Endurance time is joint-specific: a modelling and meta-analysis investigation

Improving dynamic endurance time predictions for shoulder fatigue: A comparative evaluation

Work-related musculoskeletal disorders (WMSDs) are commonplace in industry and a host of qualitative and quantitative approaches have been used to assuage the problem, including wearable sensors and biomechanical endurance models, both of which were used in the present study. Six endurance models (consumed endurance, new improved consumed endurance and the exponential and power Frey Law and Avin general and shoulder models) with four alternative maximum torque (Torquemax) quantification methods, including a novel approach to generate Torquemax, were compared. The proposed approach to quantify Torquemax, in combination with the new improved consumed endurance model produced the lowest root mean square errors (RMSE), and indicated improved performance compared to the literature. The mean RMSE was reduced from 41.08s to 19.11s for all subjects, from 26.13s to 12.16s for males, and 51.28s to 24.45s for females using the proposed method. R2 for 25% and 45% standardised intensity dynamic tasks were .459 and .314 respectively, P < .01. This research provided an optimised and individualised endurance prediction approach for loaded dynamic movements which can be applied to industry tasks and may lead to reduced upper-limb strains, and potentially WMSDs.

A predictive equation for maximum acceptable efforts based on duty cycle in repetitive back-involved tasks

INTRODUCTION

There are many different tasks involved within a workplace, and assessing the required efforts for performing them depends on factors such as the human's body posture, task nature, and number of repetitions. This study aims to develop an equation for back-involved repetitive tasks that relates the maximum acceptable effort (MAE), the maximum acceptable efforts that an individual can sustain for a specific task and is expressed as a percentage of the maximum strength, to the duty cycle, the amount of time an individual is engaged in a task relative to the total time. The equation was derived based on psychophysical data collected from previous studies on lifting, lowering, and carrying tasks. The literature search identified studies reporting maximum acceptable loads (e.g., forces and toques) for back-involved tasks.

METHOD

Data analysis was done by calculating duty cycles and for each task. Statistical tests were conducted to compare the results across different parameters, such as sex, task nature, lifting box size, box distance from the body, and population percentages.

RESULTS

The results showed a strong negative relationship between duty cycle and MAE. This relationship shows that by increasing the duty cycle, MAE should be decreased to be acceptable and prevent worker's fatigue. The developed equation was compared to existing equations for upper-limb tasks and demonstrated a close resemblance. Additionally, statistical analysis indicated that the proposed equation eliminated the effects of various parameters. The proposed equation provides an individual-specific approach for estimating MAEs and can contribute to preventing workers' fatigue and injury and reducing their associated costs.

Forearm elevation impairs local static handgrip endurance likely through reduction in vascular conductance and perfusion pressure: revisiting Rohmert's curve

Maximal isometric contraction time (MICT) is critical for most motor tasks and depends on skeletal muscle blood flow at < 40% of maximal voluntary strength (MVC). Whether limb work positions associated with reduced perfusion pressure and facilitated vessel compression affect MICT is largely unknown. In 14 healthy young men we therefore assessed bilateral handgrip MICT at 15, 20, 30, 40, and 70% of MVC in horizontal forearm positions of 0.0, + 27.5 or - 27.5 cm relative to heart level. Forearm blood flow (FBF, venous occlusion plethysmography) and brachial blood pressure were measured repetitively. MICT at 15% MVC was significantly shorter by 66.3 and 86.2 s with forearm position + 27.5 cm (389.6 ± 23.3 s) as compared to 0.0 cm (455.9 ± 34.1 s) and - 27.5 cm (475.8 ± 35.0 s) while MICT at 20-70% MVC was unaffected. Peak FBF at 15% MVC was significantly lower in position + 27.5 cm (11.11 ± 0.92 ml/min/100 ml) compared to 0.0 cm (15.55 ± 0.91 ml/min/100 ml) or - 27.5 cm (14.21 ± 0.59 ml/min/100 ml) and vascular resistance significantly higher in position + 27.5 vs 0.0 or - 27.5 cm. Working position above, but not below heart level may limit MICT at 15% MVC possibly through blood flow reduction arising from increased vascular resistance beside reduced perfusion pressure. Local isometric endurance warrants (re)evaluation regarding hydrostatic/gravitational or other hemodynamic limitations.

A Non-Weight Bearing Method for Measuring Hip Abduction Strength Overestimates Hip Abductor Muscle Fatigue During One-Leg Stance

OCCUPATIONAL APPLICATIONSAssessing workers' strength capacities is a common practice prior to return to work following injury or illness, or assessing capabilities for strenuous jobs. Because it requires 50% or more of maximum strength capacity, hip abductor muscle strength is a strong predictor of both middle- and older-aged individuals' ability to reliably balance on one leg and of their risk of falls. Our results suggest subjects were able to augment their hip abductor moment during unipedal weight stance via gluteus maximus activity. Weight-bearing hip abduction strength measures are important for assessing worker capacity for jobs requiring reliable unipedal balance whether during lateral loading, while walking in gusty winds, on slippery footing or resisting lateral deck movements on board ship or train. Measurements of hip abductor strength should be made in a full unipedal weight bearing posture; non-weight-bearing measurements significantly underpredicted hip abductor strength as well as endurance.

Muscle activity during crouched walking

OBJECTIVES

Muscle activity during crouched walking has been previously studied in the context of the evolution of hominin bipedalism and human movement disorders. However, crouched walking could also be used in approach hunting where postural height (actual height of the body from the ground to the top of the head during locomotion) is the limiting factor. Here, we aim to analyze the relationship between relative postural height (%stature), kinematics, and muscle activity during crouched walking.

MATERIALS AND METHODS

Adult males (n = 19) walked with extended limbs and at three degrees of crouch while their 3D motion capture kinematics and lower limb muscle electromyography were recorded. We measured activation of tibialis anterior, soleus, gastrocnemius medialis, gastrocnemius lateralis, vastus lateralis, rectus femoris, biceps femoris, and gluteus maximus. We analyzed the effects of postural height on kinematics and muscle activation using linear mixed effects model.

