Agonist-antagonist Muscle Pairs Research Articles

A coactivation strategy in anticipatory postural adjustments during voluntary unilateral arm movement while standing in individuals with bilateral spastic cerebral palsy

Individuals with bilateral spastic cerebral palsy (BSCP) reportedly has problems with anticipatory postural adjustments (APAs) while standing. However, the use of coactivation strategy in APAs in individuals with BSCP has conflicting evidence. Hence, this study aimed to investigate postural muscle activities in BSCP during unilateral arm flexion task in which postural perturbations occur in the sagittal, frontal, and horizontal planes. We included 10 individuals with BSCP with level II on the Gross Motor Function Classification System (BSCP group) and 10 individuals without disability (control group). The participants stood on a force platform and rapidly flexed a shoulder from 0° to 90° at their own timing. Surface electromyograms were recorded from the rectus femoris, medial hamstring, tibialis anterior, and medial gastrocnemius. The control group showed a mixture of anticipatory activation and inhibition of postural muscles, whereas the BSCP group predominantly exhibited anticipatory activation with slight anticipatory inhibition. Compared with the control group, the BSCP group tended to activate the ipsilateral and contralateral postural muscles and the agonist–antagonist muscle pairs. The BSCP group had a larger disturbance in postural equilibrium, quantified by the peak displacement of center of pressure during the unilateral arm flexion, than those without disability. Individuals with BSCP may use coactivation strategy, mainly the anticipatory activation of postural muscle activity, during a task that requires a selective postural muscle activity to maintain stable posture.

Realistic 3D human saccades generated by a 6-DOF biomimetic robotic eye under optimal control.

We recently developed a biomimetic robotic eye with six independent tendons, each controlled by their own rotatory motor, and with insertions on the eye ball that faithfully mimic the biomechanics of the human eye. We constructed an accurate physical computational model of this system, and learned to control its nonlinear dynamics by optimising a cost that penalised saccade inaccuracy, movement duration, and total energy expenditure of the motors. To speed up the calculations, the physical simulator was approximated by a recurrent neural network (NARX). We showed that the system can produce realistic eye movements that closely resemble human saccades in all directions: their nonlinear main-sequence dynamics (amplitude-peak eye velocity and duration relationships), cross-coupling of the horizontal and vertical movement components leading to approximately straight saccade trajectories, and the 3D kinematics that restrict 3D eye orientations to a plane (Listing's law). Interestingly, the control algorithm had organised the motors into appropriate agonist-antagonist muscle pairs, and the motor signals for the eye resembled the well-known pulse-step characteristics that have been reported for monkey motoneuronal activity. We here fully analyse the eye-movement properties produced by the computational model across the entire oculomotor range and the underlying control signals. We argue that our system may shed new light on the neural control signals and their couplings within the final neural pathways of the primate oculomotor system, and that an optimal control principle may account for a wide variety of oculomotor behaviours. The generated data are publicly available at https://data.ru.nl/collections/di/dcn/DSC_626870_0003_600.

Lower-Limb Myoelectric Calibration Postures for Transtibial Prostheses.

The use of an agonist-antagonist muscle pair for myoelectric control of a transtibial prosthesis requires normalizing the myoelectric signals and identifying their co-contraction signature. Extensive literature has explored the relationship between body posture and lower-limb muscle activation level using surface electromyography (EMG), but it is unknown how these relationships hold after amputation. Using a virtual tracking task, this study compares the effect of three different calibration postures (seated, standing, dynamic) on user tracking ability while in two tracking postures (seated, standing) for 18 able-bodied (AB) subjects and 9 subjects with transtibial (TT) amputation. As expected, AB subjects produced statistically significant differences in muscle activation for gastrocnemius (GAS) when seated vs. standing during calibration (p = 8.8e-4), but not for tibialis anterior (TA) (p = 0.76). TT subjects, however, showed no significant differences in GAS or TA between seated and standing (p = 0.90, 0.60 respectively). It was also determined that normalizing EMG by the global maximum signal observed (standard in biomechanic analysis) is undesirable for myoelectric control. For best general results with this framework, calibration in both seated and dynamic postures is recommended, taking the normalization information from the seated posture and the narrowest co-contraction slopes from the two.

