Intramuscular and surface electromyogram changes during muscle fatigue

Accelerated muscle fatigability of latent myofascial trigger points in humans

OBJECTIVE

Muscle fatigue is prevalent in acute and chronic musculoskeletal pain conditions in which myofascial trigger points (MTPs) are involved. The aim of this study was to investigate the association of latent MTPs with muscle fatigue.

DESIGN

Intramuscular electromyographic (EMG) recordings were obtained from latent MTPs and non-MTPs together with surface EMG recordings from the upper trapezius muscles during sustained isometric muscle contractions in 12 healthy subjects.

OUTCOME MEASURES

Normalized root mean square (RMS) EMG amplitude and mean power frequency (MNF) were analyzed. The rate of perceived exertion and pain intensity from MTP side and non-MTP side were recorded.

RESULTS

Pain intensity on the MTP side was significantly higher than the non-MTP side (P < 0.05). Intramuscular EMG from latent MTPs showed an early onset of decrease in MNF and a significant decrease at the end of fatiguing contraction as compared with non-MTPs (P < 0.05). Surface EMG from muscle fibers close to latent MTPs presented with an early onset of the increase in RMS amplitude and the increase was significantly higher than that from non-MTPs at the end of sustained isometric contraction (P < 0.05).

CONCLUSIONS

A latent MTP is associated with an accelerated development of muscle fatigue and simultaneously overloading active motor units close to an MTP. Elimination of latent MTPs and inactivation of active MTPs may effectively reduce accelerated muscle fatigue and prevent overload spreading within a muscle.

Muscle activity and pedal force profile of triathletes during cycling to exhaustion

The purpose of this study was to analyze pedaling cadence, pedal forces, and muscle activation of triathletes during cycling to exhaustion. Fourteen triathletes were assessed at the power output level relative to their maximal oxygen uptake (355 +/- 23 W). Cadence, pedal forces, and muscle activation were analyzed during start, middle, and end test stages. Normal and tangential forces increased from the start to the end of the test (-288 +/- 33 to -352 +/- 42 N and -79 +/- 45 to -124 +/- 68 N, respectively) accompanied by a decrease in cadence (96 +/- 5 to 86 +/- 6 rpm). Muscle activation increased from the start to the middle and the end in the gluteus maximus (27 +/- 5.5% and 76 +/- 9.3%) and in the vastus lateralis (13 +/- 3.5% and 27 +/- 4.4%), similar increase was observed from the start to the end in the rectus femoris and the vastus medialis (50 +/- 9.3% and 20 +/- 5.7%, respectively). Greater normal force along with enhanced activation of knee and hip extensor muscles is linked with fatigue and declines in cadence of triathletes during cycling to exhaustion.

Spike shape analysis of surface electromyographic activity in wrist flexor and extensor muscles of the world's fastest drummer

Spike shape analysis (SSA) is a method to infer motor unit (MU) activity by examining interference pattern of surface electromyography (sEMG). SSA has succeeded to assess neuromuscular adaptations after dynamic training; however, it has not been used to assess muscle activities during the dynamic movements as seen in music performance. The present study used SSA to investigate sEMG activities of wrist flexor and extensor muscles in the winner of a contest to find the world's fastest drummer (WFD) during performing rhythmic wrist flexion/extension movements with one hand using a handheld drumstick. SSA measures of the WFD were compared with those in the two control groups: non-drummers (NDs) and ordinary drummers (ODs). We found that the WFD showed significantly high mean spike frequency (MSF), short mean spike duration (MSD), and small mean number of peaks per spike (MNPPS) compared with the control groups. These results suggest that the WFD had exceptional MU activity such as higher MU discharge rate, more MU recruitment, and/or higher MU synchronization to achieve extraordinary fast 10-Hz drumming performance. SSA will be useful to investigate the muscle activity seen in music performance.

Markers of isometric training intensity and reductions in resting blood pressure

In this study, we examined the correlations between selected markers of isometric training intensity and subsequent reductions in resting blood pressure. Thirteen participants performed a discontinuous incremental isometric exercise test to volitional exhaustion at which point mean torque for the final 2-min stage (2min-torque(peak)) and peak heart rate peak (HR(peak)) were identified. Also, during 4 weeks of training (3 sessions per week, comprising 4 × 2 min bilateral leg isometric exercise at 95% HR(peak)), heart rate (HR(train)), torque (Torque(train)), and changes in EMG amplitude (ΔEMG(amp)) and frequency (ΔEMG(freq)) were determined. The markers of training intensity were: Torque(train) relative to the 2min-torque(peak) (%2min-torque(peak)), EMG relative to EMG(peak) (%EMG(peak)), HR(train) ΔEMG(amp), ΔEMG(freq), and %MVC. Mean systolic (-4.9 mmHg) and arterial blood pressure (-2.7mmHg) reductions correlated with %2min-torque(peak) (r = -0.65, P = 0.02 and r = -0.59, P = 0.03), ΔEMG(amp) (r = 0.66, P = 0.01 and r = 0.59, P = 0.03), ΔEMG(freq) (r = -0.67, P = 0.01 and r = -0.64, P = 0.02), and %EMG(peak) (systolic blood pressure only; r = -0.63, P = 0.02). These markers best reflect the association between isometric training intensity and reduction in resting blood pressure observed after bilateral leg isometric exercise training.