RESULTS

Flexion angles, individual muscle activation (except for medial gastrocnemius), and total muscle activation were negatively related to relative postural height, that is, were greater at more crouched postures. Relative postural height had a stronger effect on the activation of the thigh and gluteal muscles compared to shank muscles.

DISCUSSION

General increase in lower limb muscle activation at lower postural heights suggests a negative relationship between relative postural height and fatigue, and may indicate a possible mechanism by which short stature could benefit the hunter in approach hunting. Greater activation of thigh and gluteal muscles relative to shank muscles may help to identify crouched walking in past human populations.

ANALYSIS OF INDUCED ISOMETRIC FATIGUING CONTRACTIONS IN BICEPS BRACHII MUSCLES USING MYOTONOMETRY AND SURFACE ELECTROMYOGRAPHIC MEASUREMENTS

Viscoelastic properties of skeletal muscle tissue are known to be impacted by fatiguing contractions. In this study, an attempt has been made to utilize myotonometry for analyzing the relationship between muscle viscoelasticity and contractile behaviors in a fatiguing task. For this purpose, thirteen young healthy volunteers are recruited to perform the fatiguing isometric task and the time to task failure (TTF) is recorded. Myotonometric parameters and simultaneous surface electromyographic (sEMG) signals are recorded from the Biceps Brachii muscle of the flexed arm. The correlation between myotonometric parameters and TTF is further analyzed. Cross-validation with sEMG features is also performed. Stiffness of muscle has a positive correlation with TTF in the left hand ([Formula: see text]). Damping property of the nonfatigued muscle is positively associated with the fatigue-induced changes in amplitude features of sEMG signal in the right hand ([Formula: see text]). The normalized rate of change of mean frequency of sEMG signal has a positive correlation with stiffness values in both of the hands ([Formula: see text]). Muscle viscoelasticity is demonstrated to influence the progression of fatigue, although the difference in motor control due to handedness is also found to be an important factor. The results are promising to improve the understanding of the effect of muscle mechanics in fatigue-induced task failure.

An Empirical and Subjective Model of Upper Extremity Fatigue Under Hypogravity

In the context of extra-terrestrial missions, the effects of hypogravity (0 < G < 1) on the human body can reduce the well-being of the crew, cause musculoskeletal problems and affect their ability to perform tasks, especially during long-term missions. To date, studies of the effects of hypogravity on human movement are limited to experiments on the lower limbs. Here, we extend the knowledge base to the upper limbs, by conducting experiments to evaluate the effect of hypogravity on upper limb physical fatigue and mental workload in participants. Our hypothesis was that hypogravity would both increase participant productivity, by reducing overall physical fatigue expressed in Endurance Time, and reduce mental workload. Task Intensity-Endurance time curves are developed especially in seated positions, while performing static, dynamic, repetitive tasks. This experiment involved 32 healthy participants without chronic problems of the musculoskeletal system aged 33.59 ± 8.16 years. Using the collected data, fatigue models were constructed for tasks of varying Intensity. In addition, all participants completed the NASA – Task Load Index subjective mental workload assessment, which revealed the level of subjective workload when executing different tasks. We found two trends in the empirical fatigue models associated with the difference between the strength capabilities of males and females. The first is a significant positive (p = 0.002) relation between Endurance time and gravity level (⅙ G Moon, ⅓ G Mars, 1G) with negative coefficient for males and females for a static task. And there is marginal relation (p < 0.1) between overall mental workload and gravity level with a positive coefficient for males and females for the same task. The same trend was observed for dynamic and repetitive tasks. We concluded that the Task Intensity-Endurance Time model, adapted to hypogravity in combination with subjective mental assessment, is useful to human fatigue investigation. The combination of these methods used for ergonomic analysis and digital human modeling, could improve worker productivity. Finally, this study may help prepare astronauts for long-term missions on the Moon and Mars and improve our understanding of how we can prevent musculoskeletal disorders caused by hazardous manual handling under such extreme environments.

The Reliability and Validity of Gluteal Endurance Measures (GEMs)

BACKGROUND

The gluteals have unique morphology related to muscle endurance, including moderate fiber sizes and a majority of Type I endurance fibers. Evidence suggests gluteal endurance is related to low back pain, running kinematics, balance, posture, and more. However, reliable and valid measures specific to gluteal endurance are lacking in the literature.

HYPOTHESIS/PURPOSE

The purpose of this study was to examine the intra- and inter-rater reliability of two gluteal endurance measures (GEMs) for clinical use. It also aimed to examine validity for the two measures by using electromyography (EMG), recording reasons for task failure, and analyzing differences between demographic groups.

STUDY DESIGN

Cross-Sectional.

METHODS

Sixty-eight males and females with and without recurrent low back pain aged 18-35 years were recruited from a university population. Electromyography electrodes were placed on subjects' gluteus maximus and gluteus medius, and each subject performed three trials of GEM-A (abduction endurance) and GEM-B (bridging endurance). Hold times, EMG median frequency (MF) data, and subjective reasons for task failure were analyzed.

RESULTS

Both GEMs demonstrated high intra-rater reliability (ICC = 0.87-0.94) and inter-rater reliability (ICC = 0.99). Mean hold times were 104.83 ± 34.11 seconds for GEM-A (abduction endurance) and 81.03 ± 24.79 seconds for GEM-B (bridging endurance). No statistically significant difference was found between subjects with and without recurrent LBP. Median frequency data validated the onset of gluteal fatigue during both measures. Posterolateral hip (gluteal) fatigue was reported as the primary reason for task failure in 93% and 86% of subjects for GEM-A and GEM-B, respectively.

CONCLUSION

This seminal study of GEM-A (abduction endurance) and GEM-B (bridging endurance) found both measures to be reliable and valid measures of gluteal endurance. Further examination of the GEMs in samples with different types of LBP or hip pain is recommended.

