Muscle Co-Contraction Detection in the Time-Frequency Domain.

BACKGROUND: Muscle co-contraction plays a significant role in motion control. Available detection methods typically only provide information in the time domain. The current investigation proposed a novel approach for muscle co-contraction detection in the time-frequency domain, based on continuous wavelet transform (CWT). METHODS: In the current study, the CWT-based cross-energy localization of two surface electromyographic (sEMG) signals in the time-frequency domain, i.e., the CWT coscalogram, was adopted for the first time to characterize muscular co-contraction activity. A CWT-based denoising procedure was applied for removing noise from the sEMG signals. Algorithm performances were checked on synthetic and real sEMG signals, stratified for signal-to-noise ratio (SNR), and then validated against an approach based on the acknowledged double-threshold statistical algorithm (DT). RESULTS: The CWT approach provided an accurate prediction of co-contraction timing in simulated and real datasets, minimally affected by SNR variability. The novel contribution consisted of providing the frequency values of each muscle co-contraction detected in the time domain, allowing us to reveal a wide variability in the frequency content between subjects and within stride. CONCLUSIONS: The CWT approach represents a relevant improvement over state-of-the-art approaches that provide only a numerical co-contraction index or, at best, dynamic information in the time domain. The robustness of the methodology and the physiological reliability of the experimental results support the suitability of this approach for clinical applications.

Wavelet filtering of fetal phonocardiography: A comparative analysis.

Fetal heart rate (FHR) monitoring can serve as a benchmark to identify high-risk fetuses. Fetal phonocardiogram (FPCG) is the recording of the fetal heart sounds (FHS) by means of a small acoustic sensor placed on maternal abdomen. Being heavily contaminated by noise, FPCG processing implies mandatory filtering to make FPCG clinically usable. Aim of the present study was to perform a comparative analysis of filters based on Wavelet transform (WT) characterized by different combinations of mothers Wavelet and thresholding settings. By combining three mothers Wavelet (4th-order Coiflet, 4th-order Daubechies and 8th-order Symlet), two thresholding rules (Soft and Hard) and three thresholding algorithms (Universal, Rigorous and Minimax), 18 different WT-based filters were obtained and applied to 37 simulated and 119 experimental FPCG data (PhysioNet/PhysioBank). Filters performance was evaluated in terms of reliability in FHR estimation from filtered FPCG and noise reduction quantified by the signal-to-noise ratio (SNR). The filter obtained by combining the 4th-order Coiflet mother Wavelet with the Soft thresholding rule and the Universal thresholding algorithm was found to be optimal in both simulated and experimental FPCG data, since able to maintain FHR with respect to reference (138.7[137.7; 140.8] bpm vs. 140.2[139.7; 140.7] bpm, P > 0.05, in simulated FPCG data; 139.6[113.4; 144.2] bpm vs. 140.5[135.2; 146.3] bpm, P > 0.05, in experimental FPCG data) while strongly incrementing SNR (25.9[20.4; 31.3] dB vs. 0.7[-0.2; 2.9] dB, P < 10-14 , in simulated FPCG data; 22.9[20.1; 25.7] dB vs. 15.6[13.8; 16.7] dB, P < 10-37, in experimental FPCG data). In conclusion, the WT-based filter obtained combining the 4th-order Coiflet mother Wavelet with the thresholding settings constituted by the Soft rule and the Universal algorithm provides the optimal WT-based filter for FPCG filtering according to evaluation criteria based on both noise and clinical features.

EMG-Based Characterization of Walking Asymmetry in Children with Mild Hemiplegic Cerebral Palsy.

Hemiplegia is a neurological disorder that is often detected in children with cerebral palsy. Although many studies have investigated muscular activity in hemiplegic legs, few EMG-based findings focused on unaffected limb. This study aimed to quantify the asymmetric behavior of lower-limb-muscle recruitment during walking in mild-hemiplegic children from surface-EMG and foot-floor contact features. sEMG signals from tibialis anterior (TA) and gastrocnemius lateralis and foot-floor contact data during walking were analyzed in 16 hemiplegic children classified as W1 according to Winter' scale, and in 100 control children. Statistical gait analysis, a methodology achieving a statistical characterization of gait by averaging surface-EMG-based features, was performed. Results, achieved in hundreds of strides for each child, indicated that in the hemiplegic side with respect to the non-hemiplegic side, W1 children showed a statistically significant: decreased number of strides with normal foot-floor contact; decreased stance-phase length and initial-contact sub-phase; curtailed, less frequent TA activity in terminal swing and a lack of TA activity at heel-strike. The acknowledged impairment of anti-phase eccentric control of dorsiflexors was confirmed in the hemiplegic side, but not in the contralateral side. However, a modified foot-floor contact pattern is evinced also in the contralateral side, probably to make up for balance requirements.