Muscle fatigue-induced enhancement of corticomuscular coherence following sustained submaximal isometric contraction of the tibialis anterior muscle

Oscillatory activity of the sensorimotor cortex shows coherence with muscle activity within the 15- to 35-Hz frequency band (β-band) during weak to moderate sustained isometric contraction. We aimed to examine the acute changes in this corticomuscular coupling due to muscle fatigue and its effect on the steadiness of the exerted force. We quantified the coherence between the electroencephalogram (EEG) recorded over the sensorimotor cortex and the rectified surface electromyogram (EMG) of the tibialis anterior muscle as well as the coefficient of variance of the dorsiflexion force (ForceCV) and sum of the auto-power spectral density function of the force within the β-band (Forceβ-PSD) during 30% of maximal voluntary contraction (MVC) for 60 s before (prefatiguing task) and after (postfatiguing task) muscle fatigue induced by sustained isometric contraction at 50% of MVC until exhaustion in seven healthy male subjects. The magnitude of the EEG-EMG coherence increased in the postfatiguing task in six of seven subjects. The maximal peak of EEG-EMG coherence stayed within the β-band in both pre- and postfatiguing tasks. Interestingly, two subjects, who had no significant EEG-EMG coherence in the prefatiguing task, showed significant coherence in the postfatiguing task. Additionally, ForceCV and Forceβ-PSD significantly increased after muscle fatigue. These data suggest that when muscle fatigue develops, the central nervous system enhances oscillatory muscular activity in the β-band stronger coupled with the sensorimotor cortex activity accomplishing the sustained isometric contraction at lower performance levels.

Monitoring hand flexor fatigue in a 24-h motorcycle endurance race

Motorcycle riders must endure high levels of muscle tension for long periods of time, especially in their arms and forearms, when steering and using handlebar controls. Because the right hand operates the gas handle and front brakes, the present research focuses on fatigue in the right hand flexors. Ten adult riders, aged 32.5±5.5years, volunteered to participate in this study. During the 24h race each rider, on completion of a relay stage, visited the assessment box to do the following handgrip test sequence: (1) 10s of EMG recording at rest, (2) one 3-s maximal voluntary contraction (MVC), (3) 1min rest interval and (4) 50% MVC maintained during 10s. EMG amplitude (MP: μV) and median and mean frequency (MF and MPF: Hz) over the superficial finger flexors were recorded during the whole handgrip test sequence with adhesive surface electrodes. MVC values were maintained during the first two relays (50-60min duration in total) and dropped gradually thereafter (p<0.01). During the monitoring of the 50% MVC, mean amplitude increased (p=0.024) while median and mean frequency tended to decrease. These results suggest fatigue is produced in motorcycle riders in a 24h race. However, the expected reduction of EMG frequency was not confirmed given to a potentially large variability.

An alternative approach in muscle fatigue evaluation from the surface EMG signal

The aim of this study was to compare the intervals of time between adjacent zero crossings (ZCI), an alternative frequency-temporal parameter, with the root-mean-square (RMS) value and the median frequency (F(med)) from the surface EMG (sEMG) signal in muscle fatigue analysis. Twenty right-handed volunteers performed isometric contractions of right biceps brachii muscle while sEMG signals were collected from it at three different and arbitrary load levels until fatigue. The mean ZCI presented a significant correlation with F(med) but not with RMS value and it also presented lower coefficients of variation than others. The results pointed that mean ZCI properties can contribute more than F(med) and RMS value on the interpretation of the muscle function under fatigue conditions.

