Monoarticular Muscles Research Articles

Simulated Increase in Monoarticular Hip Muscle Strength Reduces the First Peak of Knee Compression Forces During Walking.

Reducing compressive knee contact forces (KCF) during walking could slow the progression and reduce symptoms of knee osteoarthritis. A previous study has shown that compensating for the hip flexion/extension moment could reduce the KCF peak occurring during early stance (KCFp1). Therefore, this study aimed to identify if monoarticular hip muscle could allow this compensation while considering different walking strategies. Gait trials from 24 healthy participants were used to make musculoskeletal models, and five load-cases were examined: (I) Normal, (II) with an applied external moment compensating for 100% of the hip flexion/extension moment, and (III-V) three conditions with isolated/combined 30% increase of peak isometric strength of gluteus medius and maximus. Knee contact forces, hip muscle forces, and joint moments were computed. A cluster analysis of the Normal condition was performed with hip and knee flexion/extension moment during KCFp1 as input to examine the influence of different walking strategies. The cluster analysis revealed two groups having significantly different hip and knee moments in early-stance (p < 0.01). The reduction in KCFp1 from the Normal condition, although present in both groups, was greater for the group with the highest hip and lowest knee flexion/extension moments for all conditions tested (II: -21.82 ± 8.71% versus -6.03 ± 6.68%, III: -3.21 ± 1.09% versus -1.59 ± 0.96%, IV: -3.00 ± 0.89% versus -1.76 ± 1.04%, V: -6.12 ± 1.69 versus -3.09 ± 1.95%). This reduction in KCFp1 occurred through a shift in force developed by the hamstrings during walking (biarticular) to the gluteus medius and maximus (monoarticular), whose isometric strength was increased. The differences between the groups suggest that this reduction depends on the walking strategy.

Convergent Conditions of Feedforward Control for Musculoskeletal Systems with Multi 1-DOF Joints Driven by Monoarticular and Biarticular Muscles

Muscles in the musculoskeletal system can only transmit forces in the tensile direction, thereby resulting in redundant actuation. This redundancy creates an internal force among the muscles. The musculoskeletal potential method uses a potential field generated by the internal force among muscles and performs a step input of the muscle tension balanced at the desired posture to achieve the point-to-point (PTP) position. This method is extremely simple and does not require any sensory feedback or complex real-time calculations, as long as the target muscle tension is achieved. However, it is known that convergence to a desired posture is strongly influenced by muscular arrangement. In a previous study, we limited our analysis to a specific structure with two joints and six muscles and explained the conditions for convergence to a desired posture. However, when the structure of the target system, number of joints, and number of muscles are different, the convergence conditions cannot be clarified using the previous method. In this study, we extend the previous method to a musculoskeletal system with multiple one degrees-of-freedom (DOF) joints driven by monoarticular and biarticular muscles. In this study, we clarify the conditions that must be satisfied by the muscular arrangement to converge to a desired posture in the musculoskeletal potential method and verify the results through simulation.

Contractile Work of the Soleus and Biarticular Mechanisms of the Gastrocnemii Muscles Increase the Net Ankle Mechanical Work at High Walking Speeds.

Increasing walking speed is accompanied by an increase of the mechanical power and work performed at the ankle joint despite the decrease of the intrinsic muscle force potential of the soleus (Sol) and gastrocnemius medialis (GM) muscles. In the present study, we measured Achilles tendon (AT) elongation and, based on an experimentally determined AT force-elongation relationship, quantified AT force at four walking speeds (slow 0.7 m.s-1, preferred 1.4 m.s-1, transition 2.0 m.s-1, and maximum 2.6 ± 0.3 m.s-1). Further, we investigated the mechanical power and work of the AT force at the ankle joint and, separately, the mechanical power and work of the monoarticular Sol at the ankle joint and the biarticular gastrocnemii at the ankle and knee joints. We found a 21% decrease in maximum AT force at the two higher speeds compared to the preferred; however, the net work of the AT force at the ankle joint (ATF work) increased as a function of walking speed. An earlier plantar flexion accompanied by an increased electromyographic activity of the Sol and GM muscles and a knee-to-ankle joint energy transfer via the biarticular gastrocnemii increased the net ATF mechanical work by 1.7 and 2.4-fold in the transition and maximum walking speed, respectively. Our findings provide first-time evidence for a different mechanistic participation of the monoarticular Sol muscle (i.e., increased contractile net work carried out) and the biarticular gastrocnemii (i.e., increased contribution of biarticular mechanisms) to the speed-related increase of net ATF work.

