Validation of an EMG submaximal method to calibrate a novel dynamic EMG-driven musculoskeletal model of the trunk: Effects on model estimates.

BACKGROUND: Multijoint EMG-assisted optimization models are reliable tools to predict muscle forces as they account for inter- and intra-individual variations in activation. However, the conventional method of normalizing EMG signals using maximum voluntary contractions (MVCs) is problematic and introduces major limitations. The sub-maximal voluntary contraction (SVC) approaches have been proposed as a remedy, but their performance against the MVC approach needs further validation particularly during dynamic tasks. METHODS: To compare model outcomes between MVC and SVC approaches, nineteen healthy subjects performed a dynamic lifting task with two loading conditions. RESULTS: Results demonstrated that these two approaches produced highly correlated results with relatively small absolute and relative differences (<10 %) when considering highly-aggregated model outcomes (e.g. compression forces, stability indices). Larger differences were, however, observed in estimated muscle forces. Although some model outcomes, e.g. force of abdominal muscles, were statistically different, their effect sizes remained mostly small (ηG2 ≤ 0.13) and in a few cases moderate (ηG2 ≤ 0.165). CONCLUSION: The findings highlight that the MVC calibration approach can reliably be replaced by the SVC approach when the true MVC exertion is not accessible due to pain, kinesiophobia and/or the lack of proper training.

Submaximal electromyography-driven musculoskeletal modeling of the human trunk during static tasks: Equilibrium and stability analyses.

Conventional electromyography-driven (EMG) musculoskeletal models are calibrated during maximum voluntary contraction (MVC) tasks, but individuals with low back pain cannot perform unbiased MVCs. To address this issue, EMG-driven models can be calibrated in submaximal tasks. However, the effects of maximal (when data points include the maximum contraction) and submaximal calibration techniques on model outputs (e.g., muscle forces, spinal loads) remain yet unknown. We calibrated a subject-specific EMG-driven model, using maximal/submaximal isometric contractions, and simulated different independent tasks. Both approaches satisfactorily predicted external moments (Pearson's correlation ∼ 0.75; relative error = 44%), and removing calibration tasks under axial torques markedly improved the model performance (Pearson's correlation âˆ¼ 0.92; relative error âˆ¼ 28%). Unlike individual muscle forces, gross (aggregate) model outputs (i.e., spinal loads, stability index, and sum of abdominal/back muscle forces) estimated from maximal and submaximal calibration techniques were highly correlated (r > 0.78). Submaximal calibration method overestimated spinal loads (6% in average) and abdominal muscle forces (11% in average). Individual muscle forces estimated from maximal and submaximal approaches were substantially different; however, gross model outputs (especially internal loads and stability index) remained highly correlated with small to moderate relative differences; therefore, the submaximal calibration technique can be considered as an alternative to the conventional maximal calibration approach.

A comparison of lumbar spine and muscle loading between male and female workers during box transfers.

There is a clear relationship between lumbar spine loading and back musculoskeletal disorders in manual materials handling. The incidence of back disorders is greater in women than men, and for similar work demands females are functioning closer to their physiological limit. It is crucial to study loading on the spine musculoskeletal system with actual handlers, including females, to better understand the risk of back disorders. Extrapolation from biomechanical studies conducted on unexperienced subjects (mainly males) might not be applicable to actual female workers. For male workers, expertise changes the lumbar spine flexion, passive spine resistance, and active/passive muscle forces. However, experienced females select similar postures to those of novices when spine loading is critical. This study proposes that the techniques adopted by male experts, male novices, and females (with considerable experience but not categorized as experts) impact their lumbar spine musculoskeletal systems differently. Spinal loads, muscle forces, and passive resistance (muscle and ligamentous spine) were predicted by a multi-joint EMG-assisted optimization musculoskeletal model of the lumbar spine. Expert males flexed their lumbar spine less (avg. 21.9° vs 30.3-31.7°) and showed decreased passive internal moments (muscle avg. 8.9% vs 15.9-16.0%; spine avg. 4.7% vs 7.1-7.8%) and increased active internal moments (avg. 72.9% vs 62.0-63.9%), thus producing a different impact on their lumbar spine musculoskeletal systems. Experienced females sustained the highest relative spine loads (compression avg. 7.3 N/BW vs 6.2-6.4 N/BW; shear avg. 2.3 N/BW vs 1.7-1.8 N/BW) in addition to passive muscle and ligamentous spine resistance similar to novices. Combined with smaller body size, less strength, and the sequential lifting technique used by females, this could potentially mean greater risk of back injury. Workers should be trained early to limit excessive and repetitive stretching of their lumbar spine passive tissues.

