Effect of muscle length on surface EMG wave forms in isometric contractions

Summary To elucidate the influence of muscle length on surface EMG wave form, comparisons were made of surface EMGs of the biceps and triceps brachii muscles during isometric contractions at different muscle lengths. Muscle lengths were altered by setting the elbow joint angle at several intervals between the limits of extension and flexion. The intensity of the isometric contractions was 25% of maximum voluntary contraction at the individual joint angles. Slowing was obvious in the EMG wave forms of biceps as muscle length increased. The so-called Piper rhythm appeared when the muscle was more than moderately lengthened. The slowing trend with muscle lengthening, though less marked, was also seen in triceps. Zero-cross analysis revealed quasi-linear relationships between muscle length and slowing. Frequency analysis confirmed the development of Piper rhythm. An attempt was made to interpret the slowing associated with muscle lengthening in terms of the propagation of myoelectric signals in muscle fibers. Given the effect of muscle length on EMG wave forms, a careful control of joint angle may be required in assessing local muscle fatigue when using EMG spectral indices.     

Normalisation of EMG amplitude: an evaluation and comparison of old and new methods.

The purpose of this study was to evaluate and compare four different methods of normalising the amplitude of electromyograms (EMGs), from the biceps brachii. Five males performed isotonic contractions of the elbow flexors with an external force of 50 N, 100 N, 150 N and 200 N. These were followed by a single isometric maximal voluntary contraction (MVC) and ten isokinetic MVCs at 0.35 rad s(-1) intervals between 0.35 rad s(-1) and 3.50 rad s(-1). The processed EMGs recorded from the isotonic contractions were normalised by expressing them as a percentage of: (i) the mean (Dynamic Mean Method) and (ii) the peak EMG from the same contraction (Dynamic Peak Method), (iii) the EMG from the isometric MVC (Isometric MVC Method), and (iv) the EMG from an isokinetic MVC at the same elbow angle and angular velocity (Isokinetic MVC Method). The root mean square difference (RMSD) between the outputs of the Isokinetic MVC and Dynamic Mean methods was significantly greater (P<0.05) than between the Isokinetic MVC method and the Dynamic Peak and the Isometric MVC methods. The small (10%) difference between the Isokinetic MVC and the Isometric MVC Methods was a consequence, firstly, of the lack of difference in EMG recorded from the isometric and isokinetic MVCs and, secondly, the consistency in EMG over the range of motion and at different angular velocities of isokinetic MVC. We conclude that only the Isometric and Isokinetic MVC methods should be used to normalise the amplitude of EMGs from the biceps brachii.     

The distribution of activity among the muscles of a single group during isometric contraction

Summary The study of the relations between the electrical activity of a group of synergistic muscles and the biomechanical parameters characterizing the resultant movement is generally reduced to one of the muscles of this group, this muscle being considered as equivalent. This is in particular the case for the biceps brachii in the group of elbow flexor muscles. The validity of this concept in the case of isometric, isotonic contraction can be tested by the simultaneous utilization of surface-electrode and wire-electrode detection techniques.It has been shown 1. that the quadratic relation between the global integrated EMG of the biceps brachii and the external force resulting from the action of the group of flexors can be extended to each of the muscles of this group (brachialis, brachioradialis, pronator teres); 2. that the slope of the relation between the integrated EMG of the biceps brachii and the force differs according to whether the hand is supine or prone. An explanation of a mechanical nature has been proposed for this phenomenon; 3. that in a given situation the relations between the different integrated EMG's remain constant whatever the value of the force.It follows that 1. each value of the integrated EMG of a muscle equals a coefficient times the value of the force exerted by this muscle; 2. the notion of muscle equivalent is justified for a given situation.     

Passive visco-elastic properties of the structures spanning the human elbow joint

Summary The passive elastic torque function and the passive viscous torque function of the muscles and connective tissues spanning the elbow joint have been determined in three adult male subjects. The procedure for estimating the passive elastic torque involved measurement of the torque required to passively move the forearm-hand segment, at a constant angular velocity, throughout a complete range of elbow joint motion. The suspension method (Hatze, 1975) was used to obtain the estimate of passive viscous torque. Both of these torque functions were shown to be nonlinear functions of the angular displacement of the joint. In order to assess whether myotatic reflex activity was contributing to the damping of the segment, and thereby biasing the value of the torque contributed by passive viscous elements, the surface electromyograms of the biceps brachii and brachioradialis muscles were examined while the suspended body segment was oscillated. In one subject there was firm evidence of involuntary muscle activity in the brachioradialis muscle which tended to distort the oscillogram. This activity was enhanced when the subject maintained a voluntary isometric contraction in a remote muscle group (Jendrassik's manoeuvre). Based upon these observations, recommendations have been made for reducing unwanted myotatic reflex activity while using the suspension method to obtain estimates of various biomechanical parameters.On leave from the National Research Institute for Mathematical Sciences, CSIR, Pretoria 0001     