Venous blood gas and metabolite response to low-intensity muscle contractions with external limb compression

The effect of low-intensity resistance exercise with external limb compression (100 [EC100] and 160 [EC160] mm Hg) on limb blood flow and venous blood gas-metabolite response was investigated and compared with that of high-intensity resistance exercise (no external compression). Unilateral elbow flexion muscle contractions were performed at 20% (75 repetitions, 4 sets, 30-second rest intervals) and 70% of 1-repetition maximum (1-RM; 3 sets, each set was until failure, 3-minute rest intervals). Precontraction brachial arterial blood flow (Doppler ultrasound) was reduced with EC100 or EC160 (56% and 39% of baseline value, respectively) compared with no external compression (control). At 20% 1-RM, brachial arterial blood flow increased after contractions performed with EC160 (190%), but not with the others. Decreases in venous oxygen partial pressure (P(v)O(2)) and venous oxygen saturation (S(v)O(2)) were greater during EC100 and EC160 than control (mean [SE]: P(v)O(2), 28 [3] vs 26 [2] vs 33 [2] mm Hg; S(v)O(2), 41% [5%] vs 34% [4%] vs 52% [5%], respectively). Changes in venous pH (pH(v)), venous carbon dioxide partial pressure (P(v)CO(2)), and venous lactate concentration ([L(-)](v)) were greater with EC160 than EC100 and/or control (pH(v), 7.19 [0.01] vs 7.25 [0.01] vs 7.27 [0.02]; P(v)CO(2), 72 [3] vs 64 [2] vs 60 [3] mm Hg; [L(-)](v), 5.4 [0.6] vs 3.7 [0.4] vs 3.0 [0.4] mmol/L, respectively). Seventy percent 1-RM contractions resulted in greater changes in pH(v) (7.14 [0.02]), P(v)CO(2) (91 [5] mm Hg), and [L(-)](v) (7.0 [0.5] mmol/L) than EC100 and EC160, but P(v)O(2) (30 [4] mm Hg) and S(v)O(2) (40% [3%]) were similar. In conclusion, changes in pH(v), P(v)CO(2), and [L(-)](v), but not in P(v)O(2) and S(v)O(2), are sensitive to changes in relative, "internal" intensity of low-intensity muscle contractions caused by reduced blood flow (EC160) or high-intensity muscle contractions. Given the magnitude of the changes in pH(v), P(v)CO(2), and [L(-)](v), it appears plausible that they may be involved in stimulating the observed increase in muscle activation via group III and IV afferents.

Testing of motor unit synchronization model for localized muscle fatigue

Spectral compression of surface electromyogram (sEMG) is associated with onset of localized muscle fatigue. The spectral compression has been explained based on motor unit synchronization theory. According to this theory, motor units are pseudo randomly excited during muscle contraction, and with the onset of muscle fatigue the recruitment pattern changes such that motor unit firings become more synchronized. While this is widely accepted, there is little experimental proof of this phenomenon. This paper has used source dependence measures developed in research related to independent component analysis (ICA) to test this theory.

Effects of a noncircular chainring system on muscle activation during cycling

Previous studies evaluated cycling with noncircular chainrings and suggested that changes in muscle activation would occur in response to altered pedaling mechanics throughout the crank arm revolution. However, no previous study addressed this question. The aim of this study was to compare the magnitude of muscular activity between a conventional and a noncircular crank system during an incremental maximal cycling test. Seven mountain-bike trained cyclists completed two incremental maximal tests, separated by 48 h, one for each crank system. Each test started with a workload of 100 W and was increased by 30 W every minute until exhaustion. Power output, pedaling cadence and heart rate were monitored and compared between the crank systems using paired t-tests. Surface EMG was recorded from the right rectus femoris, vastus medialis, biceps femoris and gastrocnemius medialis. EMG was compared using a general linear model considering as factors the crank system and workload with post hoc analysis at α=0.05. RMS presented effect of workload, but no effect of crank system was found for the muscles analyzed. The present results do not support effects of the noncircular crank system on variables of performance and muscle activation during incremental cycling in trained mountain bike cyclists.

Can shoulder muscle coordination during the support scale at ring height be replicated during training exercises in gymnastics?

The support scale at ring height, the swallow, is a difficult strength element, usually performed in gymnastics. Coaches try to simulate the swallow position during training to strengthen muscles, specifically in the position used for competition. However, the real effect of this position's simulation on muscle force and coordination and consequently on the muscle activity has not been determined. The purpose of the study was to compare muscle activity and coordination during a swallow performed on the rings, using a counterweight and during 2 training exercises using dumbbells or barbells, respectively. Six top-level gymnasts participated in the study. Electromyograms from the biceps brachii, triceps brachii, deltoideus (clavicular part), pectoralis major, serratus anterior, infraspinatus, trapezius (middle part), and latissimus dorsi in the right shoulder were collected during the 4 exercises and analyzed using root mean square (RMS) and mean power frequency (MPF). The RMS were normalized to the maximal voluntary contraction, and a co-activation index was also determined between biceps and triceps brachii. Our results show specific shoulder muscle coordination for each exercise. As compared with the swallow on the rings, the pectoralis major participates less in shoulder flexion during the counterweight exercise, whereas the deltoideus is more activated during the dumbbells exercise (p < 0.05). The barbell exercise reduces the participation of the serratus anterior in stabilizing the scapula (p < 0.05). Training exercises must therefore be chosen with knowledge of the specific muscle coordination induced by each. The counterweight exercise preserves the pectoralis major. The barbell exercise reduces participation of the serratus anterior. The dumbbells exercise may be useful to prepare the rotator cuff muscles carefully for use.

