Alternating activation is related to fatigue in lumbar muscles during sustained sitting

The aim of this study was to investigate the relation between variability in muscle activity and fatigue during a sustained low level contraction in the lumbar muscles. Twenty-five healthy participants (13 men 12 women) performed a 30 min sitting task with 5 degrees inclination of the trunk. Surface electromyographic (EMG) signals were recorded bilaterally from the lumbar muscles with 2 high density surface EMG grids of 9 × 14 electrodes. Median frequency (MDF) decrease, amplitude (RMS) increase and the rating of perceived exertion (RPE) were used as fatigue indices. Alternating activation and spatial and temporal variability were computed and relations with the fatigue indices were explored. During sitting, the mono- and bipolar RMS slightly increased while the MDF remained unchanged indicating no systematic muscle fatigue, although the average RPE increased from 6 to 13 on a scale ranging between 6 and 20. Higher frequency of alternating activation between the left and right side was associated with increased RPE (p = 0.03) and decreased MDF (p = 0.05). A tendency in the same direction was seen between increased spatial and temporal variation within the grids and increased RPE and decreased MDF. Present findings provide evidence for a relationship between variability in muscle activity and fatigue.     

Electromyographic analysis of the serratus anterior and trapezius muscles during push-ups on stable and unstable bases in subjects with scapular dyskinesis

The present study was performed to assess the electromyographic activity of the scapular muscles during push-ups on a stable and unstable surface, in subjects with scapular dyskinesis. Muscle activation (upper trapezius [UT]; lower trapezius [LT]; upper serratus anterior [SA_5th]; lower serratus anterior [SA_7th]) and ratios (UT/LT; UT/SA_5th; UT/ SA_7th) levels were determined by surface EMG in 30 asymptomatic men with scapular dyskinesis, during push-up performed on a stable and unstable surface. Multivariate analysis of variance with repeated measures was used for statistical analyses. The unstable surface caused a decrease in the EMG activity of the serratus anterior and an increase in EMG activity of the trapezius (p = 0.001). UT/SA_5th and UT/ SA_7th ratios were higher during unstable push-ups (p = 0.001). The results suggest that, in individuals with scapular dyskinesis, there is increased EMG activity of the trapezius and decreased EMG activity of the serratus anterior in response to an unstable surface. These results suggest that the performance of the push up exercise on an unstable surface may be more favorable to produce higher levels of trapezius activation and lower levels of serratus anterior activation. However, if the goal of the exercise program is the strengthening of the SA muscle, it is suggested to perform the push up on a stable surface.     

Scapular muscle activity in a variety of plyometric exercises

Background: Plyometric shoulder exercises are commonly used to progress from slow analytical strength training to more demanding high speed power training in the return to play phase after shoulder injury. The aim of this study was first, to investigate scapular muscle activity in plyometric exercises to support exercise selection in practice and second, to enhance understanding of how scapular muscles are recruited during the back and forth movement phase of these exercises. Methods: Thirty-two healthy subjects performed 10 plyometric exercises while surface EMG-activity of the scapular muscles (upper (UT), middle (MT) and lower trapezius (LT) and serratus anterior (SA)) was registered. A high speed camera tracked start and end of the back and forth movement. Results: Mean scapular EMG activity during the 10 exercises ranged from 14.50% to 76.26%MVC for UT, from 15.19% to 96.55%MVC for MT, from 13.18% to 94.35%MVC for LT and from 13.50% to 98.50%MVC for SA. Anova for repeated measures showed significant differences in scapular muscle activity between exercises (p < 0.001) and between the back and forth movement (p < 0.001) within exercises. Conclusion: Plyometric shoulder exercises require moderate (31–60%MVC) to high (>60%MVC) scapular muscle activity. Highest MT/LT activity was present in prone plyometric external rotation and flexion. Highest SA activity was found in plyometric external rotation and flexion with Xco and plyometric push up on Bosu. Specific exercises can be selected that recruit minimal levels of UT activity (<15%): side lying plyometric external rotation and horizontal abduction or plyometric push up on the Bosu. The results of this study support exercise selection for clinical practice.     