LEVEL OF EVIDENCE

3.

sEMG-Based Hand Posture Recognition Considering Electrode Shift, Feature Vectors, and Posture Groups

Surface electromyography (sEMG)-based gesture recognition systems provide the intuitive and accurate recognition of various gestures in human-computer interaction. In this study, an sEMG-based hand posture recognition algorithm was developed, considering three main problems: electrode shift, feature vectors, and posture groups. The sEMG signal was measured using an armband sensor with the electrode shift. An artificial neural network classifier was trained using 21 feature vectors for seven different posture groups. The inter-session and inter-feature Pearson correlation coefficients (PCCs) were calculated. The results indicate that the classification performance improved with the number of training sessions of the electrode shift. The number of sessions necessary for efficient training was four, and the feature vectors with a high inter-session PCC (r > 0.7) exhibited high classification accuracy. Similarities between postures in a posture group decreased the classification accuracy. Our results indicate that the classification accuracy could be improved with the addition of more electrode shift training sessions and that the PCC is useful for selecting the feature vector. Furthermore, hand posture selection was as important as feature vector selection. These findings will help in optimizing the sEMG-based pattern recognition algorithm more easily and quickly.

Effects of handle height and load on the endurance time for simulated demolition tasks

BACKGROUND

Manual demolition tasks are heavy physical demanding tasks which involve forceful exertion of sustained pushing. They result in muscle fatigue which could lead to musculoskeletal disorders. Assessments of maximum endurance time (MET) are essential in understanding the developing of muscle fatigue for these tasks.

OBJECTIVE

The objectives of this study were to determine the effects of handle height and load conditions on the MET, and to establish MET models for the simulated demolition tasks.

METHODS

Twenty three male participants performed simulated demolition tasks under three loads and three handle heights conditions until they could not do so any longer. Their METs and ratings of perceived exertion were recorded and analyzed.

RESULTS

The results showed that both load and handle height were significant (p < 0.0001) factors affecting the MET. Regression models to predict the MET under handle height and load conditions were established. The mean absolute deviations of these models were between 1.91 and 4.84 min.

CONCLUSION

The MET models established may be used to estimate the MET which may be adopted in work/rest arrangement for demolition tasks using a handheld demolition hammer.

Functional muscle group- and sex-specific parameters for a three-compartment controller muscle fatigue model applied to isometric contractions

The three-compartment controller with enhanced recovery (3CC-r) model of muscle fatigue has previously been validated separately for both sustained (SIC) and intermittent isometric contractions (IIC) using different objective functions, but its performance has not yet been tested against both contraction types simultaneously using a common objective function. Additionally, prior validation has been performed using common parameters at the joint level, whereas applications to many real-world tasks will require the model to be applied to agonistic and synergistic muscle groups. Lastly, parameters for the model have previously been derived for a mixed-sex cohort not considering the differece in fatigabilities between the sexes. In this work we validate the 3CC-r model using a comprehensive isometric contraction database drawn from 172 publications segregated by functional muscle group (FMG) and sex. We find that prediction errors are reduced by 19% on average when segregating the dataset by FMG alone, and by 34% when segregating by both sex and FMG. However, minimum prediction errors are found to be higher when validated against both SIC and IIC data together using torque decline as the outcome variable than when validated sequentially against hypothesized SIC intensity-endurance time curves with endurance time as the outcome variable and against raw IIC data with torque decline as the outcome variable.

A comparison of work-rest models using a "breakpoint" analysis raises questions

OCCUPATIONAL APPLICATIONSDesigning sustainable cyclic work requires attention to both the workload amplitude as well as the duty cycle, the fraction of the work cycle with active workload, that therefore also defines the recovery phase of the cycle. A number of different approaches and models have been developed to calculate the required recovery time for a given load and duty cycle. We present a comparison of three types of models at the "breakpoint" that defines the boundary of load amplitude and duty cycle where fatigue begins to accumulate faster than recovery allows within the work cycle. This comparison shows considerable variation between models of the "allowable" load or duty cycle depending on the method used. Practitioners should thus be cautious applying these models indiscriminately in job design as their results can vary substantially. In particular, differences between the tasks used for model formulation and application may compromise validity, and model application in a given context should be verified before broad application.

On the importance of the hip abductors during a clinical one legged balance test: A theoretical study

Background

The ability to balance on one foot for a certain time is a widely used clinical test to assess the effects of age and diseases like peripheral neuropathy on balance. While state-space methods have been used to explore the mechanical demands and achievable accelerations for balancing on two feet in the sagittal plane, less is known about the requirements for sustaining one legged balance (OLB) in the frontal plane.

Research question

While most studies have focused on ankle function in OLB, can age and/or disease-related decreases in maximum hip abduction strength also affect OLB ability?

Methods

A two-link frontal plane state space model was used to define and explore the ‘feasible balance region’ which helps reveal the requirements for maintaining and restoring OLB, given the adverse effects of age and peripheral neuropathy on maximum hip and ankle strengths.

Results

Maintaining quasistatic OLB required 50%-106% of the maximum hip abduction strength in young and older adults, and older patients with peripheral neuropathy. Effectiveness of a ‘hip strategy’ in recovering OLB was heavily dependent on the maximum hip abduction strength, and for healthy older women was as important as ankle strength. Natural reductions of strength due to healthy aging did not show a meaningful reduction in meeting the strength requirement of clinical OLB. However deficits in hip strength typical of patients with peripheral neuropathy did adversely affect both quasistatic OLB and recoverable OLB states.

Significance

The importance of hip muscle strength has been underappreciated in the clinical OLB test. This is partly because the passive tissues of the hip joint can mask moderate deficits in hip abduction strength until it is needed for recovering OLB. Adding a follow up OLB test with a slightly raised pelvis would be a simple way to check for adequate hip abductor muscle strength.

