Reliability and sources of variability of 3D kinematics and electromyography measurements to assess newly-acquired gait in toddlers with typical development and unilateral cerebral palsy

The aim was to 1) determine intersession and intertrial reliability and 2) assess three sources of variability (intersubject, intersession and intertrial) of lower limb kinematic and electromyographic (EMG) variables during gait in toddlers with typical development (TD) and unilateral cerebral palsy (UCP) (age <3 years, independent walking experience ≤6 months). Gait kinematics and surface EMG were recorded in 30 toddlers (19 TD and 11 UCP), during two, 3D-motion capture sessions. Standard error of measurement (SEM) between trials (gait cycles) of the same session and between sessions was calculated to assess reliability. Standard deviations (SD) between subjects, sessions and trials were calculated to estimate sources of variability. Sixty-four percent of kinematic SEM-values were acceptable (2°-5°). Frontal plane measurements were most reliable (SEM 2°-4.6°). In toddlers with UCP, EMG variables were most reliable for affected side, distal muscles. Intrinsic (intertrial and intersubject) variability was high, reflecting both motor immaturity and the high variability of toddler gait patterns. In toddlers with UCP, variability was amplified by motor impairment and delayed motor development. 3D gait analysis and surface EMG are partially reliable tools to study individual gait patterns in toddlers in clinical practice and research, although some variables must be interpreted with caution.     

Altered patterns of muscle activation during performance of four functional tasks in patients with shoulder disorders: interpretation from voluntary response index.

Altered motor control of the shoulder muscles during performance of a specific motor task in patients with shoulder disorders (SDs) has been an interesting subject to researchers. This study compared shoulder muscle activation patterns by surface electromyography (sEMG), including the upper trapezius (UT), lower trapezius (LT), and serratus anterior (SA) muscles, during four functional tasks in 25 patients with SDs and controls. A voluntary response index (VRI) was calculated, including magnitude and similarity index (SI), to quantify sEMG patterns during four functional tasks. Responsiveness and clinically meaningful levels of discrimination between patients and control for EMG magnitude and SI were determined. An altered pattern of motor control during four functional tasks was evident in the patients, in which greater EMG amplitude and abnormal EMG patterns were found. For SI among four functional tasks, normal subjects ranged from 0.80 to 1.00 while patients ranged from 0.70 to 0.99. High probabilities (97%) of discrimination between patients and normal subjects were found by SI method during an overhead height task (patients: 0.85-0.96, normal subjects: 0.95-1.00). Our results also suggest that an individual can be estimated to be abnormal when lower SI values are observed during the four functional tasks.     

Effects of different movement directions on electromyography recorded from the shoulder muscles while passing the target positions

PurposeWe compared electromyography (EMG) recorded from the shoulder joint muscles in the same position for different movement directions.MethodsFifteen healthy subjects participated. They performed shoulder elevation from 0° to 120°, shoulder depression from 120° to 0°, shoulder horizontal adduction from ?15° to 105°, and shoulder horizontal abduction from 105° to ?15°. The target positions were 90° shoulder elevation in the 0°, 30°, 60°, and 90° planes (0°, 30°, 60°, and 90° positions). EMG signals were recorded from the supraspinatus (SSP) muscle by fine-wire electrodes. EMG signals from the infraspinatus (ISP), anterior deltoid, middle deltoid, and posterior deltoid muscles were recorded using active surface electrodes.ResultsDuring elevation and horizontal abduction, the SSP showed significantly higher activity than that shown during depression and during horizontal adduction in the 0°, 30°, and 60° positions. During elevation, the ISP showed significantly higher activity than during depression and during horizontal adduction in the 90° position. During horizontal abduction, the ISP showed significantly higher activity than during depression in the 90° position.ConclusionsWhen the movement tasks were performed in different movement directions at the same speed, each muscle showed characteristic activity.     

Test-Retest reliability of the internal shoulder rotator muscles' stretch reflex in healthy men

Until now the reproducibility of the short latency stretch reflex of the internal rotator muscles of the glenohumeral joint has not been identified. Twenty-three healthy male participants performed three sets of external shoulder rotation stretches with various pre-activation levels on two different dates of measurement to assess test–retest reliability. All stretches were applied with a dynamometer acceleration of 10⁴°/s² and a velocity of 150°/s. Electromyographical response was measured via surface EMG. Reflex latencies showed a pre-activation effect (ƞ² = 0,355). ICC ranged from 0,735 to 0,909 indicating an overall “good” relative reliability. SRD 95% lay between ±7,0 to ±12,3 ms. The reflex gain showed overall poor test–retest reproducibility. The chosen methodological approach presented a suitable test protocol for shoulder muscles stretch reflex latency evaluation. A proof-of-concept study to validate the presented methodical approach in shoulder involvement including subjects with clinically relevant conditions is recommended.     

