Isometric Elbow Extension Research Articles

Inducing post-activation potentiation alters human motor unit behavior of the elbow extensors during unconstrained velocity contractions.

The purpose was to test whether inducing post-activation potentiation (PAP) altered motor unit (MU) activity during dynamic isotonic contractions. From 12 participants (3 females), 39 MUs were recorded from the anconeus (n=31) and lateral triceps brachii (n=8) with fine-wire electrodes during elbow extensions at 50 and 75% of peak power with, and without PAP. To induce PAP participants produced a 2s ramp conditioning contraction (CC) up to maximal isometric elbow extension with a 3s hold. Following the CC (~2s), independent electrical stimulation to the triceps and anconeus showed twitch torques were potentiated by 84 and 66%, respectively, (both p<0.001). Compared to baseline (i.e., without PAP), at both intensities (50 and 75%) PAP increased MU recruitment thresholds (40% and 80%, p<0.001) with lowered mean MU rates (-20 and -26%), and instantaneous rates at recruitment threshold (-26 and -25%) (all p<0.001). Firing rates increased 20% (p<0.001) from 50 to 75% power, but rates during potentiated contractions targeting 75% were lower than baseline at 50% (-10%, p<0.001). Dynamic contractions provide a more functional paradigm to assess MU activity with PAP and showed larger effects across a wider range of contractile intensities compared to previously described isometric tasks. Findings indicate that peripheral feedback from the potentiated muscle is likely not the primary mechanism in modifying MU behaviors as changes occurred at recruitment which is relatively insensitive to afferent feedback. Therefore, MU activity during dynamic contractions is responsive to activation history force potentiation and can make compensatory adjustments to optimize contractile output.

Feasibility and Reproducibility of Isokinetic Dynamometry in Children with Neuromuscular Diseases.

Background/Objectives: High-precision measurement tools are needed to measure relevant changes in strength and power in children with neuromuscular diseases. The aim of this study was to determine the feasibility (i), reproducibility (ii), and validity (iii) of isokinetic dynamometry in this population. Methods: Isometric and isokinetic knee and elbow flexion and extension were measured twice on the same day. Feasibility was based on completion rate and acceptability. Acceptability was measured with a 100 mm visual analog scale. We measured reproducibility as the intraclass correlation coefficient (ICC-agreement), standard error of measurement (SEM), and smallest detectable change (SDC). We investigated relationships between isometric strength and isokinetic power with Pearson's correlation coefficient. ROC curves were used to determine the cutoff of isometric strength to conduct isokinetic measurements. Results: Fifty children with NMDs participated with completion rates of 78-90% for isometric and 39-75% for isokinetic measurements. Acceptability was high (mean (SD) = 73 (19) mm). The ICCs for all measurements were over 0.9 (95% confidence interval (CI) = 0.932-0.998). The SEM% ranged from 5 to 14% and the SDC% from 14 to 28%. The correlations of strength and power were high (Pearson's correlation coefficient >0.9 (95% CI: 0.89-0.98)). The isometric strength needed to conduct isokinetic measurements ranged from 11.1 N in elbow flexors to 24.9 N in knee extensors. Conclusions: Isokinetic dynamometry is a feasible and reproducible method for measuring isometric strength in children with neuromuscular diseases with moderate weakness to normal strength, and isokinetic measurements are only feasible in knee extension for children with moderate weakness to normal strength. The convergent validity between isometric strength and power at low velocities is high.

Changes in brain functional connectivity and muscle strength independent of elbow flexor atrophy following upper limb immobilization in young females.

