Effect of eccentric contraction velocity on muscle damage in repeated bouts of elbow flexor exercise

Eccentric exercise induces muscle damage, but controversy exists concerning the effect of contraction velocity on the magnitude of muscle damage, and little is known about the effect of contraction velocity on the repeated-bout effect. This study examined slow (60 degrees.s(-1)) and fast (180 degrees.s(-1)) velocity eccentric exercises for changes in indirect markers of muscle damage following 3 exercise bouts that were performed every 2 weeks. Fifteen young men were divided into 2 groups based on the velocity of eccentric exercise: 7 in the Ecc60 (60 degrees.s(-1)) group, and 8 in the Ecc180 (180 degrees.s(-1)) group. The exercise consisted of 30 maximal eccentric contractions of the elbow flexors at each velocity, in which the elbow joint was forcibly extended from 60 degrees to 180 degrees (full extension) on an isokinetic dynamometer. Changes in maximal voluntary isometric contraction strength, range of motion, muscle soreness, and plasma creatine kinase activity before and for 4 days after the exercise were compared in the 2 groups using a mixed-model analysis (group x bout x time). No significant differences between groups were evident for changes in any variables following exercise bouts; however, the changes were significantly smaller (p < 0.05) after the second and third bouts than after the first bout. These results indicate that the contraction velocity does not influence muscle damage or the repeated-bout effect.

An electromyographic analysis of the deep cervical flexor muscles in performance of craniocervical flexion

Background and Purpose. This study evaluated an electromyographic technique for the measurement of muscle activity of the deep cervical flexor (DCF) muscles. Electromyographic signals were detected from the DCF, sternocleidomastoid (SCM), and anterior scalene (AS) muscles during performance of the craniocervical flexion (CCF) test, which involves performing 5 stages of increasing craniocervical flexion range of motion-the anatomical action of the DCF muscles. Subjects. Ten volunteers without known pathology or impairment participated in this study. Methods. Root-mean-square (RMS) values were calculated for the DCF, SCM, and AS muscles during performance of the CCF test. Myoelectric signals were recorded from the DCF muscles using bipolar electrodes placed over the posterior oropharyngeal wall. Reliability estimates of normalized RMS values were obtained by evaluating intraclass correlation coefficients and the normalized standard error of the mean (SEM). Results. A linear relationship was evident between the amplitude of DCF muscle activity and the incremental stages of the CCF test (F=239.04, df=36, P<.0001). Normalized SEMs in the range 6.7% to 10.3% were obtained for the normalized RMS values for the DCF muscles, providing evidence of reliability for these variables. Discussion and Conclusion. This approach for obtaining a direct measure of the DCF muscles, which differs from those previously used, may be useful for the examination of these muscles in future electromyographic applications.

Comparison of electrical nerve stimulation, electrical muscle stimulation and magnetic nerve stimulation to assess the neuromuscular function of the plantar flexor muscles.

INTRODUCTION: As it might lead to less discomfort, magnetic nerve stimulation (MNS) is increasingly used as an alternative to electrical stimulation methods. Yet, MNS and electrical nerve stimulation (ENS) and electrical muscle stimulation (EMS) have not been formally compared for the evaluation of plantar flexor neuromuscular function. METHODS: We quantified plantar flexor neuromuscular function with ENS, EMS and MNS in 10 volunteers in fresh and fatigued muscles. Central alterations were assessed through changes in voluntary activation level (VAL) and peripheral function through changes in M-wave, twitch and doublet (PS100) amplitudes. Discomfort associated with 100-Hz paired stimuli delivered with each method was evaluated on a 10-cm visual analog scale. RESULTS: VAL, agonist and antagonist M-wave amplitudes and PS100 were similar between the different methods in both fresh and fatigued states. Potentiated peak twitch was lower in EMS compared to ENS, whereas no difference was found between ENS and MNS for any parameter. Discomfort associated with MNS (1.5 ± 1.4 cm) was significantly less compared to ENS (5.5 ± 1.9 cm) and EMS (4.2 ± 2.6 cm) (p < 0.05). CONCLUSION: When PS100 is used to evaluate neuromuscular properties, MNS, EMS and ENS can be used interchangeably for plantar flexor neuromuscular function assessment as they provide similar evaluation of central and peripheral factors in unfatigued and fatigued states. Importantly, electrical current spread to antagonist muscles was similar between the three methods while discomfort from MNS was much less compared to ENS and EMS. MNS may be potentially employed to assess neuromuscular function of plantar flexor muscles in fragile populations.