Effect of a speech aid prosthesis on reducing muscle fatigue

It has been reported that the levator veli palatini muscles of speakers with velopharyngeal incompetence tend to demonstrate muscle fatigue during speech. This study examined whether a speech aid prosthesis might reduce levator muscle fatigue in such speakers. Eight individuals with post-surgical cleft palates, and who wore a speech aid prosthesis, were studied. Each person was asked to pronounce the syllable [pu] more than 50 times. Mean power frequency (MPF) of one syllable was obtained from electromyographic data from the levator muscle. The MPF regression line was calculated during the course of syllable repetition. The absolute values of the slopes of the regression lines with the prosthesis were significantly smaller than those without the prosthesis. It was shown that the prosthesis reduced the decrease in MPF during speech. These results suggested that speech aid prostheses reduce levator muscle fatigue during speech in persons with velopharyngeal incompetence.

Comparison of critical force to EMG fatigue thresholds during isometric leg extension

Theoretically, the critical force (CF) and the EMG fatigue threshold (EMGFT) tests demarcate fatiguing from nonfatiguing isometric torque levels.

PURPOSE

The purpose of this study was twofold: 1) to determine whether the mathematical model for estimating the EMGFT during cycle ergometry was applicable to isometric leg extension muscle actions and 2) to compare the mean torque level from the CF test to those of EMGFT tests for the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles during isometric muscle actions.

METHODS

The slope coefficient of the linear relationship between total "isometric work" (Wlim in newton-meters per second) and time to exhaustion (Tlim in seconds) was defined as the CF. The EMGFT was defined as the y-intercept of the isometric torque versus EMG fatigue curve slope coefficient relationship.

RESULTS

There was a significant (P < 0.05) mean difference between the CF (25.9 +/- 12.1 N.m) and the EMGFT value for the RF (41.1 +/- 20.7 N.m) muscle. There were no significant differences, however, in EMGFT values among the three superficial muscles of the quadriceps femoris. In addition, the mean CF (17.6% maximum voluntary isometric contraction [MVIC]) occurred at a percentage of MVIC that is typically not affected by circulatory occlusion (20% MVIC), whereas the mean EMGFT values for the VL (25.9% MVIC), VM (22.9% MVIC), and RF (27.8% MVIC) exceeded this threshold.

Neuromuscular Efficiency of the Triceps Surae in Induced and Voluntary Contractions: Morning and Evening Evaluations

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NMEind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (PtMmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value -10.7 +/- 5.5%, p < 0.05), whereas PtMmax was not significantly modified. NMEind (PtMmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 +/- 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (PtTmax), and the corresponding NME (NMEreflex = PtTmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 +/- 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NMEMvc was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 +/- 13.9%, p <0.05) and a trend for a lower NME25%MVC in the evening was observed, a result probably representative of a higher muscle fatigue state in the evening. Finally, to test the muscle capacities during a natural task, a NME index was calculated during a drop jump (DJ). The NMEDJ was defined as the ratio between jump height and mean amplitude of TS EMG (% of TS Mmax) between the drop and the jump. Both jump height and NMEDJ were significantly higher in the evening (mean gains of 10.9 +/- 4.5% and 15.7 +/- 7.4%, respectively, p <0.05). In conclusion, daytime changes in the efficiency of postural muscles seem to depend on both peripheral and central mechanisms. According to the experimental conditions, NME of the postural muscle could increase, remain constant, or even decrease in the evening, and this result may reflect reverse effects of better contractile capacities and higher fatigue state.

Relationships between surface EMG variables and motor unit firing rates

Although surface electromyography (sEMG) is a widely used electrophysiological technique, its physiological interpretation remains somewhat controversial. This study examined the relationship between motor unit firing rates (MUFR) and the root mean square (RMS) amplitude and mean power frequency (MPF) of the sEMG signal in the biceps brachii. Eleven subjects performed maximal isometric elbow flexion while indwelling and sEMG recordings were obtained from the biceps. The RMS amplitude and MPF of the surface signal, and the mean MUFR from the indwelling signal, were calculated over 500 ms epochs. Group means showed a strong MUFR-RMS amplitude relationship (r (2) = 0.91), but a weak MUFR-MPF relationship (r (2) = 0.20). Using all trials, the MUFR-RMS amplitude (r (2) = 0.19) and MUFR-MPF (r (2) = 0.0037) relationships were much weaker. Within individual subjects, the MUFR-RMS amplitude (mean r (2) = 0.13 +/- 0.17) and the MUFR-MPF (mean r (2) = 0.040 +/- 0.041) relationships were also weak. These results suggest that MUFR cannot be predicted from the characteristics of the sEMG signal.