Changes in normalized mutual information in response to strength training: An ancillary analysis of a quasi-randomized controlled trial.

The aim of the present investigation was twofold. (1) to assess test-retest reliability of normalized mutual information (NMI) values extracted from the surface electromyography (sEMG) signal of muscles pairs of the upper body during dynamic bench press at a high load, and (2) to assess changes in NMI values from before to after a five-week quasi-randomized controlled bench press training intervention. For test-retest reliability, 20 strength trained males (age 25 ± 2 years, height 1.81 ± 0.07 m) performed two three-repetition maximum (3RM) tests in bench press, while sEMG was recorded from six upper body muscles. Tests were separated by 8.2 ± 2.9 days. For the training intervention, 17 male participants (age 26 ± 5 years, height 1.80 ± 0.07 m) trained bench press specific strength training for 5 weeks (TRA), while 13 male participants (age 23 ± 3 years, height 1.80 ± 0.08 m) constituted a control group (CON). 3RM bench press test and sEMG recordings were carried out before and after the intervention period. The NMI values ranged from poor to almost perfect reliability, with the majority displaying substantial reliability. TRA displayed a significant decrease in NMI values during the concentric phase for two agonist-agonist muscle pairs, while one agonist-agonist and two agonist-antagonist muscle pairs increased the NMI values during the eccentric phase. The observed changes did not exceed the minimal detectable threshold, and we therefore cannot surely ascertain that the changes observed in NMI values reflect genuine neural adaptations.

The Agonist-Antagonist Myoneural Interface in a Transtibial Amputation.

The agonist-antagonist myoneural interface (AMI) technique at the time of transtibial amputation involves the use of agonist-antagonist muscle pairs to restore natural contraction-stretch relationships and to improve proprioceptive feedback when utilizing a prosthetic limb1. Utilizing the standard incision for a long posterior myofasciocutaneous flap, the lateral and medial aspects of the limb are dissected, identifying and preserving the superficial peroneal and saphenous nerve, respectively. The tendons of the tibialis anterior and peroneus longus are transected distally to allow adequate length for the AMI constructs. After ligation of the anterior tibial vessels, the deep peroneal nerve is identified and tagged to create a regenerative peripheral nerve interface (RPNI). The tibia and fibula are cut approximately 15 cm from the medial joint line, facilitating dissection of the deep posterior compartment and ligation of the peroneal and posterior tibial vessels. The tendons of the lateral gastrocnemius and tibialis posterior are transected distally, and the amputation is completed. The extensor retinaculum is harvested from the residual limb along with multiple 2 × 3-cm free muscle grafts, which will be used for the RPNI constructs. The retinaculum is secured to the tibia with suture anchors, and AMI pairs of the lateral gastrocnemius and tibialis anterior as well as the tibialis posterior and peroneus longus are constructed. Separate RPNIs of the major lower-extremity nerves are performed, and the wound is closed in a standard layered fashion. An isometric myodesis of the gastrocnemius without coaptation of agonist-antagonist muscle pairs can be performed at the time of transtibial amputation. The AMI technique restores natural agonist-antagonist relationships at the time of transtibial amputation to increase proprioceptive feedback and improve prosthetic control. These outcomes contrast with those of a traditional isometric myodesis, which prevents proprioceptive communication from the residual limb musculature to the central nervous system. Additionally, the AMI technique allows for concentric and eccentric muscular contractions, which may contribute to the maintenance of limb volume and aid with prosthetic fitting, as opposed to the typical limb atrophy observed following standard transtibial amputation1,2. With the development and availability of more advanced prostheses, the AMI technique offers more precise control and increases the functionality of these innovative devices. Early clinical outcomes of the AMI technique at the time of transtibial amputation have been promising. In a case series of the first 3 patients who underwent the procedure, complications were minor and consisted of 2 episodes of cellulitis and 1 case of delayed wound healing1. Muscle activation measured through electromyography demonstrated an improved ability to limit unintended muscular co-contraction with attempted movement of the phantom limb, as compared with patients who underwent a standard transtibial amputation1. Additionally, residual limb volume was maintained postoperatively without the need for substantial prosthetic modifications. The tendons of the tibialis anterior, peroneus longus, tibialis posterior, and lateral gastrocnemius should be transected as distal as possible to allow adequate length for creation of the AMI constructs.Approximately 2 × 3-cm free muscle grafts are harvested from the amputated extremity for RPNI3.Smooth tendon-gliding through the synovial tunnels should be confirmed before closure. If necessary, muscle debulking can improve gliding and decrease the size of the residual limb.Harvesting the extensor retinaculum for synovial tunnels has been our preferred method, although we acknowledge that other grafts options such as the tarsal tunnel are available1. RPNI = regenerative peripheral nerve interfaceAMI = agonist-antagonist myoneural interfaceEMG = electromyographic.