Joint moment trade-offs across the upper extremity and trunk during repetitive work

Individuals can coordinate small kinematic changes at several degrees of freedom simultaneously in the presence of fatigue, leaving it unclear how overall biomechanical demands at each joint are altered. The purpose of this study was to evaluate trade-offs in joint moments between the trunk, shoulder, and elbow during repetitive upper extremity work. Participants performed four simulated workplace tasks cyclically until meeting fatigue termination criteria. Emergent fatigue-induced adaptations to repetitive work resulted in task-dependent trade-offs in joint moments. In general, reduced shoulder moments were compensated for by increased elbow and trunk joint moment contributions. Although mean joint moment changes were modest (range: 1-3 Nm) across participants, a wide distribution of responses was observed, with standard deviations exceeding 10 Nm. Re-distributing biomechanical demands across joints may alleviate constant tissue loads and facilitate continued task performance with fatigue but may be at the expense of increasing demands at adjacent joints.

The finger flexors occlusion threshold in sport-climbers: an exploratory study on its indirect approximation

Blood flow partially determines specific climbing endurance (SCE) as it mediates oxygen bio-availability in the finger flexors. Blood flow is related to occlusion threshold (OT), which is defined as the contraction intensity at which intramuscular pressure exceeds perfusion blood pressure resulting in the cessation of local blood flow. The OT is represented as an inflection point on a force-time graph when isometric force is registered and applied through maximal and continuous tests. Endurance time (ET) to exhaustion is influenced by the relative isometric applied force and is different for each climber. The aim of this study was to explore whether an approximation of the finger flexoŕs OT in sport climbers through records of ET to exhaustion at different isometric relative intensities was possible. We measured maximum finger hang ETs at 6 intensities ranging from 85% to 35% maximal force in 34 sport climbers of advanced and elite level. The values obtained were analysed by two different methods in an attempt to determine a change in the shape of the curve in the intensity-ET relationship graphs that approximated the OT for each climber. The results suggest that the finger flexoŕs OT could be different among climbers, regardless of their strength and ability level. The presented methods do not accurately reflect the OT, but could indicate the intensity at which blood flow is restored in the active muscles. This is the first study to indirectly approximate the finger flexors OT in sport-climbers, a parameter that could be essential to assess SCE.

Adapting a fatigue model for shoulder flexion fatigue: Enhancing recovery rate during intermittent rest intervals

Although the rotator cuff muscles are susceptible to fatigue, shoulder fatigue studies reporting torque decline during intermittent tasks are relatively uncommon in the literature. A previous modification to the three-compartment controller (3CC) fatigue model incorporated a rest recovery multiplier (3CC-r model) to represent augmented blood flow to muscle during rest intervals (Looft et al., 2018). A rest recovery value of r = 15 was optimal for ankle, knee, and elbow joint regions, whereas r = 30 was better for hand/grip muscles. However, shoulder torque decline data was unavailable in the literature for comparison. Thus, the purpose of this study was to collect fatigue data for two different intermittent, isometric shoulder flexion fatiguing tasks and assess the 3CC-r model with r = 15 or 30 compared to the original 3CC model. Twenty healthy participants (9 M) completed two fatigue tasks: 50% maximum voluntary contraction (MVC) with 50% duty cycle (DC) and 70% MVC with 70% DC. MVCs were assessed at discrete time points (1, 3, 5, 10, and 15 min) until endurance time (MET). Mean observed percent torque decline (%TD) for the two tasks were compared to three model estimates: 3CC-r (using r = 15 and r = 30) and 3CC. Using these data, we confirmed that the addition of a rest multiplier (r = 15 somewhat better than r = 30) substantially improved predictions of shoulder fatigue using a previously validated analytical fatigue model (3CC). The relatively large reduction in model errors over the original model suggests the importance of representing augmented recovery during rest periods.

A model-based estimation of critical torques reduces the experimental effort compared to conventional testing

Purpose

Critical torque (CT) is an important fatigue threshold in exercise physiology and can be used to analyze, predict, or optimize performance. The objective of this work is to reduce the experimental effort when estimating CTs for sustained and intermittent isometric contractions using a model-based approach.

Materials and methods

We employ a phenomenological model of the time course of maximum voluntary isometric contraction (MVIC) torque and compute the highest sustainable torque output by solving an optimization problem. We then show that our results are consistent with the steady states obtained when simulating periodic maximum loading schemes. These simulations correspond to all-out tests, which are used to estimate CTs in practice. Based on these observations, the estimation of CTs can be formulated mathematically as a parameter estimation problem. To minimize the statistical uncertainty of the parameter estimates and consequently of the estimated CTs, we compute optimized testing sessions. This reduces the experimental effort even further.

Results

We estimate CTs of the elbow flexors for sustained isometric contractions to be 28% of baseline MVIC torque and for intermittent isometric contractions consisting of a 3 s contraction followed by 2 s rest to be 41% of baseline MVIC torque. We show that a single optimized testing session is sufficient when using our approach.

Conclusions

Our approach reduces the experimental effort considerably when estimating CTs for sustained and intermittent isometric contractions.

Exhaustion of Skeletal Muscle Fibers Within Seconds: Incorporating Phosphate Kinetics Into a Hill-Type Model

Initiated by neural impulses and subsequent calcium release, skeletal muscle fibers contract (actively generate force) as a result of repetitive power strokes of acto-myosin cross-bridges. The energy required for performing these cross-bridge cycles is provided by the hydrolysis of adenosine triphosphate (ATP). The reaction products, adenosine diphosphate (ADP) and inorganic phosphate (P i ), are then used-among other reactants, such as creatine phosphate-to refuel the ATP energy storage. However, similar to yeasts that perish at the hands of their own waste, the hydrolysis reaction products diminish the chemical potential of ATP and thus inhibit the muscle's force generation as their concentration rises. We suggest to use the term "exhaustion" for force reduction (fatigue) that is caused by combined P i and ADP accumulation along with a possible reduction in ATP concentration. On the basis of bio-chemical kinetics, we present a model of muscle fiber exhaustion based on hydrolytic ATP-ADP-P i dynamics, which are assumed to be length- and calcium activity-dependent. Written in terms of differential-algebraic equations, the new sub-model allows to enhance existing Hill-type excitation-contraction models in a straightforward way. Measured time courses of force decay during isometric contractions of rabbit M. gastrocnemius and M. plantaris were employed for model verification, with the finding that our suggested model enhancement proved eminently promising. We discuss implications of our model approach for enhancing muscle models in general, as well as a few aspects regarding the significance of phosphate kinetics as one contributor to muscle fatigue.

