Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.

Methods. Twelve subjects (age 25.1±3.7 years, mass 70.1±8.2 kg, mean±SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.

Results. There were no significant differences between ECC, CON and ISO peak torque output (211±63 vs 169±41 vs 177±61 Nm; ECC, CON, ISO) and IEMG activity (280±143 vs 305±146 vs 287±143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31±13% of initial values, similar to the final torque values in CON (58±15%), but significantly less than ECC (108.6±38.6%; P<0.001) values. In CON and ECC, IEMG was maintained (95±27% and 93±21%; CON and ECC), whereas IEMG for ISO decreased to 38±13% of initial values. The greatest reduction in MPFS occurred in CON (69±10%) compared to ISO (78±9%; P<0.05) and ECC (93±6%; P<0.001).

Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.     

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions.

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.     

Reliability of surface electromyography timing parameters in gait in cervical spondylotic myelopathy

The aims of this study were to validate a computerised method to detect muscle activity from surface electromyography (SEMG) signals in gait in patients with cervical spondylotic myelopathy (CSM), and to evaluate the test–retest reliability of the activation times designated by this method. SEMG signals were recorded from rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and medial gastrocnemius (MG), during gait in 12 participants with CSM on two separate test days. Four computerised activity detection methods, based on the Teager–Kaiser Energy Operator (TKEO), were applied to a subset of signals and compared to visual interpretation of muscle activation. The most accurate method was then applied to all signals for evaluation of test–retest reliability. A detection method based on a combined slope and amplitude threshold showed the highest agreement (87.5%) with visual interpretation. With respect to reliability, the standard error of measurement (SEM) of the timing of RF, TA and MG between test days was 5.5% stride duration or less, while the SEM of BF was 9.4%. The timing parameters of RF, TA and MG designated by this method were considered sufficiently reliable for use in clinical practice, however the reliability of BF was questionable.     

Comparison of electromyographic activity during eccentrically versus concentrically loaded isometric contractions

Electromyographic (EMG) amplitude and mechanical tension are directly related during isometric contraction. Maximal voluntary isometric contractions are typically elicited through two different procedures; resisting a load, which is eccentric in nature, and contracting against an immovable object, which is concentric in nature. A wealth of literature exists indicating that EMG amplitude during concentric contractions is greater than that of eccentric contractions of the same magnitude. However, the effects of different methods to elicit isometric contraction on EMG amplitude have yet to be investigated. The purpose of this study was to compare EMG amplitudes under different loading configurations designed to elicit isometric muscle contraction. Twenty healthy volunteers (10 males and 10 females, age = 23 ± 2 yrs, height = 1.7 ± 0.09 m, mass = 69.9 + 16.8 kg) performed a maximal voluntary plantarflexion effort for which the vertical ground reaction force (GRFv) sampled from a force plate and surface EMG of the soleus were recorded. Participants then performed isometric plantarflexion at 20%, 30%, 40%, and 50% GRFv_max in a seated position, from a neutral ankle position, under two different counterbalanced isometric loading conditions (concentric and eccentric). For concentric loading conditions, the subject contracted against an immovable resistance to the specified %GRFv identified via visual and auditory feedback. For eccentric loading conditions, subjects contracted against an applied load placed on the distal anterior thigh that produced the specified %GRFv. This applied load had the tendency to force the ankle into dorsiflexion. Therefore, plantarflexion force, in an attempt to maintain the ankle in a neutral position, resisted lengthening of the plantarflexor musculature, thus representing eccentric loading during an isometric contraction. Mean EMG amplitude was compared across loading levels and types using a 2 (loading type: concentric, eccentric) × 4 (loading level: 20%, 30%, 40%, 50% GRFv) repeated-measures ANOVA. The main effect for loading level was significant (p = 0.007). However, the main effect for loading type, and the loading type × loading level interaction were non-significant (p > 0.05). The present findings provide evidence that isometric muscle contractions loaded in either concentric or eccentric manners elicit similar EMG amplitudes, and are therefore comparable in research settings.     

Variability of the EMG mean power frequency: A study on the trapezius muscle

Calculation of the EMG mean power frequency (MPF) is a common procedure applied in evaluation of the frequency shift associated with local muscle fatigue. Variations of the MPF that are unrelated to muscle fatigue may jeopardize the estimation of the frequency shift. Different kinds of variation include random variation and systematic variation due to changes in posture or load. In a previous article we have evaluated the systematic linear variation of the MPF. The aim of the present study was to examine the random variation. Data sequences of 10 s, each obtained from nonfatigued trapezius muscle of 19 healthy subjects, were examined over a functional range of load and joint angles with multiple regression analysis. The random variation was evaluated with residual analysis. The residual standard deviation within the whole group was 10% for surface recordings and 13% for intramuscular recordings. If only within-subject variation was considered, the corresponding values were 5 and 8%. Based on this, confidence and prediction intervals for the regression models were calculated. Ninety-five percent confidence intervals were ±1–3% around the regression surfaces, whereas 95% prediction intervals for single measurements were as large as ±20–26% for the whole group, and ±11–20% if only within-subject variations were considered. Assessment of localized muscle fatigue using single MPF estimates should therefore be avoided. Multiple measurements and regression analysis are discussed as methods to minimize the effects of random variations.     