Maximal force during eccentric and isometric actions at different elbow angles

The purpose of this study was to examine a course of force potentiation and/or inhibition during maximal voluntary eccentric action. Maximal voluntary force (MVC) of elbow flexion of ten healthy male volunteers was measured during isometric and isokinetic eccentric action starting from 80° or 110° and ending at 140° elbow angle. Surface EMG was recorded from biceps brachii (BB) and brachioradialis (BR) muscles. Maximal voluntary eccentric force during the first 10° of the movement was higher (P<0.001) than the maximal voluntary isometric preactivation force both in 80° and in 110° starting position at all three velocities (1, 2, and 4 rad s⁻¹). The relative force potentiation was velocity dependent being smallest at the lowest stretching speed (P<0.01). Average EMG (aEMG) of BB and BR decreased as the joint angle increased both in eccentric and in isometric actions but the decrease in aEMG towards extension was somewhat higher in eccentric actions as compared to isometric. It was concluded that the force measured during the first 10° of eccentric contraction always exceeded the maximal voluntary isometric preactivation force regardless of the joint angle or of the movement velocity. When maximal voluntary preactivation preceded the stretch, the relative force potentiation seemed to be greater at higher stretching velocities (velocity dependent) while at lower preactivation levels, the velocity dependence was not observed. Decreased muscle activation and lower maximal voluntary force towards the end of the movement suggested inhibition during maximal voluntary eccentric actions.     

Behaviour of the human gastrocnemius muscle architecture during submaximal isometric fatigue

The purpose of this study was to examine whether the human gastrocnemius medialis (GM) fascicle length and pennation angle alter during a sustained submaximal isometric plantar flexion. Fourteen male subjects performed maximal voluntary plantar flexions (MVC) on a dynamometer before and after a fatiguing task. This task consisted of a sustained submaximal isometric fatiguing contraction (40% MVC) until failure to hold the defined moment. Ultrasonography was used to visualise the muscle belly of the GM. Leg kinematics were recorded (120 Hz) to calculate the joint moment using inverse dynamics. The exerted moments and the EMG signals from GM and lateralis, soleus and tibialis anterior were measured at 1,080 Hz. The root mean square (RMS) of the EMG signal of the three triceps surae muscles increased significantly (P0.05) between 17% and 28% with fatigue. Further, the fascicle length of the GM significantly decreased from 47.1±8.0 mm at the beginning to 41.8±6.7 mm at the end of fatigue and the pennation angle increased from 23.5±4.1° to 26.3±2.2° (P0.05). The changes in fascicle length and pennation angle of the GM during the contraction can influence the force potential of the muscle due to the force–length relationship and the force transmission to the tendon. This provides evidence on that an additional mechanical mechanism, namely tendon creep, can contribute to the increase in the EMG activity of the GM during submaximal isometric sustained contractions.     

The flexor function of the m. pronator teres in man: a quantitative electromyographic study

Summary The muscle pronator teres was studied by surface electromyography during elbow flexion in a horizontal plane. The forearm was in semi-pronation and movement was performed at various velocities. A quantitative comparison was made between pronator teres activity and two main elbow flexors, biceps brachii and brachioradialis.The mean timing of the onset of activity was constant: biceps brachii was activated first followed by pronator teres and brachioradialis, and the lower the velocity of flexion, the earlier was the onset of biceps brachii activity.There was a linear relationship between the integrated EMG from each muscle and the work done. However, this relationship was less exact for pronator teres and brachioradialis at low values of work, a finding which opens questions about the generality of this relationship and about the muscle equivalent concept.Pronator teres appears to participate in elbow flexion besides its role in pronation.Despite similar anatomical peculiarities, pronator teres does not behave in the same way as anconaeus or popliteus and, above all, it is not the sole muscle active in slow movement. Thus, all the stocky mucles lying close to an articulation do not behave in the same way.     