UKF Magnetometer-Free Sensor Fusion for Pelvis Pose Estimation During Treadmill Walking.

Inertial measurement units are an efficient tool to estimate the orientation of a rigid body with respect to a global or navigation frame. Thanks to their relatively small scale, these devices are often employed in clinical environments in form of wearable devices. A direct consequence of this large use of inertial sensors has been the development of many sensor fusion techniques for pose estimation in many practical applications. In this paper we study the feasibility of a nonlinear "Unscented" variant of the well-known Kalman Filter for gyroscope/accelerometer sensor fusion in pelvis pose estimation during treadmill walking. In addition, orientation estimation has been obtained without IMU magnetometer data, in order to propose a method suitable also for environments where magnetic disturbances could arise. Pelvis heading (yaw), bank (roll) and attitude (pitch) angles have been evaluated both using the proposed filter and a gold standard optometric system. The root mean square errors obtained using the proposed sensor fusion with respect to the gold standard are below 1 degree for each axis, showing also a significant high correlation (> 0.90). Findings of this study highlight the suitability of a magnetometer-free UKF approach for pose estimation of pelvis during human walking on treadmill, providing information useful also for further estimation of center of mass displacement in the same experimental conditions.

Fractal Analysis of Motor Control in Knee Arthroplasty Patients.

Knee osteoarthritis is commonly treated through total knee arthroplasty (TKA) or unicompartmental knee arthroplasty (UKA) and therefore the assessment of postoperative differences in functional capabilities between TKA and UKA patients appears of primary importance. Throughout the years, fractal analysis has been applied to several biological time-series, revealing to be particularly useful for assessing human balance and motor control by quantifying complexity and repeatability of dynamic measures. In this study, fractal dimension (FD) has been computed on ground reaction force and momentum acquired during squatting movement in two groups of TKA and UKA patients and a control group of healthy subjects (CTRL). FD resulted able to discriminate between TKA and both CTRL and UKA group, showing significant differences in all the considered measures. Outcomes of this study could help to gain further information about functional recovery after different knee arthroplasty procedures, in order to improve the choice of rehabilitative treatment.

Complexity Measures of Postural Control in Type-2 Diabetic Subjects.

Balance maintenance is commonly analyzed by evaluating the center of pressure (COP) displacement, which presents an acknowledged non-stationary behavior. The latter led to an evaluation of COP regularity through complexity measures such as the approximate (AppEn) and sample entropy (SampEn). These indexes quantify the regularity of time-series in terms of inner pattern recurrence; however, they are highly dependent on the input parameters used for their computation. Thus, this study aimed to evaluate the use of the AppEn, SampEn and a recently proposed entropy measure, the fuzzy entropy (FuzzyEn) for the analysis of COP time-series in type-2 diabetic subjects with and without neuropathy during quiet standing trials in eyes open condition. Results highlighted consistency of entropy measures for different values of input parameters, showing significant differences between the two populations in terms of COP regularity for both anterior-posterior and medial-lateral directions. Findings of this study outline low complexity in postural control of neuropathic subjects, also in the medial-lateral direction, which could indicate a limited capacity of producing adaptable responses, relying on fixed balance control patterns. Further, they support the use of complexity measures for the analysis of patients with diabetic neurological impairment.

Description of Postural Strategies through a Variable Structure Control.

Human postural strategies in balance maintenance are the results of the complex control action played by the Central Nervous System (CNS). Literature underlined that such strategies become more evident when external perturbations challenge the stance. In this study, a new model of balance maintenance under support base movement perturbation is formulated. A sliding mode approach is employed to simulate the aforementioned strategies in stabilizing a double inverted pendulum, used to describe the mechanics of the bipedal human stance. Control parameters are then optimized in order to reproduce the measured center of mass (COM) displacement in the anterior-posterior direction. Such parameters seem to be useful to distinguish different postural strategies employed by different subjects. Moreover, electromyographic data are employed to effectively support the goodness of the model.

Modeling Perturbed Posture Through An Adaptive Sliding Mode Approach.

Upright stance maintenance under perturbed condition is a complex phenomenon in which the Central Nervous System(CNS) is engaged to regulate the balance preventing subject to fall. Many models of unperturbed stance are present in literature. However, the necessity to model balance maintenance in presence of external disturbance is still an open problem. In this paper, a new model representing the human balance maintenance under perturbed condition is presented. An adaptive sliding mode approach is used to model the action played by the CNS, the control parameters are then optimized in order to match real and simulated data. The trend of optimized parameters seems to reveal the development of different postural strategies throughout the experimental trials.

