Estimation of muscle fatigue using surface electromyography and near-infrared spectroscopy.

This study looks at various parameters, derived from surface electromyography (sEMG) and Near Infrared Spectroscopy (NIRS) and their relationship in muscle fatigue during a static elbow flexion until exhaustion as well as during a semidynamic exercise.We found a linear increasing trend for a corrected amplitude parameter and a linear decreasing slope for the frequency content of the sEMG signal. The tissue oxygenation index (TOI) extracted from NIRS recordings showed a four-phase response for all the subjects. A strong correlation between frequency content of the sEMG signal and TOI was established. We can conclude that both sEMG and NIRS give complementary information concerning muscle fatigue.

Calculating gait kinematics using MR-based kinematic models.

Rescaling generic models is the most frequently applied approach in generating biomechanical models for inverse kinematics. Nevertheless it is well known that this procedure introduces errors in calculated gait kinematics due to: (1) errors associated with palpation of anatomical landmarks, (2) inaccuracies in the definition of joint coordinate systems. Based on magnetic resonance (MR) images, more accurate, subject-specific kinematic models can be built that are significantly less sensitive to both error types. We studied the difference between the two modelling techniques by quantifying differences in calculated hip and knee joint kinematics during gait. In a clinically relevant patient group of 7 pediatric cerebral palsy (CP) subjects with increased femoral anteversion, gait kinematic were calculated using (1) rescaled generic kinematic models and (2) subject-specific MR-based models. In addition, both sets of kinematics were compared to those obtained using the standard clinical data processing workflow. Inverse kinematics, calculated using rescaled generic models or the standard clinical workflow, differed largely compared to kinematics calculated using subject-specific MR-based kinematic models. The kinematic differences were most pronounced in the sagittal and transverse planes (hip and knee flexion, hip rotation). This study shows that MR-based kinematic models improve the reliability of gait kinematics, compared to generic models based on normal subjects. This is the case especially in CP subjects where bony deformations may alter the relative configuration of joint coordinate systems. Whilst high cost impedes the implementation of this modeling technique, our results demonstrate that efforts should be made to improve the level of subject-specific detail in the joint axes determination.

Instantaneous changes in heart rate regulation due to mental load in simulated office work.

The cardiac regulation effects of a mental task added to regular office work are described. More insight into the time evolution during the different tasks is created by using time-frequency analysis (TFA). Continuous wavelet transformation was applied to create time series of instantaneous power and frequency in specified frequency bands (LF 0.04-0.15 Hz; HF 0.15-0.4 Hz), in addition to the traditional linear heart rate variability (HRV) parameters. In a laboratory environment, 43 subjects underwent a protocol with three active conditions: a clicking task with low mental load and a clicking task with high mental load (mental arithmetic) performed twice, each followed by a rest condition. The heart rate and measures related to vagal modulation could differentiate the active conditions from the rest condition, meaning that HRV is sensitive to any change in mental or physical state. Differences between physical and mental stress were observed and a higher load in the combined task was observed. Mental stress decreased HF power and caused a shift toward a higher instantaneous frequency in the HF band. TFA revealed habituation to the mental load within the task (after 3 min) and between the two tasks with mental load. In conclusion, the use of TFA in this type of analysis is important as it reveals extra information. The addition of a mental load to a physical task elicited further effect on HRV parameters related to autonomic cardiac modulation.

Time-frequency heart rate variability characteristics of young adults during physical, mental and combined stress in laboratory environment.

The goal of this study was to evaluate the changes in heart rate variability (HRV) parameters due to a specific physical, mental or combined load. More specifically, the difference in effect between mental load and physical activity is studied. In addition, the effect of the combined physical and mental demand on the HRV parameters was examined and compared with the changes during the single task. In a laboratory environment, 28 subjects went through a protocol with different types of load (physical and/or mental), each followed by a period of rest. Continuous wavelet transformation was applied to create time series of instantaneous power and frequency in specified frequency bands (LF and HF). HF could distinguish the active conditions from the rest condition, meaning that HRV is sensitive to any change in mental or physical state. Differences in HRV parameters were observed between physical, mental and the combined load. In conclusion, we were able to distinguish between rest, physical and mental condition by combining different HRV characteristics. The addition of a mental load to a physical task had an extra effect on the HRV characteristics.