Amplitude and frequency measures of surface electromyography during dual task elbow torque production

Summary Studies of motor unit recruitment thresholds have demonstrated the existence of task-specific motor units within the muscles controlling the elbow. Two degree-of-freedom (df) task specificity was investigated at higher levels of elbow torque using the amplitude and frequency characteristics of surface electromyography (EMG). Flexion and supination torque data were collected together with EMG from electrode pairs on the brachioradialis (BRAD), biceps brachii short head, and medial and lateral aspects of biceps brachii long head, while subjects (n=14) performed the following four combinations of isometric tasks: (1) maximum voluntary contraction (MVC) flexion (F) and (2) MVC supination (S), each with a targeted torque of zero in the second d f; (3) MVC flexion with targeted MVC supination (FS); and (4) MVC supination with targeted MVC flexion (SF). Median power frequency (MEDF) and root mean square (RMS) amplitude under steady-state torque conditions were calculated and analyzed using ANCOVA models with planned contrasts (=0.05). A significant main effect for task was found in RMS, but not in MEDF. Contrasts showed a significant increase in RMS response in the dual MVC tasks (FS and SF) over the single MVC tasks of F and S. The lack of frequency changes with alterations in RMS data indicates that the underlying recruitment/rate coding scheme in use for dual-d f tasks may be different than in single-d f tasks, and provides possible support for the notion of motor unit task groups. Task-by-site interactions were found for both MEDF and RMS, and illustrated that the three biceps sites differed from BRAD in their responses to the F versus S tasks. These results provide further support that the synergy between biceps and BRAD is not fixed, and that the concept of flexor equivalence at the elbow does not hold under all torque task conditions     

Factors influencing quantified surface EMGs

Summary Isometric flexions of the elbow are studied in man. For the three joint angles studied, the torque, the surface EMG of the biceps brachii muscle, and the quantified EMG are recorded. The EMGs are picked up by means of bipolar electrodes located in such a way as to vary the interelectrode orientation, the interelectrode distance and the position on the muscle.Longitudinal placements collect stronger signals than transverse ones. Under certain conditions, the magnitude of the quantified EMG depends on the distance between the electrodes, and on their location on the muscle.The results are discussed in terms of conduction volume and detection volume.LA N 308, CNRS     

Cross-correlation of bilateral differences in fatigue during sustained maximal voluntary contraction

Maximal isometric force and electromyograph (EMG) activity of biceps brachii muscle during bilateral sustained elbow flexion were followed in 25 right-handed oarsmen. The percentage decline in force was greater for the left than for the right arm. Also, the mean power frequency (MPF) and the root mean square (rms) value of the EMG amplitude decreased more for the left than for the right arm. It was hypothesized that a common drive would indicate that the two forces curves would be highly correlated during the nonfatigued period, but the level of cross-correlation would decline during muscle fatigue. For the first 4 s of the contraction, the cross-correlation between the right and left force was high (r = 0.99), but thereafter it declined rapidly to a constant level. The decline of the cross-correlation was accompanied by a similar decrease in the correlation between the right and left EMG activations (MPF and rms). Thus, the decline in the cross-correlation level of force accompanied by a similar decrease in the correlation level of EMG would suggest a fatigue-induced neural derangement of the common drive.     

Motor unit recruitment during prolonged isometric contractions

Summary Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P<0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed motor unit rotation and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.     

Mechanical and electromyographic responses to stretch of the human anterior tibial muscle at different levels of contraction

Summary The EMG response and the mechanical response to 2 degree stretch of the human anterior tibial muscle was studied during contractions ranging from 0% to 80% of maximal voluntary contraction (MVC). The EMG response showed three distinct peaks M1, M2, and M3 with peak latencies of 59 ms, 86 ms, and 120 ms respectively. At low background torques M1 dominated while M2 and M3 were small or absent. M2 and M3 dominated above 40% of MVC and M2 in particular showed automatic gain compensation, i.e. it constituted a — more or less — constant proportion of the background EMG for all contraction levels. The ratio between M1 amplitude and background EMG steadily decreased with contraction level. Even though the summed contributions of M1, M2, and M3 to some degree showed automatic gain compensation, this was not the case for the mechanical response to stretch. Between 0% and 30% of MVC the reflex mediated mechanical response increased approximately in proportion to the contraction level, but the reflex mediated mechanical response peaked at 40% of MVC and declined to zero at 80% of MVC. This discrepancy between EMG and mechanical response was explained by a simple model. The regression line between rectified and filtered tibialis anterior EMG and torque was used to predict the mechanical response from the EMG response. At increasing contraction levels the twitch elicited by supramaximal electrical stimulation decreases, and we reduced the predicted mechanical response by the same factor as the twitch. This simple model predicted the mechanical response for all contraction levels, making it possible to assess the functionality of reflexes even when accurate measurements of muscle force or intrinsic muscle properties are not possible.     