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

The aim of this study was to quantify the effects of spatial reorganisation of muscle activity on task-related and tangential components of force variability during sustained contractions. Three-dimensional forces were measured from isometric elbow flexion during submaximal contractions (50 s, 5–50% of maximal voluntary contraction (MVC)) and total excursion of the centre of pressure was extracted. Spatial electromyographic (EMG) activity was recorded from the biceps brachii muscle. The centroids of the root mean square (RMS) EMG and normalised mutual information (NMI) maps were computed to assess spatial muscle activity and spatial relationship between EMG and task-related force variability, respectively. Result showed that difference between the position of the centroids at the beginning and at the end of the contraction of the RMS EMG and the NMI maps were different in the medial–lateral direction (P < 0.05), reflecting that muscle regions modulate their activity without necessarily modulating the contribution to the task-related force variability over time. Moreover, this difference between shifts of the centroids was positively correlated with the total excursion of the centre of pressure at the higher levels of contractions (>30% MVC, R² > 0.30, P < 0.05), suggesting that changes in spatial muscle activity could impact on the modulation of tangential forces. Therefore, within-muscle adaptations do not necessarily increase force variability, and this interaction can be quantified by analysing the RMS EMG and the NMI map centroids.     

Electromyographic mapping of the erector spinae muscle with varying load and during sustained contraction

The purpose of the study was to explore changes in the spatial distribution of erector spinae electromyography amplitude during static, sustained contractions and during contractions of increasing load. Surface electromyographic (EMG) signals were detected from nine healthy subjects using a grid of 13 × 5 electrodes placed unilaterally over the lumbar erector spinae musculature. Subjects stood in a 20° forward flexed position and performed: (1) six 20-s long contractions with loads ranging from 2.5 kg to 12.5 kg (2.5 kg increments) and (2) a 6 min sustained contraction with 7.5 kg load. Root mean square (RMS) and mean power spectral frequency (MNF) were computed from the recorded EMG signals. EMG RMS increased (P < 0.0001) and MNF remained constant during contractions of increased load. During the sustained contraction, MNF decreased (P < 0.0001) and RMS did not change over time. The centroid (center of activity) of the RMS map shifted caudally (P < 0.0001) with time during the sustained contraction but did not change with varying load. These results suggest a change in the distribution of erector spinae muscle activity with fatigue and a uniform distribution of muscle activation across loads.     

Posture-movement changes following repetitive motion-induced shoulder muscle fatigue

Repetitive motion-induced fatigue not only alters local motion characteristics but also provokes global reorganization of movement. However, the three-dimensional (3D) characteristics of these reorganization patterns have never been documented in detail. The goal of this study was to assess the effects of repetitive reaching-induced arm fatigue on the whole-body, 3D biomechanical task characteristics. Healthy subjects (N = 14) stood and performed a continuous reaching task (RRT) between two targets placed at shoulder height to fatigue. Whole-body kinematic (Vicon^©), kinetic (AMTI^© force platforms) and electromyographic (EMG, Noraxon^©) characteristics were recorded. Maximal voluntary isometric efforts (MVIE) of the shoulder and elbow were measured pre- and post-RRT. Post-RRT shoulder elevation MVIE was reduced by 4.9 ± 8.3% and trapezius EMG amplitude recorded during the RRT increased by 46.9 ± 49.9% from the first to last minute of the RRT, indicating that arm fatigue was effectively induced. During fatigued reaching, subjects elevated their shoulder (11.7 ± 10.5 mm) and decreased their average shoulder abduction angle by 8.3 ± 4.4°. These changes were accompanied by a lateral shift of the body’s center of mass towards the non-reaching arm. These findings suggest a compensatory strategy to decrease the load on the fatigued shoulder musculature.     