Co-activation patterns of gastrocnemius and quadriceps femoris in controlling the knee joint during walking.

Muscular co-activation is a well-known mechanism for lower limb joint stabilization in both healthy and pathological individuals. This muscular feature appears particularly important for the knee joint, not only during challenging motor tasks such as cutting and landing but also during walking, due to knee cyclic loading. Gastrocnemius acts on the knee joint with a flexor activity and co-activations with quadriceps muscles lead to greater knee ligament strain with respect to an isolated burst of either muscle. Thus, this study aimed to assess possible co-activations between gastrocnemius and quadriceps muscles during walking. Five co-activation periods were assessed: during early stance (identified in 5.7 ±â€¯5.1% of total strides), early and late foot-contact (88.9 ±â€¯8.9% and 8.9 ±â€¯8.2%), push-off (23.9 ±â€¯12.2%) and late swing (29.0 ±â€¯16.1%). Outcomes showed that late foot-contact and swing co-activations could deserve particular attention: in both cases the knee joint was close to the full extension (around 3.5° and 6°, respectively) and thus, considering also the anterior tibia translation due to the quadriceps activity, the simultaneous gastrocnemius burst could lead to an enhanced knee ligaments elongation. Findings of this study represent the first attempt to provide a reference knee joint co-activation framework, useful also for further evaluation in cohorts with knee failures.

Validity of the Nintendo Wii Balance Board for the Assessment of Balance Measures in the Functional Reach Test.

The functional reach test (FRT) is widely used for assessing dynamic balance stability in elderly and pathological subjects. Force platforms (FPs) represent a fundamental part of the instrumented FRT experimental setup due to the central role of center-of-pressure (COP) displacement in FRT analysis. Recently, the nintendo wii balance board (NBB) has been suggested as a low-cost and reliable device for ground reaction force and COP measurement in poorly dynamic motor tasks. Therefore, this paper aimed to compare NBB-COP data with those obtained from a laboratory-grade platform during FRT. Data from 48 healthy subjects were simultaneously acquired from both devices. FP-COP and NBB-COP trajectories showed a remarkable correlation in both directions ( ) and low root-mean-square error values (1.14 ± 0.88 mm and 0.55 ± 0.28 mm for anterior-posterior and medial-lateral direction). Fixed biases between COP-based parameters did not exceed 2% of the FP outcomes with high consistency throughout the present measurement range (ICC consistency always >0.950). Only the COP mean velocity exhibited a tendency toward proportional errors, which can be adjusted by a calibration of NBB data. Findings of this paper confirmed the NBB validity for COP measurement in a widely used motor task as the functional reach, supporting the feasibility of NBB in research scenarios.

Surface EMG patterns for quantification of thigh muscle co-contraction in school-age children: Normative data during walking.

Muscle co-contractions are particularly relevant in analyzing children pathologies. To interpret surface electromyography (sEMG) in pathological conditions, reliable normative data in non-pathological children are required for direct comparison. Aim of the study was the quantification of co-contraction activity between quadriceps femoris (QF) and hamstring muscles during walking in healthy children. To this aim, Statistical gait analysis was performed on sEMG signals from rectus femoris (RF), vastus medialis (VM), and lateral hamstrings (LH), in 16401 strides walked by 100 healthy school-age children. Co-contractions were assessed as overlapping period between activation intervals of considered muscles. Results showed full superimpositions of LH with both RF and VL activity from terminal swing, 80-100% of gait cycle, to successive loading response (0-15% of gait cycle), in around 90% of strides, as reported in adults. This indicates that children regularly use a cocontraction activity between QF and hamstring muscles in weight acceptance during walking, supporting the hypothesis of a regulatory role of co-contraction in providing knee joint stability. Concomitant activity of QF and hamstring muscles was detected also during push-off phase (30-50% of gait cycle), showing a large variability intra and inter subjects and a lower occurrence frequency (around 25% of strides). This could be intended for controlling rapid knee flexion and/or stabilizing pelvis during body progression. Present findings represent the first attempt to provide normative sEMG dataset on variability of QF and hamstring muscles co-contractions during child walking, useful for discriminating physiological and pathological behavior and for designing future studies on maturation of gait.

Detection of surface-EMG activity from the extensor digitorum brevis muscle in healthy children walking.

