A potential mechanism by which torque output is preserved in cerebral palsy during fatiguing contractions of the knee extensors

Introduction: The purpose of this study was to compare agonist and antagonist electromyography (EMG) during an isokinetic fatigue protocol in subjects with cerebral palsy (CP) and typical development (TD). Methods: Nine individuals with CP and 11 with TD completed 50 repetitions of maximum concentric knee extensions (KE) and flexions (KF) at 60°/second. Results: Rate of decline in peak torque for KE was significantly less in CP compared with TD. Rate of decline in agonist EMG was not significantly different between groups, but the rate of decline in antagonist EMG was significantly greater in CP. There were no differences between groups for KF. Conclusions: Declining agonist EMG occurred in parallel with declining antagonist hamstring activity in CP, decreasing the relative opposing force and resulting in a lesser decline in net torque. This finding illustrates a potential mechanism by which net torque is preserved in those with CP who are inherently weaker. Muscle Nerve 53: 297–303, 2016     

Differences between young and older women in maximal force, force fluctuations, and surface EMG during isometric knee extension and elbow flexion

The loss of muscle strength with aging appears to be greater in the lower than upper limbs, but strength and its neural control have never been compared in the same population of individuals in both upper and lower limbs. The aim of this study was to investigate differences between eight young (20-31 years) and eight older (68-76 years) healthy women in maximal voluntary contraction (MVC), force fluctuations, median frequency (MDF) of the surface electromyogram (sEMG), and muscle fiber conduction velocity (MFCV) during sustained isometric elbow flexion (EF) and knee extension (KE), performed at moderate to high force intensity. Older women showed larger fluctuations of force with endurance and changes in sEMG pointing to less fatigue, especially at high level of force, with no differences between upper and lower extremities. This may have significant implications in the design of rehabilitation programs directed to this population.     

MMG and EMG responses during fatiguing isokinetic muscle contractions at different velocities

The purpose of this study was to determine mechanomyographic (MMG) and electromyographic (EMG) responses of the superficial quadriceps muscles during repeated isokinetic contractions in order to provide information about motor control strategies during such activity, and to assess uniformity in mechanical activity (MMG) between the investigated muscles. Ten adults performed 50 maximal concentric muscle contractions at three randomly selected contraction velocities (60, 180, and 300 degrees.s(-1)) on different days. Surface electrodes and an MMG sensor were placed on the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). EMG and MMG amplitude and peak torque (PT) were calculated for each contraction, normalized, and averaged across all subjects. The results demonstrated that MMG amplitude more closely tracked the fatigue-induced decline in torque production at each velocity than did EMG amplitude. This indicates that MMG amplitude may be useful for estimating force production during fatiguing dynamic contractions when a direct measure is not available, such as during certain rehabilitative exercises. MMG amplitude responses of the VL, RF, and VM were not uniform for each velocity or across velocities, indicating that it may be possible to detect the individual contribution of each muscle to force production during repeated dynamic contractions. Therefore, MMG amplitude may be clinically useful for detecting abnormal force contributions of individual muscles during dynamic contractions, and determining whether various treatments are successful at correcting such abnormalities.     

Grip strength is a representative measure of muscle weakness in the upper extremity after stroke

Background: Muscle weakness is the most common impairment in the upper extremity after stroke, leading to a reduced ability to use the arm and the hand in daily activities. Grip strength is easier to measure than precise, but more time-consuming, isokinetic and isometric arm muscle strength measurements. It would therefore be advantageous in a clinical setting if grip strength could be used as a proxy for muscle strength in the entire upper extremity.

Objective: To investigate the association between grip strength and isometric and isokinetic arm muscle strength in persons with chronic stroke.

Methods: Forty-five persons with mild-to-moderate paresis in the upper extremity, at least 6 months post-stroke participated. Isometric grip strength was measured with a computerized grip dynamometer and arm strength (isometric shoulder abduction and elbow flexion as well as isokinetic elbow extension and flexion) with an isokinetic dynamometer. Pearson’s correlation coefficient was used to determine the association between the muscle strength measurements.

