Peculiarities of Activation of Muscles of the Shoulder Belt in Voluntary Two-Joint Movements of the Upper Limb

We studied coordination of central motor commands (СMCs) coming to muscles of the shoulder and shoulder belt in the course of single-joint and two-joint movements including flexion and extension of the elbow and shoulder joints. Characteristics of rectified and averaged EMGs recorded from a few muscles of the upper limb were considered correlates of the CMC parameters. Special attention was paid to coordination of CMCs coming to two-joint muscles that are able to function as common flexors (m. biceps brachii, caput breve, BBcb) and common extensors (m. triceps brachii, caput longum, TBcl) of the elbow and shoulder joints. Upper limb movements used in the tests included planar shifts of the arm from one spatial point to another resulting from either simultaneous changes in the angles of the shoulder and elbow joints or isolated sequential (two-stage) changes in these joint angles. As was found, shoulder muscles providing movements of the elbow with changes in the angle of the elbow joint, i.e., BBcb and TBcl, were also intensely involved in the performance of single-joint movements in the shoulder joint. The CMCs coming to two-joint muscles in the course of two-joint movements appeared, in the first approximation, as sums of the commands received by these muscles in the course of corresponding single-joint movements in the elbow and shoulder joints. Therefore, if we interpret the isolated forearm movement performed due to a change in the angle of the elbow joint as the main motor event, while the shoulder movement is considered the accessory one, we can conclude that realization of a two-joint movement of the upper-limb distal part is based on superposition of CMCs related to basic movements (main and accessory). Neirofiziologiya/Neurophysiology, Vol. 41, No. 1, pp. 48–56, January–February, 2009.     

Activation of the Shoulder-Belt and Shoulder Muscles in Two-Joint Arm Movements Performed in Humans with the Action of Opposite Loadings

In tests on four volunteers, we examined coordination of central motor commands (CMCs) controlling slow two-joint movements of the arm within the horizontal plane. Current amplitudes of EMGs recorded from six muscles of the shoulder belt and shoulder and subjected to full-wave rectifying and low-frequency filtration were considered correlates of these commands. In particular, we studied the dependence of coordination of CMCs on the direction of an external force applied to the distal forearm part. As was found, coordination of CMCs significantly depends on the direction of the force flexing the elbow joint. According to our observations, EMGs of definite muscles in the case of performance of a two-joint movement can, in a first approximation, be presented as linear combinations of the EMGs recorded in the course of separate sequential single-joint movements under conditions of shifting the reference point of the hand toward the same point of the operational space as that in the two-joint movement. These data can be interpreted as confirmation of the principle of superposition of elementary CMCs in the performance of complex movements of the extremity.     

Activation of the Shoulder Belt and Shoulder Muscles of Humans Related to Different Rates of Generation of Two-Joint Efforts by the Forearm

We studied coordination of central motor commands (CMCs) coming to the muscles that flex and extend the shoulder and elbow joints in the course of generation of voluntary isometric efforts of different directions by the forearm. Dependences of the characteristics of these commands on the direction of the effort and rate of its generation were analyzed. Amplitudes of rectified and averaged EMGs recorded from a number of shoulder belt and shoulder muscles were considered correlates of the CMC intensity. The development of the effort of a given direction and rate of rise was realized in the horizontal-plane operational space; the arm position corresponded to the 30 deg angle in the shoulder joint (external angle with respect to the frontal plane) and 90 deg angle in the elbow joint. We plotted sector diagrams of the relative changes in the level of dynamic and stationary phases of EMG activity of the studied muscles for the entire set of directions of the efforts generated with different rates of rise. In the course of formation of rapid two-joint isometric efforts, realization of nonsynergic motor tasks (extension of one joint and flexion of another one, and vice versa) required significant activation of muscles of different functional directions for both joints. Time organization of EMG activity of extensors and flexors of the shoulder and elbow joints related to the maximum and relatively rapid generation of the effort (rise time 0.12 to 0.13 and 0.25 sec, respectively) was rather complex and included dynamic and stationary phases. With these time parameters of generation of the efforts (both flexion and extension), the appearance at the stationary effort of 40 N was controlled based on coordinated interaction of dynamic phases of the activation of agonistic and antagonistic muscles. It is concluded that CMCs coming to extensors and flexors of both joints upon generation of rapid isometric efforts are rather similar in their parameters to those under conditions of realization of the forearm movements in the space in an isotonic mode.     

