Rapid acceleration of the lower arm correlates with agonist EMGs during the initial phase

Fifty-eight healthy subjects made rapid elbow extensions to a target over 54 degrees. Angular acceleration was measured and surface electromyograms (EMGs) were recorded from the antagonistic muscles using monopolar rather than bipolar electrode configurations. Marked individual differences were found in the peak value of the first derivative of acceleration (dAcc/dt_Pk). The dAcc/dt_Pk correlated with both quantitative and qualitative properties of the agonist EMGs, but not with those of the antagonist EMG. The agonist EMGs, integrated until the moment of dAcc/dt_Pk, were positively correlated with dAcc/dt_Pk. The interval between EMG onset and EMG peak decreased with increasing dAcc/dt_Pk. The duration of the initial negative phase in the EMGs, which was considered to index the time required to recruit high-threshold MUs, decreased with increasing dAcc/dt_Pk. The results indicate that the ability to rapidly accelerate the lower arm varies across subjects, probably due in part to individual differences in the neural capacity to drive the agonists.     

The role of propulsive muscles of the shoulder during quadrupedalism in vervet monkeys (Cercopithecus aethiops): implications for neural control of locomotion in primates

In comparative anatomical studies of the shoulder, the humeral retractors are often grouped together as propulsive muscles, which are important in the propulsive stroke of the forelimb during quadrupedal locomotion. Electromyographic (EMG) analyses of these muscles in opossums, cats, and dogs in general have confirmed such conclusions. An EMG study of chimpanzee shoulder muscles during knuckle-walking found, however, that the humeral retractors are either inactive or perform a function unrelated to propulsion (Larson & Stern, 1987). This contrast in muscle recruitment patterns between chimpanzees and more "typical" mammalian quadrupeds was attributed to the derived morphology of the chimpanzee shoulder. The present study examines the activity patterns of the humeral retractors in the vervet monkey, a primate more closely resembling nonprimate mammals in its shoulder morphology. The results of this EMG analysis show that despite the significant differences in anatomy between chimpanzees and vervets, the two species display very similar muscle recruitment patterns during quadrupedalism, and there is evidence for this same pattern in other species of primates. These differences in muscle activity patterns between primates and nonprimate mammals may be related to changes in the neurological control of locomotion in primates due to the evolutionary development of manipulative abilities in the primate forelimb.     

The Role of Propulsive Muscles of the Shoulder During Quadrupedalism in Vervet Monkeys (Cercopithecus aethiops)

In comparative anatomical studies of the shoulder, the humeral retractors are often grouped together as propulsive muscles, which are important in the propulsive stroke of the forelimb during quadrupedal locomotion. Electromyographic (EMG) analyses of these muscles in opossums, cats, and dogs in general have confirmed such conclusions. An EMG study of chimpanzee shoulder muscles during knuckle-walking found, however, that the humeral retractors are either inactive or perform a function unrelated to propulsion (Larson & Stern, 1987). This contrast in muscle recruitment patterns between chimpanzees and more “typical” mammalian quadrupeds was attributed to the derived morphology of the chimpanzee shoulder. The present study examines the activity patterns of the humeral retractors in the vervet monkey, a primate more closely resembling nonprimate mammals in its shoulder morphology. The results of this EMG analysis show that despite the significant differences in anatomy between chimpanzees and vervets, the two species display very similar muscle recruitment patterns during quadrupedalism, and there is evidence for this same pattern in other species of primates. These differences in muscle activity patterns between primates and nonprimate mammals may be related to changes in the neurological control of locomotion in primates due to the evolutionary development of manipulative abilities in the primate forelimb.     

Movement Related EMGs Become More Variable During Learning of Fast Accurate Movements

Human subjects performed simple flexion and extension movements about the elbow in a visual step-tracking paradigm. Movements were self-terminated. Subjects were instructed to increase movement velocity while maintaining end-point accuracy during practice. The effects of practice on the pattern and variability of EMG activity of the biceps and triceps muscles were studied. Initial movements were performed using reciprocal phasic activation of agonist and antagonist muscles as indicated by surface EMGs. With practice, increases in movement speed were associated with larger agonist and antagonist bursts and an earlier onset of the antagonist burst. Decreased duration of the premovement antagonist silence was also observed during practice.

