Time-dependence between upper arm muscles activity during rapid movements: Observation of the proportional effects predicted by the kinematic theory

Rapid human movements can be assimilated to the output of a neuromuscular system with an impulse response modeled by a Delta-Lognormal equation. In such a model, the main assumption concerns the cumulative time delays of the response as it propagates toward the effector following a command. To verify the validity of this assumption, delays between bursts in electromyographic (EMG) signals of agonist and antagonist muscles activated during a rapid hand movement were investigated. Delays were measured between the surface EMG signals of six muscles of the upper limb during single rapid handwriting strokes. From EMG envelopes, regressions were obtained between the timing of the burst of activity produced by each monitored muscle. High correlation coefficients were obtained supporting the proportionality of the cumulative time delays, the basic hypothesis of the Delta-Lognormal model. A paradigm governing the sequence of muscle activities in a rapid movement could, in the long run, be useful for applications dealing with the analysis and synthesis of human movements.     

Cross-association analysis of EEG and EMG signals according to movement intention state

Rehabilitation within three months plays a significant role in the recovery of damaged motor functions following the onset of a stroke. To increase the effectiveness of rehabilitation, it is important to perform rehabilitative exercises with movement intention. This study analyzed the association between electroencephalogram (EEG) and electromyogram (EMG) signals in healthy individuals in an attempt to verify the differences between the two signals in corticomuscular connectivity as well as the time delay in the flow of information in accordance with the presence of movement intention. To examine the relationship between the brain and muscles, coherence and mutual information analyses were performed on the EEG signals in the motor cortex and EMG signals in the flexor digitorum superficialis muscle during grasping training. Coherence and mutual information between EEG and EMG signals were significantly higher and the time delay of information flow was shorter when subjects performed active exercise with movement intention than when they performed passive exercise without movement intention. These findings could be applied to the rehabilitation of stroke patients to develop a rehabilitative training system with heightened effectiveness through verification of the presence of movement intention in the patients.     

Effects of varying load conditions on the organization of postural adjustments during voluntary arm flexion

One purpose of the experiments reported here was to further clarify the effect of varying loads on postural adjustments. Another was to reevaluate whether or not the timing of electromyographic (EMG) activity in the postural muscle is preprogrammed. To accomplish these goals, we compared the effect of the presence or absence of prior knowledge of a load on the timing of EMG activity in the postural muscle (biceps femoris [BF]) with that in the focal muscle (anterior deltoid [AD]). Although the sequence of EMG activation was similar under conditions with and without a load, the timing of postural EMG activities (BFi, ipsilateral BF; BFc, contralateral BF) in associated postural adjustments was dependent on the force of arm movement, and the latencies of postural EMG activities (BFi-BFc) were dependent on the speed of arm movement. This indicates that EMG changes in the upper (focal muscle) and lower limbs (postural muscle) were triggered by different motor programs. Moreover, similar EMG activities were observed in postural muscles when the subject had advance knowledge of the presence or the absence of a load. Thus, this suggests that BFi may be centrally preprogrammed (anticipatory regulation) and BFc may be feedback regulated. Furthermore, environmental information may be a critical source of influence on those postural responses.     

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 Varying Load Conditions on the Organization of Postural Adjustments during Voluntary Arm Flexion

One purpose of the experiments reported here was to further clarify the effect of varying loads on postural adjustments. Another was to reevaluate whether or not the timing of electromyographic (EMG) activity in the postural muscle is preprogrammed. To accomplish these goals, we compared the effect of the presence or absence of prior knowledge of a load on the timing of EMG activity in the postural muscle (biceps femoris [BF]) with that in the focal muscle (anterior deltoid [AD]). Although the sequence of EMG activation was similar under conditions with and without a load, the timing of postural EMG activities (BFi, ipsilateral BF; BFc, contralateral BF) in associated postural adjustments was dependent on the force of arm movement, and the latencies of postural EMG activities (BFi—BFc) were dependent on the speed of arm movement. This indicates that EMG changes in the upper (focal muscle) and lower limbs (postural muscle) were triggered by different motor programs. Moreover, similar EMG activities were observed in postural muscles when the subject had advance knowledge of the presence or the absence of a load. Thus, this suggests that BFi may be centrally preprogrammed (anticipatory regulation) and BFc may be feedback regulated. Furthermore, environmental information may be a critical source of influence on those postural responses.     

Muscle activation is different when the same muscle acts as an agonist or an antagonist during voluntary movement

During movement, the intrinsic muscle force-velocity property decreases the net force for the shortening muscle (agonist) and increases it for the lengthening muscle (antagonist). The authors present a quantitative analysis of the effect of that muscle property on activation and force output of the same muscle acting as agonist and antagonist in fast and medium speed goal-oriented movements. They compared biceps activation and force output when that muscle was the agonist in a series of elbow flexions and when it was the antagonist in a series of elbow extensions. They performed the same analysis for the lateral, long, and medial heads of the triceps muscle. Muscle EMG was about 2 times larger and the angular impulse developed by the modeled contractile torque was up to 3 times larger when the muscle or muscles acted as the agonist than when the same muscle or muscles acted as the antagonist in movements with similar kinematics. The large effect of the muscle force-velocity property strongly suggests that the neural controller must account for intrinsic muscle properties to generate movements with a commonly observed bell-shaped velocity profile.     