Gender differences in strength and muscle fiber characteristics

Summary Strength and muscle characteristics were examined in biceps brachii and vastus lateralis of eight men and eight women. Measurements included motor unit number, size and activation and voluntary strength of the elbow flexors and knee extensors. Fiber areas and type were determined from needle biopsies and muscle areas by computerized tomographical scanning. The women were approximately 52% and 66% as strong as the men in the upper and lower body respectively. The men were also stronger relative to lean body mass. A significant correlation was found between strength and muscle cross-sectional area (CSA; P0.05). The women had 45, 41, 30 and 25% smaller muscle CSAs for the biceps brachii, total elbow flexors, vastus lateralis and total knee extensors respectively. The men had significantly larger type I fiber areas (4597 vs 3483 m²) and mean fiber areas (6632 vs 3963 m²) than the women in biceps brachii and significantly larger type II fiber areas (7700 vs 4040 m²) and mean fiber areas (7070 vs 4290 m²) in vastus lateralis. No significant gender difference was found in the strength to CSA ratio for elbow flexion or knee extension, in biceps fiber number (180 620 in men vs 156 872 in women), muscle area to fiber area ratio in the vastus lateralis 451 468 vs 465 007) or any motor unit characteristics. Data suggest that the greater strength of the men was due primarily to larger fibers. The greater gender difference in upper body strength can probably be attributed to the fact that women tend to have a lower proportion of their lean tissue distributed in the upper body. It is difficult to determine the extent to which the larger fibers in men represent a true biological difference rather that a difference in physical activity, but these data suggest that it is largely an innate gender difference.     

Differences in central control of m. biceps brachii in movement tasks and force tasks

Summary Motor-unit activity in m. biceps brachii during isometric flexion contractions has been compared with motor-unit activity during a slow voluntary movements against constant or increasing preloads and b flexion contractions while movements were imposed by a torque motor. Recruitment levels and firing frequency behaviour of the motor units were found to be very similar when torques were generated during isometric contractions and during the imposed movements. However, these characteristics of the biceps motor units were quite different during the slow voluntary movements. It is suggested that the central activation of the and/or motoneurone pools of m. biceps brachii is different for force tasks and slow movement tasks, even if the same torques are exerted and/or movements are made.     

Modulation of corticospinal excitability dependent upon imagined force level

Motor imagery is defined as the mental execution of a movement without any muscle activity. In the present study, corticospinal excitability was assessed by motor evoked potentials (MEPs) when the subjects imagined isometric elbow flexion at various force levels. Electromyography was recorded from the right brachioradialis, the biceps brachii and the triceps brachii muscles. First, the maximum voluntary contraction (MVC) of elbow flexion was recorded in each subject. Subjects practiced performing 10, 30 and 60 % MVC using visual feedback. After the practice, MEPs were recorded during the imagery of elbow flexion with the forces of 10, 30 and 60 % MVC without any feedback. After the MEPs recording, we assigned subjects to reproduce the actual elbow flexion force at 10, 30 and 60 % MVC. The MEPs amplitudes in the brachioradialis and biceps brachii in the 60 % MVC condition were significantly greater than those in the 10 % MVC condition (p < 0.05). These findings suggest that the enhancement of corticospinal excitability during motor imagery is associated with an increase in imagined force level.     

The pressor response to voluntary and electrically evoked isometric contractions in man

Summary Blood pressure and heart rate changes during sustained isometric exercise were studied in 11 healthy male volunteers. The responses were measured during voluntary and involuntary contractions of the biceps brachii at 30% of maximal voluntary contraction (MVC), and the triceps surae at 30% and 50% MVC. Involuntary contractions were evoked by percutaneous electrical stimulation of the muscle.Measurements of the time to peak tension of maximal twitch showed the biceps brachii (67.0±7.9 ms) muscle to be rapidly contracting, and the triceps surae (118.0±10.5 ms) to be slow contracting. The systolic and diastolic blood pressures increased linearly throughout the contractions, and systolic blood pressure increased more rapidly than diastolic. There was no significant difference in response to stimulated or voluntary contractions, nor was there any significant difference between the responses to contractions of the calf or arm muscles at the same relative tension.In contrast the heart rate rose to a higher level (P<0.01) in the biceps brachii than the triceps surae at given % MVC, and during voluntary compared with the electrically evoked contractions in the two muscle groups.It was concluded that the arterial blood pressure response to isometric contractions, unlike heart rate, is primarily due to a reflex arising within the active muscles (cf. Hultman and Sjöholm 1982) which is associated with relative tension but independent of contraction time and muscle mass.     