POSITIONING DURING RESISTANCE ELBOW FLEXOR EXERCISE AFFECTS ELECTROMYOGRAPHIC ACTIVITY, HEART RATE, and PERCEIVED EXERTION

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Effects of elbow flexor muscle resistance training on strength, endurance and perceived exertion

Purpose. To verify the effects of resistance training at the electromyographic fatigue threshold (EMGFT) based on one-repetition maximum strength (1RM), heart rate (HR), rate of perceived exertion (PE) and endurance time (EndT). Methods. Nineteen subjects (training group [TG]: n = 10; control group [CG]: n = 9), performed 1-min bicep curl exercises sets at 25%, 30%, 35% and 40% 1RM. Electromyography (biceps brachii and brachiorradialis), HR and PE were registered. Biceps brachii EMGFT was used to create a load index for an eight-week resistance training programme (three sets until exhaustion/session, two sessions/week) for the TG. The CG only attended one session in the first week and another session in the last week of the eight-week training period for EndT measurement. EndT was determined from the number of repetitions of each of the three sets performed in the first and last training sessions. After training, 1RM, EMGFT, EndT, HR and PE at the different bicep curl load intensities were again measured for both groups. Results. Increases in 1RM (5.9%, p < 0.05) and EndT (> 60%, p < 0.001) after training were found. In addition, PE was reduced at all load intensities (p < 0.05), while no changes were found for HR and EMGFT after training. Conclusions. Strength-endurance training based on the EMGFT improved muscular endurance and also, to a lesser extent, muscular strength. Moreover, the reduced levels of physical exertion after training at the same intensity suggest that endurance training exercises may improve comfort while performing strength exercises.

Reduced endurance of the cervical flexor muscles in patients with concurrent temporomandibular disorders and neck disability

Subjects with temporomandibular disorders (TMDs) have been found to have clinical signs and symptoms of cervical dysfunction. Although many studies have investigated the relationship between the cervical spine and TMD, no study has evaluated the endurance capacity of the cervical muscles in patients with TMD. Thus the objective of this study was to determine whether patients with TMD had a reduced endurance of the cervical flexor muscles at any level of muscular contraction when compared with healthy subjects. One hundred and forty-nine participants provided data for this study (49 subjects were healthy, 54 had myogenous TMD, and 46 had mixed TMD). There was a significant difference in holding time at 25% MVC between subjects with mixed TMD when compared to subjects with myogenous TMD and healthy subjects. This implies that subjects with mixed TMD had less endurance capacity at a lower level of contraction (25% MVC) than healthy subjects and subjects with myogenous TMD. No significant associations between neck disability, jaw disability, clinical variables and neck flexor endurance test were found.

Electromyographic activity of the cervical flexor muscles in patients with temporomandibular disorders while performing the craniocervical flexion test: a cross-sectional study

Most patients with temporomandibular disorders (TMD) have been shown to have cervical spine dysfunction. However, this cervical dysfunction has been evaluated only qualitatively through a general clinical examination of the cervical spine.

External perturbation of the trunk in standing humans differentially activates components of the medial back muscles

During voluntary arm movements, the medial back muscles are differentially active. It is not known whether differential activity also occurs when the trunk is perturbed unpredictably, when the earliest responses are initiated by short-latency spinal mechanisms rather than voluntary commands. To assess this, in unpredictable and self-initiated conditions, a weight was dropped into a bucket that was held by the standing subject (n = 7). EMG activity was recorded from the deep (Deep MF), superficial (Sup MF) and lateral (Lat MF) lumbar multifidus, the thoracic erector spinae (ES) and the biceps brachii. With unpredictable perturbations, EMG activity was first noted in the biceps brachii, then the thoracic ES, followed synchronously in the components of the multifidus. During self-initiated perturbations, background EMG in the Deep MF increased two- to threefold, and the latency of the loading response decreased in six out of the seven subjects. In Sup MF and Lat MF, this increase in background EMG was not observed, and the latency of the loading response was increased. Short-latency reflex mechanisms do not cause differential action of the medial back muscles when the trunk is loaded. However, during voluntary tasks the central nervous system exerts a 'tuned response', which involves discrete activity in the deep and superficial components of the medial lumbar muscles in a way that varies according to the biomechanical action of the muscle component.