Intermuscular coherence of plantar and dorsiflexor muscles in older adults with Parkinson's disease and age-matched controls during bipedal and unipedal stance.

Postural instability increases with age and is exacerbated in neurological disorders such as Parkinson's disease (PD). Reducing the base of support from bipedal to unipedal stance increases center of pressure (CoP) parameters and intermuscular coherence in lower-leg muscles of healthy older adults. To further develop an understanding of postural control in an altered state of neurological impairment, we explored intermuscular coherence in lower-leg muscles and CoP displacement in older adults with PD. This study measured surface EMG from the medial (MG) and lateral (LG) gastrocnemii, soleus (SOL), and tibialis anterior (TA), and examined EMG amplitude and intermuscular coherence during bipedal and unipedal stance on a force plate with firm (no foam) and compliant (standing on foam) surface conditions in nine older adults with PD (70±5 years, 6 females) and 8 age-matched non-Parkinsonian older adults (5 females). Intermuscular coherence was analyzed between agonist-agonist and agonist-antagonist muscle pairs in the alpha (8-13 Hz) and beta (15-35 Hz) frequency bands. CoP parameters increased from bipedal to unipedal stance in both groups (p < 0.01), but did not increase from the firm to compliant surface condition (p > 0.05). During unipedal stance, CoP path length was shorter in older adults with PD (2027.9 ± 1074.1 mm) compared to controls (3128.5 ± 1198.7 mm) (p < 0.01). Alpha and beta agonist-agonist and agonist-antagonist coherence increased by 28% from bipedal to unipedal stance (p > 0.05), but did not differ between older adults with PD (0.09 ± 0.07) and controls (0.08 ± 0.05) (p > 0.05). The older adults with PD also had greater normalized EMG amplitude of the LG (63.5 ± 31.7%) and TA (60.6 ± 38.4%) during the balance tasks (p > 0.05) than the non-Parkinsonian counterparts. Older adults with PD had shorter path lengths during unipedal stance and required greater muscle activation than older adults without PD to perform the tasks, but intermuscular coherence did not differ between the groups. This may be attributable to their early disease stage and high motor function.

Changes in intermuscular connectivity during active elbow extension reveal a functional simplification of motor control after stroke.

BackgroundStroke alters muscle co-activation and notably leads to exaggerated antagonist co-contraction responsible for impaired motor function. However, the mechanisms underlying this exaggerated antagonist co-contraction remain unclear. To fill this gap, the analysis of oscillatory synchronicity in electromyographic signals from synergistic muscles, also called intermuscular coherence, was a relevant tool.ObjectiveThis study compares functional intermuscular connectivity between muscle pairs of the paretic and non-paretic upper limbs of stroke subjects and the dominant limb of control subjects, concomitantly between two muscle pairs with a different functional role, through an intermuscular coherence analysis.MethodsTwenty-four chronic stroke subjects and twenty-four healthy control subjects were included. Subjects performed twenty elbow extensions while kinematic data and electromyographic activity of both flexor and extensor elbow muscles were recorded. Intermuscular coherence was analyzed in the beta frequency band compared to the assessment of antagonist co-contraction.ResultsIntermuscular coherence was higher in the stroke subjects’ paretic limbs compared to control subjects. For stroke subjects, the intermuscular coherence of the antagonist-antagonist muscle pair (biceps brachii—brachioradialis) was higher than that of the agonist-antagonist muscle pair (triceps brachii—brachioradialis). For the paretic limb, intermuscular coherence of the antagonist-antagonist muscle pair presented a negative relationship with antagonist co-contraction.ConclusionDifferences in intermuscular coherence between the paretic limbs of stroke subjects and control subjects suggest a higher common central drive during movement. Furthermore, results highlight the association between stroke-related alteration of intermuscular functional connectivity and the alteration of motor function.