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.     

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.     

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).     

Movement refractoriness: The influence of response structure and speed

In the first experiment, refractoriness of a primary arm swing was studied using two movement speeds and three secondary responses (reversal, contralateral and ipsilateral thumb-lift). Pre-movement inter-stimulus intervals were 100, 200 or 300 msec and response-stimulus intervals ranged from zero to 200 msec. Accelerometers provided kinematic data. The second experiment repeated the reversal condition with the addition of EMG analysis.The prediction that the maximal speed condition would show a general intensification of control processes, and thus less refractoriness as compared to submaximal speed, was upheld for all response conditions. The findings also supported the prediction that the pattern of refractoriness depends on the functional relationships of the muscles concerned.The EMG analysis revealed that while the sequencing of muscle action was unchanged with movement speed, its phasing characterized the shifts in response metrics. The overall findings emphasize the need to view the refractoriness phenomenon in the context of anatomical and mechanical consequences of force changes in controlling limb movement as they interact with intentional command.     

Spatial and temporal characteristics of the spine muscles activation during walking in patients with lumbar instability due to degenerative lumbar disk disease: Evaluation in pre-surgical setting

Our purpose was to investigate the spatial and temporal profile of the paraspinal muscle activation during gait in a group of 13 patients with lumbar instability (LI) in a pre-surgical setting compared to the results with those from both 13 healthy controls (HC) and a sample of 7 patients with failed back surgery syndrome (FBSS), which represents a chronic untreatable condition, in which the spine muscles function is expected to be widely impaired.Spatiotemporal gait parameters, trunk kinematics, and muscle activation were measured through a motion analysis system integrated with a surface EMG device. The bilateral paraspinal muscles (longissimus) at L3-L4, L4-L5, and L5-S1 levels and lumbar iliocostalis muscles were evaluated.Statistical analysis revealed significant differences between groups in the step length, step width, and trunk bending and rotation. As regard the EMG analysis, significant differences were found in the cross-correlation, full-width percentage and center of activation values between groups, for all muscles investigated.Patients with LI, showed preserved trunk movements compared to HC but a series of EMG abnormalities of the spinal muscles, in terms of left-right symmetry, top-down synchronization, and spatiotemporal activation and modulation compared to the HC group. In patients with LI some of such EMG abnormalities regarded mainly the segment involved by the instability and were strictly correlated to the pain perception. Conversely, in patients with FBSS the EMG abnormalities regarded all the spinal muscles, irrespective to the segment involved, and were correlated to the disease’s severity. Furthermore, patients with FBSS showed reduced lateral bending and rotation of the trunk and a reduced gait performance and balance.Our methodological approach to analyze the functional status of patients with LI due to spine disease with surgical indications, even in more complex conditions such as deformities, could allow to evaluate the biomechanics of the spine in the preoperative conditions and, in the future, to verify whether and which surgical procedure may either preserve or improve the spine muscle function during gait.     

Effects of walking on various inclines on EMG patterns of lower limb muscles in humans

Myoelectric signals from several muscles of the lower limb were studied during treadmill locomotion over various inclines. A pattern recognition technique was used to analyse these activity patterns. The analyses revealed the following rules. These are common features among the various muscle activity patterns. The results suggest that the limb is controlled as a unit. Both phasic and average components of the muscle activity patterns are modulated to meet demands imposed by the various inclines. The distal muscles in general are more tightly controlled than the proximal muscles. The changes in average EMG values are muscle-specific, and are not similar for the stance and swing phases of the step cycle. On average, the proximal muscles show greater increases than the distal muscles. These results are compared with those found previously in the different speed and stride-length condition. Studies such as these shed light on the adaptability of the basic locomotor synergy.     

Inter-individual variability of forces and modular muscle coordination in cycling: A study on untrained subjects

The aim of this study was to investigate the muscle coordination underlying pedaling in untrained subjects by using the muscle synergies paradigm, and to connect it with the inter-individual variability of EMG patterns and applied forces. Nine subjects performed a pedaling exercise on a cycle-simulator. Applied forces were recorded by means of instrumented pedals able to measure two force components. EMG signals were recorded from eight muscles of the dominant leg, and Nonnegative Matrix Factorization was applied to extract muscle synergy vectors W and time-varying activation coefficients H. Inter-individual variability was assessed for EMG patterns, force profiles, and H. Four modules were sufficient to reconstruct the muscle activation repertoire for all the subjects (variance accounted for >90% for each muscle). These modules were found to be highly similar between subjects in terms of W (mean r = .89), while most of the variability in force profiles and EMG patterns was reflected, in the muscle synergy structure, in the variability of H. These four modules have a functional interpretation when related to force distribution along the pedaling cycle, and the structure of W is shared with that present in human walking, suggesting the existence of a modular motor control in humans.     