Peculiarities of Activation of the Upper Limb Muscles in Realization of Two-Joint Movements in Humans

In tests on humans, we recorded EMG activity from the muscles flexing and extending the forearm and shoulder in the course of realization of sequential single-joint and simultaneous two-joint movements of the upper limb. As was shown, the shoulder muscles m. biceps brachii and m. triceps brachii are involved in flexion/extension of both elbow and shoulder joints. Central commands sent to the above muscles in the course of a two-joint movement could be considered a superposition of the central commands coming to the same muscles in realization of the corresponding sequential single-joint movements with the same changes in the angles of the elbow and shoulder joints. External loadings applied in the direction of extension of the elbow and shoulder joints induced, in general, similar changes in coordination of the activity of muscles moving the forearm and shoulder under conditions of both single-joint and two-joint movements. These facts allow us to suppose that coordination of the muscle activity in two-joint movements depends to a greater extent on the forces influencing limb links than on the mode of realization of the movements (two sequential single-joint movements vs a two-joint movement corresponding to the above motor events).     

Peculiarities of Activation of the Shoulder Belt and Shoulder Muscles in Generation of Different-Direction Isometric Efforts by the Forearm

We studied central motor commands, CMCs, coming to the muscles that flex and extend the shoulder and elbow joints in the course of generation of voluntary isometric efforts of different directions by the forearm; the efforts were initiated according to a visual signal. Amplitudes of EMGs recorded from the muscles of the shoulder belt and shoulder and subjected to full-wave rectification and low-frequency filtration were considered correlates of the CMC intensity. An effort of the preset direction was developed within the operational space of the horizontal plane with angles 30 deg in the shoulder joint (external angle with respect to the frontal plane) and 90 deg in the elbow joint. We plotted sector diagrams of the logarithmic coefficient of the intensity increment of EMGs of the above muscles for the entire set of directions of generated efforts with a 15- or 20-deg step. Orientations of the maxima of EMG activity of the given muscles were rather close to the directions of the maxima of the force moments generated by these muscles. In most cases, a shift of the direction by one gradation with respect to the EMG maximum in the respective muscle resulted in a significant decrease in the level of EMG activity. It is shown that preferential activation of the muscles agonistic with respect to the examined direction of the generated effort was, as a rule, accompanied by coactivation of the antagonist muscles. When “two-joint” isometric efforts are formed, realization of the socalled synergic muscle tasks (where prevailing contractions of the muscles of the same functional direction for both joints coincide, i.e., flexion-flexion or extension-extension) is organized in a simpler manner. The programs of “nonsynergic” contractions (flexion of one joint and extension of another one, or vice versa) are more complex. In different subjects, considerably dissimilar patterns of EMG activity in muscles influencing these joints could be observed.     

A New Approach to the Study of Two-Joint Upper Limb Movements in Humans: Independent Programming of the Positioning and Force

Neurophysiology - A new approach to mechanostimulation of the human forelimbs is proposed for studying various problems of motor control. The prototype of the device is based on a modern...     

Two Agents in the Brain: Motor Control of Unimanual and Bimanual Reaching Movements

Previous studies have suggested that the left and right hands have different specialties for motor control that can be represented as two agents in the brain. This study examined how coordinated movements are performed during bimanual reaching tasks to highlight differences in the characteristics of the hands. We examined motor movement accuracy, reaction time, and movement time in right-handed subjects performing a three-dimensional motor control task (visually guided reaching). In the no-visual-feedback condition, right-hand movement had lower accuracy and a shorter reaction time than did left-hand movement, whereas bimanual movement had the longest reaction time, but the best accuracy. This suggests that the two hands have different internal models and specialties: closed-loop control for the right hand and open-loop control for the left hand. Consequently, during bimanual movements, both models might be used, creating better control and planning (or prediction), but requiring more computation time compared to the use of one hand only.     

Reaching a Better Understanding of the Control of Bimanual Movements in Older Adults

The ability to interact skilfully with the environment is essential for independent living and therefore a critical factor for the aging population. Here we investigate the differences between young and older adults in a bimanual reaching task where the goal is to bring two objects together to the same location with a synchronous placement. Older (mean age 74) and young (mean age 20) adults were asked to pick up two spatially disparate objects, one in each hand, and bring them together to place them in one of three trays laid out in front of them from left to right. The results showed that the older adults were no more detrimentally affected than the young by asymmetric bimanual movements compared to symmetric ones, and both groups completed their movements in the same time. Nevertheless, compared to the young, the older adult group produced reaches characterised by higher peak velocities (although this effect was marginal), shorter hover times, and where the movement distance varied for each hand the scaling of the kinematic profile across the two limbs diverged from that found with younger participants. They then spent longer than the young in the final adjustment phase and during this phase they made more adjustments than the young, and as a result were more synchronous in terms of the final placement of the objects. It seems that the older adults produced reach movements that were designed to reach the vicinity of the tray quite rapidly, after which time they made discreet adjustments to their initial trajectories in order to exercise the precision necessary to place the objects in the tray. These findings are consistent with the idea that older adults have problems using online control (as they wait until they can fixate both objects before making adjustments).     