Muscle Activation during the Squat Performed in Different Ranges of Motion by Women

Purpose: To analyze the muscle activation of the rectus femoris (RF), vastus lateralis (VL), gluteus maximus (GM), and biceps femoris (BF) in concentric and eccentric actions in the squat at 90° and 140° range of motion. Methods: Thirty-five women (32.9 ± 7.4 years; 64.5 ± 11.5 kg; 1.63 ± 0.1 m; BMI: 24.2 ± 2.9 kg/m2; %fat: 24.9 ± 6.5%) experienced exercise for at least eight weeks. Electrodes were positioned in standardized locations. The signals were acquired by an A/D SAS1000 V8 converter and the electromyographic activity normalized in the percentage of the highest produced value (%RMS). The data were analyzed using repeated measures two-way ANOVA, with effect size (η2) and differences calculated in percentage points (∆ p.p.). Results: The RF (p = 0.001; ∆ = 5.1 p.p.) and BF activation (p = 0.020; ∆ = 4.0 p.p.) was higher at 90° in the eccentric action. The RF showed an interaction between the range of motion and %RMS, with a large effect size (F = 37.9; p = 0.001; η2 = 0.485). The VL activation was higher at 140° (p = 0.005; ∆ = 3.9 p.p.) in the concentric action and higher at 90° (p = 0.006; ∆ = 3.7 p.p.) in the eccentric action, with a large effect size significant interaction (F = 21.3; p = 0.001; η2 = 0.485). The GM activation was higher at 90° in the concentric (p = 0.020; ∆ = 5.4 p.p.) and eccentric action (p = 0.022; ∆ = 41 p.p.). Conclusions: The biarticular muscles were influenced by the squat range only in the eccentric action of the movement, while the monoarticular muscles were influenced by the squat in both concentric and eccentric muscle action.

Three-dimensional visualization and quantitative analysis of embryonic and fetal thigh muscles using magnetic resonance and phase-contrast X-ray imaging.

The musculoskeletal system around the human hip joint has acquired a suitable structure for erect bipedal walking. However, little is known about the process of separation and maturation of individual muscles during the prenatal period, when muscle composition is acquired. Understanding the maturation process of the normal musculoskeletal system contributes to elucidating the acquisition of bipedal walking in humans and to predicting normal growth and detecting congenital muscle disorders and anomalies. In this study, we clarify the process of thigh muscle maturation from the embryonic stage to the mid-fetal stage using serial sections, phase-contrast X-ray computed tomography, and magnetic resonance imaging. We also provide a 4D atlas of human thigh muscles between 8 and 23 weeks of gestation. As a result, we first show that muscle separation in the lower thigh tends to progress from the superficial to the deep layers and that all musculoskeletal components are formed by Carnegie Stage 22. Next, we show that femur and muscle volume grow in correlation with crown-rump length. Finally, we show that the anterior, abductor, and posterior muscle groups in the thigh contain a high percentage of monoarticular muscle volume by the end of the embryonic period. This ratio approaches that of adult muscle composition during normal early fetal development and is typical of bipedal walking. This study of fetal muscle composition suggests that preparation for postnatal walking may begin in early fetal period.

Motor control strategies differ between monoarticular and biarticular quadriceps muscles during bipedal squats.

The interplay between biarticular and monoarticular muscles of the knee and hip joints during bipedal squats (SQBP ) requires adequate central-nervous control mechanisms to enable smooth and dynamic movements. Here, we investigated motor control between M. vastus medialis (VM), M. vastus lateralis (VL), and M. rectus femoris (RF) in 12 healthy male recreational athletes during SQBP with three load levels (50%, 62.5%, and 75% of 3-repetition maximum) following a standardized strength training protocol (3 sets of 10 repetitions). To quantify differences in motor control mechanisms in both time and frequency domains, we analyzed (1) muscle covariation via correlation analyses, as well as (2) common neural input via intermuscular coherence (IMC) between RF, VM, and VL. Our results revealed significantly higher gamma IMC between VM-VL compared with RF-VL and RF-VM for both legs. Correlation analyses demonstrated significantly higher correlation coefficients during ascent periods compared with descent periods across all analyzed muscle pairs. However, no load-dependent modulation of motor control could be observed. Our study provides novel evidence that motor control during SQBP is characterized by differences in common input between biarticular and monoarticular muscles. Additionally, muscle activation patterns show higher similarity during ascent compared with descent periods. Future research should aim to validate and extend our observations as insights into the underlying control mechanisms offer the possibility for practical implications to optimize training concepts in elite sports and rehabilitation.