Modelling muscle recovery from a fatigued state in isometric contractions for the ankle joint

Current models of localized muscular fatigue are capable of predicting performance in isometric tasks with reasonable accuracy. However, they do not account for the effect of continuously-varying task intensities on muscular recovery from a fatigued state. In this work, we propose and evaluate three continuous functions for modelling recovery to replace a dichotomous step-function in the three-compartment controller (3CC-r) model of muscle fatigue (Looft et al., 2018) and validate their predictions with previously collected data in the literature for intermittent and sustained isometric tasks of the ankle joint performed at different intensities. When compared to experimental data the accuracy of one of the three proposed models of recovery is found to be nearly the same as that yielded by the original step-function, but this seemingly-identical accuracy may be a limitation of the dataset used. A superelliptical curve relating recovery factor to task intensity is proposed to be the closest replacement for the step function as it depicts both the elevated value of recovery factor for near-rest activities as well as a nearly-constant value for low-to-high-intensity tasks.

Ratings of perceived fatigue predict fatigue induced declines in muscle strength during tasks with different distributions of effort and recovery

The purpose of this study was to quantify the relationship between ratings of perceived fatigue (RPF), using a modified Borg CR-10 scale, and muscle fatigue accumulation, as defined by maximal voluntary contraction strength (MVC) declines, during two complex MVC-relative tasks (conditions) that cause muscle fatigue and allow recovery. Nine female participants completed the fatiguing tasks, composed of a series of submaximal, isometric efforts (task plateaus) requiring isometric flexion at the distal interphalangeal joint of the thumb. Significant partial correlations between RPF and MVC, while controlling for task plateau intensity (%MVC), were found in 6/9 participants. A significant linear regression model, explaining 86.2% of the variance in mean MVC decline, was obtained with 3 predictor variables: mean RPF (p < 0.001), Task Plateau (p < 0.001), and the interaction between mean RPF and Task Plateau (RPF × Task Plateau; p = 0.014). The observed linear relationship between RPF and MVC declines, both at the participant and group level support, the use of RPF to estimate the instantaneous fatigue status of the muscle in tasks that allow both muscle fatigue and recovery.

Strength Decrease, Perceived Physical Exertion and Endurance Time for Backpacking Tasks

Manual material handling (MMH) tasks create a burden for workers which could result in musculoskeletal injuries. Assessments of the decrease of muscular strength and the maximum endurance time (MET) for MMH tasks are essential in studying the ergonomic risk of MMH tasks. A backpacking experiment was conducted for measuring the MET for MMH tasks. Human participants carried a load on their back and walked on a treadmill under various load, walking speed, and ramp angle conditions until they coud no longer do so. It was found that the participants were able to walk for approximately 15 min to two hours before they needed to have a pause. Their back and leg strengths declined moderately due to performing the tasks. These tasks resulted in an increase in heart rate and elevated perceived physical exertion. The rating of perceived exertion (RPE)/heart rate ratio in our backpacking tasks was 31% higher than that in the literature, implying the calibration of the RPE may be required for such tasks. A MET model incorporating the f_{MVC_back}, body weight, walking speed, and ramp angle was established. This model may be used to determine the work/rest allowance for backpacking tasks under conditions similar to this study.

Pulling strength, muscular fatigue, and prediction of maximum endurance time for simulated pulling tasks

Truck pulling is one of the common manual materials handling tasks which contribute to musculoskeletal disorders. The maximum endurance time (MET) for two-handed truck pulling tasks has been rarely discussed in the literature. The objectives of this study were to explore the development of muscular fatigue when performing two-handed pulling task and to establish models to predict the MET. A simulated pallet truck pulling experiment was conducted. Sixteen healthy adults including eight females and eight males participated. The participants pulled a handle simulating that of a pallet truck using two hands until they could not pull any longer under two postures. The forces applied for females and males were 139.65 N and 170.03 N, respectively. The maximum voluntary contractions (MVC) of the pulling strength both before and after the simulated pull were measured. After each trial, both the MET and subjective ratings of muscular fatigue on body segments were recorded. The results showed that posture significantly affected MVC of pull both before and after the trial. It was found that foot/shank of the front leg had higher subjective ratings of muscular fatigue than the other body segments. The MET equations employing both power and logarithmic functions were developed to predict the MET of the two-handed pulling tasks. Predictive models established in this study may be used to assess the MET for two-handed pulling tasks.

Modification of a three-compartment muscle fatigue model to predict peak torque decline during intermittent tasks

This study aimed to test whether adding a rest recovery parameter, r, to the analytical three-compartment controller (3CC) fatigue model (Xia and Frey Law, 2008) will improve fatigue estimates during intermittent contractions. The 3CC muscle fatigue model uses differential equations to predict the flow of muscle between three muscle states: Resting (MR), Active (MA), and Fatigued (MF). This model uses a feedback controller to match the active state to target loads and two joint-specific parameters: F, fatigue rate controlling flow from active to fatigued compartments) and R, the recovery rate controlling flow from the fatigued to the resting compartments. This model does well to predict intensity-endurance time curves for sustained isometric tasks. However, previous studies find when rest intervals are present that the model over predicts fatigue. Intermittent rest periods would allow for the occurrence of subsequent reactive vasodilation and post-contraction hyperemia. We hypothesize a modified 3CC-r fatigue model will improve predictions of force decay during intermittent contractions with the addition of a rest recovery parameter, r, to augment recovery during rest intervals, representing muscle re-perfusion. A meta-analysis compiling intermittent fatigue data from 63 publications reporting decline in peak torque (% torque decline) were used for comparison. The original model over-predicted fatigue development from 19 to 29% torque decline; the addition of a rest multiplier significantly improved fatigue estimates to 6-10% torque decline. We conclude the addition of a rest multiplier to the three-compartment controller fatigue model provides a physiologically consistent modification for tasks involving rest intervals, resulting in improved estimates of muscle fatigue.