Manifestations of shoulder fatigue in prolonged activities involving low-force contractions

Shoulder fatigue has been suggested to be a useful risk indicator for shoulder disorders in repetitive, low-force work tasks. In contrast to high-force or purely isometric tasks, it is unclear whether measurable fatigue develops in realistic low-force work. The question addressed in this review was: 'Is there evidence of objective signs of fatigue in the shoulder region in realistic, low-force work tasks?' Studies on objective measures of fatigue applied in realistic low-force work tasks were systematically reviewed, using a task duration of more than 1 h and an intensity level of less than 20% maximum voluntary contraction (MVC) for the median trapezius activation level as inclusion criteria. Thirteen studies were found to fulfil the criteria. All these studies addressed fatigue-related changes in the electromyographic signal in the descending part of the trapezius muscle. Seven did find a combination of frequency decrease and amplitude increase over time, which is generally considered as an objective manifestation of fatigue. Thus, there is evidence of objective signs of fatigue in some of the realistic, low-force tasks. The intensity level appeared to be a main determinant here. In the studies demonstrating signs of fatigue an intensity level of 15%MVC or more was used, while the intensity level in the studies with a negative result was generally lower.

A comparison of critical force and electromyographic fatigue threshold for isometric muscle actions of the forearm flexors

The purpose of this study was twofold: (1) to determine if the mathematical model used for estimating the EMG(FT) during cycle ergometry was applicable to isometric muscle actions; and (2) to compare the mean torque level from the CF test to that of the EMG(FT) test. The CF was defined as the slope coefficient of the linear relationship between total "isometric work" (W (lim) in N m s) and time to exhaustion (T (lim)). The EMG(FT) was defined as the y-intercept of the isometric torque versus EMG fatigue curve slope coefficient relationship. There was a significant (p < 0.05) mean difference between CF (6.6 +/- 3.2 N m) and EMG(FT) (10.9 +/- 4.7 N m). The results of the present study suggested that, during isometric muscle actions of the forearm flexors, fatigue thresholds estimated from the W (lim) versus T (lim) relationship (CF) are different from those estimated from electromyographic fatigue curves (EMG(FT)).

Force-EMG changes during sustained contractions of a human upper airway muscle

Human upper airway and facial muscles support breathing, swallowing, speech, mastication, and facial expression, but their endurance performance in sustained contractions is poorly understood. The muscular fatigue typically associated with task failure during sustained contractions has both central and intramuscular causes, with the contribution of each believed to be task dependent. Previously we failed to show central fatigue in the nasal dilator muscles of subjects that performed intermittent maximal voluntary contractions (MVCs). Here we test the hypothesis that central mechanisms contribute to the fatigue of submaximal, sustained contractions in nasal dilator muscles. Nasal dilator muscle force and EMG activities were recorded in 11 subjects that performed submaximal contractions (20, 35, and 65% MVC) until force dropped to <or=90% of the target force for >or=3 s, which we defined as task failure. MVC and twitch forces (the latter obtained by applying supramaximal shocks to the facial nerve) were recorded before the trial and at several time points over the first 10 min of recovery. The time to task failure was inversely related to contraction intensity. MVC force was depressed by roughly 30% at task failure in all three trials, but recovered within 2 min. Twitch force fell by 30-44% depending on contraction intensity and remained depressed after 10 min of recovery, consistent with low-frequency fatigue. Average EMG activity increased with time, but never exceeded 75% of the maximal, pretrial level despite task failure. EMG mean power frequency declined by 20-25% in all trials, suggesting reduced action potential conduction velocity at task failure. In contrast, the maximal evoked potential did not change significantly in any of the tasks, indicating that the EMG deficit at task failure was due largely to mechanisms proximal to the neuromuscular junction. Additional experiments using the interpolated twitch technique suggest that subjects can produce about 92% of the maximal evocable force with this muscle, which is not a large enough deficit to explain the entire shortfall in the EMG at task failure. These data show that the nervous system fails to fully activate the nasal dilator muscles during sustained, submaximal contractions; putative mechanisms are discussed.

Pain-induced changes in cervical muscle activation do not affect muscle fatigability during sustained isometric contraction

This study investigated whether pain-induced changes in cervical muscle activation affect myoelectric manifestations of cervical muscle fatigue. Surface EMG signals were detected from the sternocleidomastoid and splenius capitis muscles bilaterally from 14 healthy subjects during 20-s cervical flexion contractions at 25% of the maximal force. Measurements were performed before and after the injection of 0.5 ml of hypertonic (painful) or isotonic (control) saline into either the sternocleidomastoid or splenius capitis in two experimental sessions. EMG average rectified value and mean power spectral frequency were estimated throughout the sustained contraction. Sternocleidomastoid or splenius capitis muscle pain resulted in lower sternocleidomastoid EMG average rectified value on the side of pain (P < 0.01). However, changes over time of sternocleidomastoid EMG average rectified value and mean frequency (myoelectric manifestations of fatigue) during sustained flexion were not changed during muscle pain. These results demonstrate that pain-induced modifications of cervical muscle activity do not change myoelectric manifestations of fatigue. This finding has implications for interpreting the mechanisms underlying greater cervical muscle fatigue in people with neck pain disorders.