EMG-based co-contraction indexes for the assessment of motor impairments in persons post-stroke

Neuromuscular coordination can be described by activations of agonist–antagonist (AA) muscle pairs and plays an important role in the performing of daily activities. It is an important mechanism used by the central nervous system to regulate joint stability and provide movement accuracy [1]. As a result of a stroke there is an alteration in these coordination patterns that results in motor deficits of the arm function [2]. In fact, persons post-stroke have an exaggerated AA muscle co-contraction [2] associated to a higher joint stiffness [1]. The amount of co-contraction can be quantified through the use of EMG- based co-contraction indexes (CCIs) [1,2]. Therefore, the purpose of the study was to evaluate whether such EMG-based CCIs are able to assess impairment in the motor control of the upper limb in persons post-stroke. Thirty-five persons post-stroke, in chronic or sub-acute stages, were recruited in this study. Both paretic and non-paretic arms were evaluated during the execution of functional tasks related to activities of daily living (i.e., reaching, object placing and forearm pronation). The upper limb kinematics and EMG data of three AA muscle pairs (anterior/posterior deltoids, triceps/biceps and pronator/supinator) were recorded through a movement analysis system. Ten healthy subjects (HS) provided the normative data. The Rudolph’s CCI [1] was computed as proposed by Don (range 0-100%, higher values indicate a greater AA muscle co-contraction) [3]. All participants post-stroke were clinically assessed through the Fugl-Meyer Motor Assessment of Upper Extremity (FM-UE). The differences of Rudolph’s CCI between HS and subjects post-stroke, considering both the more affected (MA) and the less affected (LA) arm, were tested by ANOVA. Bonferroni post-hoc analysis was also used to establish if there were statistically significant differences among the three groups. A deviation from normative data for the MA CCIs was found in almost all functional tasks and antagonist muscle pairs. The LA CCIs also showed a significant deviation from HS. Such pattern is reported in Fig. 1 for Rudolph’s CCI during the forearm pronation task. Higher values of the EMG-based CCI were found both in LA and MA arms in persons post-stroke, indicating alterations in the neuromotor coordination for both sides. The altered AA muscle activation of the LA arm (non-paretic, ipsilateral to the brain lesion) in persons post-stroke, despite its kinematics not being compromised, could be a consequence of the over-use of the non-paretic limb in performing functional tasks. Therefore, also the non-paretic upper limb may benefit from rehabilitation, as recently suggested [4]. Our results show that EMG-based CCI is a quantitative and objective measure able to detect motor control impairment, not detectable by clinical scales.

Effects of Concurrent and Terminal Visual Feedback on Ankle Co-Contraction in Older Adults during Standing Balance.

This preliminary investigation studied the effects of concurrent and terminal visual feedback during a standing balance task on ankle co-contraction, which was accomplished via surface electromyography of an agonist–antagonist muscle pair (medial gastrocnemius and tibialis anterior muscles). Two complementary mathematical definitions of co-contraction indices captured changes in ankle muscle recruitment and modulation strategies. Nineteen healthy older adults received both feedback types in a randomized order. Following an analysis of co-contraction index reliability as a function of surface electromyography normalization technique, linear mixed-effects regression analyses revealed participants learned or utilized different ankle co-contraction recruitment (i.e., relative muscle pair activity magnitudes) and modulation (i.e., absolute muscle pair activity magnitudes) strategies depending on feedback type and following the cessation of feedback use. Ankle co-contraction modulation increased when concurrent feedback was used and significantly decreased when concurrent feedback was removed. Ankle co-contraction recruitment and modulation did not significantly change when terminal feedback was used or when it was removed. Neither ankle co-contraction recruitment nor modulation was significantly different when concurrent feedback was used compared to when terminal feedback was used. The changes in ankle co-contraction recruitment and modulation were significantly different when concurrent feedback was removed as compared to when terminal feedback was removed. Finally, this study found a significant interaction between feedback type, removal of feedback, and order of use of feedback type. These results have implications for the design of balance training technologies using visual feedback.