Electromechanical delay in human skeletal muscle under concentric and eccentric contractions

In contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The right forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of approximately equal to 0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface EMG (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p less than 0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development.     

Doublet potentiation during eccentric and concentric contractions of cat soleus muscle

Sandercock, Thomas G., and C. J. Heckman. Doubletpotentiation during eccentric and concentric contractions of cat soleusmuscle. J. Appl. Physiol. 82(4):1219-1228, 1997.The addition of an extra stimulus pulse, ordoublet, at the beginning of a low-frequency train has been shown tosubstantially increase isometric force. This study examined the effectsof muscle movement on this doublet potentiation. The soleus muscles ofanesthetized cats were stimulated at 10 Hz for 1 s, with and without anadded doublet (0.01-s interval). Isovelocity releases reduced but didnot eliminate peak and early doublet potentiation (average 0.0-0.5s after the doublet). Large releases, >0.4 s after the doublet,completely abolished sustained doublet potentiation (average0.5-1.0 s after the doublet). In contrast, early isovelocitystretches boosted peak doublet potentiation. Yet, large stretches laterin the stimulus almost completely eliminated sustained doubletpotentiation. This suggests that a different mechanism is responsiblefor early and sustained doublet potentiations. Because peak and averageinitial doublet potentiation were not strongly affected by movement,doublets still offer a viable control strategy to increase force during movement while minimizing the number of stimulus pulses.

     

The effect of a novel gait retraining device on lower limb kinematics and muscle activation in healthy adults

The Re-Link Trainer (RLT) is a modified walking frame with a linkage system designed to apply a non-individualized kinematic constraint to normalize gait trajectory of the left limb. The premise behind the RLT is that a user’s lower limb is constrained into a physiologically normal gait pattern, ideally generating symmetry across gait cycle parameters and kinematics. This pilot study investigated adaptations in the natural gait pattern of healthy adults when using the RLT compared to normal overground walking. Bilateral lower limb kinematic and electromyography data were collected while participants walked overground at a self-selected speed, followed by walking in the RLT. A series of 2-way analyses of variance examined between-limb and between-condition differences. Peak hip extension and knee flexion were reduced bilaterally when walking in the RLT. Left peak hip extension occurred earlier in the gait cycle when using the RLT, but later for the right limb. Peak hip flexion was significantly increased and occurred earlier for the constrained limb, while peak plantarflexion was significantly reduced. Peak knee flexion and plantarflexion in the right limb occurred later when using the RLT. Significant bilateral reductions in peak electromyography amplitude were evident when walking in the RLT, along with a significant shift in when peak muscle activity was occurring. These findings suggest that the RLT does impose a significant constraint, but generates asymmetries in lower limb kinematics and muscle activity patterns. The large interindividual variation suggests users may utilize differing motor strategies to adapt their gait pattern to the imposed constraint.     

Comparing electro- and mechano-myographic muscle activation patterns in self-paced pediatric gait

Electromyography (EMG) is the standard modality for measuring muscle activity. However, the convenience and availability of low-cost accelerometer-based wearables makes mechanomyography (MMG) an increasingly attractive alternative modality for clinical applications. Literature to date has demonstrated a strong association between EMG and MMG temporal alignment in isometric and isokinetic contractions. However, the EMG-MMG relationship has not been studied in gait. In this study, the concurrence of EMG- and MMG-detected contractions in the tibialis anterior, lateral gastrocnemius, vastus lateralis, and biceps femoris muscles were investigated in children during self-paced gait. Furthermore, the distribution of signal power over the gait cycle was statistically compared between EMG-MMG modalities. With EMG as the reference, muscular contractions were detected based on MMG with balanced accuracies between 88 and 94% for all muscles except the gastrocnemius. MMG signal power differed from that of EMG during certain phases of the gait cycle in all muscles except the biceps femoris. These timing and power distribution differences between the two modalities may in part be related to muscle fascicle length changes that are unique to muscle motion during gait. Our findings suggest that the relationship between EMG and MMG appears to be more complex during gait than in isometric and isokinetic contractions.     