Synergist coactivation and substitution pattern of the human masseter and temporalis muscles during sustained static contractions

OBJECTIVE

Previous reports indicated that between-muscle substitution of active motor unit pools can be found in a variety of synergist muscles, including shoulder and leg muscles, but little information is available for the masticatory muscles. We hypothesized that, during a prolonged clenching effort performed at low- to moderate-bite force levels, a substitution pattern of activity can be found also in the masseter and anterior temporal muscles.

METHODS

Ten healthy volunteers were recruited and were asked to clench unilaterally on a force transducer for 10min at 10%, 15%, and 20% of the maximum bite force. During each session, bite force, perceived muscle pain and electromyographic activity were continuously assessed. Data analyses were performed by means of cross-correlation and periodogram analyses.

RESULTS

During sustained static contractions, different contraction patterns of jaw elevator muscles could be identified. These included a coactivation pattern, a substitution pattern, and several intermediate situations between coactivation and substitution.

CONCLUSIONS

The findings support the concept that the masticatory muscles are functionally heterogeneous and provide evidence that the neuromuscular strategies used by the masticatory system to perform sustained static contractions differ between individuals.

SIGNIFICANCE

Individual neuromuscular strategies might play a role in the development of masticatory muscle pain conditions.

Aspectos relacionados à fadiga durante o ciclismo: uma abordagem biomecânica

A fadiga muscular pode ser definida como a incapacidade funcional na manutenção de um nível esperado de força. As competições de ciclismo, especialmente provas de estrada, apresentam como característica longa duração e altas intensidades. Tais características resultam na instauração do processo de fadiga, que pode estar associado a mecanismos e fatores metabólicos que afetam os músculos (fadiga periférica) e o sistema nervoso central (fadiga central). O objetivo deste trabalho é fazer uma revisão sobre aspectos relacionados com as mudanças na técnica de pedalada e na atividade elétrica dos músculos envolvidos nesse movimento durante o processo de fadiga. Alguns desses aspectos têm sido reportados na literatura e podem ter repercussão na (1) magnitude, direção e sentido de aplicação das forças no pedal; no (2) padrão de ativação muscular; na (3) geração de força e, conseqüentemente, no (4) desempenho do ciclista. No entanto, poucos estudos associam a fadiga muscular ao comportamento das forças aplicadas no pedal e ao padrão da ativação muscular. Os resultados dos estudos revisados demonstram a incapacidade dos ciclistas em manter a força desejada, perda da técnica de pedalada e mudança nos padrões de ativação elétrica sob condições de fadiga.

Quantitative contributions of the muscles of the tongue, floor-of-mouth, jaw, and velum to tongue-to-palate pressure generation

PURPOSE

The purpose of this investigation was to evaluate the relationship between tongue-to-palate pressure and the electromyography (EMG) measured from the mylohyoid, anterior belly of the digastric, geniohyoid, medial pterygoid, velum, genioglossus, and intrinsic tongue muscles. Methods Seven healthy adults performed tongue-to-palate pressure tasks at known percentages of their maximum pressure while intramuscular EMG was recorded from the muscles stated above. Multiple regression analysis was performed.

RESULTS

Predictors of pressure included the posterior fibers of the genioglossus, mylohyoid, anterior belly of digastric, medial pterygoid, and intrinsic tongue.

CONCLUSIONS

Increasing tongue-to-palate pressure coincides with increased muscle activity. Activation of the floor-of-mouth, tongue, and jaw closing muscles increased tongue-to-palate pressure. These findings support the use of a tongue-press exercise to strengthen floor-of-mouth muscles, tongue, and jaw-closing muscles.

Differential activation of regions within the biceps brachii muscle during fatigue

AIM

To examine the occurrence of repeated differential activation between the heads of the biceps brachii muscle and its relation to fatigue prevention during a submaximal contraction.

METHODS

Thirty-nine subjects carried out an isometric contraction of elbow flexion at 25% of maximal voluntary contraction (MVC) until exhaustion. A grid of 13 by 10 electrodes was used to record surface electromyographic signals from both heads of the biceps brachii. The root-mean-square of signals recorded from electrodes located medially and laterally was used to analyse activation differences. Differential activation was defined as periods of 33% different activation level between the two heads of the biceps brachii muscle.

