The relative activation of muscles during isometric contractions and low-velocity movements against a load

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, this direction was different for isometric contractions and for movements. Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.     

Anatomical model of the human trunk for analysis of respiratory muscles mechanics

A realistic two-dimensional (2D) model of the human trunk was developed for quantitative analysis of the relative contribution to breathing mechanics of seven groups of respiratory muscles. Along with noninvasive measurements of electromyography (EMG) signals from respiratory muscles near the skin surface, it provides predictions for the forces generated by inner respiratory muscles as well as the instantaneous work of each muscle. The results revealed that inspiratory muscles reach their maximal force towards the end of inspiration, while expiratory muscles reach their maximal force at mid-expiration. Inspiratory muscles contributed to the work of breathing even at low efforts, while that of the expiratory muscles was observed only at relatively high efforts. The study clearly showed that the diaphragm muscle generates forces, which are of the same order as those generated by other inspiratory muscles, but performed 60-80% of the inspiratory work. The contribution of the external intercostal muscle to inspiration was not negligible, especially at high breathing efforts.     

Coordination among the body segments during reach-to-grasp action involving the trunk

To understand the internal representations used by the nervous system to coordinate multijoint movements, we examined the coordination among the body segments during reach-to-grasp movements which involve grasping by the hand and reaching by the arm and trunk. Subjects were asked to reach and grasp an object using the arm only, the trunk only, and some combinations of both arm and trunk. Results showed that kinematic parameters related to the transport component of the arm and the trunk, such as peak velocity and time to peak velocity, varied across conditions and that the coordination pattern between the arm and trunk was different across conditions. However, parameters related to the grasp component, such as peak aperture, time to peak aperture, and closing distance, were invariant, regardless of whether the hand was delivered to the target by the arm only, the trunk only, or both. We hypothesize that a hierarchy of motor control processes exists, in which the reach and grasp components are governed by independent neuromotor synergies, which in turn are coordinated temporally and spatially by a higher-level synergy. Received: 1 April 1998 / Accepted: 25 August 1998     

Control strategies for finger movement during touch-typing The role of the extrinsic muscles during a keystroke

A single keystroke during touch-typing is a rapid, goal-directed motion of the fingertip which consists of two single-direction movements. The neural control and the role of the finger extrinsic musculature during typing have not yet been explained. The fingertip motion and force, and the intramuscular electromyographic (EMG) activity (fine-wire) of the index finger extrinsic musculature were measured during touch-typing by ten experienced typists. The motions and forces were repeatable qualitatively across keystrokes. A three-burst EMG pattern was observed during a single keystroke. The three bursts were: (1) a burst of extensor activity lifted the finger before the keystroke; (2) a burst of flexor activity followed while the fingertip was moving downward; and (3) a second burst of extensor activity occurred as the fingertip reached the end of key travel. The timing of the third burst suggests the role of the extensors is to remove the fingertip from the keyswitch rather than stop the downward motion of the finger. The collision with the end of key travel stops the downward finger motion. The timing of the finger flexor EMG activity, burst 2, suggests that the flexor contraction principally overcomes the activation force of the keyswitch rather than accelerates the finger downward as expected. Received: 1 April 1997 / Accepted: 29 December 1997     

Reprogramming of muscle activation patterns at the wrist in compensation for elbow reaction torques during planar two-joint arm movements

The relationship between wrist kinematics, dynamics and the pattern of muscle activation were examined during a two-joint planar movement in which the two joints moved in opposite directions, i.e. elbow flexion/wrist extension and elbow extension/wrist flexion. Elbow movements (ranging from 10 to 70 deg) and wrist movements (ranging from 10 to 50 deg) were performed during a visual, step-tracking task in which subjects were required to attend to the initial and final angles at each joint. As the elbow amplitude increased, wrist movement duration increased and the wrist movement trajectories became quite variable. Analysis of the torques acting at the wrist joint showed that elbow movements produced reaction torques acting in the same direction as the intended wrist movement. Distinct patterns of muscle activation were observed at the wrist joint that were dependent on the relative magnitude of the elbow reaction torque in relation to the net wrist torque. When the magnitude of the elbow reaction torque was quite small, the wrist agonist was activated first. As the magnitude of the elbow reaction torque increased, activity in the wrist agonist decreased significantly. In conditions where the elbow reaction torque was much larger than the net wrist torque, the wrist muscle torque reversed direction to oppose the intended movement. This reversal of wrist muscle torque was directly associated with a change in the pattern of muscle activation where the wrist antagonist was activated prior to the wrist agonist. Our findings indicate that motion of the elbow joint is an important consideration in planning wrist movement. Specifically, the selection of muscle activation patterns at the wrist is dependent on the relative magnitude and direction of the elbow reaction torque in relation to the direction of wrist motion.     