Muscle disuse induces a decline in muscle strength that exceeds the rate and magnitude of muscle atrophy, suggesting that factors beyond the muscle contribute to strength loss. The purpose of this study was to characterize changes in the brain and neuromuscular system in addition to muscle size following upper limb immobilization in young females. Using a within-participant, unilateral design, 12 females (age: 20.6±2.1years) underwent 14days of upper arm immobilization using an elbow brace and sling. Bilateral measures of muscle strength (isometric and isokinetic dynamometry), muscle size (magnetic resonance imaging), voluntary muscle activation capacity, corticospinal excitability, cortical thickness and resting-state functional connectivity were collected before and after immobilization. Immobilization induced a significant decline in isometric elbow flexion (-21.3±19.2%, interaction: P=0.0440) and extension (-19.9±15.7%, interaction: P=0.0317) strength in the immobilized arm only. There was no significant effect of immobilization on elbow flexor cross-sectional area (CSA) (-1.2±2.4%, interaction: P=0.466), whereas elbow extensor CSA decreased (-2.9±2.9%, interaction: P=0.0177) in the immobilized arm. Immobilization did not differentially alter voluntary activation capacity, corticospinal excitability, or cortical thickness (P>0.05); however, there were significant changes in the functional connectivity of brain regions related to movement planning and error detection (P<0.05). This study reveals that elbow flexor strength loss can occur in the absence of significant elbow flexor muscle atrophy, and that the brain represents a site of functional adaptation in response to upper limb immobilization in young females.

Reliability and validity of muscle activity analysis using wearable electromyographs.

[Purpose] The aim of this study was to develop a novel wearable surface electromyograph called NOK, and compare its reliability and validity to an existing electromyograph. [Participants and Methods] The study participants were 23 healthy university students (Seven males and 16 females; age 20.3 ± 1.1 years [mean ± standard deviation]; height 162.0 ± 6.7 cm; weight 58.4 ± 10.1 kg) who all gave informed written consent. The newly developed electromyograph (NOK) features a rubberized skin contact surface that requires no electrodes and allows the acquisition of up to 10 channels of muscle waveforms on a portable personal computer. After measuring maximal isometric elbow extension and flexion, we examined muscle waveforms during isometric contractions of elbow joint flexion and extension at approximately 50% of maximal voluntary contraction using both NOK and Delsys electromyographs and compared the results of the two devices. [Results] We found a significant moderate correlation between the measurements by the two devices for biceps and triceps. The measurements by the two devices also showed strong measure-retest reliability. Systematic errors were observed for elbow flexion and extension in the two measurements, indicating limited agreement between the two measurement methods. [Conclusion] Although the new device also has high repeatability and reliability, it is unsuitable for analyzing detailed muscle activity. However, since it can measure up to 10 channels of muscle activity, it is expected to be used in the rehabilitation and sports field in the future.

Muscle size-scaled shear elastic modulus: A muscle force index independent of maximal voluntary contraction, assessed during elbow extension

Shear elastic modulus (G) can differ among individuals due to muscle size and other factors, even for constant muscle force. Inter-individual comparisons of G usually require normalization by maximal voluntary contraction (MVC), but MVC procedures may not be appropriate for certain clinical populations including those presenting with pain or other compromised functionality. This study aimed to test whether muscle size-scaled G, which does not require MVC testing, would yield stronger correlation with absolute torque than unscaled G. Twelve-healthy males performed isometric elbow extension across a range of torque magnitudes (from 5 Nm until 60% MVC). G of the triceps brachii was measured using shear wave elastography during each trial. Cross-sectional area (CSA) and muscle thickness (MT) of the triceps brachii were measured at rest. Scaled G was calculated as a product of G and CSA or MT (“G-CSA” and “G-MT”, respectively). Within-individual linear regressions were conducted between absolute torque and the three force indicator variables. The regression slopes’ coefficient of variation (CV) was calculated for each indicator across individuals. Between-individual correlation coefficients were calculated, after pooling all data across individuals into a single regression analysis for each indicator. Linear regression found that inter-individual slope variation increased in the following order: G-CSA, G-MT, and unscaled G (CV = 0.15, 0.18, and 0.29, respectively). Pooled-individual correlation coefficients were significantly higher in G-CSA and G-MT than in unscaled G (r = 0.948, 0.924, and r = 0.783, respectively). These results suggest that muscle size-scaled G may be more appropriate than unscaled G when comparing shear moduli across individuals.