Effects of two neuromuscular fatigue protocols on landing performance

The purpose of the study was to investigate the effects of two fatigue protocols on landing performance. A repeated measures design was used to examine the effects of fatigue and fatigue protocol on neuromuscular and biomechanical performance variables. Ten volunteers performed non-fatigued and fatigued landings on two days using different fatigue protocols. Repeated maximum isometric squats were used to induce fatigue on day one. Sub-maximum cycling was used to induce fatigue on day two. Isometric squat maximum voluntary contraction (MVC) was measured before and after fatigued landings on each day. During the landings, ground reaction force (GRF), knee kinematics, and electromyographic (EMG) data were recorded. Isometric MVC, GRF peaks, loading rates, impulse, knee flexion at contact, range of motion, max angular velocity, and EMG root mean square (RMS) values were compared pre- and post-fatiguing exercise and between fatigue protocols using repeated ANOVA. Fatigue decreased MVC strength (p ? 0.05), GRF second peak, and initial impulse (p ? 0.01), but increased quadriceps medium latency stretch reflex EMG activity (p ? 0.012). Knee flexion at contact was 5.2° greater (p ? 0.05) during fatigued landings following the squat exercise compared to cycling. Several variables exhibited non-significant but large effect sizes when comparing the effects of fatigue and fatigue protocol. In conclusion, fatigue alters landing performance and different fatigue protocols result in different performance changes.     

The assessment of back muscle capacity using intermittent static contractions. Part II: Validity and reliability of biomechanical correlates of muscle fatigue

IntroductionIn a previous paper, standard surface electromyographic (EMG) indices of muscle fatigue, which are based on the lowering of the median or mean frequencies of the EMG power spectrum in time, were applied during an intermittent absolute endurance test and were evaluated relative to criterion validity and test–retest reliability. The aims of this study were to assess mechanical and alternative EMG correlates of muscle fatigue.MethodsHealthy subjects (44 males and 29 females; age: 20–55 yrs) performed three maximal voluntary contractions (MVC) and an endurance test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The test, assessing absolute endurance (90 N m torque), consisted of performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the endurance test. Mechanical indices quantifying physiological tremor and steadiness were computed from the dynamometer signals (L5/S1 extension moments) along with EMG indices presumably sensitive to variable load sharing between back muscle synergists during the endurance test.ResultsMechanical indices were significantly correlated to Tend (r range: −0.47 to –0.53) but showed deceiving reliability results. Conversely, the EMG indices were correlated to Tend (r range: −0.43 to –0.63) with some of them particularly correlated to Strength (r = − 0.72 to –0.81). In addition, their reliability results were acceptable (intra-class correlation coefficient >0.75; standard error of measurement <10% of the mean) in many cases. Finally, several analyses substantiated their physiological relevance. These findings imply that these new EMG indices could be used to predict absolute endurance as well as strength with the use of a single intermittent and time-limited (5–10 min) absolute endurance test, a practical way to assess the back capacity of chronic low back pain subjects.     

Unchanged H-reflex during a sustained isometric submaximal plantar flexion performed with an EMG biofeedback