Results: There were significant correlations (p < .0001) between grip strength and all arm strength measurements in both the more affected (r = 0.77–0.82) and the less affected upper extremity (r = 0.65–0.82).

Conclusion: This cross-sectional study showed that grip strength is strongly associated with muscle strength in the arm in persons in the chronic phase after stroke. As grip strength is easy to measure and less time-consuming than arm muscle strength measurements, this implies that grip strength can be a representative measure of muscle weakness of the entire upper extremity in the chronic phase after stroke.     

Effect of knee position on quadriceps muscle force steadiness and activation strategies

Introduction: In this study we investigated the effect of knee position on quadriceps force steadiness and activation strategies. Methods: Quadriceps force steadiness was evaluated in 22 volunteers at two knee positions by testing their ability to regulate submaximal force. Muscle activation strategies were studied in both time and frequency domains using surface electromyography. Results: Quadriceps force fluctuations and the associated agonist and antagonist activity were significantly higher at 90° than at 30° of flexion (P < 0.05). The quadriceps median frequency recorded at 30° was significantly higher than at 90° of flexion (P < 0.05). Regression analyses revealed that force steadiness was related to quadriceps activation and median frequency (P < 0.001), but not to hamstring coactivation (P > 0.05). Conclusions: The results indicate that knee position significantly affects quadriceps force steadiness and activation strategies. This finding may have important implications for designing a force control testing protocol and interpreting test results. Muscle Nerve, 2011     

Wavelet analysis of quadriceps power spectra and amplitude under varying levels of contraction intensity and velocity

Introduction: We investigated the effect of contraction intensity [100%, 75%, 50%, and 25% maximum voluntary contraction (MVC)] and movement velocity (50°, 100°, 200°, and 400°/s) on surface electromyography root mean square amplitude (SEMG_RMS) and median frequency (SEMG_MDF) of rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM). Methods: SEMGs during knee extension were resolved into their respective frequencies using wavelet transformations. Results: RF, VL, and VM muscles displayed increased SEMG_MDF as contraction intensity increased from 25% to 50% MVC and from 75% to 100% MVC, and each muscle displayed its own unique frequency shifting patterns. The SEMG_MDF was not influenced by movement velocity. SEMG_RMS increased in all 3 muscles as contraction intensity increased and was influenced by movement velocity, with the highest values observed at 400° and 200°/s. Conclusions: We infer that increasing contraction intensity facilitates greater recruitment of fast‐twitch muscle fibers, but there are differing responses in RF, VL, and VM muscles. Muscle Nerve 50 : 844–853, 2014     

女子曲棍球运动员膝关节屈伸肌群等速测试与分析

背景：等速测试系统已广泛应用来评价不同项目运动员关节肌群的力量特征。 目的：测试广东省女子曲棍球运动员的膝关节进行等速向心,观察曲棍球项目运动员膝关节屈伸肌群的等速向心测试力量特征。 方法：运用CON-TREX等速测试系统对广东省14名现役女子曲棍球运动员的膝关节进行等速向心测试。检测指标为相对峰力矩、关节肌群屈伸肌峰力矩比值、平均功率、总功和疲劳指数。 结果与结论：在60 (°/s)慢速测试时,女子曲棍球队员的两侧膝关节肌群相对峰力矩值表现为伸肌大于屈肌(P < 0.01);左膝关节屈伸肌峰力矩比值在3种不同测试速度下的范围为66%~77%,右膝关节屈伸肌峰力矩比值为82%~88%;左膝关节伸肌平均功率值均大于屈肌(P < 0.01);左右膝关节伸肌的总功值均大于屈肌(P < 0.01);膝关节屈伸肌疲劳指数范围在0.24~0.48之间。膝关节伸肌的最大力量大于屈肌,屈伸肌峰力矩比值在240 (°/s)快速测试时略微偏低,而右膝关节屈伸肌峰力矩比值在慢速测试时偏高,说明女子曲棍球队员左膝关节屈肌快速力量偏低,右膝关节伸肌最大力量比较低,伸肌的快速力量大于屈肌,伸肌的工作能力强于屈肌,屈伸肌的力量耐力水平比较低。     