Peculiarities of Activation of the Shoulder Belt and Shoulder Muscles in Generation of Different-Direction Isometric Efforts by the Forearm

We studied central motor commands, CMCs, coming to the muscles that flex and extend the shoulder and elbow joints in the course of generation of voluntary isometric efforts of different directions by the forearm; the efforts were initiated according to a visual signal. Amplitudes of EMGs recorded from the muscles of the shoulder belt and shoulder and subjected to full-wave rectification and low-frequency filtration were considered correlates of the CMC intensity. An effort of the preset direction was developed within the operational space of the horizontal plane with angles 30 deg in the shoulder joint (external angle with respect to the frontal plane) and 90 deg in the elbow joint. We plotted sector diagrams of the logarithmic coefficient of the intensity increment of EMGs of the above muscles for the entire set of directions of generated efforts with a 15- or 20-deg step. Orientations of the maxima of EMG activity of the given muscles were rather close to the directions of the maxima of the force moments generated by these muscles. In most cases, a shift of the direction by one gradation with respect to the EMG maximum in the respective muscle resulted in a significant decrease in the level of EMG activity. It is shown that preferential activation of the muscles agonistic with respect to the examined direction of the generated effort was, as a rule, accompanied by coactivation of the antagonist muscles. When “two-joint” isometric efforts are formed, realization of the socalled synergic muscle tasks (where prevailing contractions of the muscles of the same functional direction for both joints coincide, i.e., flexion-flexion or extension-extension) is organized in a simpler manner. The programs of “nonsynergic” contractions (flexion of one joint and extension of another one, or vice versa) are more complex. In different subjects, considerably dissimilar patterns of EMG activity in muscles influencing these joints could be observed.     

Recent progress in the diagnostic use of surface EMG for neurological diseases.

The different techniques to measure and analyze surface EMG are summarized with an emphasis on the clinician's point of view. The application of surface EMG in neurological disease is hampered by many inherent problems, especially the difficulties in extracting features of single motor units. However, the evolution of surface EMG from single bipolar recordings via a linear array of multiple electrodes to densely packed, multi-channel electrode arrays could in principle solve this problem. The added value of using multiple channels (up to 128) with an interelectrode distance of a few millimetres to obtain more spatial information is emphasized. At least for some muscles it is now possible to extract information from the surface EMG, conventionally thought to belong to the domain of needle EMG (for example the "electrical size" of motor units). The use of analysis techniques such as the estimation of muscle fiber conduction velocity has already proven to be of diagnostic value in several myopathies characterized by a disturbed membrane function and in metabolic myopathies with abnormal fatigue profiles. Future research should be directed at the development of analysis techniques enabling the extraction of more relevant motor unit variables from surface EMG signals.     

The comparison of trunk muscles EMG activation between subjects with and without chronic low back pain during flexion-extension and lateral bending tasks.

The purpose of the study was to compare the electromyographic (EMG) activity of the trunk muscles between normal subjects and chronic low back pain (CLBP) patients during standardized trunk movements. Thirty-three male subjects (18 normals, 15 suffering from non specific CLBP) aged between 35 and 45 yr participated. A biomechanical analysis involving the recording of EMG signals from 12 trunk muscles, the kinematics of trunk segments and the computation of L5/S1 moments was performed. The subjects performed flexion-extension and lateral bending (left and right) tasks (three complete cycles) with and without a 12 kg load. Between group comparisons were performed on the full cycle average pattern of all biomechanical variables for each task. The reliability of EMG variables was evaluated for 10 subjects (5 normals and 5 CLBP) who performed the tasks on three different days. The reliability of EMG amplitude values was generally excellent for agonist muscles but poor to moderate for antagonists. The EMG amplitude analysis revealed significant differences between groups for some muscles (left lumbar and thoracic erector spinae). The abnormal (asymmetric) EMG patterns detected among CLBP patients were not explained by postural asymmetries.     