Activity of Head Muscles During Feeding by Snakes: A Comparative Study

The adaptive radiation of colubroid snakes has involved thedevelopment of numerous prey capture specializations combinedwith conservation of a swallowing mechanism characterized byindependent movements of the right and left toothed bones ofthe skull. Synchronized electromyographic and cinematographicrecordings of swallowing in Nerodia, Elaphe, Heterodon and Agkistrodon,four diverse genera of colubroid snakes, allow a preliminaryevaluation of the relationship between prey capture and swallowing.The results indicate that the movements of the palatopterygoidbar and advance of the mandible as closing of the jaws beginsas well as patterns of muscle activity producing these movementsare similar among the four genera. Conversely, the patternsof activity of external adductors and, to some extent, the depressormandibulae differ among the four genera sampled. Analyses ofbone movements during swallowing suggest that swallowing iseffected primarily by the palatopterygoid bars. The mandiblesand their connecting soft tissues mainly press the prey againstthe palatopterygoid teeth. The mandibular teeth evidently playlittle active role in swallowing. Also, the maxilla, which displaysconsiderable morphological diversity among colubroid snakes,has little independent or direct function in swallowing, itsteeth rarely contacting the prey. The data suggest that theheads of colubroid snakes have evolved two partially separatedstructural-functional units, a medial swallowing unit and alateral prey capture unit.     

Quantitative Analysis of the Deltoid and Rotator Cuff Muscles in Humans and Great Apes

The shoulder is one of the anatomic regions differentiating orthograde primates (gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans) from the rest of the pronograde primates. Orthograde primates are characterized by a dorsal position of the scapula and a more lateral orientation of the glenoid cavity. This anatomic pattern, together with adaptations in related osteological structures and muscles, serves to facilitate the elevation of the upper extremity in the scapular plane. We quantified the proportions of the muscles comprising the principal functional and stabilizing components of the glenohumeral joint —deltoid, subscapularis, supraspinatus, infraspinatus, and teres minor— in 3 species of orthograde primates: Pongo pygmaeus, Pan troglodytes, and Homo sapiens. Our objective was to determine whether quantifiable differences in these muscles relate to the functional requirements of the types of locomotion used by these 3 species: suspension/vertical climbing, knuckle-walking, and bipedalism. We observed a close similarity between the proportional mass of these muscles in Homo sapiens and Pongo pygmaeus, whereas Pan troglodytes displayed a unique anatomic pattern, particularly in the subscapularis, which may be due to differences in how the glenohumeral joint is stabilized in a great ape knuckle-walker. Our findings may help explain the high incidence of subacromial impingement syndrome in humans.     

Control of the Elbow Extensor Muscles in Slow Targeted Extensions of the Arm in Humans

Relations between the kinematic parameters of slow (non-ballistic) targeted extension movements in the elbow joint of humans and characteristics of the movement-related EMG activity in the two heads of the m. triceps brachii were analyzed. Test movements were performed under conditions of application of non-inertional external loadings directed toward flexion. It was shown that the movement-related EMG activity of the elbow extensors, similarly to what was observed in the flexors at flexion movements with the same parameters, demonstrates a complex structure and includes dynamic and stationary phases. In the former phase, in turn, initial and main components can be differentiated. The rising edge and decay of the main component of the dynamic extensor EMG phase could be approximated by exponential functions; this component was never split into a few subcomponents. Dependences between the amplitudes of m. triceps brachii EMG phases and the amplitude of the movement (or external loading) were, as a rule, nonlinear but monotonic. An increase in the test movement velocity led to an increase in the rate of rise of the rising edge of the dynamic EMG phase, while an increment in the amplitude was less significant. Under the used test conditions, the activity of the elbow extensors was usually accompanied by some coactivation of the antagonists (m. biceps brachii). It is concluded that motor commands coming to the elbow extensors at performance of the extension test movements differ from motor commands to the flexors at analogous flexion test movements by a simpler structure and more tonic pattern. Biomechanical specificities of fixation of the mentioned muscle groups to the arm bones (stability of the moment for application of the extensor force under conditions of changing the joint angle vs variable moment of the flexor force) are considered one of the main reasons for such specificity of the patterns of the extensor and flexor motor commands.