Unique neuromuscular activation of the rectus femoris during concentric and eccentric cycling

This study investigated neuromuscular activations of thigh muscles during concentric cycling (CON_cycling) and eccentric cycling (ECC_cycling). Eleven untrained men completed 30 s of CON_cycling and ECC_cycling each at 5 power outputs of 100–300 W (every 50-W interval). During cycling, root mean square of surface electromyographic signals (RMS-EMG) were obtained from the proximal and distal regions of the rectus femoris (RFp and RFd), vastus lateralis (VL), and biceps femoris (BF). The rating of perceived exertion (RPE) was evaluated using the 6–20 Borg Scale. The RMS-EMG of VL and BF were 21.6%–67.6% higher (P < 0.05) during CON_cycling than ECC_cycling at all power outputs, while those of RFp and RFd at 100–200 W were 29.6%–40.4% lower during CON_cycling than ECC_cycling. The RPE was similar between CON_cycling at 150 W (10 ± 2) and ECC at 250 W (10 ± 2). There were no significant differences in the RMS-EMG for VL or BF between CON_cycling at 150 W and ECC_cycling at 250 W; however, the RF RMS-EMG was greater during ECC_cycling as compared with CON_cycling. There were no regional differences in RF activations. These results demonstrated the unique neuromuscular activation of RF as compared to those of other thigh muscles during CON_cycling and ECC_cycling.     

The reliability of electromechanical delay and torque during isometric and concentric isokinetic contractions

The purpose of this investigation was to examine the intra-subject reliability of electromechanical delay (EMD) and torque of the dominant and non-dominant elbow flexors during isometric and isokinetic muscle contractions repeated over five consecutive days. Eleven volunteers that were unfamiliar with isokinetic dynamometry participated in this study and were asked to attend the laboratory on five consecutive days. An isokinetic dynamometer was used to exercise the elbow flexors under isometric, slow (60° s⁻¹) and fast (210° s⁻¹) isokinetic conditions; surface electromyography was recorded from the belly of biceps brachii and the signal was synchronised with the dynamometer to determine EMD. Intra-subject reliability for all measures was good (CV range, 3.1–6.5%) with no discernable difference between the dominant and non-dominant arms during isometric and isokinetic conditions. In addition, there was little difference in EMD and torque variability between the dominant and non-dominant arms which may have applications for clinicians and future research design when monitoring and investigating human muscle function. These data provide researchers and clinicians with an indication of the magnitude of change that is required to elucidate the presence of a meaningful change to muscle function in the elbow flexors.     

Comparison of surface electromyography and myotonometric measurements during voluntary isometric contractions

Objectives: Muscle stiffness increases during muscle contraction. The purpose of this study was to determine the strength of the correlation between myotonometric measurements of muscle stiffness and surface electromyography (sEMG) measurements during various levels of voluntary isometric contractions of the biceps brachii muscle. Subjects: Eight subjects (four female; four male), with mean age of 30.6±8.23 years, volunteered to participate in this study. Methods: Myotonometer and sEMG measurements were taken simultaneously from the right biceps brachii muscle. Data were obtained: (1) at rest, (2) while the subject held a 15 lb (6.8 kg) weight isometrically and, (3) during a maximal voluntary isometric contraction. Myotonometer force–displacement curves (amount of tissue displacement to a given unit of force applied perpendicular to the muscle) were compared with sEMG measurements using Pearson’s product–moment correlation coefficients. Results: Myotonometer and sEMG measurement correlations ranged from −0.70 to −0.90. The strongest correlations to sEMG were from Myotonometer force measurements between 1.00 and 2.00 kg. Conclusions: Myotonometer and sEMG measurements were highly correlated. Tissue stiffness, as measured by the Myotonometer, appears capable of assessing changes in muscle activation levels.     

Combining muscle synergies and biomechanical analysis to assess gait in stroke patients

The understanding of biomechanical deficits and impaired neural control of gait after stroke is crucial to prescribe effective customized treatments aimed at improving walking function. Instrumented gait analysis has been increasingly integrated into the clinical practice to enhance precision and inter-rater reliability for the assessment of pathological gait. On the other hand, the analysis of muscle synergies has gained relevance as a novel tool to describe the neural control of walking. Since muscle synergies and gait analysis capture different but equally important aspects of walking, we hypothesized that their combination can improve the current clinical tools for the assessment of walking performance.To test this hypothesis, we performed a complete bilateral, lower limb biomechanical and muscle synergies analysis on nine poststroke hemiparetic patients during overground walking. Using stepwise multiple regression, we identified a number of kinematic, kinetic, spatiotemporal and synergy-related features from the paretic and non-paretic side that, combined together, allow to predict impaired walking function better than the Fugl-Meyer Assessment score. These variables were time of peak knee flexion, VAF_total values, duration of stance phase, peak of paretic propulsion and range of hip flexion. Since these five variables describe important biomechanical and neural control features underlying walking deficits poststroke, they may be feasible to drive customized rehabilitation therapies aimed to improve walking function.This paper demonstrates the feasibility of combining biomechanical and neural-related measures to assess locomotion performance in neurologically injured individuals.     

