMG, and AEMG at the endurance limit did not change across sessions for the maintain-position task. These findings indicate that the net activation of the muscle, which represents the balance of the excitatory and inhibitory input into the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, the capacity to change endurance time across sessions is related to the type of load supported and hence the control strategy used during the fatiguing contraction.
Consistent with these observations, there were no changes in bursting activity of the interference EMG, the RPE scores, or the fluctuations in acceleration across sessions. The absence of a change in the bursting activity across sessions, which contrasts with that observed for the maintain-force task across sessions (15), suggests an inability to reduce the transient increase in motor unit activity with practice across sessions for the maintain-position task. Furthermore, the rate of rise in RPE and amplitude of fluctuations in accelerations, which are modulated by descending drive (5, 21, 22), did not change across sessions for the maintain-position task. Similarly, although there was a substantial increase in the pressor response (mean arterial pressure and heart rate) during each fatiguing contraction, the rate of increase did not change across sessions. These results indicate that the plasticity of the activation signals received by the motor neurons and sent from the spinal cord to muscle varies with the type of load supported during the fatiguing contraction.
Increase in AEMG Differed Between Synergist Muscles
The increase in AEMG during a low-force contraction is largely due to the recruitment of larger motor units as the muscle becomes progressively fatigued (4, 6, 7, 12). Any deviation from this gradual increase, therefore, likely represents a change in motor unit recruitment. In the present study, there were differences in the rate of increase in the AEMG among the elbow flexor muscles: the short head of biceps brachii did not show the characteristic increase in activation that was common to the long head of biceps brachii, brachioradialis, and brachialis muscles. In contrast, the short head of biceps brachii had similar increases in AEMG to that of the long head of biceps brachii and brachioradialis muscles during a maintain-force task performed at 20% of MVC force (Fig. 8) (14). This difference in the AEMG for the short head of biceps brachii between the two tasks suggests a task-specific activation of motor unit populations within the biceps brachii, which has been demonstrated previously in single motor unit experiments (31-33). Furthermore, the rate of increase in activation of the short head of biceps brachii was lower during a high-intensity task when the arm was suspended compared with a restrained position (8). The progressive increase in EMG of the short head of biceps brachii for the maintain-position task observed previously (16), occurred with a load at 15% of MVC, compared with a load at 20% of MVC that was used in the present study. Thus the lack of increase in motor unit activity of the biceps brachii short head during a fatiguing contraction may be specific to conditions of reduced restraint at higher loads. Because the endurance time of a submaximal contraction can be influenced by the distribution of muscle activation across a group of synergist muscles (17, 28-30), the differences in activation among the elbow flexor muscles likely contributes to the difference in endurance time for the maintain-force task (1,451 ± 970 s) (15) and maintain-position task (320 ± 149 s, present study) at 20% MVC.
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In addition to the inhomogeneity of biceps brachii activity, the brachialis activity during the fatiguing contraction differed in two ways: 1) the normalized AEMG for the brachialis was greater during the fatiguing contraction than that for the short and long heads of biceps brachii and brachioradialis, and 2) the brachialis had twice the EMG bursting activity compared with the other elbow flexor muscles. The greater AEMG of the brachialis was observed in both the maintain-force and maintain-position tasks in other studies (14, 16, 28). Furthermore, there was greater bursting activity in the interference EMG for brachialis during the maintain-position task compared with the maintain-force task when performed at 15% of MVC force (16). These results underscore the differences that can occur in the distribution of activity among a group of synergist muscles during an isometric fatiguing contraction with variation in the demands of the task.
Endurance Time and Pressor Response Were Similar for Men and Women
A significant finding in the present study was the similarity of the pressor response for men and women. In contrast, a previous study involving a maintain-force task performed at 20% of MVC force found an association between endurance time, target force, and the increase in MAP (14). The longer endurance time for women when performing a submaximal contraction sustained to the endurance limit is a common finding, but the mechanism is unknown (13, 20, 26, 34). The lower absolute target force exerted by the women in the previous study (14) was interpreted to indicate less mechanical compression on the feed arteries, a reduced buildup of metabolites within the muscle, and a reduced rate of increase in MAP during the fatiguing contraction (2, 18, 25, 27). In the present study, however, the endurance time for the maintain-position task did not differ for men and women even though the men supported a load that was twice as large. Consequently, some other mechanism that was independent of the target load, but involved a similar pressor response, must have limited the fatiguing contraction of men and women similarly. Given that the pressor response during a fatiguing contraction can be mediated by both a peripheral reflex and central command (1, 11, 23, 24), the mechanism responsible for the interaction between task and sex warrants further investigation.In summary, the endurance time and the EMG activity of the elbow flexor muscles for a maintain-position task did not vary across sessions, in contrast to a prior observation for a maintain-force task (15). Accordingly, there was no change across sessions in other indexes of neural activity, including RPE scores, pressor response, and the fluctuations in acceleration. However, the amplitude and rate of increase in EMG activity varied among the elbow flexor muscles during the fatiguing contraction for the maintain-position task. This included no change in the EMG for the short head of biceps brachii and greater bursting activity in brachialis. These patterns of EMG activity differed from those reported previously for a maintain-force task (15) and may have contributed to the briefer endurance time for the maintain-position task. Taken together, these results indicate that the type of load supported by the elbow flexor muscles influences the physiological adjustments that occur during a fatiguing contraction and the adaptations that are possible across repeat performances of the task.
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ACKNOWLEDGEMENTS |
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We thank Ohmeda (Madison, WI) for donation of the Finapres 2300.
This research was supported by National Institute of Neurological Disorders and Stroke Grant NS-43275 (to R. M. Enoka).
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FOOTNOTES |
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Address for reprint requests and other correspondence: R. M. Enoka, Dept. of Kinesiology and Applied Physiology, University of Colorado, Boulder, CO 80309-0354 (E-mail: roger.enoka{at}colorado.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.
First published January 24, 2003;10.1152/japplphysiol.01038.2002
Received 13 November 2002; accepted in final form 17 January 2003.
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REFERENCES |
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TOP
ABSTRACT
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METHODS
RESULTS
DISCUSSION
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