Comparison of fascicle behaviors between superficial and deeper muscles of triceps brachii during isometric contractions

PurposeWe assessed fascicle behaviors of the upper extremities during isometric contractions at different joint angles in this study. MethodsThirteen healthy men and women performed isometric elbow extension tasks at 50% and 75% of maximal voluntary contraction (MVC) at 60°, 90°, and 120° of elbow extension (full extension = 180°). Extended field-of-view B-mode ultrasonography was used to obtain sagittal plane panoramic images of the long head (TB-Long) and medial head (TB-Med) of the triceps brachii at rest and during contraction; fascicle length and pennation angle were measured. ResultsIn the TB-Long, significant fascicle shortening from rest was found during 50% and 75%MVC at 60° and during 75%MVC at 90° of extension. There was no significant fascicle shortening in the TB-Med muscle under any conditions. There was no significant pennation angle change from rest in either muscle. The pennation angle of the TB-Long was significantly greater than that of the TB-Med under all conditions. ConclusionsThese results suggest that fascicle shortening in the TB-Long muscle occurs in flexion; however, no change was found in the TB-Med. In the upper extremity muscle–tendon complex, the superficial and deeper muscles may have different force-transmission efficiency at flexed joint angles.

Antagonistic muscle prefatigue weakens the functional corticomuscular coupling during isometric elbow extension contraction.

During muscle fatigue, acute changes in the interaction between the sensorimotor cortex and peripheral neurons have been widely studied. However, it is still unclear about the effect of antagonist muscle prefatigue on corticomuscular coupling and central modulation. The purpose of this study was to investigate the changes in the magnitude of electroencephalogram-electromyography (EEG-EMG) coherence and phase synchronization index (PSI) induced by antagonistic muscle prefatigue. Twelve young male volunteers conducted a 30-s long, nonfatiguing isometric elbow extension with a target force level of 20% maximum voluntary contraction (MVC) before and after a fatiguing sustained elbow flexion contraction at 20% MVC until task failure. Coherence and PSI between the EEG recorded over the sensorimotor cortex and the surface EMG of the triceps brachii (TB) muscle were quantified for the pre- and post-fatigue elbow extension contractions. Coherence and PSI in the gamma frequency band (35-60 Hz) were found significantly decreased in the postfatigue elbow extension contraction than the prefatigue contraction. The power of the EEG in the beta and gamma band were significantly increased, while the EMG power showed no significant changes when the antagonistic muscle was prefatigued. PSI in the gamma frequency band between the EMG of the TB muscle and the EEG were found significantly decreased during postfatigue elbow extension contraction compared with prefatigue contraction. Antagonistic muscle prefatigue led to significantly lower gamma band corticomuscular coherence and phase coupling during an isometric elbow extension position task. The lower corticomuscular coupling may indicate a central modulation mechanism of antagonist muscle prefatigue that related to decreased descending common drive or joint instability compensation modulation mechanism.

The effect of five isometric exercises on glenohumeral translations in healthy subjects and patients with the hypermobility type of the ehlers-danlos syndrome (heds) or hypermobility spectrum disorder (hsd) with multidirectional shoulder instability: an observational study

ObjectivesTo examine whether patients, diagnosed with the hypermobility type of the Ehlers-Danlos syndrome (hEDS) or Hypermobility Spectrum Disorder (HSD), with multidirectional shoulder instability (MDI) have increased humeral head translations compared to healthy controls and to describe the direction of the humeral translations during five exercises. DesignObservational study. SettingGhent University Hospital. ParticipantsTwenty-seven female patients (aged mean (SD) 35 (13) years) with hEDS/HSD and MDI and 20 female healthy controls (aged 34 (11) years) participated in this study. InterventionsThe acromiohumeral (AHD) and humeralglenoid distance (HGD) were measured using ultrasound during five isometric exercises: shoulder external rotation, shoulder extension, shoulder flexion, elbow extension and holding a 2kg dumbbell. Main outcome measuresUltrasound measures of the AHD and HGD. ResultsDuring isometric shoulder extension, elbow extension and dumbbell loading, patients had a significantly larger change in AHD compared to controls. In patients, the AHD was significantly smaller during isometric shoulder flexion, extension and elbow extension compared to the AHD measured in rest. By contrast, the AHD was significantly larger during isometric external rotation and dumbbell loading compared to the AHD measured in rest. Regarding the HGD, no significant differences between patients and controls were observed. However, significantly smaller HGD values were found in patients during isometric shoulder flexion compared to the HGD in rest. ConclusionsIsometric external rotation and holding a 2kg dumbbell caused an inferior translation in patients with hEDS/HSD and MDI, whereas isometric shoulder flexion and shoulder/elbow extension respectively led to an anterior-superior and superior translation.