Atlas-based non-rigid image registration to automatically define line-of-action muscle models: a validation study.

Research has raised a growing concern about the accuracy of rescaled generic musculoskeletal models for estimating a subject's musculoskeletal geometry. Information extracted from magnetic resonance (MR) images can improve the subject-specific detail and accuracy of musculoskeletal models. Nevertheless, methods that allow efficient, automated definition of subject-specific muscular models for use in biomechanical analysis of gait have not yet been published to the best of our knowledge. We report a novel method for automated definition of subject-specific muscle paths using non-rigid image registration between an atlas image and the subject's MR images. We validated this approach quantitatively by measuring the distance between automatically and manually defined coordinates of muscle attachment sites. Data was collected for 34 muscles in each lower limb of 5 paediatric subjects diagnosed with diplegic cerebral palsy and presenting varying degrees of increased femoral anteversion. Distances showed an overall median Euclidean error of 6.1mm: 2.0mm along the medio-lateral direction, 1.8mm along the anterior-posterior direction and 3.8mm along the superior-inferior direction. A qualitative validation between automatically defined muscle points and the muscular geometry observed in the subject's medical image data corroborated the quantitative validation. This automated approach followed by visual inspection and, if needed, correction to the muscle paths, reduced the time required for defining 34 lower-limb muscle paths from around 3.5 to 1h. Furthermore, the method was also applicable to aberrant skeletal geometry. Using the proposed method, defining MR-based musculoskeletal models becomes a time efficient and more accurate alternative to rescaling generic models.

Calculated moment-arm and muscle-tendon lengths during gait differ substantially using MR based versus rescaled generic lower-limb musculoskeletal models.

Biomechanical analysis of gait relies on the use of lower-limb musculoskeletal models. Most models are based on a generic model which takes into account the subject's skeletal dimensions by isotropic or anisotropic rescaling. Alternatively, personalized models can be built based on information from magnetic resonance (MR) images. We have studied the effect of these approaches on muscle-tendon lengths (MTLs) and moment-arm lengths (MALs) for 16 major muscles of the lower limb of a normal adult during both normal and pathologic gait. For most muscles, the MTL and MAL calculated using the rescaled generic models showed high correlation values, but large offsets when compared to values calculated using personalized models. MTL and MAL differences with the personalized model are only slightly smaller for an anisotropic than for an isotropic rescaled model. Gait kinematics influenced the observed inter-model differences and correlations due to an altered range of joint angles in both gait patterns. In conclusion, both generic rescaling methods failed to accurately estimate absolute values for MTL and MAL calculated using the personalized model. However, the magnitude of MTL and MAL changes during normal and pathologic gait corresponded between all three models for most muscles. Since rescaling depends strongly on modelling assumptions and cannot fully take into account subject-specific musculoskeletal geometry, interpretation of MTL and MAL even in normal adult subjects requires extreme caution.

Personalized MR-based musculoskeletal models compared to rescaled generic models in the presence of increased femoral anteversion: effect on hip moment arm lengths.

Advanced biomechanical analysis of muscle function during gait relies on the use of a musculoskeletal model. In clinical practice, personalization of the model is usually limited to rescaling a generic model to approximate the patient's anthropometry, even in the presence of bony deformities, as in the case of cerebral palsy (CP). However, the current state of the art in biomechanics allows highly detailed subject-specific models to be built based on magnetic resonance (MR) images. We hypothesized that moment arm length (MAL) calculations from MR-based models would be more accurate than those from rescaled generic musculoskeletal models. Our study compared hip muscle MAL estimated by (1) a personalized model based on full-leg MR scans and (2) a rescaled generic model of both lower limbs in six children presenting with increased femoral anteversion. Personalized MR-based models were created using a custom-built workflow. Rescaled generic models were created based on three-dimensional positions of anatomical markers measured during a standing trial. For all 12 lower limb models, the hip flexion, adduction and rotation MAL of 13 major muscles were analyzed over a physiological range of hip motion using Software for interactive musculoskeletal modelling (SIMM) (Motion Analysis Corporation, USA). Our results showed that rescaled generic models, which do not take into account the subject's femoral geometry, overestimate MAL for hip flexion, extension, adduction, abduction and external rotation, but underestimate MAL for hip internal rotation. The differences in MAL introduced by taking the aberrant femoral geometry into account in the MR-based model were consistent with major gait characteristics presented in CP patients.