Design and Joint Position Control of Bionic Jumping Leg Driven by Pneumatic Artificial Muscles

Using the skeletal structure and muscle distribution of the hind limbs of a jumping kangaroo as inspiration, a bionic jumping leg was designed with pneumatic artificial muscles (PAMs) as actuators. Referring to the position of biarticular muscles in kangaroos, we constructed a bionic joint using biarticular and monoarticular muscle arrangements. At the same time, the problem of the joint rotation angle limitations caused by PAM shrinkage was solved, and the range of motion of the bionic joint was improved. Based on the output force model of the PAM, we established a dynamic model of the bionic leg using the Lagrange method. In view of the coupling problem caused by the arrangement of the biarticular muscle, an extended state observer was used for decoupling. The system was decoupled into two single-input and single-output systems, and angle tracking control was carried out using active disturbance rejection control (ADRC). The simulation and experimental results showed that the ADRC algorithm had a better decoupling effect and shorter adjustment time than PID control. The jumping experiments showed that the bionic leg could jump with a horizontal displacement of 320 mm and a vertical displacement of 150 mm.

Stability analysis of multi-serial-link mechanism driven by antagonistic multiarticular artificial muscles

Artificial multiarticular musculoskeletal systems consisting of serially connected links driven by monoarticular and multiarticular muscles, which are often inspired by vertebrates, enable robots to elicit dynamic, elegant, and flexible movements. However, serial links driven by multiarticular muscles can cause unstable motion (e.g., buckling). The stability of musculoskeletal mechanisms driven by antagonistic multiarticular muscles depends on the muscle configuration, origin/insertion of muscles, spring constants of muscles, contracting force of muscles, and other factors. We analyze the stability of a multi-serial-link mechanism driven by antagonistic multiarticular muscles aiming to avoid buckling and other undesired motions. We theoretically derive the potential energy of the system and the stable condition at the target point, and validate the results through dynamic simulations and experiments. This paper presents the static stability criteria of serially linked robots, which are redundantly driven by monoarticular and multiarticular muscles, resulting in the design and control guidelines for those robots.

Human-in-the-Loop Weight Compensation in Upper Limb Wearable Robots Towards Total Muscles’ Effort Minimization

In this letter: (1) We present a novel human-in-the-loop adaptation method for whole arm muscles&#x2019; effort minimization by means of weight compensation in the face of an object with an unknown mass. (2) This adaptation rule can also be used as a cognitive model for the identification of mass value using muscle activation signals. (3) This adaptation rule utilizes the activation signal of only four muscles in the upper limb to minimize the whole muscles&#x2019; effort. We analytically discuss the stability, optimality, and convergence of the proposed method. The effectiveness of this method for whole muscles&#x2019; effort reduction is studied by simulations (OpenSim software) on a generic and realistic model of the human arm, a model with 7-DOF and 50 Hill-type muscles. The simulation results show the presented method&#x2019;s performance and applicability for weight compensation and mass estimation in upper limb assistive robots. In addition, the simulations in OpenSim completely support that the suggested set of mono-articular muscles is sufficient for whole muscles&#x2019; effort reduction.

A study on effect of virtual actuator in optimization calculation for muscle force estimation

The reserve actuators used in the Residual Reduction Algorithm of the musculoskeletal software OpenSim generate forces instead of muscles. In this study, we verified range of reserve actuator settings that can appropriately estimate muscle forces. A healthy adult man walked during the experiment. The results of the estimated lower limb muscle forces during walking indicated that the biarticular muscles, including rectus femoris and biceps femoris LH, and the monoarticular muscles which have many muscle attachment points, including gluteus maximus and psoas muscles, were not affected by the reserve actuators. While the monoarticular muscles, including soleus muscle, gastrocnemius muscle, tibialis anterior muscle, and biceps femoris SH, which have few muscle attachment points were easily affected by the reserve actuators.