Peak torque, rate of torque development and average torque of isometric ankle and elbow contractions show excellent test-retest reliability.

Background:Peak Torque (PT), Rate of Torque Development (RTD) and Average Torque (AT) over a single contraction assess the three components of muscle function during isometric contractions. Surprisingly, AT has never been reported or its reliability confirmed.Objectives:This study aims to establish protocol reliability for ankle dorsiflexion and elbow extension isometric muscle function (PT, RTD, AT) in healthy participants using the Biodex System 3 Dynamometer.Methods:Twelve participants (6 male, 6 female, mean age years) performed four maximal isometric contractions on two occasions. Intraclass Correlation Coefficient (ICC), Typical Error (TE) and Coefficient of Variation (CV) for PT, RTD and AT were reported.Results:The ICC for all strength parameters varied from 0.98–0.92. TE for ankle dorsiflexion PT was 1.38 Nm, RTD 7.43 Nm/s and AT 1.33 Nm, CV varied from to . For elbow extension, TE was 3.36 Nm for PT, 14.87 Nm/s for RTD and 3.03 Nm for AT, CV varied from to .Conclusion:Maximal isometric ankle dorsiflexion and elbow extension PT, RTD and AT can be evaluated with excellent reliability when following the described protocol. This testing procedure, including the application of AT, can be confidently applied in research, exercise or clinical settings.

Spastic co-contraction, rather that spasticity, is associated with impaired active function in adults with acquired brain injury: A pilot study.

To elucidate the adverse consequences of spasticity and spastic co-contraction of elbow flexors on motor impairment and upper limb functional limitation. A pilot case-controlled prospective observational study. Ten brain-injured adults, and 10 healthy controls. The co-contraction index was computed from electromyographic recordings of elbow flexors during sub-maximal (25% Maximal Voluntary Contraction) isometric elbow extension. Spasticity was assessed with the Tardieu scale, upper limb limitation using a goniometer during active elbow extension, motor selectivity with the Fugl-Meyer Assessment for the upper limb, and motor function with the Action Research Arm Test. Greater co-contraction occurred in patients with brain injury compared with controls. In contrast to spasticity, strong associations were found between the co-contraction index, the limitation of active elbow extension, the Fugl-Meyer Assessment, and the Action Research Arm Test. This pilot study suggests that spastic co-contraction rather than spasticity is an important factor in altered upper limb motricity in subjects with brain injury, leading to abnormal restricting arm movement patterns in subjects with more severe motor impairment. Practical applications directly concern the pre- and post-therapeutic evaluation of treatments aimed at improving motor skills in subjects with brain injury.

Characterization of the coordination of agonist and antagonist muscles among stroke patients, healthy late middle-aged and young controls using a myoelectric-controlled interface

Objective. The coordination of agonist and antagonist muscles around a single joint plays an important role in daily activities. The aim of this study was to apply a myoelectric-controlled interface (MCI) with different dimensions to investigate stroke- and aging-induced deteriorations in the coordination of agonist and antagonist muscles. Approach. Eight stroke patients (affected sides), ten healthy late middle-aged controls and eighteen healthy young controls were enrolled to perform tracking tasks during voluntary isometric elbow flexion and extension by modulating their biceps and triceps activities with 1D or 2D MCI. The root mean square error (RMSE) between actual and target agonist activations, normalized agonist and antagonist activations, and co-contraction index (CI) and normalized elbow torque were used to quantify the movement performance. Main results. During elbow extension, significant increases in RMSE were identified in stroke patients with increasing MCI dimensionality, whereas significant decreases in normalized agonist and antagonist activations and normalized elbow torque were observed in all three groups. In addition, significant decreases in CI were observed in both control groups (P < 0.05). During elbow flexion and extension, RMSE increased in the following order: young controls < late middle-aged controls < stroke patients. By contrast, CI was significantly higher in stroke patients and late middle-aged controls than in young controls, possibly due to stroke- and aging-induced loss of skill in modulating the coordination of agonist and antagonist muscles when meeting the demands of a changing environment. Significance. This study suggests that 2D MCI might be applied as a rehabilitation tool to achieve fine control of abnormal muscle coordination.