Decreases in variability of movements during practice were not accompanied by equivalent decreases in variability of the associated EMGs. Surprisingly, both agonist and antagonist EMGs were more variable in faster, practiced movements. The combined agonist-antagonist EMG variability depended on both movement speed and trajectory variability. Lower variability in movements in the presence of greater variability in the related EMGs occurred because of linked variations in agonist and antagonist muscle activities. Variations in the first agonist burst were often compensated for by associated variations in the antagonist and late agonist bursts. These linked variations maintained the limb trajectory relatively constant in spite of large variations in the first agonist burst. Modifications to impulse-variability models are therefore needed to explain compensations for variability in accelerative impulses (produced by the first agonist burst) by linked variations in impulses for deceleration (produced by the antagonist and late agonist bursts).     

Movement related EMGs become more variable during learning of fast accurate movements

Human subjects performed simple flexion and extension movements about the elbow in a visual step-tracking paradigm. Movements were self-terminated. Subjects were instructed to increase movement velocity while maintaining end-point accuracy during practice. The effects of practice on the pattern and variability of EMG activity of the biceps and triceps muscles were studied. Initial movements were performed using reciprocal phasic activation of agonist and antagonist muscles as indicated by surface EMGs. With practice, increases in movement speed were associated with larger agonist and antagonist bursts and an earlier onset of the antagonist burst. Decreased duration of the premovement antagonist silence was also observed during practice. Decreases in variability of movements during practice were not accompanied by equivalent decreases in variability of the associated EMGs. Surprisingly, both agonist and antagonist EMGs were more variable in faster, practiced movements. The combined agonist-antagonist EMG variability depended on both movement speed and trajectory variability. Lower variability in movements in the presence of greater variability in the related EMGs occurred because of linked variations in agonist and antagonist muscle activities. Variations in the first agonist burst were often compensated for by associated variations in the antagonist and late agonist bursts. These linked variations maintained the limb trajectory relatively constant in spite of large variations in the first agonist burst. Modifications to impulse-variability models are therefore needed to explain compensations for variability in accelerative impulses (produced by the first agonist burst) by linked variations in impulses for deceleration (produced by the antagonist and late agonist bursts).     

Velocity-dependent EMG activity of masseter and sternocleidomastoideus muscles during a ballistic arm thrusting movement

Two experiments were conducted to investigate the functional relationship between the general somatic motor function and the oral motor function. In Experiment 1, we analyzed the relationship between the amount of masseter muscle (MSS) activity and the velocity of a ballistic, 'karate-do' arm thrusting movement (ThrMov). ThrMov velocity was measured from video images taken with a high-speed CCD camera at a frequency of 500Hz. EMGs of MSS and sternocleidomastoideus (SCM) muscles as well as other related muscles were recorded simultaneously with video images in 6 varsity 'karate-do' athletes. Pearson's correlation coefficients were calculated between EMG amplitude and movement velocity. EMG activity of MSS as well as the other muscles increased as a function of ThrMov velocity in all participants, as evidenced by highly significant (p<.01) correlation coefficients, ranging from .64 to .87 (mean: .75). MSS EMG activity attained during ThrMovs performed at maximum velocity ranged between 14.6% and 113.8% of this muscle's MVC (45.7+/-39.3% MVC, mean+/-SD). SCM was also strongly active and closely associated with MSS. Besides changes in amount of EMG activity, it was further found that R-MSS EMG onset progressively shifted to the earlier phase of the ThrMov as ThrMov velocity increased. EMG onset time of R-MSS as well as R- and L-SCMs was negatively correlated with ThrMov velocity; when performed at maximum velocity MSS activation preceded the start of ThrMov by more than 100ms, whereas MSS was recruited last at approximately 150ms after the start of ThrMov when performed at moderate speed ( approximately 50% of maximum). In Experiment 2, the effects of head movement relative to the trunk on R-MSS and SCMs EMG activity were tested in both gazing and sidelong glancing conditions. A much smaller head rotation relative to the trunk was necessary during the ThrMov in the sidelong glancing condition compared to the gazing condition. R-MSS EMG activity was affected significantly by the difference between these conditions and decreased by 5.2% MVC in the sidelong glancing condition compared to the gazing condition. In association with the change in requirement for head movement between those conditions, EMG balance between the bilateral SCMs changed substantially. Finally, marked muscle activity during ThrMov was found in the MSS that was not directly involved in performing this movement, indicating a form of 'remote facilitation'.     

Effects of time uncertainty and instructed muscle tension on fractionated reaction time

Abstract: An experiment was conducted to examine the effects of time uncertainty and instructed muscle tension on the reaction time of elbow flexion. Twenty-two right-handed subjects were asked to respond to an audio stimulus by flexing their right elbow under four conditions (2 time uncertainty × 2 instructed muscle tension). Electromyograms (EMGs) were recorded from the biceps and triceps on the subject's right side. Reaction time was divided into premotor time and motor time, based on the difference between the EMG and elbow flexion response. Analysis of reaction time showed that the effects of time uncertainty and instructed muscle tension were additive. Time uncertainty affected premotor time only, and instructed muscle tension affected motor time only. These results are discussed in terms of the assumption that premotor time is a reflection of the central nervous system and motor time is a reflection to the peripheral muscle system.     