RESULTS

Differential muscle activation was demonstrated in 30 of 33 subjects with appropriate data quality. The frequency of differential activation increased from 4.9 to 6.6 min(-1) at the end of the contractions with no change in duration of the differential activations (about 1.4 s). Moreover, the frequency of differential activation was, in general, negatively correlated with time to exhaustion.

CONCLUSION

The observed differential activation between the heads of the biceps brachii can be explained by an uneven distribution of synaptic input to the motor neurone pool. The findings of this study indicate that differential activation of regions within a muscle does not prevent fatigue at a contraction level of 25% of MVC.

Fatigue analysis before and after shaker exercise: physiologic tool for exercise design

Recent studies suggest that the Shaker exercise induces fatigue in the upper esophageal sphincter (UES) opening muscles and sternocleidomastoid (SCM), with the SCMs fatiguing earliest. The aim of this study was to measure fatigue induced by the isometric portion of the Shaker exercise by measuring the rate of change in the median frequency (MF rate) of the power spectral density (PSD) function, which is interpreted as proportional to the rate of fatigue, from surface electromyography (EMG) of suprahyoid (SHM), infrahyoid (IHM), and SCM. EMG data compared fatigue-related changes from 20-, 40-, and 60-s isometric hold durations of the Shaker exercise. We found that fatigue-related changes were manifested during the 20-s hold. The findings confirm that the SCM fatigues initially and as fast as or faster than the SHM and IHM. In addition, upon completion of the exercise protocol, the SCM had a decreased MF rate, implying improved fatigue resistance, while the SHM and IHM showed increased MF rates, implying that these muscles increased their fatiguing effort. We conclude that the Shaker exercise initially leads to increased fatigue resistance of the SCM, after which the exercise loads the less fatigue-resistant SHM and IHM, potentiating the therapeutic effect of the Shaker exercise regimen with continued exercise performance.

Electromyographic measures of muscle activation and changes in muscle architecture of human elbow flexors during fatiguing contractions

The study compared changes in intramuscular and surface recordings of EMG amplitude with ultrasound measures of muscle architecture of the elbow flexors during a submaximal isometric contraction. Ten subjects performed a fatiguing contraction to task failure at 20% of maximal voluntary contraction force. EMG activity was recorded in biceps brachii, brachialis, and brachioradialis muscles using intramuscular and surface electrodes. The rates of increase in the amplitude of the surface EMG for the long and short heads of biceps brachii and brachioradialis were greater than those for the intramuscular recordings measured at different depths. The amplitude of the intramuscular recordings from three muscles increased at a similar rate (P = 0.13), as did the amplitude of the three surface recordings from two muscles (P = 0.83). The increases in brachialis thickness (27.7 +/- 5.7 to 30.9 +/- 3.5 mm; P < 0.05) and pennation angle (10.9 +/- 3.5 to 16.5 +/- 4.8 degrees ; P = 0.003) were not associated with the increase in intramuscular EMG amplitude (P > 0.58). The increase in brachioradialis thickness (22.8 +/- 4.8 to 25.5 +/- 3.4 mm; P = 0.0075) was associated with the increase in the amplitude for one of two intramuscular EMG signals (P = 0.007, r = 0.79). The time to failure was more strongly associated with the rate of increase in the amplitude of the surface EMG than that for the intramuscular EMG, which suggests that the surface measurement provides a more appropriate measure of the change in muscle activation during a fatiguing contraction.

Effects of creatine loading on electromyographic fatigue threshold during cycle ergometry in college-aged women

The purpose of this study was to examine the effects of 5 days of Creatine (Cr) loading on the electromyographic fatigue threshold (EMG_FT) in college-aged women. Fifteen healthy college-aged women (mean ± SD = 22.3 ± 1.7 yrs) volunteered to participate in this double-blind, placebo-controlled study and were randomly placed into either placebo (PL – 10 g of flavored dextrose powder; n = 8) or creatine (Cr – 5 g di-creatine citrate plus 10 g of flavored dextrose powder; n = 7; Creatine Edge, FSI Nutrition) loading groups. Each group ingested one packet 4 times per day (total of 20 g/day) for 5 days. Prior to and following supplementation, each subject performed a discontinuous incremental cycle ergometer test to determine their EMG_FT value, using bipolar surface electrodes placed on the longitudinal axis of the right vastus lateralis. Subjects completed a total of four, 60 second work bouts (ranging from 100–350 W). The EMG amplitude was averaged over 10 second intervals and plotted over the 60 second work bout. The resulting slopes from each successive work bouts were used to calculate EMG_FT. A two-way ANOVA (group [Cr vs. PL] × time [pre vs. post]) resulted in a significant (p = 0.031) interaction. Furthermore, a dependent samples t-test showed a 14.5% ± 3.5% increase in EMG_FT from pre- to post-supplementation with Cr (p = 0.009), but no change for the PL treatment (-2.2 ± 5.8%; p = 0.732). In addition, a significant increase (1.0 ± 0.34 kg; p = 0.049) in weight (kg) was observed in the Cr group but no change for PL (-0.2 kg ± 0.2 kg). These findings suggest that 5 days of Cr loading in women may be an effective strategy for delaying the onset of neuromuscular fatigue during cycle ergometry.