Comparison of isometric triceps brachii force measurement in different elbow positions.

Objective and reliable force measurement is necessary to monitor the rehabilitation after triceps brachii pathology, injuries, and posterior approach-based surgery. It is unclear at which amount of extension the triceps is best tested and if comparison to the uninjured sided is reliable. This study aims to identify the most reliable elbow position at which elbow extension force is measured using a dynamometer. Furthermore, it aims to compare the extension strength of the dominant arm with that of the nondominant arm. Isometric elbow extension force of the dominant and nondominant arms of healthy subjects was measured. The measurements were taken in three sequences per arm in 0, 30, 60, 90, and 120 degrees of flexion. A subgroup repeated the measurements to analyze test-retest reliability using intraclass correlation. We included a total of 176 volunteers. The repeated measures analysis of variance for within-subject effect showed the lowest variation coefficient at 30 degrees of flexion. Extension forces showed a mean difference of 3.2-6.9 N in advantage of the dominant arm, resulting in ratios from 1.05 to 1.09. Learning curve analysis showed that during the first session in dominant and nondominant arms, less forces were exerted. The most reliable isometric triceps brachii muscle strength measurement was at 30 degrees of flexion of the elbow. Considering the learning curve, a first tryout session for both arms is indicated. Then, a second measurement suffices as no further learning curve is observed.

Dimensionality effect of myoelectric-controlled interface on the coordination of agonist and antagonist muscles during voluntary isometric elbow flexion and extension

Abstract This study aimed to investigate the dimensionality effect of myoelectric-controlled interface (MCI) on the coordination of agonist and antagonist muscles during voluntary isometric elbow flexion and extension. Eighteen healthy subjects were recruited to control a controllable cursor to track a target cursor by real-time modulating the biceps and triceps activities within one-dimensional and two-dimensional MCIs. Electromyographic (EMG) signals were collected to calculate the normalized muscle activation, while the slope of the best-fitting linear relationship between the normalized agonist and antagonist activations was used to quantify the muscle co-activation. The tracking error and the normalized net torque of the elbow joint were also calculated. Results showed that no significant difference was found in the tracking error between one-dimensional and two-dimensional MCIs. The normalized antagonist activation, the muscle co-activation and the normalized net torque were significantly lower within two-dimensional MCI than within one-dimensional MCI. In addition, significant decrease in the normalized agonist activation was also found during elbow extension. These results implied that within two-dimensional MCI, subjects were able to modulate the coordination of agonist and antagonist precisely by inhibiting unnecessary muscle activities. Therefore, two-dimensional MCI might be applied as a rehabilitation tool aiming at fine control of abnormal muscle coordination.

Voluntary activation of biceps-to-triceps and deltoid-to-triceps transfers in quadriplegia

The biceps or the posterior deltoid can be transferred to improve elbow extension function for many individuals with C5 or C6 quadriplegia. Maximum strength after elbow reconstruction is variable; the patient’s ability to voluntarily activate the transferred muscle to extend the elbow may contribute to the variability. We compared voluntary activation during maximum isometric elbow extension following biceps transfer (n = 5) and deltoid transfer (n = 6) in three functional postures. Voluntary activation was computed as the elbow extension moment generated during maximum voluntary effort divided by the moment generated with full activation, which was estimated via electrical stimulation. Voluntary activation was on average 96% after biceps transfer and not affected by posture. Individuals with deltoid transfer demonstrated deficits in voluntary activation, which differed by posture (80% in horizontal plane, 69% in overhead reach, and 70% in weight-relief), suggesting inadequate motor re-education after deltoid transfer. Overall, individuals with a biceps transfer better activated their transferred muscle than those with a deltoid transfer. This difference in neural control augmented the greater force-generating capacity of the biceps leading to increased elbow extension strength after biceps transfer (average 9.37 N-m across postures) relative to deltoid transfer (average 2.76 N-m across postures) in our study cohort.

Antagonist Muscle Prefatigue Increases the Intracortical Communication between Contralateral Motor Cortices during Elbow Extension Contraction.