Suppression of Multiphoton Resonances in Driven Quantum Systems via Pulse Shape Optimization.

This Letter demonstrates control over multiphoton absorption processes in driven two-level systems, which include, for example, superconducting qubits or laser-irradiated graphene, through spectral shaping of the driving pulse. Starting from calculations based on Floquet theory, we use differential evolution, a general purpose optimization algorithm, to find the Fourier coefficients of the driving function that suppress a given multiphoton resonance in the strong field regime. We show that the suppression of the transition probability is due to the coherent superposition of high-order Fourier harmonics which closes the dynamical gap between the Floquet states of the two-level system.

Coherent destruction of tunneling in graphene irradiated by elliptically polarized lasers.

Photo-induced transition probabilities in graphene are studied theoretically from the viewpoint of Floquet theory. Conduction band populations are computed for a strongly, periodically driven graphene sheet under linear, circular, and elliptic polarization. Features of the momentum spectrum of excited quasi-particles can be directly related to the avoided crossing of the Floquet quasi-energy levels. In particular, the impact of the ellipticity and the strength of the laser excitation on the avoided crossing structure-and on the resulting transition probabilities-is studied. It is shown that the ellipticity provides an additional control parameter over the phenomenon of coherent destruction of tunneling in graphene, allowing one to selectively suppress multiphoton resonances.

A biomechanical comparison between expert and novice manual materials handlers using a multi-joint EMG-assisted optimization musculoskeletal model of the lumbar spine.

Expertise is a key factor modulating the risk of low back disorders (LBD). Through years of practice in the workplace, the typical expert acquires high level specific skills and maintains a clean record of work-related injuries. Ergonomic observations of manual materials handling (MMH) tasks show that expert techniques differ from those of novices, leading to the idea that expert techniques are safer. Biomechanical studies of MMH tasks performed by experts/novices report mixed results for kinematic/kinetic variables, evoking potential internal effect of expertise. In the context of series of box transfers simulated by actual workers, detailed internal loads predicted by a multiple-joint EMG-assisted optimization lumbar spine model are compared between experts and novices. The results confirmed that the distribution of internal moments are modulated by worker expertise. Experts flexed less their lumbar spine and exerted more active muscle forces while novices relied more on passive resistance of the muscles and ligamentous spine. More specifically for novices, the passive contributions came from global extensor muscles, selected local extensor muscles, and passive structures of the lumbar spine (ligaments and discs). The distinctive distribution of internal forces was not concomitant with a similar effect on joint forces, these forces being dependent on external loading which was equivalent between experts and novices. From a safety standpoint, the present results suggest that experts were more efficient than novices in partitioning internal moment contributions to balance net (external) loading. Thus, safer handling practices might be seen as a result of experts׳ experience.

Optimization of integrated polarization filters.

This study reports on the design of small footprint, integrated polarization filters based on engineered photonic lattices. Using a rods-in-air lattice as a basis for a TE filter and a holes-in-slab lattice for the analogous TM filter, we are able to maximize the degree of polarization of the output beams up to 98% with a transmission efficiency greater than 75%. The proposed designs allow not only for logical polarization filtering, but can also be tailored to output an arbitrary transverse beam profile. The lattice configurations are found using a recently proposed parallel tabu search algorithm for combinatorial optimization problems in integrated photonics.

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.

Low amplitude characterization tests conducted at regular intervals can affect tendon mechanobiological response.

In bioreactor studies of tissue mechanobiology, characterizing changes in tissue quality is essential for understanding and predicting the response to mechanical stimuli. Unfortunately, current methods are often destructive and cannot be used at regular intervals on the same sample to characterize progression over time. Non-destructive methods such as low amplitude stress relaxation tests could be used, but then, the following dilemma comes into play: how can we accurately measure live tissue progression over time if the tissue is reacting to our measurement methods? In this study, we investigated the hypothesis that stress relaxation tests at physiological amplitudes conducted at regular intervals between stimulation periods do not modify tissue progression over time. Live, healthy tendons were subjected to mechanical stimuli inside a bioreactor for 3 days. The tendons were grouped based on the daily characterization protocol (24 or 0 stress relaxation tests) and their progression over time were compared. Stress relaxation tests at physiological amplitudes modified the tendon response to mechanical stimulation as observed through mechanical and histologic analyses. Possible solutions to eliminate or minimize the effect of stress relaxation tests are to use the mechanical stimuli to characterize tissue progression or to limit the number of stress relaxation tests.