Feedforward activity of the cervical flexor muscles during voluntary arm movements is delayed in chronic neck pain

The objective of this study was to compare onset of deep and superficial cervical flexor muscle activity during rapid, unilateral arm movements between ten patients with chronic neck pain and 12 control subjects. Deep cervical flexor (DCF) electromyographic activity (EMG) was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid (SCM) and anterior scalene (AS) muscles. While standing, subjects flexed and extended the right arm in response to a visual stimulus. For the control group, activation of DCF, SCM and AS muscles occurred less than 50 ms after the onset of deltoid activity, which is consistent with feedforward control of the neck during arm flexion and extension. When subjects with a history of neck pain flexed the arm, the onsets of DCF and contralateral SCM and AS muscles were significantly delayed (p<0.05). It is concluded that the delay in neck muscle activity associated with movement of the arm in patients with neck pain indicates a significant deficit in the automatic feedforward control of the cervical spine. As the deep cervical muscles are fundamentally important for support of the cervical lordosis and the cervical joints, change in the feedforward response may leave the cervical spine vulnerable to reactive forces from arm movement.

Patients with neck pain demonstrate reduced electromyographic activity of the deep cervical flexor muscles during performance of the craniocervical flexion test

Study Design. Cross-sectional study. Objective. The present study compared activity of deep and superficial cervical flexor muscles and craniocervical flexion range of motion during a test of craniocervical flexion between 10 patients with chronic neck pain and 10 controls. Summary of Background Data. Individuals with chronic neck pain exhibit reduced performance on a test of craniocervical flexion, and training of this maneuver is effective in management of neck complaints. Although this test is hypothesized to reflect dysfunction of the deep cervical flexor muscles, this has not been tested. Methods. Deep cervical flexor electromyographic activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the superficial neck muscles ( sternocleidomastoid and anterior scalene). Root mean square electromyographic amplitude and craniocervical flexion range of motion was measured during five incremental levels of craniocervical flexion in supine. Results. There was a strong linear relation between the electromyographic amplitude of the deep cervical flexor muscles and the incremental stages of the craniocervical flexion test for control and individuals with neck pain ( P = 0.002). However, the amplitude of deep cervical flexor electromyographic activity was less for the group with neck pain than controls, and this difference was significant for the higher increments of the task ( P < 0.05). Although not significant, there was a strong trend for greater sternocleidomastoid and anterior scalene electromyographic activity for the group with neck pain. Conclusions. These data confirm that reduced performance of the craniocervical flexion test is associated with dysfunction of the deep cervical flexor muscles and support the validity of this test for patients with neck pain.

A new method of isometric dynamometry for the craniocervical flexor muscles

Background and Purpose. A new method of dynamometry has been developed to measure the performance of the craniocervical (CC) flexor muscles by recording the torque that these muscles exert on the cranium around the CC junction. This report describes the method, the specifications of the instrument, and the preliminary reliability data. Subjects and Methods. For the reliability study, 20 subjects (12 subjects with a history of neck pain, 8 subjects without a history of neck pain) performed, on 2 occasions, maximal voluntary isometric contraction (MVIC) tests of CC flexion in 3 positions within the range of CC flexion and submaximal sustained tests (20% and 50% of MVIC) in the middle range of CC flexion (craniocervical neutral position). Reliability coefficients were calculated to establish the test-retest reliability of the measurements. Results. The method demonstrated good reliability over 2 sessions in the measurement of MVIC (intraclass correlation coefficient [ICC] =.79-.93, SEM=0.6-1.4 N-m) and in the measurement of steadiness (standard deviation of torque amplitude) of a sustained contraction at 20% of NMC (ICC=.74-.80, SEM=0.01 N-m), but not at 50% of MVIC (ICC=.07-.76, SEM=0.04-0.13 N-m). Discussion and Conclusion. The new dynamometry method appears to have potential clinical application in the measurement of craniocervical flexor muscle performance.