Wavelet-independent component analysis to remove electrocardiography contamination in surface electromyography.

Removing artifacts from biomedical signals, such as surface electromyography (sEMG), has become a major research topic in biomedical signal processing. In electromyography signals, a source of contamination is the electrophysiological signal of the heart (ECG signals). This contamination influences features extracted from the sEMG, especially during low-activity measurements of the muscles such as during mental stress. As the heart is a muscle, the frequency content of the heart signals overlaps the frequency content of the muscle signals, so basic frequency filtering is not possible. In this paper, we present the results of a recently developed algorithm: wavelet-independent component analysis. We compare these results with the widely described algorithm of ECG template subtraction for removing ECG contamination.

Textile integrated contactless EMG sensing for stress analysis.

Stress has become an important issue in today's society. Forty to fifty percent of the work related illnesses are directly or indirectly related to stress. In the ConText project, a biofeedback shirt for daily use is being developed to register muscle activity. The user receives feedback about muscle fatigue and/or the level of stress in order to lower the risk on musculoskeletal disorders. Comfort is an important factor to lower the acceptance threshold to wear the shirt. To achieve optimal comfort, the project aims for unobtrusive measurements with contactless sensors which are textile integrated. Working with contactless sensors induces new challenges for instance the displacement of the sensor in the shirt relative to the anatomical position of the muscles. This could affect the recorded signal and lead to errors in the signal. In this paper, we present the results of the quantification of this misalignment. Secondly, we present the first tests with the embroidered sensor.

Evaluation of the effect of backpack load and position during standing and walking using biomechanical, physiological and subjective measures.

Recommendations on backpack loading advice restricting the load to 10% of body weight and carrying the load high on the spine. The effects of increasing load (0%-5%-10%-15% of body weight) and changing the placement of the load on the spine, thoracic vs. lumbar placement, during standing and gait were analysed in 20 college-aged students by studying physiological, biomechanical and subjective data. Significant changes were: (1) increased thorax flexion; (2) reduced activity of M. erector spinae vs. increased activation of abdominals; (3) increased heart rate and Borg scores for the heaviest loads. A trend towards increased spinal flexion, reduced pelvic anteversion and rectus abdominis muscle activity was observed for the lumbar placement. The subjective scores indicate a preference for the lumbar placement. These findings suggest that carrying loads of 10% of body weight and above should be avoided, since these loads induce significant changes in electromyography, kinematics and subjective scores. Conclusions on the benefits of the thoracic placement for backpack loads could not be drawn based on the parameter set studied.

Musculo-tendon length and lengthening velocity of rectus femoris in stiff knee gait.

Inappropriate activity of M. rectus femoris (RF) during swing is believed to contribute to stiff knee gait in cerebral palsy. This study used musculoskeletal modeling techniques to analyze rectus femoris musculo-tendon (MT) length and lengthening velocity during stiff knee gait in 35 children with diplegic cerebral palsy (CP). Duncan Ely test scores were used to categorize the patients into four groups with increasing levels of rectus femoris spasticity. Knee kinematics confirmed a significant reduction and delay of maximal peak knee flexion during swing in the patient groups compared to reference values. Maximal musculo-tendon length of M. rectus femoris was reduced and occurred prematurely in swing. Musculo-tendon lengthening velocity was significantly reduced and the timing of the maximal lengthening velocity was shifted into stance phase. The present study demonstrates altered dynamic behavior of the M. rectus femoris in stiff knee gait and the results indicate that maximal knee flexion in swing was not a valid reference for the MT length of the M. rectus femoris. Furthermore, in the patient group maximal musculo-tendon lengthening velocity of the muscle related to the stance phase rather than the stance-swing transition.

Exercise programs for older men: mode and intensity to induce the highest possible health-related benefits.