Neural interfacing architecture enables enhanced motor control and residual limb functionality postamputation

Despite advancements in prosthetic technologies, patients with amputation today suffer great diminution in mobility and quality of life. We have developed a modified below-knee amputation (BKA) procedure that incorporates agonist-antagonist myoneural interfaces (AMIs), which surgically preserve and couple agonist-antagonist muscle pairs for the subtalar and ankle joints. AMIs are designed to restore physiological neuromuscular dynamics, enable bidirectional neural signaling, and offer greater neuroprosthetic controllability compared to traditional amputation techniques. In this prospective, nonrandomized, unmasked study design, 15 subjects with AMI below-knee amputation (AB) were matched with 7 subjects who underwent a traditional below-knee amputation (TB). AB subjects demonstrated significantly greater control of their residual limb musculature, production of more differentiable efferent control signals, and greater precision of movement compared to TB subjects (P < 0.008). This may be due to the presence of greater proprioceptive inputs facilitated by the significantly higher fascicle strains resulting from coordinated muscle excursion in AB subjects (P < 0.05). AB subjects reported significantly greater phantom range of motion postamputation (AB: 12.47 ± 2.41, TB: 10.14 ± 1.45 degrees) when compared to TB subjects (P < 0.05). Furthermore, AB subjects also reported less pain (12.25 ± 5.37) than TB subjects (17.29 ± 10.22) and a significant reduction when compared to their preoperative baseline (P < 0.05). Compared with traditional amputation, the construction of AMIs during amputation confers the benefits of enhanced physiological neuromuscular dynamics, proprioception, and phantom limb perception. Subjects' activation of the AMIs produces more differentiable electromyography (EMG) for myoelectric prosthesis control and demonstrates more positive clinical outcomes.

High muscle co-contraction does not result in high joint forces during gait in anterior cruciate ligament deficient knees.

The mechanism of knee osteoarthritis development after anterior cruciate ligament injuries is poorly understood. The objective of this study was to evaluate knee gait variables, muscle co-contraction indices and knee joint loading in young subjects with anterior cruciate ligament deficiency (ACLD, n = 36), versus control subjects (n = 12). A validated, electromyography-informed model was used to estimate joint loading. For the involved limb of ACLD subjects versus control, muscle co-contraction indices were higher for the medial (p = 0.018, effect size = 0.93) and lateral (p = 0.028, effect size = 0.83) agonist-antagonist muscle pairs. Despite higher muscle co-contraction, medial compartment contact force was lower for the involved limb, compared to both the uninvolved limb (mean difference = 0.39 body weight, p = 0.009, effect size = 0.70) as well as the control limb (mean difference = 0.57 body weight, p = 0.007, effect size = 1.14). Similar observations were made for total contact force. For involved versus uninvolved limb, the ACLD group demonstrated lower vertical ground reaction force (mean difference = 0.08 body weight, p = 0.010, effect size = 0.70) and knee flexion moment (mean difference = 1.32% body weight * height, p = 0.003, effect size = 0.76), during weight acceptance. These results indicate that high muscle co-contraction does not always result in high knee joint loading, which is thought to be associated with knee osteoarthritis. Long-term follow-up is required to evaluate how gait alterations progress in non-osteoarthritic versus osteoarthritic subjects. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

The effects of feedback on stability and maneuverability of a phase-reduced model for cockroach locomotion.

In previous work, we built a neuromechanical model for insect locomotion in the horizontal plane, containing a central pattern generator, motoneurons, muscles actuating jointed legs, and rudimentary proprioceptive feedback. This was subsequently simplified to a set of 24 phase oscillators describing motoneuronal activation of agonist-antagonist muscle pairs, which facilitates analyses and enables simulations over multi-dimensional parameter spaces. Here we use the phase-reduced model to study dynamics and stability over the typical speed range of the cockroach Blaberus discoidalis, the effects of feedback on response to perturbations, strategies for turning, and a trade-off between stability and maneuverability. We also compare model behavior with experiments on lateral perturbations, changes in body mass and moment of inertia, and climbing dynamics, and we present a simple control strategy for steering using exteroceptive feedback.

