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We accept a definition of synergy introduced by Nikolai Bernstein and develop it for various actions, from those involving the whole body to those involving a single muscle. Furthermore. we use two major theoretical developments in the field of motor control-the idea of hierarchical control with spatial referent coordinates and the uncontrolled manifold hypothesis-to discuss recent studies of syncrgies within spaces of individual motor units (MUs) recorded within a single muscle. During the accurate finger force production tasks, MUs within hand extrinsic muscles form robust groups, with parallel scaling of the firing frequencies. The loading factors at individual MUs within each of the two main groups link them to the reciprocal and coactivation commands. Furthermore, groups are recruited in a task-specific way with gains that covary to stabilize muscle force. Such force-stabilizing synergies are seen in MUs recorded in the agonist and antagonist muscles but not in the spaces of MUs combined over the two muscles. These observations reflect inherent trade-offs between synergies at different levels of a control hierarchy. MU-based synergies do not show effects of hand dominance. whereas such effects are seen in multifinger synergies. Involuntary, reflex-based, force changes are stabilized by intramuscle synergies but not by multifinger synergies. These observations suggest that multifinger (inultimuscle synergies) are based primarily on supraspinal circuitry, whereas intramuscle synergies reftect spinal circuitry. Studies of intra- and multimuscle synergies promise a powerful tool for exploring changes in spinal and supraspinal circuitry across patient populations. [ABSTRACT FROM AUTHOR]

When stopping a closing door or catching an object, humans process the motion of inertial objects and apply reactive limb force over short period to interact with them. One way in which the visual system processes motion is through extraretinal signals associated with smooth pursuit eye movements (SPEMs). We conducted three experiments to investigate how SPEMs contribute to anticipatory and reactive hand force modulation when interacting with a virtual object moving in the horizontal plane. We hypothesized that SPEM signals are critical for timing motor responses, anticipatory control of hand force, and task performance. Participants held a robotic manipulandum and attempted to stop an approaching simulated object by applying a force impulse (area under force-time curve) that matched the object’s virtual momentum upon contact. We manipulated the object’s momentum by varying either its virtual mass or its speed under free gaze or constrained gaze conditions. We examined gaze variables, the timing of hand motor responses, anticipatory force control, and overall task performance. Our results show that when participants were fixated at a designated location instead of following objects with SPEM, anticipatory modulation of hand force before contact decreased. However, constraining gaze by asking participants to fixate did not seem to affect the timing of the motor response or the task performance. Together, these results suggest that SPEMs may be important for anticipatory control of hand force before contact and may also play a critical role in anticipatory stabilization of limb posture when humans interact with moving objects. NEW & NOTEWORTHY We show for the first time that smooth pursuit eye movements (SPEMs) play a role in the modulation of anticipatory control of hand force to stabilize posture against contact forces. SPEMs are critical for tracking moving objects, facilitate processing motion of moving objects, and are impacted during aging and in many neurological disorders, such as Alzheimer’s disease and multiple sclerosis. These results provide a novel basis to probe how changes in SPEMs could contribute to deficient limb motor control in older adults and patients with neurological disorders. [ABSTRACT FROM AUTHOR]

We explored the synergic organization of motor units in extrinsic finger muscles, flexor digitorum superficialis (FDS), and extensor digitorum communis (EDC). Healthy subjects produced accurate cyclical force by pressing with the middle phalanges of one of the three fingers (Index, Middle, and Ring) and all three together. Two wireless sensor arrays were used to record and identify motor unit action potentials in FDS and EDC. Stable motor unit groups were identified within each muscle and across both muscles. Analysis of motor units combined over the two muscles showed one of the first two motor unit groups with consistently opposite signs of the loading factors for the FDS and EDC motor units, and the other group with consistently same signs of the loading factors for the two muscles. We interpret the two motor unit groups as reflections of the reciprocal and co-activation commands within the theory of control with spatial referent coordinates. Force changes within the cycle were primarily associated with the modulation of the co-activation motor unit group. Analysis of inter-cycle variance within the spaces of motor unit groups defined for FDS and EDC separately showed force-stabilizing synergies across both single-finger and three-finger tasks. In contrast, analysis within the motor unit groups defined across both muscles failed to show force-stabilizing synergies. We interpret these results as a reflection of the trade-off across levels within a hierarchical control system. [ABSTRACT FROM AUTHOR]