To investigate the cortico-cortical coupling changes related to antagonist muscle prefatigue, we recorded EEG at FC3, C3, FC4, and C4 electrodes of twelve young male volunteers during a 30-second-long, nonfatiguing isometric elbow extension contraction with a target force level of 20% MVC before and after a sustained fatiguing elbow flexion contraction until task failure. EEG-EEG phase synchronization indices in alpha and beta frequency bands were calculated for the pre- and postfatigue elbow extension contractions. The phase synchronization index in the beta frequency band was found significantly increased between EEG of FC3-C3. The increased phase synchronization index may reflect an enhanced intracortical communication or integration of the signals between contralateral motor cortices with antagonist muscle prefatigue, which may be related to the central modulation so as to compensate for the antagonist muscle prefatigue-induced joint instability.

Investigating Aging-Related Changes in the Coordination of Agonist and Antagonist Muscles Using Fuzzy Entropy and Mutual Information

Aging alters muscular coordination patterns. This study aimed to investigate aging-related changes in the coordination of agonist and antagonist muscles from two aspects, the activities of individual muscles and the inter-muscular coupling. Eighteen young subjects and 10 elderly subjects were recruited to modulate the agonist muscle activity to track a target during voluntary isometric elbow flexion and extension. Normalized muscle activation and fuzzy entropy (FuzzyEn) were applied to depict the activities of biceps and triceps. Mutual information (MI) was utilized to measure the inter-muscular coupling between biceps and triceps. The agonist activation decreased and the antagonist activation increased significantly during elbow flexion and extension with aging. FuzzyEn values of agonist electromyogram (EMG) were similar between the two age groups. FuzzyEn values of antagonist EMG increased significantly with aging during elbow extension. MI decreased significantly with aging during elbow extension. These results indicated increased antagonist co-activation and decreased inter-muscular coupling with aging during elbow extension, which might result from the reduced reciprocal inhibition and the recruitment of additional cortical-spinal pathways connected to biceps. Based on FuzzyEn and MI, this study provided a comprehensive understanding of the mechanisms underlying the aging-related changes in the coordination of agonist and antagonist muscles.

Exercise and gait training in persons with paraplegia and its effect on muscle properties.

Upper extremity strengthening and gait training with orthoses form a major part of inpatient rehabilitation of paraplegic patients in developing countries. This helps to overcome architectural barriers and limited wheelchair accessible environment in the community. To evaluate the changes in physiological properties of the Triceps Brachii muscle following exercise training in individuals with paraplegia. The authors also explored the correlation between muscle property changes and gait parameters using orthoses in paraplegic persons. Twelve subjects with complete paraplegia and neurological level of injury (NLI) from T9 to L1, underwent exercise training for a mean 64.1 ± 4.1 days. Triceps brachii was chosen as the sample muscle. Variables like arm circumference, time to fatigue and mean power frequency (MF) (surface EMG parameter), were recorded at the beginning and the end of training, during a sub-maximal isometric elbow extension. Non-parametric tests were used to assess statistical significance between the two recordings. Additionally, gait parameters like walking speed and distance (with the help of orthoses) were obtained and compared with the above variables, to determine impact of upper extremity strengthening on gait improvements in such patients. Statistically significant changes were noted in bilateral arm circumferences (p= 0.003 bilaterally) and MF drop, expressed as percentage (right p= 0.04, left p= 0.01), indicative of better muscle resilience and adaptation. Significant positive correlation was observed between `time to fatigue' and the orthoses-aided total walking distance (right ρ = 0.65, left ρ = 0.69). Exercise training induces noticeable changes in the muscles of upper extremities favoring better muscle adaptation. Furthermore, positive correlation between `time to fatigue' and (orthotic) aided walking distance highlights the positive impact of strengthening program on gait parameters in paraplegic patients. These findings are important and relevant in developing countries with environmental barriers. Upper extremity strengthening should be included in the rehabilitation of paraplegic patients who are being trained for ambulation with orthoses.