Trapezius muscle rest time during standardised computer work--a comparison of female computer users with and without self-reported neck/shoulder complaints.

Work related musculoskeletal disorders (WMSDs) in the shoulder/neck area are a common and increasing problem among European computer workers, especially women. Long-term low-level workloads with low degree of muscle rest are a potential risk factor for developing WMSDs. The purpose of the present study of female computer users (age 45-65 years) in Denmark and Sweden was to investigate if subjects with self-reported neck/shoulder complaints (cases, N=35) show less trapezius muscle relative rest time (RRT) than controls (N=44) when performing standardised short-term computer work tasks in controlled laboratory conditions. Surface electromyography (EMG) signals were recorded bilaterally from the upper trapezius muscles during a type, edit, precision and colour word stress task. Besides RRT, 10th percentile RMS values were calculated. On the average, 15 of the cases and 18 of the controls showed analysable EMG files per task. For the colour word stress task, the results showed lower RRT values and higher 10th percentile RMS amplitude levels among cases compared to controls. No such signs could be found for the other tasks performed. The present results indicate an increased motor response to a psychological stressor among subjects with self-reported neck/shoulder complaints.     

Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue.

Intramuscular pressure (IMP) and electromyography (EMG) mirror muscle force in the nonfatigued muscle during static contractions. The present study explores whether the constant IMP-EMG relationship with increased force may be extended to dynamic contractions and to fatigued muscle. IMP and EMG were recorded from shoulder muscles in three sessions: 1). brief static arm abductions at angles from 0 to 90 degrees, with and without 1 kg in the hands; 2). dynamic arm abductions at angular velocities from 9 to 90 degrees /s, with and without 1 kg in the hands; and 3). prolonged static arm abduction at 30 degrees for 30 min followed by recovery. IMP and EMG increased in parallel with increasing shoulder torque during brief static tasks. During dynamic contractions, peak IMP and EMG increased to values higher than those during static contractions, and EMG, but not IMP, increased significantly with speed of abduction. In the nonfatigued supraspinatus muscle, a linear relationship was found between IMP and EMG; in contrast, during fatigue and recovery, significant timewise changes of the IMP-to-EMG ratio occurred. The results indicate that IMP should be included along with EMG when mechanical load sharing between muscles is evaluated during dynamic and fatiguing contractions.     

An application of pattern recognition for the comparison of trunk muscles EMG waveforms between subjects with and without chronic low back pain during flexion-extension and lateral bending tasks.

The purpose of the study was twofold: (1) to evaluate the reliability of distance measures computed from a principal component analysis (PCA) of electromyographic (EMG) waveforms of trunk muscles recorded during standardized trunk movements and (2) to evaluate their sensitivity to the load lifted, the trunk range of motion (ROM) and to low back status. Thirty-three male subjects (18 normals, 15 suffering from non specific CLBP) aged between 35-45 years participated. The EMG signals from 12 trunk muscles and the kinematics of trunk segments were recorded during 12 tasks. The subjects performed flexion-extension and lateral bending (left and right) tasks (three complete cycles) with and without a 12 kg load and at different trunk ROM (maximal or at defined submaximal angles). Distance measures locating each subject relative to a reference PCA model were computed for each muscle and task. The reliability of these distance measures was evaluated for 10 subjects (five normals and five CLBP) who performed two tasks on three different days. The reliability of distance measures was acceptable for agonist muscles only. The distance measures were sensitive to the load lifted and to the trunk ROM for different muscles and tasks but poorly sensitive to low back status. Several reasons that could explain the low sensitivity of these measures to low back status are discussed and potential solutions are proposed. A procedure based on a reliability analysis is proposed to select the number of principal components to include in the reference PCA model. It is expected that the refinement of the method used in this study could provide an effective clinical tool to assess EMG waveforms of trunk muscles during dynamic tasks.     

Reproducibility of eight lower limb muscles activity level in the course of an incremental pedaling exercise.