Motor unit synchronization is increased in biceps brachii after exercise-induced damage to elbow flexor muscles

The purpose of this study was to determine the effect of eccentric exercise on correlated motor unit discharge (motor unit synchronization and coherence) during low-force contractions of the human biceps brachii muscle. Eight subjects (age, 25 +/- 7 yr) performed three tasks involving isometric contraction of elbow flexors while EMG (surface and intramuscular) records were obtained from biceps brachii. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; and 3) sustained discharge of pairs of concurrently active motor units for 2-5 min. These tasks were performed before, immediately after, and 24 h after fatiguing eccentric exercise. MVC force declined 46% immediately after eccentric exercise and remained depressed (31%) 24 h later, which is indicative of muscle damage. For the constant-force task, biceps brachii EMG ( approximately 100% greater) and force fluctuations ( approximately 75% greater) increased immediately after exercise, and both recovered by approximately 50% 24 h later. Motor unit synchronization, quantified by cross-correlation of motor unit pairs during low-force (1-26% MVC) contractions, was 30% greater immediately after (n = 105 pairs) and 24 h after exercise (n = 92 pairs) compared with before exercise (n = 99 pairs). Similarly, motor unit coherence at low (0-10 Hz) frequencies was 20% greater immediately after exercise and 34% greater 24 h later. These results indicate that the series of events leading to muscle damage from eccentric exercise alters the correlated behavior of human motor units in biceps brachii muscle for > or =24 h after the exercise.     

不同关节角度肌肉等长最大自主收缩抗阻力训练的神经适应分析

目的 研究不同关节角度下肌肉等长最大自主收缩(maximum voluntary contraction,MVC)抗阻力训练神经适应特点.方法 将30名健康男性大学生随机分成3组,每组分别做肘关节角度在45°、90°、135° MVC等长收缩抗阻力训练,测量训练前后肱二头肌的表面肌电(surface electromy...     

Intramuscular laser-Doppler flowmetry in the supraspinatus muscle during isometric contractions

To study the regulation of microvascular blood flow in a compartment muscle, laser-Doppler measurements of muscle microcirculation were recorded in the supraspinatus muscle in eight volunteers during and following submaximal isometric muscle contractions. The subjects performed isometric shoulder abductions at five contraction levels from 5% to 50% maximal voluntary contraction for I min each and a sustained 30° shoulder abduction for 20 min. The subjects' perceived exertion increased from no perceived exertion to near maximal exertion during the 20-min period with 30° shoulder abduction. Microcirculation increased during all 1-min contractions. Following the contractions at 20%, 30% and 50% MVC post-exercise reactive hyperaemia was seen for a period of at least 1 min. The reactive hyperaemia increased in magnitude in response to increasing contraction level. The results showed the same time-history of the blood flow at microvascular level as previously seen in larger peripheral vessels in response to muscle contractions. During the 20-min contraction microcirculation increased in line with the findings during the brief contractions. However, in contrast to the brief contractions no postexercise reactive hyperaemia occurred following the prolonged contraction. Lack of postexercise reactive hyperaemia following the prolonged shoulder abduction would suggest insufficient regulation of the vascular resistance. Alternatively, lack of hyperaemia could be taken as an indication of sufficient microcirculation during the preceding contraction. From previous studies on intramuscular pressure and metabolism the latter alternative would seem unlikely.     

Habitual level of physical activity and muscle fatigue of the elbow flexor muscles in older men

Reduced muscle performance, related to the loss of muscle mass and strength, is a common and natural part of ageing. Nevertheless, it is generally believed that regular participation in activities of moderate intensity may slow down these age-related changes. This study investigated the relationship between the habitual level of physical activity (PA), assessed by the modified Baecke Questionnaire, and the mechanical and fatigue characteristics of the right elbow flexor muscles, m. biceps brachii and m. brachioradialis, in men over the age of 55 years. Muscle fatigue was quantified both by measuring the maximal voluntary contraction (MVC) torque before and after a sustained isometric contraction at 25% MVC until exhaustion, and also by the temporal changes observed in the surface electromyographic (EMG) signal recorded during the fatigue task. Results showed a decreased MVC torque at the end of the fatiguing contraction. After 20 min of recovery, the MVC force was still significantly lower than the pre-fatigue value, except for the most active subjects. Typical myoelectrical indications of fatigue were also observed: a shift in the frequency spectrum of the signal towards lower frequencies accompanied by an increase in the EMG amplitude. We concluded from this study that the level of PA was related to the absolute isometric MVC values and the measurement of neuromuscular efficiency after 20 min of recovery, but did not influence the indications of muscle fatigue during an isometric fatigue task. Electronic Publication