BACKGROUND: Although health-related benefits of fitness training in older men are well established, it is not clear yet which mode and intensity of a exercise program is most effective. This study addresses whether the combination of endurance (ED) and resistance training in older men have supplementary health-related benefits in addition to profits attained through endurance training alone. Additionally, effects of moderate- and low-intensity resistance training are compared. METHODS: Men, 55-75 years of age, were randomly assigned to a control group (N = 13) or one of three exercise groups (20 weeks, two to three times per week): endurance plus moderate resistance (MR) training (N = 22), endurance plus low resistance (LR) training (N = 22) and endurance training only (N = 22). Cardiovascular (CV) risk factors, muscular fitness and postural control were assessed before and after training. RESULTS: All exercise groups revealed significant (P < 0.05) improvements in resting heart rate, work capacity and recovery, waist girth, insulin response and knee-extensor strength with no differences among groups. Body composition, resting metabolic rate (RMR), VO2peak and postural control did not change in exercise groups. CONCLUSION: In older men, a fitness program consisting of 20 weeks endurance training combined with resistance training is equally effective as endurance training alone. Moderate vs. low resistance training added to endurance training yields similar health-related benefits.

Muscle fatigue of the elbow flexor muscles during two intermittent exercise protocols with equal mean muscle loading.

OBJECTIVE: Whether different muscle-loading patterns with the same mean load have a similar influence on the resultant muscle fatigue is still unknown. This study investigated the influence of two intermittent exercise protocols, with equal mean muscle loading, on the mechanical and myoelectrical signs of fatigue in the elbow flexor muscles. DESIGN: Ten subjects performed two 20-min long intermittent isometric elbow flexion tasks. The exercise period load for the low force intermittent test was 25% of the maximum voluntary contraction, with a work-cycle time of 20 s and a duty cycle of 0.5. The high force intermittent exercise protocol had an exercise period load of 50% maximum voluntary contraction, a cycle time of 20 s and a duty cycle of 0.25. METHODS: Muscle fatigue was quantified both by measuring the maximal voluntary contraction torque before and after the intermittent exercises, and also by the temporal changes observed in the electromyography signal. RESULTS: Both muscle-loading patterns induced a 15% decrease in maximum voluntary contraction. However, the electromyographic spectral changes during the intermittent static contractions varied between the two protocols. CONCLUSIONS: Variation in muscle load without loss in productivity can affect the physiological responses. The results also showed that different methods to assess muscle fatigue sometimes provide different information on the resultant fatigue.

Posture, muscle activity and muscle fatigue in prolonged VDT work at different screen height settings.

With the increasing use of video display terminals (VDTs), there is growing concern over the corresponding increase in the number of health problems reported. Although much research has focused on identifying the optimal screen height, there is to date no consensus. This study aimed to investigate the effect of prolonged (89 min) VDT work at four different screen heights on head-neck posture, muscle activity and the development of muscle fatigue. The results show that lowering screen height, starting from 15 cm above the baseline (i.e. top of the screen level with eye height while sitting), decreased the ear-eye angle, increased the viewing angle, increased the viewing angle relative to the ear-eye line, and increased the muscle activity of the neck extensor muscles. There were also some significant time effects on postural angles and muscle activity. In this study there were only rare occurrences of muscle fatigue, defined as a simultaneous increase in EMG amplitude and a shift of the EMG power spectrum to lower frequencies. Muscle activity increased significantly in some muscles and for certain screen heights.

The complementary role of the plantarflexors, hamstrings and gluteus maximus in the control of stance limb stability during gait.

This paper focuses on the contributions of the gluteus maximus, biceps femoris, gastrocnemius and soleus in maintaining the stability of the stance limb in the sagittal plane during the mid-stance phase of gait. In the absence of any one of these muscles, the potential compensatory changes in muscle activation are explored, the aim being to restore stability to the stance limb. The investigation was carried out by integrating musculoskeletal modelling, forward simulation and optimization techniques. We concluded that maintenance of stance limb stability requires a subtle interplay of muscle activations. Weakness in a single muscle is unlikely to be adequately compensated for by increasing or decreasing the activation of one muscle alone.