The aim of this study was to assess H-reflex plasticity and activation pattern of the plantar flexors during a sustained contraction where voluntary EMG activity was controlled via an EMG biofeedback. Twelve healthy males (28.0 ± 4.8 yr) performed a sustained isometric plantar flexion while instructed to maintain summed EMG root mean square (RMS) of gastrocnemius lateralis (GL) and gastrocnemius medialis (GM) muscles fixed at a target corresponding to 80% maximal voluntary contraction torque via an EMG biofeedback. Transcutaneous electrical stimulation of the posterior tibial nerve was evoked during the contraction to obtain the maximal H-reflex amplitude to maximal M-wave amplitude ratio (H_sup/M_sup ratio) from GL, GM and soleus (SOL) muscles. Neuromuscular function was also assessed before and immediately after exercise. Results showed a decrease in SOL activation during sustained flexion (from 65.5 ± 6.4% to 42.3 ± 3.8% maximal EMG, p < 0.001), whereas summed EMG RMS of GL and GM remained constant (59.7 ± 4.8% of maximal EMG on average). No significant change in the H_sup/M_sup ratio was found for SOL, GL and GM muscles. Furthermore, it appears that the decrease in maximal voluntary contraction torque (?20.4 ± 2.9%, p < 0.001) was related to both neural and contractile impairment. Overall, these findings indicate that the balance between excitation and inhibition affecting the motoneuron pool remains constant during a sustained contraction where myoelectrical activity is controlled via an EMG biofeedback or let free to vary.     

Between-day reliability of a hand-held dynamometer and surface electromyography recordings during isometric submaximal contractions in different shoulder positions

Functional shoulder assessments require the use of objective and reliable standardized outcome measures. Therefore, the aim of this study was to examine the between-day reliability of a hand-held dynamometer when measuring muscle strength during flexion, abduction, and internal and external rotation as well as surface electromyography (EMG) when measuring muscle activity from m. trapezius superior and deltoideus anterior. Twenty-four healthy subjects participated and performed four isometric contractions measured with a hand-held dynamometer and EMG. Both relative and absolute reliability were calculated based on the mean of the last three of the four repetitions. EMG amplitude was assessed calculating both absolute and normalized root-mean-square (RMS) values. The reliability of the hand-held dynamometer was high (LOA = 3.2–7.6% and ICC = 0.89–0.98). The absolute reliability for EMG showed similar results for absolute RMS values (LOA = 20.0–68.4%) and normalized RMS values (LOA = 42.4–66.5%). However, the results concerning the relative reliability showed higher ICC for absolute RMS values (ICC = 0.82–0.92) compared with normalized values (ICC = 0.57–0.72).The outcome measurements of this study with healthy subjects were found reliable and, therefore, have the potential to detect changes in muscle strength and muscle activity.     

Effect of prolonged walking with backpack loads on trunk muscle activity and fatigue in children

This study investigated the effect of prolonged walking with load carriage on muscle activity and fatigue in children. Fifteen Chinese male children (age = 6 years, height = 120.0 ± 5.4 cm, mass = 22.9 ± 2.6 kg) performed 20-min walking trials on treadmill (speed = 1.1 m s⁻¹) with different backpack loads (0%, 10%, 15% and 20% body weight). Electromyography (EMG) signals from upper trapezius (UT), lower trapezius (LT) and rectus abdominis (RA) were recorded at several time intervals (0, 5, 10, 15 and 20 min), and were normalized to the signals collected during maximum voluntary contraction. Integrated EMG signal (IEMG) was calculated to evaluate the muscle activity. Power spectral frequency analysis was applied to evaluate muscle fatigue by the shift of median power frequency (MPF). Results showed that a 15% body weight (BW) load significantly increased muscle activity at lower trapezius when the walking time reached 15 min. When a 20% BW load was being carried, increase in muscle activity was found from 5 min, and muscle fatigue was found from 15 min. In upper trapezius, increase of muscle activity was not found within the 20-min period, however, muscle fatigue was found from 10 min. No increased muscle activity or muscle fatigue was found in rectus abdominis. It is suggested that backpack loads for children should be restricted to no more than 15% body weight for walks of up to 20 min duration to avoid muscle fatigue.     