Muscle architecture of the vastus lateralis and rectus femoris in the production of knee extensor torque in physically inactive women

Aim: This study aimed to verify the data reliability of muscle architecture (MA) variables, and the relationship between MA and the isometric peak torque (PT) of the monoarticular and biarticular knee extensor (KE) muscle in physically inactive women. Methods: Ten physically inactive women (24.0 ± 1.64 years; 162.9 ± 5.34 cm; 63.5 ± 11.90 kg) participated in the study. An ultrasound device assessed the MA variables (muscle thickness, fascicle length, and pennation angle) of the Vastus Lateralis (VL) and Rectus Femoris (RF), and an isokinetic dynamometer assessed the PT. Pearson correlation evaluated the relationship between PT and MA variables, with a significance level of 5%. Additionally, the intraclass correlation coefficient, coefficient of variation, and standard error of measurement. Results: Excellent reliability between images was observed, and no significant relationships were observed between the PT and MA variables of the VL and RF. Conclusion: Isolated variables of the MA of a monoarticular or a biarticular muscle do not influence the production of the isometric PT of the KE.

Development and Evaluation of a Prosthetic Ankle Emulator With an Artificial Soleus and Gastrocnemius

Abstract In individuals with transtibial limb loss, a contributing factor to mobility-related challenges is the disruption of biological calf muscle function due to transection of the soleus and gastrocnemius. Powered prosthetic ankles can restore primary function of the mono-articular soleus muscle, which contributes to ankle plantarflexion. In effect, a powered ankle acts like an artificial soleus (AS). However, the biarticular gastrocnemius connection that simultaneously contributes to ankle plantarflexion and knee flexion torques remains missing, and there are currently no commercially available prosthetic ankles that incorporate an artificial gastrocnemius (AG). The goal of this work is to describe the design of a novel emulator capable of independently controlling artificial soleus and gastrocnemius behaviors for transtibial prosthesis users during walking. To evaluate the emulator's efficacy in controlling the artificial gastrocnemius behaviors, a case series walking study was conducted with four transtibial prosthesis users. Data from this case series showed that the emulator exhibits low resistance to the user's leg swing, low hysteresis during passive spring emulation, and accurate force tracking for a range of artificial soleus and gastrocnemius behaviors. The emulator presented in this paper is versatile and can facilitate experiments studying the effects of various artificial soleus and gastrocnemius dynamics on gait or other movement tasks. Using this system, it is possible to address existing knowledge gaps and explore a wide range of artificial soleus and gastrocnemius behaviors during gait and potentially other activities of daily living.

Simulation-based biomechanical assessment of unpowered exoskeletons for running

Due to the complexity and high degrees of freedom, the detailed assessment of human biomechanics is necessary for the design and optimization of an effective exoskeleton. In this paper, we present full kinematics, dynamics, and biomechanics assessment of unpowered exoskeleton augmentation for human running gait. To do so, the considered case study is the assistive torque profile of I-RUN. Our approach is using some extensive data-driven OpenSim simulation results employing a generic lower limb model with 92-muscles and 29-DOF. In the simulation, it is observed that exoskeleton augmentation leads to 4.62% metabolic rate reduction for the stiffness coefficient of alpha ^*=0.6. Moreover, this optimum stiffness coefficient minimizes the biological hip moment by 26%. The optimum stiffness coefficient (alpha ^*=0.6) also reduces the average force of four major hip muscles, i.e., Psoas, Gluteus Maximus, Rectus Femoris, and Semimembranosus. The effect of assistive torque profile on the muscles’ fatigue is also studied. Interestingly, it is observed that at alpha ^{#}=0.8, both all 92 lower limb muscles’ fatigue and two hip major mono-articular muscles’ fatigue have the maximum reduction. This result re-confirm our hypothesis that ”reducing the forces of two antagonistic mono-articular muscles is sufficient for involved muscles’ total fatigue reduction.” Finally, the relation between the amount of metabolic rate reduction and kinematics of hip joint is examined carefully where for the first time, we present a reliable kinematic index for prediction of the metabolic rate reduction by I-RUN augmentation. This index not only explains individual differences in metabolic rate reduction but also provides a quantitative measure for training the subjects to maximize their benefits from I-RUN.