EMG activation patterns during force production in precision grip

Electromyographic (EMG) activity was analyzed for the occurrence of synergistic patterns during the steady hold periods of force in the precision grip. To establish the presence of muscle synergies in the amplitude (spatial) domain, the EMG activation levels of pairs of simultaneously active muscles were linearly correlated. Cross-correlations of EMG activity were computed to quantify muscle synergies in the spatiotemporal domain (synchronization). A muscle pair was defined to be synergistically coupled or synchronously activated when the correlation (amplitude domain) or cross-correlation (time domain) was significant for at least two of the three steady state force levels. Muscle synergies in the amplitude domain were found in one-third of the 213 muscle pairs tested, distributed among 47 of the 82 tested muscle combinations. Coactivation was the predominant synergistic pattern, whereas trade-off comprised not more than 23% of the synergies. Cross-correlation peak size varied between 5 and 39% of the autocorrelation size, with delays in the range of ±8 ms and base width between 12 and 20 ms. Synchronization was found in one-fourth of the 213 muscle pairs tested and among 35 of the 82 muscle combinations, i.e., less frequently than covariation of EMG activity levels. However, the interindividual prevalence was higher for synchronization than for synergies in the amplitude domain, since, for the synergistic muscle combinations, almost twice as many muscle pairs were found to be synchronized than coupled in the amplitude domain. Synergies in the two domains occurred independently in some pairs and concurrently in other cases, and were observed between muscles moving the thumb, the index finger, or both digits. Synchronization was more frequent in pairs of muscles supplied by branches of the same peripheral nerve (46%) than in those innervated by different nerves (18%). Synergies in the amplitude domain were distributed in similar proportions across intrinsic, extrinsic, and combinations of both types of muscles, whereas synchronization mainly occurred in pairs of intrinsic muscles. When the task was repeated with slightly lower target forces, there were fewer synergies in the amplitude domain (in 52 of the 213 pairs, distributed among 35 of 82 muscle combinations) and their distribution changed, indicating a flexible, force-dependent mechanism. In conclusion, no strictly coherent interindividual pattern of synergies in the spatial domain could be established.     

Afferent feedback in the triphasic EMG pattern of leg muscles associated with rapid body sway

Electromyographic (EMG) patterns of leg muscles associated with rapid body sway were studied in relation to displacement of the center of foot pressure (CFP). Standing subjects were instructed to shift the CFP by swaying their bodies, pivoting at the ankle as rapidly and accurately as possible after an auditory signal. CFP position was designated as N when the subject maintained a relaxed bending posture and as F when a maximally forward-leaning posture was maintained. A serial, stereotyped triphasic EMG pattern was observed in the rapid shift of CFP from N to F: cessation of EMG activity in the gastrocnemius (GC) muscle was followed by a burst in the tibialis anterior (TA) muscle (acceleration phase), and then resumed discharge occurred in the GC muscle with cessation of activity in the TA muscle (deceleration and stop). When the subject shifted the CFP from N to F to different degrees, the duration and amount of EMG activity in the TA muscle during acceleration and the GC muscle in deceleration were proportionate to the amount of CFP displacement associated with forward body sway. To determine the functional roles of sensory inputs from the foot on the triphasic EMG pattern, body sway was studied under the condition of sensory block in the feet induced by ischemia from tourniquets placed bilaterally just above the ankle joints. The triphasic EMG pattern persisted during ischemia. The time of GC cessation and the onset of TA burst at acceleration remained unchanged, but the times of TA cessation and resumption of GC discharge at deceleration were altered during ischemia. Moreover, subjects were unable to stop at F and eventually fell. These results indicate that both amount and duration of EMG activity associated with rapid body sway are functions of the amount of CFP displacement. Somatic sensation from the feet is important for control of burst and cessation timing and duration in leg muscle activity.     

Healthy humans use sex-specific co-ordination patterns of trunk muscles during gait

Human gait patterns differ considerably between the sexes. Therefore sex specific trunk muscle activation patterns can be expected. Healthy volunteers of both sexes (51 women, 55 men) walked on a treadmill at speeds from 2 to 6 km/h. Surface electormyography was recorded from five pairs of trunk muscles. Grand averaged root mean square (rms) curves and amplitude normalised curves were calculated. Mean amplitudes and relative amplitudes were calculated as well. Mean amplitudes as well as relative amplitude levels were not generally sex specific, but differed for single muscles. Grand averaged rms curves of all investigated muscles differed between sexes. At low walking speeds, differences mostly originated from mean amplitude level differences, alternating between sexes. At higher walking speeds, amplitude curves became more phasic, differences again alternated between sexes. Therefore, trunk muscle co-ordination during gait is sex-specific. Any interpretation of trunk muscle co-ordination patterns during gait requires sex specific normatives.     