Habitual level of physical activity and muscle fatigue of the elbow flexor muscles in older men.

Reduced muscle performance, related to the loss of muscle mass and strength, is a common and natural part of ageing. Nevertheless, it is generally believed that regular participation in activities of moderate intensity may slow down these age-related changes. This study investigated the relationship between the habitual level of physical activity (PA), assessed by the modified Baecke Questionnaire, and the mechanical and fatigue characteristics of the right elbow flexor muscles, m. biceps brachii and m. brachioradialis, in men over the age of 55 years. Muscle fatigue was quantified both by measuring the maximal voluntary contraction (MVC) torque before and after a sustained isometric contraction at 25% MVC until exhaustion, and also by the temporal changes observed in the surface electromyographic (EMG) signal recorded during the fatigue task. Results showed a decreased MVC torque at the end of the fatiguing contraction. After 20 min of recovery, the MVC force was still significantly lower than the pre-fatigue value, except for the most active subjects. Typical myoelectrical indications of fatigue were also observed: a shift in the frequency spectrum of the signal towards lower frequencies accompanied by an increase in the EMG amplitude. We concluded from this study that the level of PA was related to the absolute isometric MVC values and the measurement of neuromuscular efficiency after 20 min of recovery, but did not influence the indications of muscle fatigue during an isometric fatigue task.

The study of muscle action during single support and swing phase of gait: clinical relevance of forward simulation techniques.

Individual muscle function during single stance and swing phase of gait were analyzed using muscle driven forward simulation. The activation of each of 22 muscles in a musculoskeletal model with seven degrees of freedom were excluded from the forward simulation and the resulting changes in joint angles studied. A classification of muscle function during single support and swing phase of gait is presented. Altered joint kinematics due to the absence of individual muscle action is discussed in the light of pathological gait kinematics and clinical decision-making.

Reduction of spastic hypertonia during repeated passive knee movements in stroke patients.

OBJECTIVES: To quantify changes in spastic hypertonia during repeated passive isokinetic knee movements in stroke patients and to assess the role of muscle activity. DESIGN: A between-groups design with repeated measures. SETTING: Rehabilitation center for stroke patients. PARTICIPANTS: Ten stroke patients with hypertonia and 10 healthy subjects matched for age and gender. INTERVENTION: With an isokinetic apparatus, movements were imposed on the knee in series of 10 repetitions at speeds of 60 degrees /s, 180 degrees /s, and 300 degrees /s. MAIN OUTCOME MEASURES: Spastic hypertonia was assessed on the basis of torque measurement and electromyographic activity of the quadriceps, hamstrings, and gastrocnemius muscles. RESULTS: Compared with the controls, stroke patients presented a significantly stronger torque reduction during the mid- and endphases of movements at all speeds tested (P<.05). The strongest torque decline occurred during knee flexion and during the first movements. The effect increased toward the end phase of movements and with increasing speeds. The effect of movement repetitions on torque measurements was unchanged after electromyographic activity was included in the statistical analysis, except during extension movements at 180 degrees /s and 300 degrees /s. CONCLUSION: Passive movements of the knee induced a decrease of spastic hypertonia in stroke patients through a combination of reflexive and mechanical factors. The role of these mechanisms is velocity dependent and differs for flexion and extension movements.

Perceived Discomfort and Electromyographic Activity of the Upper Trapezius While Working at a VDT Station.

Ten female participants performed work at a video display terminal (VDT) station over a whole working day. Subjective local muscular fatigue was evaluated by means of the Category Ratio 10 scale. Electromyographic activity of the upper right and left trapezius was measured. A comparison was made between 5 participants who had previous complaints and 5 participants who reported no musculoskeletal problems in the shoulder-neck region. The subjective scores for the shoulder differed significantly between the two groups, being higher for the group with complaints. Both groups showed a decrease in discomfort after the lunch break. The activity of the trapezius increased significantly for both groups, in a more pronounced way for the group with disorders. Although it is found in literature that VDT work is a task with very low static loads, it seems from this study that the EMG activity increase can be an indication of muscle fatigue: More effort was required to accomplish the same VDT task at the end of the day.