Epimuscular myofascial force transmission from biarticular rectus femoris elongation increases shear modulus of monoarticular quadriceps muscles

This study aimed to examine the effect of rectus femoris (RF) elongation with passive hip angle change on the shear moduli of the vastus lateralis (VL) and medialis (VM) to verify whether Epimuscular myofascial force transmission (EMFT) occurs in the human quadriceps. Fourteen healthy men participated in this study. The shear moduli of the RF, VL, and VM were measured in four hip positions: flexion (Flex), extension and abduction (Ext-Abd), extension (Ext), and extension and adduction (Ext-Add). As the behavior of shear moduli may differ depending on the parts of the vasti muscles, we measured the medial and lateral parts of the VL (Medial-VL and Lateral-VL) and VM (Medial-VM and Lateral-VM). The shear moduli at the Ext and Ext-Add positions were higher than at the Flex position in the RF, VL, and VM. The shear moduli during Ext and Ext-Add were higher than at the Ext-Abd in the RF, VL, and Lateral-VM. Moreover, the shear modulus of the Lateral-VM was higher than of the Medial-VM (Flex: 8.5% higher; Ext-Abd: 15.6%; Ext: 30.2%; Ext-Add: 32.6%). The shear moduli of the VL and VM, which are monoarticular muscles of the knee, increased with passive hip extension or adduction with extension, even when the knee angle was kept constant. The results suggest that EMFT occurs in the quadriceps, and EMFT had a great impact in the Lateral-VM, which is anatomically adjacent to the RF, but it had little effect in the Medial-VM, which is further away from the RF.

Jump control of a robot with a redundant musculoskeletal system including biarticular muscles

Understanding the functions of a coordinated drive of biarticular and monoarticular muscles in locomotion is fundamental to establishing a control strategy of a musculoskeletal robot. However, our current understanding is inadequate due to the complexity of the musculoskeletal structure. To tackle this issue, in this study, we developed a monopod robot with a musculoskeletal system including three biarticular muscles. We investigated the relationship between the height of a jump and the drive timing of two muscles: the gastrocnemius and soleus. As a result, we found that the relationship of the drive timing determined the height of a jump. We found that driving the gastrocnemius muscle earlier than the soleus muscle improved the jump height compared to the case where it was not. We also confirmed that the robot could jump three times consecutively with the parameters achieving the highest jump in this study.

Cosine tuning determines plantarflexors' activities during human upright standing and is affected by incomplete spinal cord injury.

Plantarflexors such as the soleus (SOL) and medial gastrocnemius (MG) play key roles in controlling bipedal stance; however, how the central nervous system controls the activation levels of these plantarflexors is not well understood. Here we investigated how the central nervous system controls the plantarflexors' activation level during quiet standing in a cosine tuning manner where the maximal activation is achieved in a preferred direction (PD). Furthermore, we investigated how spinal cord injury affects these plantarflexors' activations. Thirteen healthy adults (AB) and thirteen individuals with chronic, incomplete spinal cord injury (iSCI) performed quiet standing trials. Their body kinematics and kinetics as well as electromyography signals from the MG and SOL were recorded. In the AB group, we found that the plantarflexors followed the cosine tuning manner during quiet standing. That is, MG was most active when the ratio of plantarflexion torque to knee extension torque was ~2:-3, whereas SOL was most active when the ratio was ~2:1. This suggests that the SOL muscle, despite being a monoarticular muscle, is sensitive to both ankle plantarflexion and knee extension during quiet standing. The difference in the PDs accounts for the phasic activity of MG and for the tonic activity of SOL. Unlike the AB group, the MG's activity was similar to the SOL's activity in the iSCI group, and the SOL PDs were similar to those in the AB group. This result suggests that chronic iSCI affects the control strategy, i.e., cosine tuning, for MG, which may affect standing balance in individuals with iSCI.NEW & NOTEWORTHY Soleus muscle shows a tonic activity whereas medial gastrocnemius muscle shows a phasic activity during quiet standing. Cosine tuning and their preferred direction account for the different muscle activation patterns between these two muscles. In individuals with chronic incomplete spinal cord injury, the preferred direction of gastrocnemius medial head is affected, which may result in their deteriorated standing balance.