Force path curvature and conserved features of muscle activation

This study examined the patterns of muscle activity that subserve the production of dynamic isometric forces in various directions. The isometric condition provided a test for basic features of neuromuscular control, since the task was analogous to reaching movement, but the behavior was not necessarily shaped by the anisotropy of inertial and viscoelastic resistance to movement. Electromyographic (EMG) activity was simultaneously recorded from nine elbow and/or shoulder muscles, and force pulses, steps, and ramps were monitored using a transducer fixed to the constrained wrists of human subjects. The force responses were produced by activating shoulder and elbow muscles; response direction was controlled by the relative intensity of activity in muscles with different mechanical actions. The primary objective was to characterize the EMG temporal pattern. Ideally, synchronous patterns of phasic muscle activation (and synchronous dynamic elbow and shoulder torques) would result in a straight force path; asynchronous muscle activation could result in substantial force path curvature. For both pulses and steps, asynchronous muscle activation was observed and was accompanied by substantial force path curvature. A second objective was to compare phasic and tonic EMG activity. The spatial tuning of EMG intensity was similar for the phasic and tonic activities of each muscle and also similar to the spatial tuning of tonic activity in a previous study where the arm was stationary but unconstrained.     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Summary Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71±31.3 to 505±188 V,p<0.01) and mean power frequency (89±13.3 to 123±23.5 Hz,p<0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike ampltitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412±79 to 972±117 V,p<0.01) and mean firing frequency (17.8±5.4 to 24.7±4.1 Hz,p<0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Activity patterns of upper arm muscles in relation to direction of rapid wrist movement in man

Summary Adjustment of arm posture associated with rapid wrist movements was studied by EMG analysis. Seven healthy adults, seated and holding their right arm with the shoulder in a neutral position with the elbow in 90° flexion and the wrist position neutral, were instructted to flex or extend the wrist as fast as possible. To examine whether the activity patterns of the upper arm muscles were related to the prime mover or the direction of the movement in space, the forearm was in two postures, supinate and pronate. The surface EMGs of biceps brachii, brachialis, triceps brachii and the prime movers were recorded along with the angular displacement of the wrist. The sequences of the upper arm muscle activities changed in relation to the direction of the movement. The earliest activities of the upper arm muscles were considered to counteract the dynamic perturbation induced by the rapid wrist movement. The onsets of the earliest activity of the upper arm muscles preceded the movement onset by 50–60 ms. These results revealed that the activity patterns of the arm muscles associated with the rapid wrist movements were functionally compatible with the anticipatory postural adjustment and were controlled according to the direction of the movement in space.     

EMG activation patterns during force production in precision grip. III. Synchronisation of single motor units

Motor unit (MU) synchronisation during isometric force production in the precision grip was analysed in five subjects performing a visually guided steptracking motor task with three different force levels. With this aim multi-unit electromyographic (EMG) activity of 14 intrinsic and extrinsic finger muscles from 15 experimental sessions was decomposed into the potentials of single MUs. The behaviour of 62 intrinsic and 30 extrinsic MUs in the motor task was quantified. Most MUs displayed a positive correlation between firing rate and grip force. Compared to MUs in extrinsic muscles, intrinsic MUs had steeper regression lines with negative intercepts indicating higher force sensitivity and higher recruitment thresholds. A cross-correlation analysis was performed for 69 intra- and 166 intermuscular MU pairs while steady grip force was exerted at the three force levels. Synchronisation, for at least one force level, was found in 78% of the intra- and 45% of the intermuscular pairs. The occurrence of synchronisation was not stable over the force range tested. Factors influencing the fluctuations in occurrence and strength of synchronisation were investigated. Force increase was not paralleled by increased synchronisation; in contrast, in most MU pairs, especially intermuscular pairs, synchronisation occurred preferentially at the lower force levels. The recruitment threshold appeared to play a determining role in synchronisation: the more similar the thresholds of two MUs, the greater the probability of them being synchronised at this force level. Synchronised MUs fired on average at a lower frequency than non-synchronised ones. Finally, synchronisation at the multi-unit EMG level does not indicate that all underlying MUs are synchronised, nor does the absence of temporal coupling at the multi-unit level indicate that none of the MUs is synchronised.     