Reorganised anticipatory postural adjustments due to experimental lower extremity muscle pain

Automated movements adjusting postural control may be hampered during musculoskeletal pain leaving a risk of incomplete control of balance. This study investigated the effect of experimental muscle pain on anticipatory postural adjustments by reaction task movements. While standing, nine healthy males performed two reaction time tasks (shoulder flexion of dominant side and bilateral heel lift) before, during and after experimental muscle pain. On two different days experimental pain was induced in the m. vastus medialis (VM) or the m. tibialis anterior (TA) of the dominant side by injections of hypertonic saline (1 ml, 5.8%). Isotonic saline (1 ml, 0.9%) was used as control injection. Electromyography (EMG) was recorded from 13 muscles. EMG onset, EMG amplitude, and kinematic parameters (shoulder and ankle joint) were extracted. During shoulder flexion and VM pain the onset of the ipsilateral biceps femoris was significantly faster than baseline and post injection sessions. During heels lift in the VM and TA pain conditions the onset of the contralateral TA was significantly faster than baseline and post injection sessions in bilateral side. VM pain significantly reduced m. quadriceps femoris activity and TA pain significantly reduced ipsilateral VM activity and TA activity during bilateral heel lift. The EMG reaction time was delayed in bilateral soleus muscles during heels lift with VM and TA pain. The faster onset of postural muscle activity during anticipatory postural adjustments may suggest a compensatory function to maintain postural control whereas the reduced postural muscle activity during APAs may indicate a pain adaptation strategy to avoid secondary damage.     

A comparison of EMG and SCL in normal and depressed subjects

Skin conductance level (SCL) and electromyograms (EMGs) recorded from three muscle sites were compared for 15 female hospitalized psychiatric patients meeting diagnostic criteria for primary affective disorder and 15 age- and race-matched controls. The depressed subjects had significantly lower SCL and significantly higher resting EMG levels recorded from one muscle site. Electromyogram and SCL were not found to be significantly correlated with each other or with measures of anxiety or depression for the patients with primary affective disorder. Normal controls, however, showed significant correlations between a measure of depression and both EMG and SCL. These results are discussed with suggestions for future research.     

Effect of Age and Gender in the Control of Elbow Flexion Movements

In previous studies of rapid elbow movements in young healthy men, characteristic task-dependent changes in the patterns of muscle activation when movement speed or distance was varied have been reported. In the present study, the authors investigated whether age or gender is associated with changes in the patterns of muscle activity previously reported in young men. Arm movements of 10 healthy older and 10 healthy younger participants (5 men and 5 women in each group) were studied. Surface electromyograms (EMGs) from agonist (biceps) and antagonist (triceps) muscles, kinematic and kinetic parameters, as well as anthropometric and strength measures were recorded. All 4 groups of participants showed similar task- (distance or speed) dependent changes in biphasic EMG activity. Similar modulation of the initial rate of rise of the EMG, integrated agonist and antagonist EMG activity, as well as their relative timing were observed in all 4 groups. Those results suggest that older individuals of both genders retain the control strategies for elbow movements used by young individuals. Despite the qualitative similarities in the patterns of muscle activation, the men moved more quickly than the women, and younger participants moved more quickly than older participants. Those performance differences could not be explained in terms of differences in body size and strength alone.     

Effects of scapular retraction/protraction position and scapular elevation on shoulder girdle muscle activity during glenohumeral abduction