The supporting role of the teres major muscle, an additional component in glenohumeral stability? An anatomical and radiological study

Muscle coordination plays an important role in glenohumeral stability. The rotator cuff and the long head of the biceps are considered the primary dynamic stabilizers muscles. However, the fact that a subgroup of patients with a massive tear in the rotator cuff were able to keep a normal function, should make us question this traditional view.We hypothesize that the teres major which is also a monoarticular scapulohumeral muscle, although it is not part of the conjoined tendon of the rotator cuff, can play a role in glenohumeral stability by a direct support of the humeral head generated by the particular posteroanterior location of this muscle under the humeral head and which, as far as we know, has not been written up previously. This particular effect could appear while the arm is being lifted and the humeral head could be leaning on against the teres major muscle belly underneath it.An anatomical a radiological study was carried out to substantiate our hypothesis. Two cadaver specimens were used for the anatomical study. Frist body was studied through conventional dissection. The second body was analysed through sectional anatomy. Then a radiological study was carried out using magnetic resonance imaging in a healthy male volunteer.Both anatomically and radiologically, the anteroinferior surface of the humeral head was showed firmly resting against the muscle belly of the teres major, to the point of misshaping it from 110 degrees of arm elevation with external rotation. The specific contribution of this effect to the glenohumeral stability needs to be confirmed by further studies and can help us to prevent the high incidence of glenohumeral dislocations.

Coordination amongst quadriceps muscles suggests neural regulation of internal joint stresses, not simplification of task performance

Many studies have demonstrated covariation between muscle activations during behavior, suggesting that muscles are not controlled independently. According to one common proposal, this covariation reflects simplification of task performance by the nervous system so that muscles with similar contributions to task variables are controlled together. Alternatively, this covariation might reflect regulation of low-level aspects of movements that are common across tasks, such as stresses within joints. We examined these issues by analyzing covariation patterns in quadriceps muscle activity during locomotion in rats. The three monoarticular quadriceps muscles (vastus medialis [VM], vastus lateralis [VL], and vastus intermedius [VI]) produce knee extension and so have identical contributions to task performance; the biarticular rectus femoris (RF) produces an additional hip flexion. Consistent with the proposal that muscle covariation is related to similarity of muscle actions on task variables, we found that the covariation between VM and VL was stronger than their covariations with RF. However, covariation between VM and VL was also stronger than their covariations with VI. Since all vastii have identical actions on task variables, this finding suggests that covariation between muscle activity is not solely driven by simplification of overt task performance. Instead, the preferentially strong covariation between VM and VL is consistent with the control of internal joint stresses: Since VM and VL produce opposing mediolateral forces on the patella, the high positive correlation between their activation minimizes the net mediolateral patellar force. These results provide important insights into the interpretation of muscle covariations and their role in movement control.

Biomechanical factors affecting energy cost during running utilising different slopes

ABSTRACTThis study aimed to examine the characteristics of electromyography (EMG) and kinematics of the supporting leg affecting energy cost while running at incline, level, and decline slopes. Twelve male Japanese middle- and long-distance runners volunteered for this study. The subjects were asked to run at 13.5 km·h−1 on a treadmill under three slope conditions. Sagittal plane kinematics and the EMG of the lower limb muscles, respiratory gases were recorded. Energy cost differed significantly between slopes, being the lowest in decline slope and the greatest in incline slope. Integrated EMG (iEMG) of leg extensor muscles was greater in the incline slope than in the decline slope, and iEMG of the gastrocnemius and soleus muscles correlated positively with energy cost. The knee and ankle joint kinematics were associated with energy cost during running. In incline slope, the knee and ankle joints were more extended (plantarflexed) to lift the body. These movements may disturb the coordination between the ankle and knee joints. The gastrocnemius muscle would do greater mechanical work to plantarflex the ankle joint rather than transfer mechanical energy as well as greater mechanical work of mono-articular muscles. These muscular activities would increase energy cost.

Feedback From Mono-Articular Muscles is Sufficient for Exoskeleton Torque Adaptation.

In this paper, by using a biomechanical model of the human body, we prove that (1) due to the existence of bi-articular muscles and compliant-elements, blind full-torque-compensation at joint level leads to muscles' activity amplification and consequently online adaptation methods are required for exoskeleton torque optimization. Moreover, (2) we state a new hypothesis that "reducing the net torque of two antagonistic mono-articular muscles is sufficient for involved muscles' total effort reduction" and analytically discuss its validity condition. Using this hypothesis, (3) we develop an adaptation rule which optimizes the exoskeleton torque using EMG signals of only two antagonistic mono-articular muscles. Furthermore, (4) the stability, convergence, optimality, and robustness of our adaptation method are proved in the presence of electromyography's intrinsic noisy behavior. Finally, (5) we experimentally validate our EMG-based adaptation method on six healthy subjects. We show that adaptation of the elbow compliance in a 2-DOF semi-active assistive arm in a cyclic task results in significant muscles activity reduction in all our subjects.