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Purpose: The aim of this study was to examine the effect of vibration on motor unit (MU) firing behavior and physical performance of antagonist muscles in healthy young adult males. Methods: Fourteen males (age = 24.3 ± 3.6 years) were included in this study. There were two conditions, one in which participants received 80 Hz vibration in the distal tendon of the hamstring for 30 s and the control condition (no vibration). High-density surface electromyography (HD-SEMG) signals and maximal voluntary contraction (MVC) of knee extensor muscles were evaluated before and after the respective conditions and recorded from the vastus lateralis muscle during submaximal ramp-up and sustained contractions at 30% MVC. Convolution blind source separation was used to decompose the HD-SEMG signals into individual MU firing behaviors. Results: In total, 739 MUs were detected (control; 360 MUs and vibration; 379 MUs), and a total of 312 matched MUs were identified across both submaximal contraction conditions (control: 150 MUs; vibration: 162 MUs). Vibration significantly increased the discharge rate (p = 0.047) and decreased the recruitment threshold before and after intervention (p = 0.001) but not in the control condition. Furthermore, the recruitment threshold is a factor that influences discharge rate. Significant correlations were observed between the recruitment threshold and both the ∆ discharge rate and the ∆ recruitment threshold under the vibration condition (p < 0.001). Conclusion: Vibration increased in the discharge rate and decreased the recruitment threshold of the antagonist muscle. These findings suggested that vibration contributes to immediate changes in the neural control of antagonist muscles. [ABSTRACT FROM AUTHOR]

Position sense is arguably more important than any of the other proprioceptive senses, because it provides us with information about the position of our body and limbs in relationship to one another and to our surroundings; it has been considered to contribute to our self‐awareness. There is currently no consensus over the best method of measuring position sense. We have recently measured position sense with three commonly used methods. These were two‐arm matching, one‐arm pointing and one‐arm repositioning, all carried out by blindfolded subjects with their lightly loaded forearms moving in the sagittal plane. It is currently believed that muscle spindles are the principal position sensors. We posed the question, was there evidence for spindles participating in the generation of position sense with each method? The indicator of spindle activity we used was the presence of thixotropic errors in the position signal, in response to conditioning voluntary contractions of forearm muscles. Based on this criterion, there was evidence of spindles contributing to position sense with all three methods. It was concluded that the spindle contribution to the position signal and the extent to which this was processed centrally was different with each method. It is argued that a case could be made for the existence of more than one position sense. Differences between the methods have implications for their meaning in a clinical setting. [ABSTRACT FROM AUTHOR]

The purpose of this cross-sectional study was to compare explosive strength and underpinning contractile, hypertrophic, and neuromuscular activation characteristics of long-term maximum strength-trained (LT-MST; ie, ≥3 years of consistent, regular knee extensor training) and untrained individuals. Sixty-three healthy young men (untrained [UNT] n = 49, and LT-MST n = 14) performed isometric maximum and explosive voluntary, as well as evoked octet knee extension contractions. Torque, quadriceps, and hamstring surface EMG were recorded during all tasks. Quadriceps anatomical cross-sectional area (QACSAMAX; via MRI) was also assessed. Maximum voluntary torque (MVT; +66%) and QACSAMAX (+54%) were greater for LT-MST than UNT ([both] p < 0.001). Absolute explosive voluntary torque (25–150 ms after torque onset; +41 to +64%; [all] p < 0.001; 1.15≤ effect size [ES]≤2.36) and absolute evoked octet torque (50 ms after torque onset; +43, p < 0.001; ES = 3.07) were greater for LT-MST than UNT. However, relative (to MVT) explosive voluntary torque was lower for LT-MST than UNT from 100 to 150 ms after contraction onset (−11% to −16%; 0.001 ≤ p ≤ 0.002; 0.98 ≤ ES ≤ 1.11). Relative evoked octet torque 50 ms after onset was lower (−10%; p < 0.001; ES = 1.14) and octet time to peak torque longer (+8%; p = 0.001; ES = 1.18) for LT-MST than UNT indicating slower contractile properties, independent from any differences in torque amplitude. The greater absolute explosive strength of the LT-MST group was attributable to higher evoked explosive strength, that in turn appeared to be due to larger quadriceps muscle size, rather than any differences in neuromuscular activation. In contrast, the inferior relative explosive strength of LT-MST appeared to be underpinned by slower intrinsic/ evoked contractile properties. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the muscle activation and co-activation of selected muscles during the kettlebell single arm swing exercise. To the best of our knowledge, this is the first study investigating the muscle co-activation of a kettlebell single arm swing exercise. Nine volunteers participated in the present study (age: 22.6 ± 3.8 years; body mass: 80.4 ± 9.2 kg; height: 175.6 ± 7.5 cm). The electrical muscle activity of eight right agonist/antagonist muscles (AD/PD, ESL/RA, ESI/EO, and GM/RF) were recorded using a surface EMG system (Myon m320RX; Myon, Switzerland) and processed using the integrated EMG to calculate a co-activation index (CoI) for the ascending and descending phases. A significant effect of the ascending and descending phases on the muscles' CoI was observed. Post hoc analyses showed that the co-activation was significantly higher in the descending phase compared to that in the ascending phase of AD/PD CoI (34.25 ± 18.03% and 24.75 ± 13.03%, p < 0.001), ESL/RA CoI (34.97 ± 17.86% and 24.19 ± 10.32%, p < 0.001), ESI/EO CoI (41.14 ± 10.72% and 30.87 ± 11.26%, p < 0.001), and GM/RF CoI (27.49 ± 12.97% and 34.98 ± 14.97%, p < 0.001). In conclusion, the co-activation of the shoulder muscles varies within the kettlebell single arm swing. The highest level of co-activation was observed in the descending phase of AD/PD and GM/RF CoI, and the lowest level of co-activation was observed during the descending phase, ESL/RA and ESI/EO CoI. In addition, the highest level of co-activation was observed in the ascending phase of ESL/RA and ESI/EO CoI, and the lowest level of co-activation was observed during the ascending phase, AD/PD and GM/RF CoI. The co-activation index could be a useful method for the interpretation of the shoulder and core muscles' co-activity during a kettlebell single arm swing. [ABSTRACT FROM AUTHOR]