Multiobjective optimization in integrated photonics design.

We propose the use of the parallel tabu search algorithm (PTS) to solve combinatorial inverse design problems in integrated photonics. To assess the potential of this algorithm, we consider the problem of beam shaping using a two-dimensional arrangement of dielectric scatterers. The performance of PTS is compared to one of the most widely used optimization algorithms in photonics design, the genetic algorithm (GA). We find that PTS can produce comparable or better solutions than the GA, while requiring less computation time and fewer adjustable parameters. For the coherent beam shaping problem as a case study, we demonstrate how PTS can tackle multiobjective optimization problems and represent a robust and efficient alternative to GA.

Criterion validity and between-day reliability of an inertial-sensor-based trunk postural stability test during unstable sitting.

INTRODUCTION: Adequate neuromuscular control of the lumbar spine is required to prevent lumbar injuries. A trunk postural stability test has been proposed earlier, using a chair wobbling on a central pivot and four springs with adjustable positions to modulate task difficulty. An inertial sensor is fixed on the chair to measure postural sway. The aim of this study is to assess the criterion validity and between-day reliability of the calibration and testing components. METHODS: Thirty six subjects (with and without low back pain) followed a calibration procedure, four practice trials and three 60-s trials on 2days. The criterion validity of the inertial sensor was tested against an optoelectronic system and a force platform. The reliability of 38 body sway measures obtained from the inertial sensor angular measures was estimated. RESULTS: The inertial sensor led to valid estimates of postural sway. The reliability of the calibration procedure was moderate. Practically no learning effect was detected except for a few body sway measures in patients with CLBP. Three 60-s trials provided acceptable reliability for approximately half of the body sway measures, although this is more difficult to achieve in patients with CLBP. DISCUSSION: The use of an easy to use inertial sensor led to valid measures of postural sway. A number of body sway measures were identified as reliable tools for individual follow-ups but inter-subject comparisons were anticipated as more difficult when patients with CLBP are involved.

Beam shaping using genetically optimized two-dimensional photonic crystals.

We propose the use of two-dimensional (2D) photonic crystals (PhCs) with engineered defects for the generation of an arbitrary-profile beam from a focused input beam. The cylindrical harmonics expansion of complex-source beams is derived and used to compute the scattered wave function of a 2D PhC via the multiple scattering method. The beam shaping problem is then solved using a genetic algorithm. We illustrate our procedure by generating different orders of Hermite-Gauss profiles, while maintaining reasonable losses and tolerance to variations in the input beam and the slab refractive index.

Relative importance of expertise, lifting height and weight lifted on posture and lumbar external loading during a transfer task in manual material handling.

The objective of this study was to measure the effect size of three important factors in manual material handling, namely expertise, lifting height and weight lifted. The effect of expertise was evaluated by contrasting 15 expert and 15 novice handlers, the effect of the weight lifted with a 15-kg box and a 23-kg box and the effect of lifting height with two different box heights: ground level and a 32 cm height. The task consisted of transferring a series of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables (moments) while expertise had low impact. On the other hand, expertise showed a significant effect of posture variables on the lumbar spine and knees. All three factors are important, but for a reduction of external back loading, the focus should be on the lifting height and weight lifted. PRACTITIONER SUMMARY: The objective was to measure the effect size of three important factors in a transfer of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables but expertise was a major determinant in back posture.

An improved multi-joint EMG-assisted optimization approach to estimate joint and muscle forces in a musculoskeletal model of the lumbar spine.

Muscle force partitioning methods and musculoskeletal system simplifications are key modeling issues that can alter outcomes, and thus change conclusions and recommendations addressed to health and safety professionals. A critical modeling concern is the use of single-joint equilibrium to estimate muscle forces and joint loads in a multi-joint system, an unjustified simplification made by most lumbar spine biomechanical models. In the context of common occupational tasks, an EMG-assisted optimization method (EMGAO) is modified in this study to simultaneously account for the equilibrium at all lumbar joints (M-EMGAO). The results of this improved approach were compared to those of its conventional single-joint equivalent (S-EMGAO) counterpart, the latter method being applied to the same lumbar joints but one at a time. Despite identical geometrical configurations and passive contributions used in both models, computed outcomes clearly differed between single- and multi-joint methods, especially at larger trunk flexed postures and during asymmetric lifting. Moreover, muscle forces predicted by L5-S1 single-joint analyses do not maintain mechanical equilibrium at other spine joints crossed by the same muscles. Assuming that the central nervous system does not attempt to balance the external moments one joint at a time and that a given muscle cannot exert different forces at different joints, the proposed multi-joint method represents a substantial improvement over its single-joint counterpart. This improved approach, hence, resolves trunk muscle forces with biological integrity but without compromising mechanical equilibrium at the lumbar joints.