Impact of stretched position of a shortened muscle on active command in spastic paresis

In spastic paresis, factors limiting active movement include antagonist contracture, agonist paresis, and spastic cocontraction. We studied agonist and antagonist muscle activation around the elbow to determine the role of the degree of muscle stretch in modifying paresis and cocontraction. Eighteen healthy (age 47 ± 10) and 15 hemiparetic (age 42 ± 16) subjects performed maximal isometric elbow flexion and extension 5-second efforts, with elbow flexed and extended. Using biceps and triceps brachii surface electromyography, we determined for each muscle: (i) the Mean Rectified Voltage (MRV) during the 500 ms peak voluntary agonist recruitment elbow flexed (MRV ago500F ) and elbow extended (MRV ago500E ); (ii) the coefficient of stretch-sensitivity (CSS) of agonist recruitment, calculated as (MRV ago500F -MRV ago500E )/MRV ago500F ; (iii) the cocontraction index (CCI) in each elbow position, ratio of the MRV from each muscle during an opposite movement, MRV antago5 to MRV ago500 . We compared the dominant arm in healthy subjects with the non-paretic and paretic arms in hemiparetic subjects. X V1 was 175 ± 11° in the elbow flexors (maximal passive elbow extension; coefficient of shortening, 2.7%) and 163 ± 7° in the elbow extensors (no shortening) . CSS was not significantly different from 0 in the healthy and non-paretic arm; however, CSS in the paretic arm was −21 ± 10% in the paretic elbow extensors vs −4 ± 7% in the non-paretic arm ( P vs extended, P = 0.013). Triceps brachii CCI with the elbow flexed was 0.10 ± 0.05 [95%CI, 0.03–0.17], 0.15 ± 0.10 [0.07–0.23] and 0.26 ± 0.24 [0.18–0.34], in the healthy, non-paretic and paretic arm respectively (between-group difference P = 0.011), and 0.12 ± 0.06 [95%CI, 0.03–0.21], 0.15 ± 0.08 [0.06–0.25], and 0.38 ± 0.32 [0.28–0.48] with the elbow extended (between-group difference P vs extended, NS). In hemiparesis, elbow extended position deteriorates active command at both agonist and antagonist levels: decreasing voluntary recruitment of agonist MNs–a phenomenon defined as stretch-sensitive paresis–and increasing antagonist co-contraction. This may occur through physiological effects of high-threshold afferent fiber recruitment in contractured elbow flexors.

Agonist-to-antagonist dependency during target-directed isometric elbow flexion and extension.

Muscle coordination during isometric elbow contraction was investigated in this study in order to understand the agonist-to-antagonist dependency. 15 healthy subjects were recruited and instructed to perform target-directed isometric elbow flexion and extension. Electromyography (EMG) signals from both biceps and triceps were captured. Root mean square (RMS) of the EMG signals was applied to reflect the muscle activation level and fuzzy approximate entropy (fApEn) was utilized to represent the complexity of EMG signals. Mutual information (MI) was applied to analyze the inter-dependency between biceps brachii and triceps brachii. The differences in RMS and fApEn between elbow flexion and extension were non-significant. And the MI between the biceps and triceps during isometric extension was significantly higher than that during isometric flexion. Our results suggested that the lack of proficiency in using triceps may result in more dependency towards the use of antagonist during isometric elbow extension than elbow flexion.

Increased antagonist muscle activity in cervical SCI patients suggests altered reciprocal inhibition during elbow contractions

ObjectiveAfter spinal cord injury (SCI), the antagonist muscles activation is increased during voluntary contractions and reflex conditioning protocols. This increase can be the result of both muscle atrophy and reciprocal facilitation mechanism. It remains however unclear to what extent increased antagonist muscles activation could be rather attributable to central vs. peripheral changes during voluntary contractions achieved by SCI participants. MethodsWe investigated the activations of elbow extensors and flexors during isometric elbow flexion and extension contractions performed at 3 force levels by 10 healthy participants and 8 participants with cervical SCI (cSCI). ResultsAt similar force level and absolute net torque in flexion, the antagonist muscles activation was increased for the participants with cSCI. At similar absolute net torque in extension, the activations of agonist and antagonist muscles were increased for the participants with cSCI. ConclusionDuring flexion contractions, increased antagonist muscles activation may be explained by extensors atrophy or reciprocal facilitation. During extension contractions, increased antagonist muscles activation may reflect the importance of reciprocal facilitation as antagonist muscles were evaluated as intact by clinical testing and maximal net joint torque recording. SignificanceThese results in cSCI participants revealed an increased activation of antagonist muscles, which may reflect a reorganization of the spinal reflexes and their supraspinal control involved during isometric elbow contractions.