Anterior cruciate ligament tear induces a sustained loss of muscle fiber force production

Introduction: Patients with anterior cruciate ligament (ACL) tears have persistent quadriceps strength deficits that are thought to be due to altered neurophysiological function. Our goal was to determine the changes in muscle fiber contractility independent of the ability of motor neurons to activate fibers. Methods: We obtained quadriceps biopsies of patients undergoing ACL reconstruction, and additional biopsies 1, 2, and 6 months after surgery. Muscles fiber contractility was assessed in vitro, along with whole muscle strength testing. Results: Compared with controls, patients had a 30% reduction in normalized muscle fiber force at the time of surgery. One month later, the force deficit was 41%, and at 6 months the deficit was 23%. Whole muscle strength testing demonstrated similar trends. Discussion: While neurophysiological dysfunction contributes to whole muscle weakness, there is also a reduction in the force generating capacity of individual muscle cells independent of alpha motor neuron activation. Muscle Nerve 58 : 145–148, 2018     

The effects of facioscapulohumeral dystrophy and dynamic arm support on upper extremity muscle coordination in functional tasks

This study's objective is to understand the effect of muscular weakness in persons with facioscapulohumeral dystrophy as well as the effect of a dynamic arm support on muscle coordination and activity performance, during activities of daily living. People with facioscapulohumeral dystrophy (n=12, 56.0±14.5 years) and healthy controls (n=12, 55.5±13.4 years) performed five simulated daily activity tasks, while unsupported and supported by the Gowing dynamic arm support. Surface electromyography, kinematics, and maximum force output were recorded. Outcomes were calculated for muscle coordination (muscle synergies), maximum muscle activity, movement performance indicators, and upper limb muscular weakness (maximum force output). Muscle coordination was altered and less consistent in persons with facioscapulohumeral dystrophy compared with healthy controls. The dynamic arm support alleviated muscle efforts and affected muscle coordination in both populations. While populations became more similar, the internal consistency of persons with facioscapulohumeral dystrophy remained unaffected and lower than that of healthy controls. Furthermore, the support affected movements’ performance in both groups. The maximum force outputs were lower in persons with facioscapulohumeral dystrophy than controls. Muscle coordination differences were presumably the result of individual-specific in muscle weakness and compensatory strategies for dealing with gravity compensation and movement constraints.     

Factors influencing the relation between corticospinal output and muscle force during voluntary contractions

The mechanisms by which voluntary forces of different strengths are produced in human muscles are not clear. We studied the relation between force and surface electromyography (sEMG) variables over a wide range of voluntary contraction strengths of biceps brachii (BIC) and abductor digiti minimi (ADM). The relation between force and motor evoked potentials (MEPs) to transcranial magnetic stimulation of motor cortex was also assessed. The root mean square of sEMG and median frequency (Mf) of the sEMG power spectrum as well as the MEP area of ADM and BIC were calculated up to the maximum voluntary contraction (MVC). The root mean square of ADM and BIC increased with increasing force levels up to the MVC. The Mf of BIC increased with force levels up to 70% MVC after which it rapidly declined. The Mf of ADM peaked at 40% MVC to slowly decline thereafter. The MEP changes with force were similar to Mf changes. Thus, corticospinal output, as tested by the Mf and MEPs, does not parallel force increments across the contraction range. This decline, which is contingent on the relative contribution of motor unit recruitment and rate coding to force production in each muscle, may depend on reduced motoneurone responsiveness at high firing rates. We suggest that, under controlled conditions, the frequency content of the sEMG signal may be taken to indicate motor unit recruitment range. This information may improve the utility of the Mf to enable evaluation of voluntary activation under different conditions.     