Despite the wide use of surface electromyography (EMG) recorded during dynamic exercises, the reproducibility of EMG variables has not been fully established in a course of a dynamic leg exercise. The aim of this study was to investigate the reproducibility of eight lower limb muscles activity level during a pedaling exercise performed until exhaustion. Eight male were tested on two days held three days apart. Surface EMG was recorded from vastus lateralis, rectus femoris (RF), vastus medialis, semimembranosus, biceps femoris, gastrocnemius lateral, gastrocnemius medianus and tibialis anterior during incremental exercise test. The root mean square, an index of global EMG activity, was averaged every five crank revolutions (corresponding to about 3 s at 85 rpm) throughout the tests. Despite inter-subjects variations, we showed a high reproducibility of the activity level of lower limb muscles during a progressive pedaling exercise performed until exhaustion. However, RF muscle seemed to be the less reproducible of the eight muscles investigated during incremental pedaling exercise. These results suggest that each subject adopt a personal muscle activation strategy in a course of an incremental cycling exercise but fatigue phenomenon can induce some variations in the most fatigable muscles (RF).     

Activation of the Shoulder Belt and Shoulder Muscles of Humans Related to Different Rates of Generation of Two-Joint Efforts by the Forearm

We studied coordination of central motor commands (CMCs) coming to the muscles that flex and extend the shoulder and elbow joints in the course of generation of voluntary isometric efforts of different directions by the forearm. Dependences of the characteristics of these commands on the direction of the effort and rate of its generation were analyzed. Amplitudes of rectified and averaged EMGs recorded from a number of shoulder belt and shoulder muscles were considered correlates of the CMC intensity. The development of the effort of a given direction and rate of rise was realized in the horizontal-plane operational space; the arm position corresponded to the 30 deg angle in the shoulder joint (external angle with respect to the frontal plane) and 90 deg angle in the elbow joint. We plotted sector diagrams of the relative changes in the level of dynamic and stationary phases of EMG activity of the studied muscles for the entire set of directions of the efforts generated with different rates of rise. In the course of formation of rapid two-joint isometric efforts, realization of nonsynergic motor tasks (extension of one joint and flexion of another one, and vice versa) required significant activation of muscles of different functional directions for both joints. Time organization of EMG activity of extensors and flexors of the shoulder and elbow joints related to the maximum and relatively rapid generation of the effort (rise time 0.12 to 0.13 and 0.25 sec, respectively) was rather complex and included dynamic and stationary phases. With these time parameters of generation of the efforts (both flexion and extension), the appearance at the stationary effort of 40 N was controlled based on coordinated interaction of dynamic phases of the activation of agonistic and antagonistic muscles. It is concluded that CMCs coming to extensors and flexors of both joints upon generation of rapid isometric efforts are rather similar in their parameters to those under conditions of realization of the forearm movements in the space in an isotonic mode.     

Reproducibility of surface EMG variables and peak torque during three sets of ten dynamic contractions.

The interpretation of the electromyogram (EMG) of dynamic contractions might be difficult because the movement per se introduces additional factors that could affect its characteristics. There is a lack of studies concerning the reproducibility of surface EMG registrations during dynamic contractions. The aim was to investigate the during-the-day reproducibility (using intra-class correlation; ICC) of the peak torque (PT) and the EMG variables (without removing the electrodes) of dynamic contractions. Ten healthy subjects performed three sets of 10 dynamic maximum right-knee extensions with a one-hour interval in between, using an isokinetic dynamometer and the PT was determined. EMG signals were recorded from the right vastus lateralis, rectus femoris and vastus medialis muscles using surface electrodes and the mean frequency of the power spectrum (MNF [Hz]) and the signal amplitude (RMS [microV]), were computed. The ability to relax in-between the maximum extensions was calculated as a ratio of the RMS during the passive flexion phase and the RMS during the active extension phase of each contraction cycle: the signal amplitude ratio (SAR). Both PT (ICC = 0.99) and RMS (ICC = 0.83-0.98) had good reproducibility. The reproducibility of MNF was good for all muscles when the mean of contraction nos.: 1-10 was used. Vastus lateralis had the highest ICC among the three muscles. The reproducibility of SAR was generally poor (ICC < 0.60). The present study showed good reproducibility for common EMG variables (MNF and RMS) obtained during maximum isokinetic contractions.     

Quadriceps and hamstring isokinetic strength and electromyographic activity measured at different ranges of motion: a reproducibility study.