Modeling of maximum endurance time for static pulling tasks

BACKGROUND

Pallet truck pulling is one of the common manual materials handling tasks which could result in musculoskeletal injuries. The endurance time for sustained truck pulling tasks has been rarely discussed in the literature.

OBJECTIVE

The objectives of this study were to measure the muscular fatigue after performing a pulling task and to establish models to predict the maximum endurance time for truck pulling tasks.

METHODS

A simulated truck pulling experiment was conducted. Ten human participants were recruited. The participants pulled a handle simulating that of a pallet truck under two loading conditions until they could not pull any longer.

RESULTS

The results indicated that hand/wrist and elbow had higher subjective ratings of muscular fatigue than the other body segments. A subjective rating of 5 or more was found on at least two of the body segments for all trials except one. An exponential model and a power model were established to predict the maximum endurance time of the pulling tasks.

CONCLUSION

Hand/wrist and elbow were the body segments most likely to suffer muscular fatigue for the simulated truck pulling tasks. The endurance time models established may be adopted in job designs for one-handed truck pulling tasks.

Performance Fatigability: Mechanisms and Task Specificity

Performance fatigability is characterized as an acute decline in motor performance caused by an exercise-induced reduction in force or power of the involved muscles. Multiple mechanisms contribute to performance fatigability and originate from neural and muscular processes, with the task demands dictating the mechanisms. This review highlights that (1) inadequate activation of the motoneuron pool can contribute to performance fatigability, and (2) the demands of the task and the physiological characteristics of the population assessed, dictate fatigability and the involved mechanisms. Examples of task and population differences in fatigability highlighted in this review include contraction intensity and velocity, stability and support provided to the fatiguing limb, sex differences, and aging. A future challenge is to define specific mechanisms of fatigability and to translate these findings to real-world performance and exercise training in healthy and clinical populations across the life span.

Modelling endurance and resumption times for repetitive one-hand pushing

This study's objective was to develop models of endurance time (ET), as a function of load level (LL), and of resumption time (RT) after loading as a function of both LL and loading time (LT) for repeated loadings. Ten male participants with experience in construction work each performed 15 different one-handed repetaed pushing tasks at shoulder height with varied exerted force and duration. These data were used to create regression models predicting ET and RT. It is concluded that power law relationships are most appropriate to use when modelling ET and RT. While the data the equations are based on are limited regarding number of participants, gender, postures, magnitude and type of exerted force, the paper suggests how this kind of modelling can be used in job design and in further research. Practitioner Summary: Adequate muscular recovery during work-shifts is important to create sustainable jobs. This paper describes mathematical modelling and presents models for endurance times and resumption times (an aspect of recovery need), based on data from an empirical study. The models can be used to help manage fatigue levels in job design.

Experimental validation of a subject-specific maximum endurance time model

This study aimed at experimentally validating a subject-specific maximum endurance time (MET) model. Thirty health participants (15 males and 15 females; Age: mean = 21.5 years, SD = 1.6 years) volunteered to conduct an isometric elbow flexion task until exhaustion. The endurance times of each participant were measured under relative exertion levels ranging from 30% MVC (Maximum Voluntary Contraction) to 70% MVC at 10% intervals. Assessment of the model showed that the intensity-endurance time relationship for each studied individual could be well fitted by the subject-specific MET model (R² > 0.89). The fatigue rates identified from the model fitting were normally distributed (Mean = 0.96 min^-1, SD = 0.29 min^-1). In addition, the fatigue rates of the male group were significantly higher than the female group. The subject-specific MET model can be used to predict the MET for individual workers, and further support physical task design, based on the fatigability data of a targeted worker population. Practitioner Summary: Ergonomists have extensively used MET models in physical fatigue assessment and physical task design. A subject-specific MET model could be used to predict the MET at individual levels, and also to support work design for a target worker population, based on the fatigability data distribution obtained from sampled workers.

Exercise tolerance during muscle contractions below and above the critical torque in different muscle groups

The objective of this study was to test the hypotheses that end-test torque (ET) (expressed as % maximal voluntary contraction; MVC) is higher for plantar flexors (PF) than knee extensors (KE) muscles, whereas impulse above ET (IET) is higher for KE than PF. Thus, we expected that exercise tolerance would be longer for KE than PF only during the exercise performed above ET. After the determination of MVC, 40 men performed two 5-min all-out tests to determine ET and IET. Eleven participants performed a further 4 intermittent isometric tests, to exhaustion, at ET + 5% and ET – 5%, and 1 test for KE at the exercise intensity (%MVC) corresponding to ET + 5% of PF. The IET (7243.2 ± 1942.9 vs. 3357.4 ± 1132.3 N·m·s) and ET (84.4 ± 24.8 vs. 73.9 ± 19.5 N·m) were significantly lower in PF compared with KE. The exercise tolerance was significantly longer for PF (300.7 ± 156.7 s) than KE (156.7 ± 104.3 s) at similar %MVC (∼60%), and significantly shorter for PF (300.7 ± 156.7 s) than KE (697.0 ± 243.7 s) at ET + 5% condition. However, no significant difference was observed for ET – 5% condition (KE = 1030.2 ± 495.4 s vs. PF = 1028.3 ± 514.4 s). Thus, the limit of tolerance during submaximal isometric contractions is influenced by absolute MVC only during exercise performed above ET, which seems to be explained by differences on both ET (expressed as %MVC) and IET values.