Cardiovascular changes during whole body hyperthermia treatment of advanced malignancy

Summary Cardiovascular studies were carried out on patients subjected to whole body hyperthermia treatment for advanced malignancy in order to assess the magnitude of the changes occurring and the degree of strain imposed on the system. The subjects, who were anaesthetised with a nitrous oxide/oxygen and relaxant sequence, were heated in a modified Siemens hyperthermia cabin and maintained at a body temperature of 41.8 C for 2 h. The results of 30 treatments are presented. Large increases in cardiac output and heart rate were accompanied by large decreases in peripheral resistance in both the systemic and pulmonary vascular beds. The pulmonary arterial pressure rose whereas that in the systemic circulation fell. This caused right ventricular work to increase proportionately more than left ventricular work. Care should be exercised when subjecting patients with limited right ventricular function to this treatment.This work was supported by K. W. F. Grant: EUR 77-4     

Gender differences in stability of the instantaneous patterns of body surface potentials during ventricular repolarisation

Women have a higher risk of developing torsade de pointes under QT-prolonging conditions. The electrophysiological differences between the sexes that could account for this are largely unknown. The objective of the work was to evaluate gender differences in repolarisation potentials using a method that is independent of the specific electrical properties of the thorax. 1410 normal recordings from the Glasgow 12-lead ECG database and 52 normal ECG maps obtained separately in Milan were analysed. The average difference between 1 and the correlation coefficient of the instantaneous pattern at the peak of T with that at every other instant is called the early repolarisation deviation index (ERDI) for J-T peak and the late repolarisation deviation index (LRDI) for T peak-T end. In standard ECG recordings, the ERDI was 0.42±0.22 in females compared with 0.19±0.16 in males (p<10⁻⁶). The LRDI was higher in males under the age of 50. In body surface maps, the ERDI was 0.32±0.21 in females against 0.16±0.17 in males (p<0.01), and the LRDI was non-significantly higher in males. The pattern of instantaneous body surface potentials showed gender differences during repolarisation with a method that is independent of the electrical properties of the thorax.     

Eye position modulates the electromyographic responses of neck muscles to electrical stimulation of the superior colliculus in the alert cat

Rapid gaze shifts are often accomplished with coordinated movements of the eyes and head, the relative amplitude of which depends on the starting position of the eyes. The size of gaze shifts is determined by the superior colliculus (SC) but additional processing in the lower brain stem is needed to determine the relative contributions of eye and head components. Models of eye–head coordination often assume that the strength of the command sent to the head controllers is modified by a signal indicative of the eye position. Evidence in favor of this hypothesis has been recently obtained in a study of phasic electromyographic (EMG) responses to stimulation of the SC in head-restrained monkeys (Corneil et al. in J Neurophysiol 88:2000–2018, 2002b). Bearing in mind that the patterns of eye–head coordination are not the same in all species and because the eye position sensitivity of phasic EMG responses has not been systematically investigated in cats, in the present study we used cats to address this issue. We stimulated electrically the intermediate and deep layers of the caudal SC in alert cats and recorded the EMG responses of neck muscles with horizontal and vertical pulling directions. Our data demonstrate that phasic, short latency EMG responses can be modulated by the eye position such that they increase as the eye occupies more and more eccentric positions in the pulling direction of the muscle tested. However, the influence of the eye position is rather modest, typically accounting for only 10–50% of the variance of EMG response amplitude. Responses evoked from several SC sites were not modulated by the eye position.     

Fixed patterns of rapid postural responses among leg muscles during stance

Summary The aim of this study has been to present firmer evidence that during stance functionally related postural muscles in the legs are activated according to fixed patterns. The importance of fixed patterns of activation for stabilization, balance, and movement control has received considerable theoretical and experimental attention. With regard to postural adjustment in humans, however, evidence for fixed activation patterns has been circumstantial only. Previous studies could not rule out the possibility that fixed patterns were caused by the mechanical coupling of rotatory movements among the joints of the body.This study has shown that in subjects employing FSR adjustments during stance activation patterns among leg muscles at FSR latency (functional stretch response, 100–120 msec) are preprogrammed prior to a response and are, on the average, fixed, independent of the associated motions among the ankle, knee, and hip joints. The identical fixed patterns were produced by sway rotation about the ankle joints and by direct rotation of the ankles. A pattern was characterized as fixed when, during a 1 hr session, the ratios of estimated force between pairs of functionally related leg muscles remained constant. In addition, the sequence of activation among muscles was fixed and followed a course beginning at the ankle muscle and proceeding proximally.The discussion of these results considered the functional implications of fixed contractile patterns during stance posture control.     