T80. Alteration of intermuscular coherence in synergistic muscle pairs during actual elbow flexion contractions after cervical spinal cord injury

Introduction During isometric contractions, the level of antagonist muscles activation is increased in people with cervical spinal cord injury (cSCI) in comparison to healthy participants. This alteration of antagonists activation after cSCI has been primarily associated with a lower cortical control of the spinal mechanisms – especially reciprocal inhibition – regulating agonists-antagonists co-activation. Through time-frequency intermuscular coherence analysis, which is likely to reflect the strength of the common neural drive across multiple muscles, this study investigates the influence of a cSCI on the common neural drive regulating the activity of agonist and antagonist muscles pairs. Methods The EMG of elbow flexors (biceps brachii, brachioradialis) and elbow extensors (long and short heads of the triceps brachii) was recorded while 8 participants with cSCI (SCI group) and 10 able-bodied participants (AB group) performed 21 isometric 6-s elbow isometric flexion contractions at 3 force levels (20%, 40% and 60% of MVC). The magnitude of intermuscular coherence between the EMG signals from elbow flexors (AGO-AGO), elbow extensors (ANTAGO-ANTAGO) and all possible pairs of elbow flexors-extensors (AGO-ANTAGO) was quantified in alpha (8–12 Hz), beta (15–30 Hz) and gamma (35–60 Hz) frequency bands. The magnitude of the intermuscular coherence and the mean net joint torque produced around the elbow joint were analyzed over a 2-s stable force production window for all contractions. The effects of the Force level and Group were tested using adequate statistical tests according to the distribution of the dependent variables. Results Results revealed that the net joint torque significantly increased with the Force level in both groups with no significant Group difference. In the beta-band, AGO-ANTAGO intermuscular coherence magnitude was significantly lower in the SCI group than in the AB group. In the gamma-band, the ANTAGO-ANTAGO intermuscular coherence magnitude significantly increased concomitantly with the Force level in both groups. No significant intermuscular coherence was found in the alpha-band, whatever the group and the muscle pair. Conclusion The alteration of AGO-ANTAGO intermuscular coherence magnitude in the beta-band after cSCI could reflect reduced common neural drive influencing the spinal inhibitory mechanisms. This finding may suggest lower common drive to agonist-antagonist muscle pairs in SCI participants, which could take part in the alteration of antagonist muscles co-activation. Increased ANTAGO-ANTAGO coherence magnitude with the force level revealed in the gamma-band could be related to the Piper rhythm phenomenon, which contributes to synchronize muscle activations to gain in efficiency, especially at high force levels. Despite lower intermuscular coupling between agonist and antagonist muscles, our results taken together support that the central mechanisms controlling the modulation of the force level are preserved during actual contractions after cSCI.

Static analysis of a torsion motor generating flexion - extension motions of the elbow

The paper presents the construction and static analysis of a rotation module responsible for conducting flexion-extension motions as part of a medical rehabilitation device of the elbow. The rotation module is actuated by a torsion motor consisting of a pair of agonist-antagonist pneumatic muscles. The study concerning the static behaviour of the rotation module draws upon similarity to the biological model, considering the influence of the neuronal control quantities on the forces developed by the two muscles.

Establishing the torsional rigidity of a rotation module responsible for the flexion-extension motions of the elbow

The paper presents and discusses a system devised for the mobilisation of the elbow including a pair of agonist-antagonist pneumatic muscles aimed mainly at ensuring the accuracy of the rotation angle. This system is of SISO type (single input - single output) with the variable Δp (the pressure by that one of the muscles is charged symmetrically in the detriment of the other) as its input value. Further discussed are the dependency of the equilibrium angle on Δp and the possibility of controlling the torsional rigidity by adjusting the sum of the feed pressures.

Effects of hand configuration on muscle force coordination, co-contraction and concomitant intermuscular coupling during maximal isometric flexion of the fingers.