青年男女膝关节屈伸肌群达到峰值力矩时关节角度变化规律的等速肌力测试

背景：人体各关节灵活程度不同,但屈伸运动都是具备的,在日常活动当中,屈伸运动也是最为常见的,但关节屈伸时在何角度产生的力矩是最大的呢？ 目的：掌握膝关节屈伸时发挥最大肌力的关节角度,为运动健身、训练提供理论指导。 设计、时间及地点：对比观察,试验于2008-04/06在郑州大学体育学院实验室进行。 对象：自愿参加测试的大学生男女各30名。 方法：测试仪器使用澳大利亚产Kinitech等速肌力测试系统,按照Kinitech等速肌力测试系统手册要求对参试者膝关节进行固定并测试,3种测试速度为慢速60 (°)/s、中速120 (°)/s和快速240 (°)/s。 主要观察指标：膝关节屈伸肌群的峰值力矩、达到峰值力矩时关节角度。 结果：在速度为60 (°)/s时,男女左屈肌小于右屈肌达到峰值力矩的关节角度,且左伸肌小于右伸肌;男子右屈肌达到峰值力矩的关节角度大于左屈肌,且女子与男子相同。在速度为120(°)/s时,男子屈伸肌达到峰值力矩的关节角度差异不显著,女子右伸肌达到峰值力矩的关节角度大于左伸肌。在速度为240 (°)/s时,男子左屈肌达到峰值力矩的关节角度大于右屈肌,而左伸肌和右伸肌差异不存在显著性。女子左右屈伸肌差异也不存在显著性。 结论：自然发育下人体存在优势腿,在不同速度下达到峰值力矩的关节角度有所不同,在日常运动健身时尽可能的全面训练,使屈伸肌群得到全面发展。 关键词：等速;膝关节;峰值力矩;关节角度     

Variability and interrelationships of surface EMG parameters during local muscle fatigue

The inter-and intraindividual variability of the frequency power density spectral and surface EMG amplitude parameters and of the muscle fiber conduction velocity (MFCV) is studied in 26 healthy volunteers during fatiguing isometric ischemic intermittent exercise of the m, biceps brachii at 80% of the maximal voluntary contraction level, with a contraction rate of 30/min. No significant age effects were found. Males were significantly stronger compared with females. The higher initial SEMG amplitude and the strongershift of the frequency power density spectrum (PDS) to lower frequencies appear to be significantly correlated with males. Fatigue induces an almost proportional compression of the SEMG frequency content. The muscle fiber conduction velocity has the highest intraindividual reproducibility (r = 0.81). Despite the definite and strong influence of the MFCV on the PDS, the shift of the PDS can not be explained by a change of MFCV alone.© 1993 John Wiley & Sons, Inc.     

Use of hand-held dynamometry in the evaluation of lower limb muscle strength in people with Huntington’s disease

Purpose   Sub-clinical muscle involvement, including myopathic changes and mitochondrial dysfunction of skeletal muscle, has been reported in people with Huntington’s disease (HD). Muscle strength was evaluated using a hand-held dynamometer. Reliability and validity in people with HD were determined. Method   Isometric muscle strength of 6 lower limb muscle groups was measured in 20 people with HD and matched healthy controls. People with HD were evaluated with the Unified Huntington’s Disease Rating Scales (UHDRS). Within session reliability using intra-class correlation coefficients (ICC) was calculated. Discriminant and convergent validity was also evaluated. Results   UHDRS motor scores of people with HD ranged from 28 to 80. Reliability of strength testing was excellent (ICC 0.86 to 0.98). People with HD had on average about half the strength of healthy matched controls. UHDRS motor scores and strength scores were significantly correlated (convergent) providing a further indication of validity of strength testing. Conclusions   The hand-held dynamometer is a reliable and valid measurement tool to detect strength differences between people with HD and a matched control group. There is significant reduction in lower limb muscle strength in HD which does not appear to have been described previously.     