Bilateral impairments of shoulder abduction in chronic hemiparesis: Electromyographic patterns and isokinetic muscle performance

ObjectiveTo analyze electromyographic (EMG) patterns and isokinetic muscle performance of shoulder abduction movement in individuals who sustained a cerebrovascular accident (CVA).DesignTwenty-two individuals who sustained a CVA and 22 healthy subjects volunteered for EMG activity and isokinetic shoulder abduction assessments. EMG onset time, root mean square (RMS) for upper trapezius and deltoid muscles, as well as the isokinetic variables of peak torque, total work, average power and acceleration time were compared between limbs and groups.ResultsThe paretic side showed a different onset activation pattern in shoulder abduction, along with a lower RMS for both muscles (21.8 ± 13.4% of the maximal voluntary isometric contraction (MVIC) for the deltoid and 25.9 ± 15.3% MVIC for the upper trapezius, about 50% lower than the control group). The non-paretic side showed a delay in both muscles activation and a lower RMS for the deltoid (32.2 ± 13.7% MVIC, about 25% lower than the control group). Both sides of the group of individuals who sustained a CVA presented a significantly lower isokinetic performance compared to the control group (paretic side ～60% lower; non-paretic side ～35% lower).ConclusionsShoulder abduction muscle performance is impaired in both paretic and non-paretic limbs of individuals who sustained a CVA.     

Differential patterns of muscle modification in women with episodic and chronic tension-type headache revealed using surface electromyographic analysis

Tension-type headache (TTH) is a prototypical disorder in which muscular factors play a key role in the pathogenesis. This study was designed to understand muscular dysfunction in patients with episodic (ETTH) and chronic TTH (CTTH) using surface electromyography analysis (SEMG). Women with frequent ETTH (n = 14), CTTH (n = 14) and age-matched controls (n = 13) were recruited. SEMG data were recorded from the masseter, sternocleidomastoid, and upper trapezius muscles during maximum voluntary contraction and sustained voluntary isometric clenching, the neck flexion endurance test and shoulder elevation for 30 s. The root mean square (RMS) and median frequency (MDF) of the SEMG signal were measured throughout the test. The fatigue index, which is the MDF slope during sustained muscle contraction, decreased significantly faster in the ETTH and CTTH groups compared with that in the control (p < 0.05). The mean absolute RMS and relative percentage values at the initial and final period during sustained isometric contraction decreased significantly in the CTTH group (p < 0.05). Furthermore, headache clinical parameters (frequency and duration) were negatively correlated with the amplitude values (p < 0.05). A different muscle firing pattern or some muscle modifications in patients with CTTH may reflect reorganization of the motor-control strategy.     

Rotator cuff tendinopathy alters the muscle activity onset and kinematics of scapula

Athletes with rotator cuff (RC) tendinopathy demonstrate an aberrant pattern of scapular motion which might relate to deficits in the scapular muscles. This study aimed to determine whether alteration in scapular kinematics is associated with deficits in the activity onset of scapular muscles. Forty-three male volleyball players (17 asymptomatic and 26 with RC tendinopathy) joined the study. Three-dimensional scapular kinematics was quantified using an acromial marker cluster method. The activity onset of the upper (UT), middle (MT), and lower trapezius (LT), and serratus anterior (SA) during arm abduction was assessed with electromyography. Athletes with RC tendinopathy demonstrated less scapular upward rotation (6.6 ± 2.3 vs. 8.2 ± 1.1°, p = 0.021) in the early phase of shoulder abduction from 0° to 30° when compared to asymptomatic athletes. The tendinopathy group had delayed activity onset of LT (14.1 ± 31.4 ms vs. 74.4 ± 45.1 ms, p < 0.001) and SA (−44.9 ± 26.0 ms vs. 23.0 ± 25.2 ms, p < 0.001) relative to UT when compared to the asymptomatic group. In asymptomatic athletes, earlier activity onset of MT and LT relative to UT was associated with more scapular upward rotation during 0–30° of abduction (r = 0.665, p = 0.021) and 30–60° of abduction (r = 0.680, p = 0.015), respectively. Our findings showed the control of the scapular upward rotation is related to the activity onset of the scapular muscles in athletes.     