The mechanisms governing the control of musculoskeletal redundancy remain to be fully understood. The hand is highly redundant, and shows different functional role of extensors according to its configuration for a same functional task of finger flexion. Through intermuscular coherence analysis combined with hand musculoskeletal modelling during maximal isometric hand contractions, our aim was to better understand the neural mechanisms underlying the control of muscle force coordination and agonist-antagonist co-contraction. Thirteen participants performed maximal isometric flexions of the fingers in two configurations: power grip (Power) and finger-pressing on a surface (Press). Hand kinematics and force/moment measurements were used as inputs in a musculoskeletal model of the hand to determine muscular tensions and co-contraction. EMG-EMG coherence analysis was performed between wrist and finger flexors and extensor muscle pairs in alpha, beta and gamma frequency bands. Concomitantly with tailored muscle force coordination and increased co-contraction between Press and Power (mean difference: 48.08%; p < 0.05), our results showed muscle-pair-specific modulation of intermuscular coupling, characterized by pair-specific modulation of EMG-EMG coherence between Power and Press (p < 0.05), and a negative linear association between co-contraction and intermuscular coupling for the ECR/FCR agonist-antagonist muscle pair (r = - 0.65; p < 0.05). This study brings new evidence that pair-specific modulation of EMG-EMG coherence is related to modulation of muscle force coordination during hand contractions. Our results highlight the functional importance of intermuscular coupling as a mechanism contributing to the control of muscle force synergies and agonist-antagonist co-contraction.

Abstract P25: A Caprine Model of a Novel Amputation Paradigm for Bi-directional Neural Control of a Bionic Limb

PURPOSE: No significant changes have been made to limb amputation surgical paradigms since the Civil War era. Traditional approaches are plagued by poor nerve treatment, which leads to neuromas and phantom pain, and complicates neural communication with advanced limb prostheses. In this study we present a caprine model of the Linked Residual Musculature Peripheral Nerve Interface (LRM-PNI), a surgical approach designed to reduce phantom pain and improve bi-directional neural control of a bionic limb. The key advancement in this architecture is the surgical coaptation of natively-innervated agonist-antagonist muscle pairs within the residuum. The benefits of this approach are two-fold. First, preservation of the mechanical coupling between agonist contraction and antagonist stretch allows physiologically-relevant muscle state feedback from a prosthesis through mechanical activation of native mechanoreceptors within these linked muscles. Second, by preserving functionality of these native mechanoreceptors, we limit the presence of non-meaningful phantom sensation, including phantom pain. We hypothesize that these advancements will allow for afferent proprioceptive sensation of a limb prosthesis through native neural pathways, which is crucial in both reflexive and volitional lower extremity control during gait. METHODS: A transtibial amputation was performed on the hindlimb of a goat, during which the lateral tarsal tunnel from the amputated ankle was grafted transversely to the medial surface of the tibia, serving as a “synovial pulley”. Distal ends of the lateral gastrocnemius and tibialis cranialis were then coapted to either end of this synovial pulley, allowing coupled motion of the agonist-antagonist muscle pair. The residual musculature was sensorized with implantable epimysial and intramuscular electrodes, as well as sonomicrometry crystals. Over a two-month period, electromyography and muscle-state measurements were recorded from the residuum while the animal ambulated without a prosthesis, and fluoroscopic evaluations of coupled motion in the presence of artificial muscle stimulation were carried out under anesthesia. The animal was then sacrificed, and the residuum was examined. RESULTS: Electromyography from residual musculature during gait indicate that the animal continued to send neural commands to the coapted agonist-antagonist muscle pair in the absence of a prosthetic limb device. Muscle-state measurements and fluoroscopic evaluations show clear coupled motion of the agonist-antagonist pair in the presence of both natural neural commands and artificial muscle stimulation. Integrated EMG from the agonist muscle was cross-correlated against muscle state recordings from the antagonist; it was determined that the average time delay from agonist muscle activation to antagonist muscle stretch was 93 ms, with a correlation coefficient of .73. Post-mortem examination of the residual tissues showed no signs of necrosis and no abundance of scarring. CONCLUSIONS: These findings indicate that the LRM-PNI architecture has the potential to provide coupled motion of agonist-antagonist muscle pairs, as in the pre-amputation physiological milieu. Guided by these findings, it is our expectation that further development of the LRM-PNI architecture will improve bi-directional neural control of advanced limb prostheses.