There is a significant influence of muscle fatigue on the coupling of antagonistic muscles while patients with post-stroke spasticity are characterized by abnormal antagonistic muscle coactivation activities. This study was designed to verify whether the coupling of antagonistic muscles in patients with post-stroke spasticity is influenced by muscle fatigue. Ten patients with chronic hemipare and spasticity and 12 healthy adults were recruited to participate in this study. Each participant performed a fatiguing isometric elbow flexion of the paretic side or right limb at 30% maximal voluntary contraction (MVC) level until exhaustion while surface electromyographic (sEMG) signals were collected from the biceps brachii (BB) and triceps brachii (TB) muscles during the sustained contraction. sEMG signals were divided into the first (minimal fatigue) and second halves (severe fatigue) of the contraction. The power and coherence between the sEMG signals of the BB and TB in the alpha (8–12 Hz), beta (15–35 Hz), and gamma (35–60 Hz) frequency bands associated with minimal fatigue and severe fatigue were calculated. The coactivation ratio of the antagonistic TB muscle was also determined during the sustained fatiguing contraction. The results demonstrated that there was a significant decrease in maximal torque during the post-fatigue contraction compared to that during the pre-fatigue contraction in both stroke and healthy group. In the stroke group, EMG-EMG coherence between the BB and TB in the alpha and beta frequency bands was significantly increased in severe fatigue compared to minimal fatigue, while coactivation of antagonistic muscle increased progressively during the sustained fatiguing contraction. In the healthy group, coactivation of the antagonistic muscle showed no significant changes during the fatiguing contraction and no significant coherence was found in the alpha, beta and gamma frequency bands between the first and second halves of the contraction. Therefore, the muscle fatigue significantly increases the coupling of antagonistic muscles in patients with post-stroke spasticity, which may be related to the increased common corticospinal drive from motor cortex to the antagonistic muscles. The increase in antagonistic muscle coupling induced by muscle fatigue may provide suggestions for the design of training program for patients with post-stroke spasticity.

In a review of the topic, Pollock, Baer, Pomeroy, and Langhorne (2009) concluded that surface electromyography (SEMG) biofeedback does not appear to have any positive benefit for recovery for loss of motor function. One would expect that a modality that can provide a window onto muscle functioning would be widely used as a means to restore motor function after injury. However, in 2019 little had changed regarding the use of SEMG. It is not commonly found in the armamentarium of treatment options in the typical physical/occupational therapy clinic. The reasons for this are multifaceted, but can be summarized as lack of knowledge or application of the conditions required for success. This article will attempt to encourage the reader to engage in further study for one very important reason—it enables successful treatment. [ABSTRACT FROM AUTHOR]