Physiological and electromyographic responses during 40-km cycling time trial: relationship to muscle coordination and performance

The purpose of this study was to compare the oxygen uptake (VO(2)), respiratory exchange ratio (RER), cadence and muscle activity during cycling a 40-km time trial (TT), and to analyse the relationship between muscle activity and power output (PO). Eight triathletes cycled a 40-km TT on their own bicycles, which were mounted on a stationary cycle simulator. The VO(2), RER and muscle activity (electromyography, EMG) from tibialis anterior (TA), gastrocnemius medialis (GA), biceps femoris (BF), rectus femoris (RF) and vastus lateralis (VL) of the lower limb were collected. The PO was recorded from the cycle simulator. The data were collected at the 3rd, 10th, 20th, 30th and 38th km. The root mean square envelope (RMS) of EMG was calculated. The VO(2) and PO presented a significant increase at the 38th km (45.23+/-8.35 ml kg min(-1) and 107+/-7.11% of mean PO of 40-km, respectively) compared to the 3rd km (38.12+/-5.98 ml kg min(-1) and 92+/-8.30% of mean PO of 40-km, respectively). There were no significant changes in cadence and RER throughout the TT. The VL was the only muscle that presented significant increases in the RMS at the 10th km (22.56+/-3.05% max), 20th km (23.64+/-2.52% max), 30th km (25.27+/-3.00% max), and 38th km (26.28+/-3.57%max) when compared to the 3rd km (21.03+/-1.88%max). The RMS of VL and RF presented a strong relationship to PO (r=0.89 and 0.86, respectively, p<0.05). The muscular steady state reported for cycling a 30-min TT seems to occur in the 40-km TT, for almost all assessed muscles, probably in attempt to avoid premature muscle fatigue.

Effect of oral administration of sodium bicarbonate on surface EMG activity during repeated cycling sprints

The purpose of this study was to determine the effect of oral administration of sodium bicarbonate (NaHCO3) on surface electromyogram (SEMG) activity from the vastus lateralis (VL) during repeated cycling sprints (RCS). Subjects performed two RCS tests (ten 10-s sprints) interspersed with both 30-s and 360-s recovery periods 1 h after oral administration of either NaHCO3 (RCSAlk) or CaCO3 (RCSPla) in a random counterbalanced order. Recovery periods of 360 s were set before the 5th and 9th sprints. The rate of decrease in plasma HCO3- concentration during RCS was significantly greater in RCSAlk than in RCSPla, but the rates of decline in blood pH during the two RCS tests were similar. There was no difference between change in plasma lactate concentration in RCSAlk and that in RCSPla. Performance during RCSAlk was similar to that during RCSPla. There were no differences in oxygen uptake immediately before each cycling sprint (preVO2) and in SEMG activity between RCSAlk and RCSPla. In conclusion, oral administration of NaHCO3 did not affect SEMG activity from the VL. This suggests that the muscle recruitment strategy during RCS is not determined by only intramuscular pH.

Myoelectric manifestations of fatigue at low contraction levels in subjects with and without chronic pain

The aim of the present study was to investigate differences in myoelectric responses to fatigue development between cases with chronic neck-shoulder pain (n=10) and healthy controls (n=10) during a low force level sustained contraction. Subjects performed a 15-min isometric shoulder elevation at a force level of 40 N (sustained contraction), preceded and followed by a step contraction, consisting of five force levels from 20 to 100 N. EMG recordings were made with a two-dimensional electrode array on the upper trapezius of the dominant side. Root-mean-square (RMS(G)), median power frequency (FMED(G)), conduction velocity (CV), number of motor unit action potentials per second (MUAP Rate) and MUAP shape properties were estimated. Changes over time and differences between the groups were statistically evaluated with a linear mixed model. During the sustained contraction, cases showed less increase in RMS(G) than controls (controls: 58.5%, cases: 33.0%). FMED(G) and CV decreased in controls (FMED(G): -6.3%, CV: -5.3%) and stayed constant (FMED(G)) or slightly increased (CV, 3.15%) in cases. Overall, cases showed a less pronounced myoelectric response to the fatiguing task than controls, which may be related to additional recruitment of higher-threshold MUs. A possible explanation might be that cases were already (chronically) fatigued before the experiment started.