A murine model of a novel surgical architecture for proprioceptive muscle feedback and its potential application to control of advanced limb prostheses

Objective. Proprioceptive mechanisms play a critical role in both reflexive and volitional lower extremity control. Significant strides have been made in the development of bionic limbs that are capable of bi-directional communication with the peripheral nervous system, but none of these systems have been capable of providing physiologically-relevant muscle-based proprioceptive feedback through natural neural pathways. In this study, we present the agonist–antagonist myoneural interface (AMI), a surgical approach with the capacity to provide graded kinesthetic feedback from a prosthesis through mechanical activation of native mechanoreceptors within residual agonist–antagonist muscle pairs. Approach. (1) Sonomicrometery and electroneurography measurement systems were validated using a servo-based muscle tensioning system. (2) A heuristic controller was implemented to modulate functional electrical stimulation of an agonist muscle, using sonomicrometric measurements of stretch from a mechanically-coupled antagonist muscle as feedback. (3) One AMI was surgically constructed in the hindlimb of each rat. (4) The gastrocnemius-soleus complex of the rat was cycled through a series of ramp-and-hold stretches in two different muscle architectures: native (physiologically-intact) and AMI (modified). Integrated electroneurography from the tibial nerve was compared across the two architectures. Main results. Correlation between stretch and afferent signal demonstrated that the AMI is capable of provoking graded afferent signals in response to ramp-and-hold stretches, in a manner similar to the native muscle architecture. The response magnitude in the AMI was reduced when compared to the native architecture, likely due to lower stretch amplitudes. The closed-loop control system showed robustness at high stretch magnitudes, with some oscillation at low stretch magnitudes. Significance. These results indicate that the AMI has the potential to communicate meaningful kinesthetic feedback from a prosthetic limb by replicating the agonist–antagonist relationships that are fundamental to physiological proprioception.

On the Origin of Muscle Synergies: Invariant Balance in the Co-activation of Agonist and Antagonist Muscle Pairs

Investigation of neural representation of movement planning has attracted the attention of neuroscientists, as it may reveal the sensorimotor transformation essential to motor control. The analysis of muscle synergies based on the activity of agonist–antagonist (AA) muscle pairs may provide insight into such transformations, especially for a reference frame in the muscle space. In this study, we examined the AA concept using the following explanatory variables: the AA ratio, which is related to the equilibrium-joint angle, and the AA sum, which is associated with joint stiffness. We formulated muscle synergies as a function of AA sums, positing that muscle synergies are composite units of mechanical impedance. The AA concept can be regarded as another form of the equilibrium-point (EP) hypothesis, and it can be extended to the concept of EP-based synergies. We introduce, here, a novel tool for analyzing the neurological and motor functions underlying human movements and review some initial insights from our results about the relationships between muscle synergies, endpoint stiffness, and virtual trajectories (time series of EP). Our results suggest that (1) muscle synergies reflect an invariant balance in the co-activation of AA muscle pairs; (2) each synergy represents the basis for the radial, tangential, and null movements of the virtual trajectory in the polar coordinates centered on the specific joint at the base of the body; and (3) the alteration of muscle synergies (for example, due to spasticity or rigidity following neurological injury) results in significant distortion of endpoint stiffness and concomitant virtual trajectories. These results indicate that muscle synergies (i.e., the balance of muscle mechanical impedance) are essential for motor control.

Training-related changes in the EMG–moment relationship during isometric contractions: Further evidence of improved control of muscle activation in strength-trained men?

The possibility of using electromyography (EMG) to track muscle activity has raised the question of its relationship with the effort exerted by the muscles around the joints. However, the EMG–moment relationship is yet to be fully defined, and increasing knowledge of this topic could contribute to research in motor control and to the development of EMG-based algorithms and devices. With regards the training-related adaptations at the peripheral and central level, the present study investigated the effect of strength training on EMG–moment relationship. Our aim was to clarify its nature and gain further understanding of how morphological and neural factors may affect its form. The EMG–moment relationship was determined during knee flexion and extension isometric contractions performed by strength-trained male athletes and untrained male participants. The results showed that strength training induced linearity of the EMG–moment relationship concomitantly with enhanced maximum force production capacity and decreased co-activation of knee agonist–antagonist muscle pair. These results clarified discordant results regarding the linear or curved nature of the EMG–moment in isometric conditions and suggested that the remarkable linearity of the EMG–moment found in trained participants could indicate improved control of muscle activation.