In this study, we aimed to analyze control mechanisms of short-latency afferent inhibition (SAI) during motor output exertion from an agonist or antagonist muscle. The motor task involved index finger abduction (agonist) and adduction (antagonist). In Experiment 1, motor-evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) muscle with and without SAI at three output force levels. In Experiment 2, MEPs were recorded with and without SAI at various time points immediately before the muscle output. Experiment 1 showed that inhibition decreased with an increase in muscle output in the agonist muscle but increased in the antagonist muscle. Experiment 2 showed a decreasing trend of inhibition in the agonist muscle immediately before contraction but showed no significant change in the antagonist muscle. MEPs without electrical stimulation during the reaction time increased in both directions of movement as compared to those in the resting state. These results suggest that SAI modulation strongly influences smooth motor output. Analyzing the inhibitory or enhanced mechanisms during the performance of motor output by SAI in patients with motor impairment and comparing them with the mechanisms seen in healthy participants will improve our understanding of the neurophysiological mechanisms relevant to various situations (e.g., rehabilitation and sports). • We analyzed regulation of short-latency afferent inhibition during motor output. • Inhibition decreased in the agonist muscle with increasing muscle output. • Inhibition increased in the agonist muscle with increasing muscle output. • Inhibition tended to decrease in the agonist immediately before contraction. • SAI modulation strongly influences smooth motor output. [ABSTRACT FROM AUTHOR]

The prediction of muscle forces is very important in order to evaluate the physical loads during human motion. Although a number of musculoskeletal models, which can predict muscle forces during motion, have been proposed, these models use the optimization method, in which the sum of the predicted muscle forces is minimized. However, unfortunately, the optimization method does not usually consider the role of antagonistic muscles, which act in a direction opposite to the prime motion or for restriction of rotational joint motion. Therefore, the present study proposes a new method to predict muscle forces considering the role of the antagonistic muscle during human motion. The present study proposes a new model, in which the agonist muscle is assumed to be connected to the antagonistic muscle by a coupled spring. Joint torque is defined as the summation of both the torques derived from the agonist muscles and the torques derived from the antagonistic muscles. Each muscle force can be predicted in order to maintain balance among the torques generated by the agonist muscles and the antagonistic muscles, respectively. Experiments were conducted in order to validate the proposed method of predicting muscle forces. Surface electromyograms (sEMGs) were measured for comparison with the predicted muscle forces. The experimental results showed that the predicted muscle forces agreed well with the sEMGs of muscles.

Previous studies have suggested that skilled athletes may show a specific muscle activation pattern with a lower antagonist coactivation level. Based on the point, we hypothesize that the coupling of antagonistic muscles may be different between badminton players and non-skilled individuals during exercises. The current work was designed to verify the hypothesis. Ten male college students and eight male badminton players performed three maximal voluntary isometric contractions (MVC) and a set of three maximal concentric ankle dorsiflexion and plantar flexions at an angular velocity of 30°, 60°, 120°, and 180°/s. Surface electromyography (EMG) was recorded from the tibialis anterior (TA) and lateral gastrocnemius (LG) muscles during the test. Normalized average EMG amplitude and phase synchronization index (PSI) between surface EMG of TA and LG were calculated. Antagonist muscle coactivation was significantly lower (from 22.1% ± 9.4 and 10.7% ± 3.7 at 30°/s to 22.4% ± 9.7 and 10.6% ± 2.5 at 180°/s for non-players and badminton players group, respectively), and PSI in beta frequency band was significantly higher (from 0.42 ± 0.06 and 0.47 ± 0.15 at 30°/s to 0.35 ± 0.12 and 0.49 ± 0.14 at 180°/s) in the badminton player group compared with the non-player group during isokinetic ankle dorsiflexion contraction. No significant difference was found in antagonist muscle coactivation and PSI between two group subjects during ankle plantar flexion. The decrease of antagonist coactivation may indicate an optimal motor control style to increase the contraction efficiency, while the increase coupling of antagonistic muscles may help to ensure joint stability to compensate for the decrease of antagonist coactivation. Graphical abstract Significant difference of observed indexes between non-players and badminton players. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the muscle activation and co-activation of selected muscles during the kettlebell single arm swing exercise. To the best of our knowledge, this is the first study investigating the muscle co-activation of a kettlebell single arm swing exercise. Nine volunteers participated in the present study (age: 22.6 ± 3.8 years; body mass: 80.4 ± 9.2 kg; height: 175.6 ± 7.5 cm). The electrical muscle activity of eight right agonist/antagonist muscles (AD/PD, ESL/RA, ESI/EO, and GM/RF) were recorded using a surface EMG system (Myon m320RX; Myon, Switzerland) and processed using the integrated EMG to calculate a co-activation index (CoI) for the ascending and descending phases. A significant effect of the ascending and descending phases on the muscles’ CoI was observed. Post hoc analyses showed that the co-activation was significantly higher in the descending phase compared to that in the ascending phase of AD/PD CoI (34.25 ± 18.03% and 24.75 ± 13.03%, p < 0.001), ESL/RA CoI (34.97 ± 17.86% and 24.19 ± 10.32%, p < 0.001), ESI/EO CoI (41.14 ± 10.72% and 30.87 ± 11.26%, p < 0.001), and GM/RF CoI (27.49 ± 12.97% and 34.98 ± 14.97%, p < 0.001). In conclusion, the co-activation of the shoulder muscles varies within the kettlebell single arm swing. The highest level of co-activation was observed in the descending phase of AD/PD and GM/RF CoI, and the lowest level of co-activation was observed during the descending phase, ESL/RA and ESI/EO CoI. In addition, the highest level of co-activation was observed in the ascending phase of ESL/RA and ESI/EO CoI, and the lowest level of co-activation was observed during the ascending phase, AD/PD and GM/RF CoI. The co-activation index could be a useful method for the interpretation of the shoulder and core muscles’ co-activity during a kettlebell single arm swing.