OBJECTIVE: The purpose of the study was the assessment of activation patterns of the extensor digitorum brevis (EDB) muscle in healthy children, during walking at self-selected speed and cadence. APPROACH: To this end, statistical gait analysis was performed on surface electromyographic (sEMG) signals of the EDB, in a large number (hundreds) of strides per subject. sEMG data from the tibialis anterior (TA) and gastrocnemius lateralis (GL) were also investigated for comparative purposes. MAIN RESULTS: Results from 23 healthy children showed a large variability in the number of muscle activations, occurrence frequency, and onset-offset instants across considered strides. The assessment of different modalities of muscle activation allowed the identification of a single activity pattern, common to all the modalities and we were able to characterize the behavior of the EDB during the gait of healthy children. The pattern of EDB activity centered in two main regions of the gait cycle: in the second half of the stance phase (detected in 100% of subjects) and in the final swing phase (50%). Comparison with the TA and GL regions of activity suggested that the EDB and TA worked mainly as antagonist muscles for the ankle joint, while the EDB and GL did not oppose each other in action, but acted in synergy for the control of the ankle joint during walking. SIGNIFICANCE: The 'Normality' pattern for the EDB activity reported here represents the first attempt to develop a reference for dynamic sEMG of the EDB in healthy children, enabling us to include the physiological variability of the phenomenon. Present results could be useful for discriminating physiological and pathological behavior in children and for deepening the maturation of the gait.

Antagonist thigh-muscle activity in 6-to-8-year-old children assessed by surface EMG during walking.

Analysis of muscle co-contractions seems to be relevant in the characterization of children pathologies such as spastic cerebral palsy. The aim of the study was the quantification of thigh-muscle co-contractions during walking in healthy children. To this aim, the Statistical Gait Analysis, a recent methodology providing a statistical characterization of gait, was performed on surface EMG signals from Vastus Medialis (VM) and Lateral Hamstrings (LH) in 30 healthy 6-to-8-year-old children. Muscular co-contraction was assessed as the overlapping period between activation intervals of agonist and antagonist muscles. As in adults, VM activity occurring from terminal swing to the following loading response superimposed LH activity in the same percentage of the gait cycle. This co-contraction occurred in order to control knee joint stability during weight acceptance. It was acknowledged in the totality (100 %) of the considered strides. Concomitant activity of VM and LH was detected also in the second half of stance phase in 17.1 ± 4.8 % of the considered strides. Working VM and LH on different joints, this concomitant activity of antagonist muscles should not be considered as an actual co-contraction. Present findings provide new information on the variability of the reciprocal role of VM and LH during child walking, useful for comparison between normal and pathological walking in the clinical context and for designing future studies on maturation of gait.

A new parameter for quantifying the variability of surface electromyographic signals during gait: The occurrence frequency.

Natural variability of myoelectric activity during walking was recently analyzed considering hundreds of strides. This allowed assessing a parameter seldom considered in classic surface EMG (sEMG) studies: the occurrence frequency, defined as the frequency each muscle activation occurs with, quantified by the number of strides when a muscle is recruited with that specific activation modality. Aim of present study was to propose the occurrence frequency as a new parameter for assessing sEMG-signal variability during walking. Aim was addressed by processing sEMG signals acquired from Gastrocnemius Lateralis, Tibialis Anterior, Rectus Femoris and Biceps femoris in 40 healthy subjects in order to: (1) show that occurrence frequency is not correlated with ON/OFF instants (Rmean=0.11±0.07; P>0.05) and total time of activation (Rmean=0.15±0.08; P>0.05); (2) confirm the above results by two handy examples of application (analysis of gender and age) which highlighted that significant (P<0.05) gender-related and age-related differences within population were detected in occurrence frequency, but not in temporal sEMG parameters. In conclusion, present study demonstrated that occurrence frequency is able to provide further information, besides those supplied by classical temporal sEMG parameters and thus it is suitable to complement them in the evaluation of variability of myoelectric activity during walking.

Is child walking conditioned by gender? Surface EMG patterns in female and male children.

EMG-based differences between females and males during walking are generally acknowledged in adults. Aim of the study was the quantification of possible gender differences in myoelectric activity of gastrocnemius lateralis (GL) and tibialis anterior (TA) during walking in school-age children. Gender-related comparison with adults was also provided to get possible novel insight in maturation of gait. To this aim, Statistical gait analysis, a recent methodology performing a statistical characterization of gait by averaging spatial-temporal and surface-EMG-based parameters over hundreds of strides, was performed in100 healthy school-age children (C-group) and in 33 healthy young adults (YA-group). On average, 301±110 consecutive strides were analyzed for each subject. In C-group, no significant differences (p>0.05) were observed between females and males in GL and TA, considering mean onset/offset instants of activation and occurrence frequency. Stratifying the C-group for age, small differences between females and males in occurrence frequency of GL arose in oldest children. In YA-group, females showed a significant propensity for a more complex recruitment of TA and GL (higher number of activations during gait cycle, quantified by occurrence frequency) compared to males. These outcomes suggest that gender-related differences in sEMG parameters do not characterize the recruitment of GL and TA during child walking in early years (6-8 years), start occurring when adolescence is approaching (10-12 years), and are acknowledged in both ankle muscles only in adults. Present findings seem to support previous studies on maturation of gait which indicate adolescence as the time-range where gait is completing its maturation path.