Directionality of anticipatory activation of trunk muscles in a lifting task depends on load knowledge

We investigated to what extent subjects base anticipatory activity patterns of trunk muscles before lifting a load on knowledge of the inertial properties of the load. Eight healthy male subjects performed rapid arm lifts of a load with a varying center of mass position in the frontal plane. In one set of trials subjects were familiar with the center of mass position, in another set of trials they were not. In both cases trunk extensor muscles were active before the onset of lift force applied to the load. In the trials with load knowledge this anticipatory activity was specific with respect to center of mass position. In the absence of load knowledge left and right extensor muscles were equally active before the lift and the rate of lifting was reduced. Thus anticipatory control of trunk muscles appears specifically tuned to counteract the expected perturbation. In the absence of load knowledge trunk stiffness is increased by bilateral activity and the perturbation is attenuated since the rate of lifting is reduced. Received: 25 August 1998 / Accepted: 24 March 1999     

The development of goal-directed reaching in infants II. Learning to produce task-adequate patterns of joint torque

 Nine young infants were followed longitudinally from 4 to 15 months of age. They performed multijoint reaching movements to a stationary target presented at shoulder height. Time-position data of the hand, shoulder, and elbow were collected using an optoelectronic measurement system. In addition, we recorded electromyographic activity (EMG) from arm extensors and flexors. This paper documents how control problems of proximal torque generation may account for the segmented hand paths seen during early reaching. Our analysis revealed the following results: first, muscular impulse (integral of torque) increased significantly between the ages of 20 (reaching onset) and 64 weeks. That is, as infants got older they produced higher levels of mean muscular flexor torque during reaching. Data were normalized by body weight and movement time, so differences are not explained by anthropometric changes or systematic variations in movement time. Second, while adults produced solely flexor muscle torque to accomplish the task, infants generated flexor and extensor muscle torque at shoulder and elbow throughout a reach. At reaching onset more than half of the trials revealed this latter kinetic profile. Its frequency declined systematically as infants got older. Third, we examined the pattern of muscle coordination in those trials that exhibited elbow extensor muscle torque. We found that during elbow extension coactivation of flexor and extensor muscles was the predominant pattern in 67% of the trials. This pattern was notably absent in comparable adult reaching movements. Fourth, fluctuations in force generation, as measured by the rate of change of total torque (NET) and muscular torque (MUS), were more frequent in early reaching (20–28 weeks) than in the older cohort (52–64 weeks), indicating that muscular torque production became increasingly smoother and task-efficient. Our data demonstrate that young infants have problems in generating smooth profiles of proximal joint torques. One possible reason for this imprecision in infant force control is their inexperience in predicting the magnitude and direction of external forces. That infants learned to consider external forces is documented by their increasing reliance on these forces when performing voluntary elbow extensions. The patterns of muscle coordination underlying active elbow extensions were basically the same as during the prereaching phase, indicating that the formation of functional synergies is based on a basal repertoire of innervation patterns already observable in very early, spontaneous movements. Received: 5 January 1996 / Accepted: 19 August 1996     

Age-related fatigability of the ankle dorsiflexor muscles during concentric and eccentric contractions

This study compares the fatigability of the ankle dorsiflexors during five sets of 30 maximal concentric and eccentric contractions in young and elderly adults. The torque produced by the ankle dorsiflexors and the average surface electromyogram (aEMG) of the tibialis anterior were continuously recorded. The contribution of central and peripheral mechanisms to muscle fatigue was tested before, after each set of contractions, and during a 30 min recovery period by the superimposed electrical stimulation method. The compound muscle action potential (M-wave), the mechanical response to single (twitch) and paired (doublet) stimulation, and the postactivation potentiation were also recorded. Compared with young subjects, elderly adults exhibited a greater loss of torque for concentric (50.2 vs. 40.9%; P<0.05) and eccentric (42.1 vs. 27.1%; P < 0.01) contractions. Although young subjects showed a lesser decrease in torque during the eccentric compared with concentric contractions, elderly adults experienced similar fatigability for the two types of contractions despite a comparable depression in the EMG activity of both groups and contraction types (10–20%). As tested by the interpolated-twitch method and aEMG/M-wave ratio, voluntary activation was not altered during either type of contraction or for either age group. During the two fatigue tasks, only elderly adults experienced a decrease in M-wave area (26.4–35.4%; P < 0.05). All together, our results suggest that the fatigue exhibited by both young and elderly adults during maximal concentric and eccentric contractions mainly involved peripheral alterations and that elderly adults may also have experienced a decline in neuromuscular propagation.