We explored the organization of motor units recorded in the flexor digitorum superficialis into stable groups (MU-modes) and force-stabilizing synergies in spaces of MU-modes. Young, healthy participants performed one-finger and three-finger accurate cyclical force production tasks. Two wireless sensor arrays (Trigno Galileo, Delsys, Inc.) were placed over the proximal and distal portions of the muscle for surface recording and identification of motor unit action potentials. Principal component analysis with Varimax rotation and factor extraction was used to identify MU-modes. The framework of the uncontrolled manifold hypothesis was used to analyze inter-cycle variance in the space of MU-modes and compute the index of force-stabilizing synergy. Multiple linear regression between the first MU-mode in the three-finger task and the first MU-modes in the three single-finger tasks showed no differences between the data recorded by the two electrodes suggesting that MU-modes were unlikely to be synonymous with muscle compartments. Multi-MU-mode synergies stabilizing task force were documented across all tasks. In contrast, there were no force-stabilizing synergies in the three-finger task analyzed in the space of individual finger forces. Our results confirm the synergic organization of motor units in single-finger tasks and, for the first time, expand this result to multi-finger tasks. We offer an interpretation of the findings within the theoretical scheme of control with spatial referent coordinates expanded to the analysis of individual motor units. The results confirm trade-offs between synergies at different hierarchical levels and expand this notion to intra-muscle synergies. [ABSTRACT FROM AUTHOR]

Key points: We used the idea of synergic control and the framework of the uncontrolled manifold hypothesis to explore the synergic control of a single muscle.Individual motor units in flexor digitorum superficialis formed two‐three groups (MU‐modes) with parallel changes in firing frequency, robust over force‐up and force‐down segments.There were strong force‐stabilizing synergies in the MU‐mode space during accurate cyclical force production.The results show, for the first time, that the idea of synergic control is applicable to individual muscles.The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. In the present study, for the first time, we have used the idea of synergic control and the framework of the uncontrolled manifold (UCM) hypothesis to test two hypotheses: (i) individual motor units are organized into stable groups (MU‐modes) with parallel scaling of firing rates with changes in the muscle force, and (ii) the gains of MU‐mode involvement co‐vary to stabilize the desired force magnitude. Young healthy subjects performed accurate cyclical force tracing tasks at 1 Hz by pressing with intermediate phalanges; only one finger was instructed to produce force at a time. Surface electromyographic signal from flexor digitorum superficialis was recorded and used to identify individual motor units and their firing frequencies. Principal component analysis with rotation and factor extraction was used to identify MU‐modes, which showed similar compositions over the force‐up and force‐down task segments. Inter‐cycle variance analysis in the MU‐mode space confirmed the existence of strong synergies stabilizing finger force. There were no synergies stabilizing MU‐mode magnitude in the space of individual motor units. This is the first application of the UCM framework to the neural control of a single muscle. It extends the applicability of this approach to analysis of individual muscles. We discuss the importance of the findings for the idea of hierarchical control and the notion of muscle compartments. The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. Key points: We used the idea of synergic control and the framework of the uncontrolled manifold hypothesis to explore the synergic control of a single muscle.Individual motor units in flexor digitorum superficialis formed two‐three groups (MU‐modes) with parallel changes in firing frequency, robust over force‐up and force‐down segments.There were strong force‐stabilizing synergies in the MU‐mode space during accurate cyclical force production.The results show, for the first time, that the idea of synergic control is applicable to individual muscles.The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. [ABSTRACT FROM AUTHOR]