Motor fatigability in persons with multiple sclerosis: Relation between different upper limb muscles, and with fatigue and the perceived use of the arm in daily life

BACKGROUND

Motor fatigability is increasingly acknowledged in persons with MS (pwMS). It is unknown whether fatigability is generalized across upper limb muscles and relates to fatigue and perceived difficulties in upper limb use.

METHODS

This observational case-controlled study included twenty PwMS (median EDSS = 3, range 1.5-6.5) and twenty healthy controls who performed 30″ sustained maximal muscle contractions for index finger abduction, hand grip, elbow flexion and shoulder abduction. A static fatigue index (SFI) was calculated to assess motor fatigability for each muscle group. PwMS completed the Fatigue Severity Scale (FSS) and Modified Fatigue Index Scale (MFIS), to quantify severity and perceived impact of fatigue and the Manual Ability Measure (MAM-36) reflecting perceived difficulty in using the upper limbs. Comparisons between groups and muscles was made by t-tests. Associations between outcomes were calculated with correlation coefficients.

RESULTS

Fatigue was highest in pwMS. PwMS showed preserved muscle strength and a greater motor fatigability in elbow flexors compared to healthy controls. SFI of elbow flexors and shoulder abductors were associated, and contributed to FSS and MFIS. SFI of elbow flexors and finger abductors predicted half of the variation in MAM-36.

CONCLUSION

Increased motor fatigability was only present in elbow flexors of PwMS, indicating that expression of motor fatigability is not generalized. Fatigability was associated with perceived fatigue (impact) and daily life upper limb use. Results are preliminary given the small sample size with predominantly persons with mild MS.

Predicted endurance times during overhead work: influences of duty cycle and tool mass estimated using perceived discomfort

A need for overhead work remains in several industries and such work is an important risk factor for shoulder musculoskeletal problems. In this study, we evaluated the effects of duty cycle and tool mass on endurance times during overhead work. A psychophysical approach was used, via a new methodology that was implemented to more efficiently estimate endurance times (rather than through direct measurements). Participants performed a simulated overhead task in specified combinations of tool mass and duty cycle. Both duty cycle and tool mass have substantial effects on the development of fatigue and estimated endurance times, though the former was more substantial and an interactive effect was evident. Gender differences were not substantial, except when using the largest tool mass. We recommend that, for two-hour periods of overhead work, tool masses greater than 1.25 kg should be avoided, as should duty cycles greater than 50%. Practitioner Summary: The current results may facilitate enhanced design and evaluation of overhead work tasks. In addition, the new estimation approach that was employed may enhance the efficiency of future studies using a psychophysical approach (ie using extrapolation of patterns of reported discomfort to predict longer term outcomes).

A motor unit-based model of muscle fatigue

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.

Is There a Deficit After Nonoperative Versus Operative Treatment of Shortened Midshaft Clavicular Fractures in Adolescents?

BACKGROUND

Recent clinical studies in adults have reported a higher incidence of symptomatic malunions and functional deficits in nonoperatively treated shortened midshaft clavicular fractures. We sought to determine whether functional or subjective deficits are found in adolescents after operative versus nonoperative treatment of clavicle fractures.

METHODS

Adolescents with displaced midshaft clavicle fractures, >15 mm of shortening, and a minimum of 9 months of follow-up were recruited. Exclusion criteria included concomitant upper extremity injuries or abnormalities that would affect biomechanical strength testing. Sixteen patients, equally divided between nonoperative and operative plate fixation, met inclusion criteria and consented to testing. The average age (±SD) at the time of injury was 14±2 years (range, 10 to 16 y), time from injury was 22±10 months (range, 10 to 41 mo), and shortening was 24±6 mm (range, 16 to 35 mm). Consenting subjects completed a QuickDASH Score, Constant Shoulder Score, and questions regarding satisfaction with treatment. Quantitative isometric strength, range of motion, and abduction fatigue testing was performed on the involved and uninvolved sides for comparison.

RESULTS

Treatment groups did not differ in age, time from injury, or fracture shortening. QuickDASH and Constant Shoulder Scores were perfect in all but 1 patient in the operative group who actively complained of persistent symptomatic hardware. This patient and one other in the operative group underwent symptomatic hardware removal. There were no cases of symptomatic malunions in the nonoperative group. All patients expressed satisfaction with their treatment. Two in each treatment group were unsatisfied with the appearance of the clavicle. With the exception of a 3% decrease in abduction strength in the operative group (P=0.03) there were no differences in range of motion, isometric strength, or abduction fatigue of the involved shoulder in either treatment group.

CONCLUSIONS

Shortened midshaft clavicular fractures had excellent outcomes after both operative and nonoperative treatments. No subjective or objective differences were observed between treatment groups.

LEVEL OF EVIDENCE

Level III-therapeutic, retrospective comparative study.

The Effects of Performance Fatigability on Postural Control and Rehabilitation in the Older Patient

Fatigue is common in older adults and has a significant effect on quality of life. Despite the high prevalence of fatigue in older individuals, several aspects are poorly understood. It is important to differentiate subjective fatigue complaints from fatigability of motor performance because the two are independent constructs with potentially distinct consequences on mobility. Performance fatigability is the magnitude of change in a performance criterion over a given time of task performance. Performance fatigability is a compulsory element of any strength training program, yet strength training is an important component of rehabilitation programs for older adults. The consequences of fatigability for older adults suggest that acute exercise of various types may result in acute impairments in postural control. The effects of performance fatigability on postural control in older adults are evaluated here to aid the rehabilitation clinician in making recommendations for evaluation of fall risks and exercise prescription.