The purpose of this study was to compare the effect of resistance training (RT) duration, including years of exposure, on agonist and antagonist neuromuscular activation throughout the knee extension voluntary torque range. Fifty‐seven healthy men (untrained [UNT] n = 29, short‐term RT [12WK] n = 14, and long‐term RT [4YR] n = 14) performed maximum and sub‐maximum (20%‐80% maximum voluntary torque [MVT]) unilateral isometric knee extension contractions with torque, agonist and antagonist surface EMG recorded. Agonist EMG, including at MVT, was corrected for the confounding effects of adiposity (ie, muscle‐electrode distance; measured with ultrasonography). Quadriceps maximum anatomical cross‐sectional area (QACSAMAX; via MRI) was also assessed. MVT was distinct for all three groups (4YR +60/+39% vs UNT/12WK; 12WK +15% vs UNT; 0.001 < P ≤ 0.021), and QACSAMAX was greater for 4YR (+50/+42% vs UNT/12WK; [both] P < 0.001). Agonist EMG at MVT was +44/+33% greater for 4YR /12WK ([both] P < 0.001) vs. UNT, but did not differ between RT groups. The torque‐agonist EMG relationship of 4YR displayed a right/down shift with lower agonist EMG at the highest common torque (196 Nm) compared to 12WK and UNT (0.005 ≤ P ≤ 0.013; Effect size [ES] 0.90 ≤ ES ≤ 1.28). The torque‐antagonist EMG relationship displayed a lower slope with increasing RT duration (4YR < 12WK < UNT; 0.001 < P ≤ 0.094; 0.56 ≤ ES ≤ 1.31), and antagonist EMG at the highest common torque was also lower for 4YR than UNT (−69%; P < 0.001; ES = 1.18). In conclusion, 4YR and 12WK had similar agonist activation at MVT and this adaptation may be maximized during early months of RT. In contrast, inter‐muscular coordination, specifically antagonist coactivation was progressively lower, and likely continues to adapt, with prolonged RT. [ABSTRACT FROM AUTHOR]

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The aim of this study was to investigate if static and dynamic stretching exercises of the knee muscles (quadriceps and hamstring muscles) have any effects on concentric and eccentric isokinetic peak torques and electromyographic amplitudes (EMG) of the antagonist muscles. Twenty healthy male athletes (age between 18-30 years) voluntarily participated in this study. All of the subjects visited the laboratory to complete the following intervention in a randomized order on 5 separate days; (a) non-stretching (control), (b) static stretching of the quadriceps muscles, (c) static stretching of the hamstring muscles, (d) dynamic stretching of the quadriceps muscles, and (e) dynamic stretching of the hamstring muscles. Static stretching exercises either for the quadriceps or the hamstring muscles were carried out at the standing and sitting positions. Subjects performed four successive repetitions of each stretching exercises for 30 seconds in both stretching positions. Similar to static stretching exercises two different stretching modes were designed for dynamic stretching exercises. Concentric and eccentric isokinetic peak torque for the non-stretched antagonist quadriceps or hamstring muscles at angular velocities of 60°/sec and 240°/sec and their concurrent electromyographic (EMG) activities were measured before and immediately after the intervention. Isokinetic peak torques of the non-stretched agonist hamstring and quadriceps muscles did not represent any significant (p > 0.05) differences following static and dynamic stretching of the antagonist quadriceps and hamstring muscles, respectively. Similarly, the EMG activities of the agonist muscles exhibited no significant alterations (p > 0.05) following both stretching exercises of the antagonist muscles. According to the results of the present study it is possible to state that antagonist stretching exercises either in the static or dynamic modes do not affect the isokinetic peak torques and the EMG activities of the non-stretched agonist quadriceps or hamstring muscles. [ABSTRACT FROM AUTHOR]