Further evaluation of an EMG technique for assessment of the deep cervical flexor muscles

A novel surface electromyographic (EMG) technique was recently described for the detection of deep cervical flexor muscle activity. Further investigation of this technique is warranted to ensure EMG activity from neighbouring muscles is not markedly influencing the signals recorded. This study compared deep cervical flexor (DCF) muscle activity with the activity of surrounding neck and jaw muscles during various anatomical movements of the neck and jaw in 10 volunteer subjects. DCF EMG activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid, anterior scalene, masseter and suprahyoid muscles. Positioned in supine, subjects performed isometric cranio-cervical flexion, cervical flexion, right and left cervical rotation,jaw clench and resisted jaw opening. Across all movements examined, EMG amplitude of the DCF muscles was greatest during neck movements that would require activity of the DCF muscles, particularly during cranio-cervical flexion, their primary anatomical action. The actions of jaw clench and resisted jaw opening demonstrated significantly less DCF EMG activity than the cranio-cervical flexion action (p < 0.05). Across all other movements, the neighbouring neck and jaw muscles demonstrated greatest EMG amplitude during their respective primary anatomical actions, which occurred in the absence of increased EMG amplitude recorded from the DCF muscles. The finding of substantial EMG activity of the DCF muscles only during neck actions that would require their activity, particularly cranio-cervical flexion, and not during actions involving the jaw, provide further assurance that the majority of myoelectric signals detected from the nasopharyngeal electrode are from the DCF muscles. (C) 2005 Elsevier Ltd. All rights reserved.

Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths

We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and,27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths (P < 0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle. (C) 2003 Elsevier Ltd. All rights reserved.

Neural influences on sprint running - Training adaptations and acute responses

Performance in sprint exercise is determined by the ability to accelerate, the magnitude of maximal velocity and the ability to maintain velocity against the onset of fatigue. These factors are strongly influenced by metabolic and anthropometric components. Improved temporal sequencing of muscle activation and/or improved fast twitch fibre recruitment may contribute to superior sprint performance. Speed of impulse transmission along the motor axon may also have implications on sprint performance. Nerve conduction velocity (NCV) has been shown to increase in response to a period of sprint training. However, it is difficult to determine if increased NCV is likely to contribute to improved sprint performance. An increase in motoneuron excitability, as measured by the Hoffman reflex (H-reflex), has been reported to produce a more powerful muscular contraction, hence maximising motoneuron excitability would be expected to benefit sprint performance. Motoneuron excitability can be raised acutely by an appropriate stimulus with obvious implications for sprint performance. However, at rest reflex has been reported to be lower in athletes trained for explosive events compared with endurance-trained athletes. This may be caused by the relatively high, fast twitch fibre percentage and the consequent high activation thresholds of such motor units in power-trained populations. In contrast, stretch reflexes appear to be enhanced in sprint athletes possibly because of increased muscle spindle sensitivity as a result of sprint training. With muscle in a contracted state, however, there is evidence to suggest greater reflex potentiation among both sprint and resistance-trained populations compared with controls. Again this may be indicative of the predominant types of motor units in these populations, but may also mean an enhanced reflex contribution to force production during running in sprint-trained athletes. Fatigue of neural origin both during and following sprint exercise has implications with respect to optimising training frequency and volume. Research suggests athletes are unable to maintain maximal firing frequencies for the full duration of, for example, a 100m sprint. Fatigue after a single training session may also have a neural manifestation with some athletes unable to voluntarily fully activate muscle or experiencing stretch reflex inhibition after heavy training. This may occur in conjunction with muscle damage. Research investigating the neural influences on sprint performance is limited. Further longitudinal research is necessary to improve our understanding of neural factors that contribute to training-induced improvements in sprint performance.

Comparação da análise miográfica sonora com a força muscular

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