According to scapulohumeral rhythm, shoulder abduction is followed through scapular upward rotation to ensure joint mobility and stability. Of interest, the shoulder abduction can be performed holding the scapula in different positions and in association with scapular elevation, with possible effects on shoulder muscle activity. Therefore, the aim of the study was to analyze the activity of relevant shoulder muscles and the activity ratios between the scapulothoracic muscles, during shoulder abduction performed in different combinations of scapular position (neutral, retracted, protracted) and scapular elevation.The electromyographic activity of middle deltoid, serratus anterior, upper, middle and lower fibers of trapezius was recorded during shoulder abduction movements executed holding the scapula in neutral, retracted and protracted position, and subsequently a shoulder elevation movement. The activation of each muscle and the scapulothoracic muscles activity ratios were determined every 15 degrees, from 15° to 120° of abduction.Scapular retraction led to higher activation of the entire trapezius muscle, whereas protraction induced higher upper trapezius, middle deltoid and serratus anterior activity, along with lower activity of middle and lower trapezius. Shoulder elevation led to higher activity of the upper trapezius and middle deltoid. Moreover, it induced lower activation of the serratus anterior and middle and lower trapezius, thus leading to high ratios between the upper trapezius and the other scapulothoracic muscles, especially between 15 and 75 degrees of abduction.This study highlights that shoulder abduction performed with scapular protraction and in combination with scapular elevation leads to increased activity of the middle deltoid and upper trapezius, resulting in imbalances between the scapulothoracic muscles that could hamper the optimal scapulohumeral rhythm. The abduction performed in the aforementioned scapular conditions also induce potential reciprocal inhibition effects between the movers and stabilizers muscles of scapula, suggesting different motor control strategies of integrating a common shoulder movement with various modification of the scapular position.     

Three-dimensional kinematics of the scapula and trunk,and associated scapular muscle timing in individuals with stroke

Poor scapulothoracic control is a risk for developing shoulder pathology, but has received little attention so far in individuals with stroke (IwS). Trunk and scapular kinematics and surface muscle activity were measured in 15 healthy controls and 18 IwS during a low and high forward flexion (FF). Group-differences in trunk and scapular kinematics were assessed during low and high FF using a t-test (independent samples). Differences in muscle onset and offset time relative to movement start (both FF tasks) were determined using a mixed model taking into account the different groups and muscles. Recruitment patterns per group and task were described based on significant differences between muscles. In IwS, earlier lower trapezius and late infraspinatus offset were found during low FF, as well as a later onset and earlier offset of serratus anterior. For low FF, significantly more trunk axial rotation was found in IwS during both elevation and lowering. During high FF, IwS showed significantly less scapular posterior tilt during elevation and more scapular lateral rotation during lowering. IwS demonstrated adaptive muscle timing with earlier initiation and late inactivation of lower trapezius and infraspinatus, possibly to compensate for a late activation and early deactivation of the serratus anterior and to establish as such the correct pattern of scapulothoracic movement.     

Simulation Studies of Descending and Reflex Control of Fast Movements

Several neurological control strategies for fast head movements are considered using computer simulations of a stretch reflex model. Each control strategy incorporates a different amount of proprioceptive feedback contributing to braking and/or clamping the movement. The model behavior for each control strategy is qualitatively compared to experimental data that includes the agonist and antagonist EMGs, and the head position, velocity, and acceleration. Significance of the study is discussed with respect to the characteristic tri-phasic EMG pattern for fast voluntary movements and the possible roles that the stretch reflex may have in contributing to this pattern of activation.     

Influence of perturbation induced by an anticipated load on human motor regulatory systems.

To investigate how human motor regulatory systems are modified by prior knowledge of a predictable external perturbation, six normal human subjects, each when sitting on a chair, were required to maintain a stable elbow flexion angle (90 degrees) while different weight perturbations were applied (0.5 kg or 2-kg loads). Loads were applied either by the experimenter Without Anticipation or With Anticipation by the subject's own contralateral hand. Acceleration of the forearm movement (elbow extension and flexion) by loads and electromyograms (EMGs) of the biceps brachii (BB) and the triceps brachii (TB) muscle were recorded. Under With Anticipation conditions, preceding EMG activities of BB and TB muscles prior to the onset time of perturbation were clearly observed. Furthermore, the amount of these preceding EMG activities was larger in the heavy load perturbation than in the light load perturbation. Under Without Anticipation conditions, however, these preceding EMG activities were not observed. In the preceding EMG activities, EMG bursts (latency 20 msec.) of a presumed stretch reflex induced by the perturbation were clearly observed. Thus, the function of anticipatory adjustment of mainitaining the elbow angle definitely appears to optimize limb stability in the case of the mechanical self-applied perturbation. Furthermore, the extent of the anticipatory adjustment of the elbow angle was dependent on the predicted magnitude of load.     