Gait patterns comparison of children with Duchenne muscular dystrophy to those of control subjects considering the effect of gait velocity.

3D analysis of the gait of children with Duchenne muscular dystrophy (DMD) was the topic of only a few studies and none of these considered the effect of gait velocity on the gait parameters of children with DMD. Gait parameters of 11 children with DMD were compared to those of 14 control children while considering the effect of gait velocity using 3D biomechanical analysis. Kinematic and kinetic gait parameters were measured using an Optotrak motion analysis system and AMTI force plates embedded in the floor. The data profiles of children with DMD walking at natural gait velocity were compared to those of the control children who walked at both natural and slow gait velocities. When both groups walked at similar velocity, children with DMD had higher cadence and shorter step length. They demonstrated a lower hip extension moment as well as a minimal or absent knee extension moment. At the ankle, a dorsiflexion moment was absent at heel strike due to the anterior location of the center of pressure. The magnitude of the medio-lateral ground reaction force was higher in children with DMD. Despite this increase, the hip abductor moment was lower. Hip power generation was also observed at the mid-stance in DMD children. These results suggest that most of the modifications observed are strategies used by children with DMD to cope with possible muscle weakness in order to provide support, propulsion and balance of the body during gait.

Offering proper feedback to control for out-of-plane lumbar moments influences the activity of trunk muscles during unidirectional isometric trunk exertions.

The assessment of trunk muscle activation and coordination using dynamometric measurements made in one anatomical plane has hardly minimized the production of out-of-plane coupled moments (CMs). This absence of control may add much variability in moment component partition as well as in recorded muscle activation. The aim of the study was to assess whether providing these CMs as visual feedback efficiently reduces them and whether this feedback influences trunk muscle activation. Twenty men performed five 5-s static ramp submaximal contractions, ranging from 0% to 55% of the maximal voluntary contraction (MVC), in six different directions while standing in a static dynamometer measuring L5/S1 moments. For each direction, four feedback conditions were offered, ranging from simple 1D-feedback in the primary plane of exertion, to full 3D-feedback. Surface electromyographic signals were collected for eight back and six abdominal muscles. Muscle activation amplitudes and CMs were extracted at each 10% force level from 10% to 50% maximum voluntary contraction (MVC). Providing 3D-feedback significantly reduced the CMs, at 50% MVC, by about 1-6%, 1-8% and 2-10% MVC in the sagittal, frontal and transverse planes, respectively. Providing 3D-feedback produced relatively small systematic effects (2-7%) on trunk muscle activation. However, the subjects responded differently to adequately control the coupled moments, leading in some cases to relatively high inter-individual differences in muscle activation. Interestingly, the statistical differences, and size of the effects, were mainly observed when the primary exertions were performed in the frontal and transverse planes. The implications of these findings are discussed.

A submaximal test to assess back muscle capacity: evaluation of construct validity.

A functional endurance test more specific to common occupational tasks is proposed for assessing back muscle capacity. The test involves static intermittent contractions (8-s work-rest cycles) using a predefined absolute load (90 Nm) across subjects. Since the test involved an absolute endurance task, it was hypothesized that performance would be influenced by both the strength and relative endurance of the subjects, thus demonstrating the construct validity of this new test. Fifteen males and 17 females were assessed on three different days to allow familiarization and to measure their Strength as well as their absolute and relative endurance. Absolute and relative endurance were defined as the time to reach exhaustion (Tend(abs) and Tend(rel), respectively) during a fatigue protocol including both an absolute (90 Nm) and a relative (40% of individual strength) load (extension moment at the L5/S1 joint). Strength and Tend(rel) each explained an almost equivalent portion of Tend(abs) (total variance explained: 61.5%), thus confirming the construct validity of the functional endurance test. This new test should better identify the back muscle impairments (weakness, fatigability) often observed in chronic low-back-pain patients.

Toward the development of predictive equations of back muscle capacity based on frequency- and temporal-domain electromyographic indices computed from intermittent static contractions.