Isokinetic strength measurements of the quadriceps and hamstring that are commonly conducted using a 90 degrees range of motion (RoM) may involve some risk to specific knee patient groups. Testing these muscles at a much shorter RoM may reduce the risk but in order to render this method clinically acceptable the reproducibility of the derived test findings has to be established. Therefore the main objective of this study was to assess the reproducibility of isokinetic peak torque and normalized EMG scores of these muscles based on 90 degrees (0-90 degrees flexion, LR) and three successive short RoMs: 0-30 degrees (SR1), 30-60 degrees (SR2) and 60-90 degrees (SR3). Eight healthy subjects were tested three times with a 2 week between-session interval. All tests were performed on the dominant limb and consisted of maximal concentric and eccentric exertions. The velocities applied were 90 degrees /s for LR and 30 degrees /s for each of the SRs. Findings indicated no between-session improvement in strength. Based on the coefficient of variation the measurement error for all isokinetic strength scores remained stable throughout the testing sessions ranging 0.6-13.9% with the absolute majority of instances less than 10%. The reproducibility of the EMG scores was poorer ranging 1.5-25% and 0.5-19% for the quadriceps and hamstring, respectively. It is concluded that testing of knee muscles at short (30 degrees ) RoMs does not compromise the reproducibility of the strength or EMG scores derived from the commonly used RoM of 90 degrees . However, whereas strength was reproducible to within the accepted clinical standards, the corresponding EMG scores were characterized by a wider error band.     

Reliability of telemetric electromyography and near-infrared spectroscopy during high-intensity resistance exercise

This study quantified the inter- and intra-test reliability of telemetric surface electromyography (EMG) and near infrared spectroscopy (NIRS) during resistance exercise. Twelve well-trained young men performed high-intensity back squat exercise (12 sets at 70–90% 1-repetition maximum) on two occasions, during which EMG and NIRS continuously monitored muscle activation and oxygenation of the thigh muscles. Intra-test reliability for EMG and NIRS variables was generally higher than inter-test reliability. EMG median frequency variables were generally more reliable than amplitude-based variables. The reliability of EMG measures was not related to the intensity or number of repetitions performed during the set. No notable differences were evident in the reliability of EMG between different agonist muscles. NIRS-derived measures of oxyhaemoglobin, deoxyhaemoglobin and tissue saturation index were generally more reliable during single-repetition sets than multiple-repetition sets at the same intensity. Tissue saturation index was the most reliable NIRS variable. Although the reliability of the EMG and NIRS measures varied across the exercise protocol, the precise causes of this variability are not yet understood. However, it is likely that biological variation during multi-joint isotonic resistance exercise may account for some of the variation in the observed results.     

Reproducibility of surface EMG variables in isometric sub-maximal contractions of jaw elevator muscles.

The aims of this study were: (1) to develop and assess reproducibility of a new method for measuring masticatory force in the intercuspal position; (2) to test the reproducibility of surface EMG signal amplitude and spectral variables in constant force contractions of jaw elevator muscles and its dependency on inter-electrode distance. The study was performed on the masseter and temporalis anterior muscles of both sides of nine healthy volunteers. An intraoral compressive-force sensor was used to measure maximal voluntary contraction forces in the intercuspal position and to provide a visual feedback on sub-maximal forces to the subject. Three experimental sessions were performed in three days. In each session, three isometric contractions at 80% of the maximal force were sustained by the subjects for 30s. The intra-class correlation coefficient (ICC) of the maximal force measure was 71.9%. ICC of average rectified value and mean power spectral frequency of the EMG signal increased with inter-electrode distance, with values larger than 70% with 30 mm inter-electrode distance. It was concluded that surface EMG variables measured in isometric contractions of the jaw elevator muscles with the proposed force recording system show good reproducibility for clinical applications when a 30 mm inter-electrode distance is considered.     

Reproducibility of a new signal processing technique to assess joint sway during standing

Postural control strategies can be investigated by kinematic analysis of joint movements. However, current research is focussing mainly on the analysis of centre of pressure excursion and lacks consensus on how to assess joint movement during postural control tasks. This study introduces a new signal processing technique to comprehensively quantify joint sway during standing and evaluates its reproducibility. Fifteen patients with non-specific low back pain and ten asymptomatic participants performed three repetitions of a 60-second standing task on foam surface. This procedure was repeated on a second day. Lumbar spine movement was recorded using an inertial measurement system. The signal was temporally divided into six sections. Two outcome variables (mean absolute sway and sways per second) were calculated for each section. The reproducibility of single and averaged measurements was quantified with linear mixed-effects models and the generalizability theory. A single measurement of ten seconds duration revealed reliability coefficients of .75 for mean absolute sway and .76 for sways per second. Averaging a measurement of 40 seconds duration on two different days revealed reliability coefficients higher than .90 for both outcome variables. The outcome variables’ reliability compares favourably to previously published results using different signal processing techniques or centre of pressure excursion. The introduced signal processing technique with two outcome variables to quantify joint sway during standing proved to be a highly reliable method. Since different populations, tasks or measurement tools could influence reproducibility, further investigation in other settings is still necessary. Nevertheless, the presented method has been shown to be highly promising.     