Effect of age and gender in the control of elbow flexion movements

In previous studies of rapid elbow movements in young healthy men, characteristic task-dependent changes in the patterns of muscle activation when movement speed or distance was varied have been reported. In the present study, the authors investigated whether age or gender is associated with changes in the patterns of muscle activity previously reported in young men. Arm movements of 10 healthy older and 10 healthy younger participants (5 men and 5 women in each group) were studied. Surface electromyograms (EMGs) from agonist (biceps) and antagonist (triceps) muscles, kinematic and kinetic parameters, as well as anthropometric and strength measures were recorded. All 4 groups of participants showed similar task- (distance or speed) dependent changes in biphasic EMG activity. Similar modulation of the initial rate of rise of the EMG, integrated agonist and antagonist EMG activity, as well as their relative timing were observed in all 4 groups. Those results suggest that older individuals of both genders retain the control strategies for elbow movements used by young individuals. Despite the qualitative similarities in the patterns of muscle activation, the men moved more quickly than the women, and younger participants moved more quickly than older participants. Those performance differences could not be explained in terms of differences in body size and strength alone.     

Anticipatory and compensatory postural adjustments in sitting in children with cerebral palsy

The aim of this study was to examine postural control in children with cerebral palsy performing a bilateral shoulder flexion to grasp a ball from a sitting posture. The participants were 12 typically developing children (control) without cerebral palsy and 12 children with cerebral palsy (CP). We analyzed the effect of ball mass (1 kg and 0.18 kg), postural adjustment (anticipatory, APA, and compensatory, CPA), and groups (control and CP) on the electrical activity of shoulder and trunk muscles with surface electromyography (EMG). Greater mean iEMG was seen in CPA, with heavy ball, and for posterior trunk muscles (p < .05). The children with CP presented the highest EMG and level of co-activation (p < .05). Linear regression indicated a positive relationship between EMG and aging for the control group, whereas that relationship was negative for participants with CP. We suggest that the main postural control strategy in children is based on corrections after the beginning of the movement. The linear relationship between EMG and aging suggests that postural control development is affected by central nervous disease which may lead to an increase in muscle co-activation.     

A linked muscular activation model for movement generation and control

A new model for movement control is presented which incorporates characteristics of impulse-variability and mass-spring models. Movements in the model were controlled with phasic torque impulses in agonist and antagonist muscles and a tonic agonist torque. Characteristics of the phasic agonist and antagonist torque profiles were based on observed properties of movement-related EMGs and muscle isometric torques. Variability of the phasic impulses depended on impulse magnitude as in impulse-variability models. The model therefore predicted a speed-accuracy tradeoff for limb movement. The time of onset and magnitude of the antagonist torque depended on the magnitude of the preceding agonist torque as indicated in studies of movement-related EMGs. This led to the new concept of linkage between the agonist and antagonist muscle forces which was shown to be important for reducing variability of fast movements. Progressive development of linkage during practice could explain the previous findings of decreased movement variability with practice coupled with increased variability of movement-related EMGs. It was concluded that an inherently variable motor system deals with the variability associated with generation of large muscle forces by linking the forces produced by opposing muscles. In this way, variability in net joint torques and in movements can be decreased without the need for the nervous system to closely regulate the individual torques.     

A Linked Muscular Activation Model for Movement Generation and Control

A new model for movement control is presented which incorporates characteristics of impulse-variability and mass-spring models. Movements in the model were controlled with phasic torque impulses in agonist and antagonist muscles and a tonic agonist torque.

Characteristics of the phasic agonist and antagonist torque profiles were based on observed properties of movement-related EMGs and muscle isometric torques. Variability of the phasic impulses depended on impulse magnitude as in impulse-variability models. The model therefore predicted a speed-accuracy tradeoff for limb movement. The time of onset and magnitude of the antagonist torque depended on the magnitude of the preceding agonist torque as indicated in studies of movement-related EMGs. This led to the new concept of linkage between the agonist and antagonist muscle forces which was shown to be important for reducing variability of fast movements. Progressive development of linkage during practice could explain the previous findings of decreased movement variability with practice coupled with increased variability of movement-related EMGs.

It was concluded that an inherently variable motor system deals with the variability associated with generation of large muscle forces by linking the forces produced by opposing muscles. In this way, variability in net joint torques and in movements can be decreased without the need for the nervous system to closely regulate the individual torques.     

Speech-motor equivalence in aging subjects'

The peak amplitude of EMG activity was measured from the orbicularis oris superior (OOS), orbicularis oris inferior (OOI), and masseter muscles for three normal, geriatric women (range 70 to 75 yr.) and compared with prior data for a group of normal, 4- and 8-yr.-old children and young adults (range 21 to 29 yr.). The elderly groups' variability across the three muscles paralleled that of the 4-yr.-olds, suggesting that speech-motor equivalence returns to an earlier level of operation in aging speakers. Also, the elderly subjects evidenced reduced levels of average peak EMG activity as compared to those of the other groups. This finding was interpreted as reflecting a loss of general muscle function, a possible concomitant of facial muscle atrophy that accompanies advanced age.