BACKGROUND CONTEXT: Back muscles capacity is impaired in chronic low back pain patients but no motivation-free test exists to measure it. A functional endurance test (FET) was used to assess capacity of back muscles using surface electromyographic (EMG) indices as outcome measures. PURPOSE: The main objective of the present study was to explore the possibility of combining different types of EMG indices to predict absolute endurance and strength. STUDY DESIGN/SETTING: A cross-sectional study using a repeated measures design in laboratory setting. METHODS: Healthy subjects (44 men and 29 women) performed maximal voluntary contractions (Strength criterion: extension moment at L5/S1) and a fatigue test involving intermittent static extension contractions to exhaustion (Tend endurance criterion: time to reach exhaustion). Surface EMG signals were collected from four pairs of back muscles. From the first 5 minutes (women) or 10 minutes (men) of EMG data, frequency- and temporal-domain analyses were applied to compute various EMG indices. RESULTS: Strength values ranged from 153 to 508Nm, whereas Tend values ranged from 3 to 57 minutes across the subjects. Gender-specific multiple regression equations were developed, using the retained EMG indices from the four electrode sites, to predict Tend (men: R(2)=0.76, error=9%; women: R(2)=0.70, error=17%) and Strength (men: R(2)=0.72, error=9%; women: R(2)=0.25, error=13%). CONCLUSIONS: It appears to be possible to predict the capacity of back muscles using an intermittent and time-limited (submaximal) fatigue task. Frequency- and temporal-domain EMG indices were shown to provide complementary information in this respect. This FET has potential to better infer back muscle capacity for realistic occupational tasks because more specific muscle fatigue mechanisms are involved.

The assessment of back muscle capacity using intermittent static contractions. Part II: validity and reliability of biomechanical correlates of muscle fatigue.

INTRODUCTION: In a previous paper, standard surface electromyographic (EMG) indices of muscle fatigue, which are based on the lowering of the median or mean frequencies of the EMG power spectrum in time, were applied during an intermittent absolute endurance test and were evaluated relative to criterion validity and test-retest reliability. The aims of this study were to assess mechanical and alternative EMG correlates of muscle fatigue. METHODS: Healthy subjects (44 males and 29 females; age: 20-55 yrs) performed three maximal voluntary contractions (MVC) and an endurance test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The test, assessing absolute endurance (90 Nm torque), consisted of performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the endurance test. Mechanical indices quantifying physiological tremor and steadiness were computed from the dynamometer signals (L5/S1 extension moments) along with EMG indices presumably sensitive to variable load sharing between back muscle synergists during the endurance test. RESULTS: Mechanical indices were significantly correlated to Tend (r range: -0.47 to -0.53) but showed deceiving reliability results. Conversely, the EMG indices were correlated to Tend (r range: -0.43 to -0.63) with some of them particularly correlated to Strength (r=-0.72 to -0.81). In addition, their reliability results were acceptable (intra-class correlation coefficient >0.75; standard error of measurement <10% of the mean) in many cases. Finally, several analyses substantiated their physiological relevance. These findings imply that these new EMG indices could be used to predict absolute endurance as well as strength with the use of a single intermittent and time-limited (5-10min) absolute endurance test, a practical way to assess the back capacity of chronic low back pain subjects.

The assessment of back muscle capacity using intermittent static contractions. Part I - Validity and reliability of electromyographic indices of fatigue.

INTRODUCTION: Back muscle capacity is impaired in chronic low back pain patients but no motivation-free test exists to measure it. The aims of this study were to assess the reliability and criterion validity of electromyographic indices of muscle fatigue during an intermittent absolute endurance test. METHODS: Healthy subjects (44 males and 29 females; age: 20-55 yrs) performed three maximal voluntary contractions (MVC) and a fatigue test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The fatigue test, assessing absolute endurance (90-Nm torque), consisted in performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the fatigue test. From the first five min (females) or ten min (males) of EMG data, frequency and time-frequency domain analyses were applied to compute various spectral indices of muscle fatigue. RESULTS: The EMG indices were more reliable when computed from the time-frequency domain than when computed from the frequency domain, but showed comparable correlation results (criterion validity) with Tend and Strength. Some EMG indices reached moderate to good correlation (range: 0.64-0.69) with Tend, lower correlations (range: 0.39-0.55) with Strength, and good to excellent between-day test-retest reliability results (intra-class correlation range: 0.75-0.83). The quantification of the spectral content more locally in different frequency bands of the power spectrum was less valid and reliable than the indices computed from the entire power spectrum. Differences observed among muscles were interpreted in light of specific neuromuscular activation levels that were observed during the endurance test. These findings supported the use of an intermittent and time-limited (5-10min) absolute endurance test, that is a practical way to assess the back capacity of chronic low back pain subjects, to assess absolute endurance as well as strength with the use of electromyographic indices of muscle fatigue.