Long-term electromyographic activity in upper trapezius and low back muscles of women with moderate physical activity.

The habitual activity patterns of trapezius and postural back muscles (multifidus, iliocostalis, longissimus) of 23 female subjects with moderate physical activity were studied. Bilateral surface electromyographic (sEMG) recordings from start of work until bedtime were analyzed. The activity level was calibrated as percentage of root mean square-detected muscle activity at maximal voluntary contraction (EMG(max)). Sixty-six previous trapezius recordings of women with moderate physical activity were included in some analyses to pursue the full range of variation in trapezius activity. Twenty-six of these were recorded twice, separated by 16-28 mo. Median activity level and duration of periods with sEMG activity of <0.5% EMG(max) ("rest time"; only trapezius) and exceeding 2 ("burst time"), 10, 30, and 50% EMG(max) was determined. The trapezius median activity level ranged from 0.6 to 8.8% EMG(max), burst time from 9 to 84%, and rest time from 2 to 84%. The activity patterns of the back muscles showed similar large interindividual variation. Repeated trapezius recordings of the same subject showed high consistency; intraclass correlation coefficients ranged from 0.62 to 0.79 for different sEMG variables. Periods with high sEMG amplitude were of short duration; 7% of the trapezius recordings did not present time intervals (0.2-s duration) above 50% EMG(max). The activity patterns of the postural muscles, despite large interindividual variability, were distinctly different from activity patterns of upper and lower limb muscles reported by others (e.g., mean burst time 40-50 vs. 10-20%). We conclude that postural trunk muscles show idiosyncratic activity patterns with large interindividual variation. High-threshold motor units are activated to a very minor extent.     

Muscular fatigue during repeated isokinetic shoulder forward flexions in young females

Peak torque, work, mean power and electromyographic (EMG) activity were recorded for each of 150 repeated isokinetic maximal shoulder flexions (45 degrees-90 degrees) in 23 healthy females. From the EMG signals of trapezius, deltoid, infraspinatus and biceps brachii the mean power frequency and the signal amplitude were determined in real time. The mechanical output showed a steep decrease during the first 40 contractions, followed by a plateau maintained until the end. In all muscles, except the biceps brachii, significant decreases in mean power frequency occurred during the first 40 contractions, showing a tendency to stabilize around the same absolute frequency value. Signal amplitude increased in the trapezius, the deltoid and the infraspinatus, but was constant in the biceps brachii. For some individuals rather high EMG activity was recorded in the muscles during the time the arm was supposed to be passively extended to the starting position, and this was found to be associated with lower strength and endurance levels. Longitudinal analyses showed that the mean power frequencies correlated better than the signal amplitudes with the three mechanical variables. The results suggest that the initial steep decrease in mechanical performance and mean power frequency is caused by fatiguing of type 2 motor units.     

Controlling for out-of-plane lumbar moments during unidirectional trunk efforts: Learning and reliability issues related to trunk muscle activation estimates

To assess the electromyographic (EMG) activation of trunk muscle during exertions performed in one primary plane (sagittal, frontal, transverse), we previously proposed a protocol allowing minimizing out-of-plane efforts (coupled moments – CMs) with the use of a static dynamometer combined with a visual feedback system. The aims of this study were to go further by testing motor learning and reliability issues related to such a protocol. Three identical sessions were conducted, where maximal voluntary contractions and submaximal ramp contractions were performed in six different directions while standing in the dynamometer. Two feedback conditions were tested, the simple 1D-feedback in the primary plane and the full 3D-feedback in all planes simultaneously. Surface EMG signals were collected from back and abdominal muscles and EMG amplitude and CMs were computed during the ramp contractions. Providing a 3D feedback to minimize CMs did not improve EMG reliability or in other words, did not reduce the within-subject variability. Providing three assessment days had practically no effect (no learning) on CMs and EMG variables. Overall, the reliability of EMG was at best moderate. However, although this limits its use on an individual basis, it still allows within- and between-group comparisons for research applications.