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.     

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.     

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).     

Modifications of EEG Related to Repeated Static Grasp Efforts Developed by the Hand in Humans

The characteristics of EEG recorded before, in the course, and after three sequential static grasp efforts developed by the right hand were analyzed in 14 healthy volunteers of both sexes, 19 to 56 years old. The grasps were 3 min long, and intervals between the sequential trials were 5 min long. The mean intensity was, as a rule, the greatest in the first trial and decreased, because of fatigue, in the second and third trials. In the course of the grasp effort, significant (P ≤ 0.05) or close to significant decreases in the spectral power were observed in all trials within the alpha2 subrange in some frontal, central, and central/temporal leads. This reaction of desynchronization of alpha2 oscillations probably reflects activation of the sensorimotor cortical zones. Within the above period, the power of alpha2 oscillations increased in the occipital leads. In a few leads of the frontal group, the powers of delta, theta1, theta2, and alpha1 oscillations increased significantly (P < 0.05) in the course of the first trial. Such reaction was not observed in the second and third trials, while the powers of delta and alpha1 ranges increased in the occipital sites (P < 0.05). The powers of delta and theta oscillations increased, as compared with the control values, after the second and third grasps. Such specific EEG modifications in sequential trials can be related to the actions of a novelty factor in the first trial and a fatigue factor in the subsequent trials. The coherence coefficient (CC) of alpha and beta oscillations for symmetric leads usually increased in the course of the grasp. This effect reached a significance level (P < 0.05) for alpha2 oscillations in central, posterotemporal, parietal, and occipital pairs. The CC for beta2 oscillations increased in both temporal lead pairs. A drop in the interhemisphere coherence was more typical of the delta and theta1 ranges. Therefore, changes in the CC values show that the structure of interhemisphere interaction undergoes modifications with the development of the grasp static efforts. Neirofiziologiya/Neurophysiology, Vol. 37, No. 4, pp. 362–371, July–August, 2005.     

Central Activation of the Upper Limb Muscles in Humans Related to Creation of an Isometric Effort: Dependence on the Position of the Point of Force Application within the Operational Space

We examined the peculiarities of central coordination of motor commands coming to the muscles of the shoulder belt and shoulder in the course of generation of targeted isometric efforts by the arm. The dependence of these commands on changes in the effort direction and position of the forearm within the working space were analyzed. The intensity of the central commands was estimated according to the amplitudes of rectified and averaged EMGs recorded from the corresponding muscles. Sector diagrams of EMG activity of the above muscles depending on the direction of the effort vector, EV, were plotted [1]. Preferential sectors of activity where the efforts were formed due to activation of definite functional muscle groups were identified. As was found, the direction of these sectors depends significantly on the EV orientation. Differences between the patterns of coactivation of the examined muscles were demonstrated. Organization of the motor commands under conditions of creation of extensor efforts is distinguished by a more complex pattern than that related to flexor efforts. In the former case, the activity of extensor muscles is accompanied by more significant activation of the flexors.     

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...     

Trunk Muscle Activation at the Initiation and Braking of Bilateral Shoulder Flexion Movements of Different Amplitudes

The aim of this study was to investigate if trunk muscle activation patterns during rapid bilateral shoulder flexions are affected by movement amplitude. Eleven healthy males performed shoulder flexion movements starting from a position with arms along sides (0°) to either 45°, 90° or 180°. EMG was measured bilaterally from transversus abdominis (TrA), obliquus internus (OI) with intra-muscular electrodes, and from rectus abdominis (RA), erector spinae (ES) and deltoideus with surface electrodes. 3D kinematics was recorded and inverse dynamics was used to calculate the reactive linear forces and torque about the shoulders and the linear and angular impulses. The sequencing of trunk muscle onsets at the initiation of arm movements was the same across movement amplitudes with ES as the first muscle activated, followed by TrA, RA and OI. All arm movements induced a flexion angular impulse about the shoulders during acceleration that was reversed during deceleration. Increased movement amplitude led to shortened onset latencies of the abdominal muscles and increased level of activation in TrA and ES. The activation magnitude of TrA was similar in acceleration and deceleration where the other muscles were specific to acceleration or deceleration. The findings show that arm movements need to be standardized when used as a method to evaluate trunk muscle activation patterns and that inclusion of the deceleration of the arms in the analysis allow the study of the relationship between trunk muscle activation and direction of perturbing torque during one and the same arm movement.     

Expression of Myosin Heavy Chain Isoforms in the Supraspinatus Muscle of Different Primate Species: Implications for the Study of the Adaptation of Primate Shoulder Muscles to Different Locomotor Modes

The supraspinatus muscle is a key component of the soft tissues of the shoulder. In pronograde primates, its main function, in combination with the other rotator cuff muscles (subscapularis, infraspinatus, and teres minor), is to stabilize the glenohumeral joint, whereas in orthograde primates it functions together with the deltoid, to elevate the upper extremity in the scapular plane. To determine whether these functional differences are also reflected in the molecular biochemistry of the supraspinatus muscles involved in these different locomotor modes, we used real-time polymerase chain reaction (RT-PCR) to analyze the expression of the myosin heavy chain (MHC) isoforms in supraspinatus muscles from modern humans and 12 species of pronograde and orthograde primates. The MHC expression pattern in the supraspinatus muscle of pronograde primates was consistent with its function as a tonic and postural muscle, whereas the MHC expression pattern observed in the supraspinatus muscle of nonhuman orthograde primates was that of a muscle that emphasizes speed, strength, and less resistance to fatigue. These findings are consistent with the role of the supraspinatus in the posture and locomotor modes of these groups of nonhuman primates. The humans included in the study had an expression pattern similar to that of the nonhuman orthograde primates. In conclusion, molecular analysis of skeletal muscles via RT-PCR can contribute to a better understanding of the morphological and functional characteristics of the primate musculoskeletal system.     

Toward the Restoration of Hand Use to a Paralyzed Monkey: Brain-Controlled Functional Electrical Stimulation of Forearm Muscles

Loss of hand use is considered by many spinal cord injury survivors to be the most devastating consequence of their injury. Functional electrical stimulation (FES) of forearm and hand muscles has been used to provide basic, voluntary hand grasp to hundreds of human patients. Current approaches typically grade pre-programmed patterns of muscle activation using simple control signals, such as those derived from residual movement or muscle activity. However, the use of such fixed stimulation patterns limits hand function to the few tasks programmed into the controller. In contrast, we are developing a system that uses neural signals recorded from a multi-electrode array implanted in the motor cortex; this system has the potential to provide independent control of multiple muscles over a broad range of functional tasks. Two monkeys were able to use this cortically controlled FES system to control the contraction of four forearm muscles despite temporary limb paralysis. The amount of wrist force the monkeys were able to produce in a one-dimensional force tracking task was significantly increased. Furthermore, the monkeys were able to control the magnitude and time course of the force with sufficient accuracy to track visually displayed force targets at speeds reduced by only one-third to one-half of normal. Although these results were achieved by controlling only four muscles, there is no fundamental reason why the same methods could not be scaled up to control a larger number of muscles. We believe these results provide an important proof of concept that brain-controlled FES prostheses could ultimately be of great benefit to paralyzed patients with injuries in the mid-cervical spinal cord.     

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.