Assessment of functional recovery in tennis elbow

Objectives: (a) To investigate changes in muscular strength, fatigue and activity in recovered tennis elbow (RTE); (b) to assess the appropriateness of EMG and strength measurements in monitoring functional recovery in tennis elbow (TE).Methods: Study included three age-matched female groups of Control (C) (n = 8, no history of musculoskeletal problems), TE (n = 7, local tenderness at the lateral epicondyle and pain with resisted wrist and middle finger extension) and RTE (n = 6, asymptomatic for at least 6 months, no lateral epicondyle tenderness). Measurements included metacarpophalangeal (MCP), wrist, shoulder and grip isometric strength and EMG measures of muscle fatigue and activity for five forearm muscles (wrist extensors and flexors).Results: Strength was greater (p < 0.05) for all measurements in C compared to RTE and TE except for MCP extension. Only MCP extension was stronger in RTE than TE. EMG revealed increased activity of extensor carpi radialis (ECR) in RTE, decreased in TE.Conclusions: Despite attenuation of pain, global upper limb weakness in RTE indicated incomplete functional recovery. Increased strength of MCP extension may protect weakened wrist extensors from further injury. Monitoring the ECR activity as well as strength measurements may provide a useful assessment of functional recovery in TE.     

Adaptations of upper trapezius muscle activity during sustained contractions in women with fibromyalgia

The study compared the distribution of electromyographic (EMG) signal amplitude in the upper trapezius muscle in 10 women with fibromyalgia and in 10 healthy women before and after experimentally-induced muscle pain. Surface EMG signals were recorded over the right upper trapezius muscle with a 10 × 5 grid of electrodes during 90° shoulder abduction sustained for 60 s. The control subjects repeated the abduction task following injections of isotonic and hypertonic (painful) saline into the upper trapezius muscle. The EMG amplitude was computed for each electrode pair and provided a topographical map of the distribution of muscle activity. The pain level rated by the patients at the beginning of the sustained contraction was 5.9 ± 1.5. The peak pain intensity for the control group following the injection of hypertonic saline was 6.0 ± 1.6. During the sustained contractions, the EMG amplitude increased relatively more in the cranial than caudal region of the upper trapezius muscle for the control subjects (shift in the distribution of EMG amplitude: 2.3 ± 1.3 mm; P < 0.01). The patient group showed lower average EMG amplitude than the controls during the contraction (P < 0.05) and did not show different changes in EMG amplitude between different regions of the upper trapezius. A similar behavior was observed for the control group following injection of hypertonic saline. The results indicate that muscle pain prevents the adaptation of upper trapezius activity during sustained contractions as observed in non-painful conditions, which may induce overuse of similar muscle compartments with fatigue.     

Behaviour of a surface EMG based measure for motor control: Motor unit action potential rate in relation to force and muscle fatigue

Surface electromyography parameters such as root-mean-square value (RMS) and median power frequency (FMED) are commonly used to assess the input of the central nervous system (CNS) to a muscle. However, RMS and FMED are influenced not only by CNS input, but also by peripheral muscle properties. The number of motor unit action potentials (MUAPs) per second, or MUAP Rate (MR), being the sum of the firing rates of the active motor units, would reflect CNS input solely. This study explored MR behaviour in relation to force and during a fatiguing contraction in comparison to RMS and FMED.In the first experiment (n = 10) a step contraction of shoulder elevation force (20–100 N) was performed while multi-channel array EMG was recorded from the upper trapezius muscle. The sensitivity of MR for changes in force (1.8%/N) was almost twice as high as that of RMS (0.97%/N), indicating that MR may be more suitable for monitoring muscle force. The second experiment (n = 6) consisted of a 15-min isometric contraction of the biceps brachii. MR increased considerably less than RMS (0.9% vs. 4.1%), suggesting that MR selectively reflects central motor control whereas RMS also reflects peripheral changes. These results support that, at relatively low force levels, MR is a suitable parameter for non-invasive assessment of the input of the CNS to the muscle.     