Some characteristics of EMG patterns during locomotion: implications for the locomotor control process

Myoelectric signals from several muscles of the lower limb were studied under various speed and stride length conditions. The main purpose was to determine invariant and variant features among these myoelectric patterns. A pattern recognition algorithm was used to analyze these activity patterns. Within-condition analysis revealed some common features among the EMG patterns. This suggests that the nervous system does not have to generate all the muscle activity patterns, only the common features that can, in appropriate combination, produce the necessary activity patterns. From the across condition analysis, the following rules emerged. First, both phasic component and magnitude (d.c. level) of the muscle activity patterns have to be modulated to meet the demands imposed by the various conditions. Second, the variability in the proximal muscle activity patterns across conditions are higher than the distal muscle activity patterns. Within each group, the extensor muscles and double-jointed muscles show greater variability than the flexor muscles and single-jointed muscles. And finally, the changes in the average value (d.c. level) of the muscle activity patterns across conditions are not uniform but show muscle and task specificity. For example, within the speed condition, the increase in d.c. level of the extensors with speed of locomotion show a proximal to distal trend. Based on these results, a conceptual model for the human locomotor control process is proposed.     

Some Characteristics of EMG Patterns During Locomotion

Myoelectric signals from several muscles of the lower limb were studied under various speed and stride length conditions. The main purpose was to determine invariant and variant features among these myoelectric patterns. A pattern recognition algorithm was used to analyze these activity patterns. Within-condition analysis revealed some common features among the EMG patterns. This suggests that the nervous system does not have to generate all the muscle activity patterns, only the common features that can, in appropriate combination, produce the necessary activity patterns. From the across condition analysis, the following rules emerged. First, both phasic component and magnitude (d.c. level) of the muscle activity patterns have to be modulated to meet the demands imposed by the various conditions. Second, the variability in the proximal muscle activity patterns across conditions are higher than the distal muscle activity patterns. Within each group, the extensor muscles and double-jointed muscles show greater variability than the flexor muscles and single-jointed muscles. And finally, the changes in the average value (d.c. level) of the muscle activity patterns across conditions are not uniform but show muscle and task specificity. For example, within the speed condition, the increase in d.c. level of the extensors with speed of locomotion show a proximal to distal trend. Based on these results, a conceptual model for the human locomotor control process is proposed.     

Prediction of maximum speed of human movement by two selected muscular coordination mechanisms and by maximum static strength

This study was undertaken to test the predictive value of two selected muscular coordination mechanisms, the sequential order of muscle activation and a specific acceleration-deceleration point of inflection, and of the maximum static strength of agonist and antagonist muscles for maximum speed of human movement. For 22 male subjects 77% of the variance associated with maximum speed of human movement was accounted for by the two mechanisms of muscular coordination investigated and by maximum isometric strength. The results suggest that separate neuromotor systems control human speed and strength and that the interplay between agonist and antagonist muscles is important for maximum speed of human movement.     

Principal postural acceleration and myoelectric activity: Interrelationship and relevance for characterizing neuromuscular function in postural control

One approach to investigating sensorimotor control is to assess the accelerations that produce changes in the kinematic state of the system. When assessing complex whole-body movements, structuring the multi-segmental accelerations is important. A useful structuring can be achieved through a principal component analysis (PCA) performed on segment positions followed by double-differentiation to obtain “principal accelerations” (PAs). In past research PAs have proven sensitive to altered motor control strategies, however, the interrelationship between PAs and muscle activation (surface electromyography, sEMG) have never been determined. The purpose of the current study was therefore to assess the relationship between PAs and sEMG signals recorded from muscles controlling the ankle joint during one-leg standing trials. It was hypothesized that medium correlation should be observed when accounting for neurophysiologic latencies (electro-mechanical delay). Unipedal balancing on a level-rigid ground was performed by 25 volunteers. sEMG activities were recorded from the tibialis anterior, peroneus longus, gastrocnemius medialis, and soleus muscles of the stance leg. The first eight PA-time series were determined from kinematic marker data. Then, a cross-correlation analysis was performed between sEMG and PA time series. We found that peak correlation coefficients for many participants aligned at time delays between 0.116 and 0.362 s and were typically in the range small to medium (|r| = 0.1 to 0.6). Thus, the current study confirmed a direct association between many principal accelerations PA(t) and muscle activation signals recorded from four muscles crossing the ankle joint complex. The combined analysis of PA and sEMG signals allowed exploring the neuromuscular function of each muscle in different postural movement components.