Surface-EMG analysis for the quantification of thigh muscle dynamic co-contractions during normal gait.

The research purpose was to quantify the co-contraction patterns of quadriceps femoris (QF) vs. hamstring muscles during free walking, in terms of onset-offset muscular activation, excitation intensity, and occurrence frequency. Statistical gait analysis was performed on surface-EMG signals from vastus lateralis (VL), rectus femoris (RF), and medial hamstrings (MH), in 16315 strides walked by 30 healthy young adults. Results showed full superimpositions of MH with both VL and RF activity from terminal swing, 80 to 100% of gait cycle (GC), to the successive loading response (≈0-15% of GC), in around 90% of the considered strides. A further superimposition was detected during the push-off phase both between VL and MH activation intervals (38.6±12.8% to 44.1±9.6% of GC) in 21.9±13.6% of strides, and between RF and MH activation intervals (45.9±5.3% to 50.7±9.7 of GC) in 32.7±15.1% of strides. These findings led to identify three different co-contractions among QF and hamstring muscles during able-bodied walking: in early stance (in ≈90% of strides), in push-off (in 25-30% of strides) and in terminal swing (in ≈90% of strides). The co-contraction in terminal swing is the one with the highest levels of muscle excitation intensity. To our knowledge, this analysis represents the first attempt for quantification of QF/hamstring muscles co-contraction in young healthy subjects during normal gait, able to include the physiological variability of the phenomenon.

Dynamic knee muscle co-contraction quantified during walking.

The purpose of the present study was the quantification of the co-activation patterns of the knee extensor and flexor muscles during walking at self-selected speed and cadence. To this aim, the Statistical Gait Analysis, a recent methodology providing a statistical characterization of gait, was performed on surface EMG signals from Vastus Lateralis (VL) and Medial Hamstrings (MH) in 14 healthy young adult subjects. Muscular co-contraction was assessed as the overlapping period between activation intervals of agonist and antagonist muscles. Superimpositions between VL and MH activity were detected from terminal swing to the following loading response in 100% of the considered strides. This superimposition could be intended as an actual co-contraction of VL and MH, working across the same joint, the knee. It occurs in this gait phase likely in order to assist knee extension, developing muscle tension for weight acceptance during loading response, and to control knee flexion. A further less frequent (28.9±13.6% of the strides, P<;0.001) superimposition was detected in terminal stance; this superimposition, however, should not be considered a real co-contraction, because VL and MH work on different joints. These findings have the merit to provide a novel data on the variability of the reciprocal role of VL and MH during walking, allowing a deeper insight in the physiological mechanisms that regulate the knee flexion/extension.

Co-activation periods of gastrocnemius and vastus lateralis during walking evaluated by surface electromyography.

"In vivo" studies reported that the co-activation of gastrocnemius and quadriceps femoris (QF) muscles produces ACL strain values greater than those caused by an isolated activation of either muscle. Aim of this study was to assess the co-activation of gastrocnemius (lateral head, GL) and vastus lateralis (VL) in healthy and young adults during walking. To this purpose the Statistical Gait Analysis was performed, that allows a characterization of gait considering hundreds of strides belonging to the same walking trial. Three GL/VL co-activations were detected during a single gait cycle: in foot-contact phase, from 6.8±8.5% to 22.9±23.3% of gait cycle, (FC co-activation), in push-off phase, from 33.0±11.9% to 41.5±13.4% (PO co-activation), and in swing phase, from 86.5±6.7% to 93.2±5.9% (SW co-activation). FC co-activation was the most recurrent (in 100% of the strides, P<;0.001) and longest (16% of gait cycle) one. Thus, the ACL strain due to the co-activation between GL and VL is longer and more frequently during FC phase, than in all the others gait phases. Moreover, the position of the knee and the amount of the weight-bearing on knee, achieved in this gait phase, suggested that FC co-activation is the one that produces a highest strain value of anterior cruciate ligament (ACL). These findings could help to better understand risk factors of the ACL injuries and to design more focused preventive and rehabilitative strategies.