Objective To address the roles and mechanisms of co-activation in two flexor/extensor pairs during elbow extension in children with cerebral palsy (CP). Methods 13 Typically Developing (TD) and 13 children with unilateral spastic CP performed elbow extension/flexion at different speeds. Elbow angle and velocity were recorded using a 3D motion analysis system. The acceleration and deceleration phases of extension were analyzed. Co-activation of the brachioradialis/triceps and biceps/triceps pairs was computed for each phase from surface electromyographic signals. Statistical analysis involved linear mixed effects models and Spearman rank correlations. Results During the acceleration phase, there was strong co-activation in both muscle pairs in the children with CP, which increased with speed. Co-activation was weak in the TD children and it was not speed-dependent. During the deceleration phase, co-activation was strong and increased with speed in both groups; co-activation of brachioradialis/triceps was stronger in children with CP, and was negatively correlated with extension range and positively correlated with flexor spasticity. Conclusions Abnormal patterns of co-activation in children with CP were found throughout the entire movement. Co-activation was specific to the movement phase and to each flexor muscle. Significance Co-activation in children with CP is both physiological and pathological. [ABSTRACT FROM AUTHOR]

Antagonist coactivation is the simultaneous activation of agonist and antagonist muscles during a motor task. Age-related changes in coactivation may contribute to observed differences in muscle performance between children and adults. Our aim was to systematically summarize age-related differences in antagonist muscle coactivation during multi-joint dynamic and single-joint isometric and isokinetic contractions. Electronic databases were searched for peer-reviewed studies comparing coactivation in upper or lower extremity muscles between healthy children and adolescents/young adults. Of the 1083 studies initially identified, 25 met eligibility criteria. Thirteen studies examined multi-joint dynamic movements, 10 single-joint isometric contractions, and 2 single-joint isokinetic contractions. Of the studies investigating multi-joint dynamic contractions, 83% (11/13 studies) reported at least one significant age-related difference: In 84% (9/11 studies) coactivation was higher in children, whereas 16% (2/11 studies) reported higher coactivation in adults. Among single-joint contractions, only 25% (3/12 studies) reported significantly higher coactivation in children. Fifty six percent of studies examined females, with no clear sex-related differences. Child-adult differences in coactivation appear to be more prevalent during multi-joint dynamic contractions, where generally, coactivation is higher in children. When examining child–adult differences in muscle function, it is important to consider potential age-related differences in coactivation, specifically during multi-joint dynamic contractions. [ABSTRACT FROM AUTHOR]

The present study aimed to investigate corticospinal excitability changes during muscle relaxation with a tracking task. The motor-evoked potential, which was elicited from the flexor carpi radialis (agonist) and extensor carpi radialis (antagonist) muscles, was analyzed in terms of both the relaxation velocity and phase. Our results suggest that increasing corticospinal excitability in the antagonist muscle plays an important role in controlling the relaxation of the agonist muscle during gradual relaxation with a tracking task. [ABSTRACT FROM AUTHOR]

In addition to the role of muscle coactivation, a major question in the field is how antagonist activation is controlled to minimize its opposing effect on agonist muscle performance. Muscle fatigue is an interesting condition to analyze the neural adjustments in antagonist muscle activity and to gain more insights into the control mechanisms of coactivation. In that context, previous studies have reported that although the EMG activity of agonists and antagonists increase in parallel, the ratio between EMG activities in the two sets of muscles during a fatiguing submaximal contraction decreased progressively and contributed to a reduction in the time to task failure. In contrast, more recent studies using a novel normalization procedure indicated that the agonist/antagonist ratio remained relatively constant, suggesting that the fatigue-related increase in coactivation does not impede performance. Current knowledge also indicates that peripheral mechanisms cannot by themselves mediate the intensity of antagonist coactivation during fatiguing contractions, implying that supraspinal mechanisms are involved. The unique modulation of the synaptic input from Ia afferents to the antagonist motor neurones during a fatiguing contraction of the agonist muscles further suggests a separate control of the two sets of muscles. [ABSTRACT FROM AUTHOR]