Functional significance of upper and lower motor neuron impairment in amyotrophic lateral sclerosis

The objective of this study was to examine the contribution of lower motor neuron (LMN) and upper motor neuron (UMN) dysfunction to weakness and impaired motor control in 27 patients with amyotrophic lateral sclerosis (ALS). Isometric strength was measured by dorsiflexor maximum voluntary contraction force (MVC). LMN function was measured by tetanic force, twitch force, and the amplitude of the compound muscle action potential. UMN function was measured by the speed of rapid foot taps, the maximum rate of rise of voluntary force, and the central activation ratio [CAR = MVC/(MVC + superimposed tetanic force)]. The results suggest that (1) LMN loss appears to be the primary cause of progressive weakness in ALS; while (2) UMN impairment in ALS leads to slowing of contraction speed and rapid movements, and modest decreases in central activation; and (3) during 6 months of progression, LMN changes were greater than UMN changes. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:762–768, 1998.     

Combined electromyography-31P-magnetic resonance spectroscopy study of human muscle fatigue during static contraction

Metabolic changes measured by ³¹P-magnetic resonance spectroscopy and surface electromyograms were simultaneously recorded during isometric contraction of forearm flexor muscles sustained at 60% of maximal force until exhaustion. Throughout the fatigue trial, energy in the low-frequency (L) band continuously increased whereas energy in the high-frequency (H) band first increased and then fell only prior to exhaustion. PCr content decreased linearly. Intracellular pH (pHi) transiently increased during the first 22 s of trial. The triggering of acidosis was associated with critical PCr values (35–70% of initial content) and decreased electromyogram (EMG) energy in the H band. Linear relationships were only found between energy in the L band, pHi, and PCr content. The interindividual variability of metabolic and EMG changes was high despite standardized conditions of contraction. Maximal PCr consumption was correlated with the maximal pHi decrease measured at the end of the trial. Overall, there was no correlation between H/L EMG ratio and changes in muscle metabolism. © 1996 John Wiley & Sons, Inc.     

Pathological and physiological muscle co-activation during active elbow extension in children with unilateral cerebral palsy

Objective

To address the roles and mechanisms of co-activation in two flexor/extensor pairs during elbow extension in children with cerebral palsy (CP).

眼轮匝肌肌电信号在不同运动模式下的定量特征信息

Objective: To research function of rbicularis oculi muscle (OOM) and explore the relationship between the electromyography (EMG) signals and the functional actions of the muscle via analyzing the EMG signals features of OOM of rabbit in different movement states. Methods: The EMG signals were extracted from OOM of normal rabbits by the implanted microelectrodes. Then the features of these EMG signals were analyzed in the time domain and the frequency domain. Results: The average peak values of the absolute EMG amplitude for natural continuous eye-opening event, natural continuous eye-closing state, natural eye-blinking movement and evoked eye-closing state were 29.3μV, 34.7μV, 435.2μV and 737.3μV respectively. The peak frequency values of the EMG power spectrum for the four modes of OOM were 99Hz, 143Hz, 193Hz and 374Hz respectively. The electrical activities of OOM during the natural eye-opening event and the natural continuous eye-closing state were stable and displayed low amplitudes. During the spontaneous blink state and the evoked eye-closing state, the EMG amplitudes markedly increased, and the increased level in the latter state was stronger than that in the former state. When rabbits continuously closed eye or opened eye, all of the peak values of the absolute voltage amplitude were less than 50μV.The absolute amplitude values of the starting site were between 50μV and 60μV during the spontaneous blink and the evoked eye-closing movements. The whole frequency band of the energy of the power spectrum density (PSD) of OOM was between 0Hz and 500Hz, and the primary frequency range was between 20Hz and 350Hz.In the time domain, the difference was not significant for the EMG signals of OOM between the continuous eye-opening state and the continuous eye-closing movement, but there were statistically significant in the other states for their pairwise comparisons. In the frequency domain, there was no significant difference for the peak frequency of PSD about the EMG signals when compared the continuous eye-opening state with the continuous eye-closing event or compared the continuous eye-closing state with the spontaneous blink movement. When compared that in the other movements with each other, however, the differences were significant. Conclusions: OOM relaxes when eyelid keeps continuously opening. The action of eyelid-closing is due to contraction of this muscle. Each state has its own features of the EMG signals for OOM, these features can be as foundation for computer to judge and identify movement states of the muscle.