Predicting muscle forces in gait from EMG signals and musculotendon kinematics

An EMG-driven muscle model for determining muscle force-time histories during gait is presented. The model, based on Hill's equation (1938), incorporates morphological data and accounts for changes in musculotendon length, velocity, and the level of muscle excitation for both concentric and eccentric contractions. Musculotendon kinematics were calculated using three-dimensional cinematography with a model of the musculoskeletal system. Muscle force-length-EMG relations were established from slow isokinetic calibrations. Walking muscle force-time histories were determined for two subjects. Joint moments calculated from the predicted muscle forces were compared with moments calculated using a linked segment, inverse dynamics approach. Moment curve correlations ranged from r = 0.72 to R = 0.97 and the root mean square (RMS) differences were from 10 to 20 Nm. Expressed as a relative RMS, the moment differences ranged from a low of 23% at the ankle to a high of 72% at the hip. No single reason for the differences between the two moment curves could be identified. Possible explanations discussed include the linear EMG-to-force assumption and how well the EMG-to-force calibration represented excitation for the whole muscle during gait, assumptions incorporated in the muscle modeling procedure, and errors inherent in validating joint moments predicted from the model to moments calculated using linked segment, inverse dynamics. The closeness with which the joint moment curves matched in the present study supports using the modeling approach proposed to determine muscle forces in gait.     

Normalizing upper trapezius EMG amplitude: Comparison of ramp and constant force procedures

The present study compared three procedures for normalization of upper trapezius surface electromyographic (EMG) amplitudes: (a) a ramp procedure (providing data in per cent of maximal voluntary contraction, MVC); (b) a constant force procedure based on two reference contractions (two-force procedure) (%MVC) and (c) a procedure expressing muscle activation in per cent of a reference voluntary electrical activity (%RVE). The study also evaluated the repeatability of the ramp and the RVE procedures and estimated the force exertion (%MVC) corresponding to the RVE. To illustrate the ergonomic effect of different normalization procedures, trapezius EMG during two work tasks was compared after normalization by the two-force and the RVE procedures. Fifteen subjects participated in the whole study. We found that force estimates obtained by the ramp procedure equation could be translated to force estimates obtained by the two-force procedure by the equation: %MVC_2force = − 0.6 + 0.9*%MVC_ramp, although with a considerable imprecision due to large inter-individual differences. In the ramp procedure, the intra-individual test-retest coefficient of variation (CV) depended on the force level; it was 45% at 5% MVC and 10% at 30% MVC. The CV of the RVE was 15%. The reference contraction used in the RVE procedure corresponded from 13–79% MVC (median 33%MVC). The load reducing effect of an ergonomic intervention was less obvious with the RVE procedure than with the two-force procedure due to a larger inter-individual variation. The advantages and disadvantages of the different procedures are discussed.     

Essential role of TRPV2 ion channel in the sensitivity of dystrophic muscle to eccentric contractions

Duchenne myopathy is a lethal disease due to the absence of dystrophin, a cytoskeletal protein. Muscles from dystrophin-deficient mice (mdx) typically present an exaggerated susceptibility to eccentric work characterized by an important force drop and an increased membrane permeability consecutive to repeated lengthening contractions. The present study shows that mdx muscles are largely protected from eccentric work-induced damage by overexpressing a dominant negative mutant of TRPV2 ion channel. This observation points out the role of TRPV2 channel in the physiopathology of Duchenne muscular dystrophy.     

The effects of high heeled shoes on female gait: A review

Walking is the most common form of human locomotion. From a motor control perspective, human bipedalism makes the task of walking extremely complex. For parts of the step cycle, there is only one foot on the ground, so both balance and propulsion are required in order for the movement to proceed smoothly. One condition known to compound the difficulty of walking is the use of high heeled shoes, which alter the natural position of the foot–ankle complex, and thereby produce a chain reaction of (mostly negative) effects that travels up the lower limb at least as far as the spine. This review summarises recent studies that have examined acute and chronic effects of high heels on balance and locomotion in young, otherwise healthy women. Controversial issues, common study limitations and directions for future research are also addressed in detail.     

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.