Comparison between muscle activation measured by electromyography and muscle thickness measured using ultrasonography for effective muscle assessment

In this study, we aimed to compare the intrarater reliability and validity of muscle thickness measured using ultrasonography (US) and muscle activity via electromyography (EMG) during manual muscle testing (MMT) of the external oblique (EO) and lumbar multifidus (MF) muscles. The study subjects were 30 healthy individuals who underwent MMT at different grades. EMG was used to measure the muscle activity in terms of ratio to maximum voluntary contraction (MVC) and root mean square (RMS) metrics. US was used to measure the raw muscle thickness, the ratio of muscle thickness at MVC, and the ratio of muscle thickness at rest. One examiner performed measurements on each subject in 3 trials. The intrarater reliabilities of the % MVC RMS and raw RMS metrics for EMG and the % MVC thickness metrics for US were excellent (ICC = 0.81–0.98). There was a significant difference between all the grades measured using the % MVC thickness metric (p < 0.01). Further, this % MVC thickness metric of US showed a significantly higher correlation with the EMG measurement methods than with the others (r = 0.51–0.61). Our findings suggest that the % MVC thickness determined by US was the most sensitive of all methods for assessing the MMT grade.     

Electromyographic analysis of upper limb muscles during standardized isotonic and isokinetic robotic exercise of spastic elbow in patients with stroke

Although it has been reported that strengthening exercise in stroke patients is beneficial for their motor recovery, there is little evidence about which exercise method is the better option. The purpose of this study was to compare isotonic and isokinetic exercise by surface electromyography (EMG) analysis using standardized methods.Nine stroke patients performed three sets of isotonic elbow extensions at 30% of their maximal voluntary isometric torque followed by three sets of maximal isokinetic elbow extensions with standardization of mean angular velocity and the total amount of work for each matched set in two strengthening modes. All exercises were done by using 1-DoF planner robot to regulate exact resistive torque and speed. Surface electromyographic activity of eight muscles in the hemiplegic shoulder and elbow was recorded. Normalized root mean square (RMS) values and co-contraction index (CCI) were used for the analysis.The isokinetic mode was shown to activate the agonists of elbow extension more efficiently than the isotonic mode (normalized RMS for pooled triceps: 96.0 ± 17.0 (2nd), 87.8 ± 14.4 (3rd) in isokinetic, 80.9 ± 11.0 (2nd), 81.6 ± 12.4 (3rd) in isotonic contraction, F[1, 8] = 11.168; P = 0.010) without increasing the co-contraction of muscle pairs, implicating spasticity or synergy.     

EMG amplitude and frequency parameters of muscular activity: Effect of resistance training based on electromyographic fatigue threshold

The present study aimed to evaluate the effect of a resistance training program based on the electromyographic fatigue threshold (EMG_FT, defined as the highest exercise intensity performed without EMG alterations), on the EMG amplitude (root mean square, RMS) and frequency (median frequency, MF) values for biceps brachii (BB), brachioradialis (BR), triceps brachii (TB) and multifidus (MT). Twenty healthy male subjects, (training group [TG], n = 10; control group [CG], n = 10), firstly performed isometric contractions, and after this, dynamic biceps curl at four different loads to determine the EMG_FT. The TG training program used the BB EMG_FT value (8 weeks, 2 sessions/week, 3 exhaustive bouts/session, 2 min rest between bouts). No significant differences were found for the isometric force after the training. The linear regression slopes of the RMS with time during the biceps curl presented significant decrease after training for the BB, BR and TB muscles. For the MT muscle, the slope and MF intercept values changed with training. The training program based on the EMG_FT influenced EMG the amplitude more than EMG frequency, possibly related to the recruitment patterns of the muscles, although the trunk extensor muscles presented changes in the frequency parameter, showing adaptation to the training program.