Major League Baseball pace-of-play rules and their influence on predicted muscle fatigue during simulated baseball games

Major League Baseball (MLB) has proposed rule changes to speed up baseball games. Reducing the time between pitches may impair recovery from fatigue. Fatigue is a known precursor to injury and may jeopardise joint stability. This study examined how fatigue accumulated during baseball games and how different pace of play initiatives may influence fatigue. Pitcher data were retrieved from a public database. A predictive model of muscle fatigue estimated muscle fatigue in 8 arm muscles. A self-selected pace (22.7 s), 12 s pace (Rule 8.04 from the MLB) and a 20 s rest (a pitch clock examined in the 2014 Arizona Fall League (AFL)) were examined. Significantly more muscle fatigue existed in both the AFL and Rule 8.04 conditions, when compared to the self-selected pace condition (5.01 ± 1.73%, 3.95 ± 1.20% and 3.70 ± 1.10% MVC force lost, respectively). Elevated levels of muscle fatigue are predicted in the flexor-pronator mass, which is responsible for providing elbow stability. Reduced effectiveness of the flexor-pronator mass may reduce the active contributions to joint rotational stiffness, increasing strain on the ulnar collateral ligament (UCL) and possibly increasing injury risk.

Effect of alternative video displays on postures, perceived effort, and performance during microsurgery skill tasks

Physical work demands and posture constraint from operating microscopes may adversely affect microsurgeon health and performance. Alternative video displays were developed to reduce posture constraints. Their effects on postures, perceived efforts, and performance were compared with the microscope. Sixteen participants performed microsurgery skill tasks using both stereo and non-stereoscopic microscopes and video displays. Results showed that neck angles were 9-13° more neutral and shoulder flexion were 9-10° more elevated on the video display than the microscope. Time observed in neck extension was higher (30% vs. 17%) and neck movements were 3x more frequent on the video display than microscopes. Ratings of perceived efforts did not differ among displays, but usability ratings were better on the microscope than the video display. Performance times on the video displays were 66-110% slower than microscopes. Although postures improved, further research is needed to improve task performance on video displays.

Endurance time, muscular activity and the hand/arm tremor for different exertion forces of holding

This study aimed to examine the effects of exertion force on endurance time, muscular activity and hand/arm tremor during holding. Fifteen healthy young males were recruited as participants. The independent variable was exertion force (20%, 40%, 60% and 80% maximum holding capacity). The dependent variables were endurance time, muscular activity and hand/arm tremor. The results showed that endurance time decreased with exertion force while muscular activity and hand/arm tremor increased with exertion force. Hand/arm tremor increased with holding time. Endurance time of 40%, 60% and 80% maximum holding capacity was approximately 22.7%, 12.0% and 5.6% of that of 20% maximum holding capacity, respectively. The rms (root mean square) acceleration of hand/arm tremor of the final phase of holding was 2.27-, 1.33-, 1.20- and 1.73-fold of that of the initial phase of holding for 20%, 40%, 60% and 80% maximum holding capacity, respectively.

Mathematical Models of Localized Muscle Fatigue: Sensitivity Analysis and Assessment of Two Occupationally-Relevant Models

Muscle fatigue models (MFM) have broad potential application if they can accurately predict muscle capacity and/or endurance time during the execution of diverse tasks. As an initial step toward facilitating improved MFMs, we assessed the sensitivity of selected existing models to their inherent parameters, specifically that model the fatigue and recovery processes, and the accuracy of model predictions. These evaluations were completed for both prolonged and intermittent isometric contractions, and were based on model predictions of endurance times. Based on a recent review of the literature, four MFMs were initially chosen, from which a preliminary assessment led to two of these being considered for more comprehensive evaluation. Both models had a higher sensitivity to their fatigue parameter. Predictions of both models were also more sensitive to the alteration of their parameters in conditions involving lower to moderate levels of effort, though such conditions may be of most practical, contemporary interest or relevance. Although both models yielded accurate predictions of endurance times during prolonged contractions, their predictive ability was inferior for more complex (intermittent) conditions. When optimizing model parameters for different loading conditions, the recovery parameter showed considerably larger variability, which might be related to the inability of these MFMs in simulating the recovery process under different loading conditions. It is argued that such models may benefit in future work from improving their representation of recovery process, particularly how this process differs across loading conditions.

Predictive Neuromuscular Fatigue of the Lower Extremity Utilizing Computer Modeling

This paper studies the modeling of lower extremity muscle forces and their correlation to neuromuscular fatigue. Two analytical fatigue models were combined with a musculoskeletal model to estimate the effects of hamstrings fatigue on lower extremity muscle forces during a side step cut. One of the fatigue models (Tang) used subject-specific knee flexor muscle fatigue and recovery data while the second model (Xia) used previously established fatigue and recovery parameters. Both fatigue models were able to predict hamstrings fatigue within 20% of the experimental data, with the semimembranosus and semitendinosus muscles demonstrating the largest (11%) and smallest (1%) differences, respectively. In addition, various hamstrings fatigue levels (10-90%) on lower extremity muscle force production were assessed using one of the analytical fatigue models. As hamstrings fatigue levels increased, the quadriceps muscle forces decreased by 21% (p < 0.01), while gastrocnemius muscle forces increased by 36% (p < 0.01). The results of this study validate the use of two analytical fatigue models in determining the effects of neuromuscular fatigue during a side step cut, and therefore, this model can be used to assess fatigue effects on risk of lower extremity injury during athletic maneuvers. Understanding the effects of fatigue on muscle force production may provide insight on muscle group compensations that may lead to altered lower extremity motion patterns as seen in noncontact anterior cruciate ligament (ACL) injuries.

Mechanical and electromyographic responses during the 3-min all-out test in competitive cyclists

While the 3-min all-out test is an ideal exercise paradigm to study muscle fatigue during dynamic whole-body exercise, so far it has been used mainly to provide insight into the bioenergetic determinants of performance. To shed some light into the development of peripheral muscle fatigue during the 3-min all-out test, we investigated the time course of muscle-fibre conduction velocity (MFCV). Twelve well-trained cyclists (23 ± 3 yrs) performed an incremental test, a 3-min all-out familiarization trial and a 3-min all-out test. Surface electromyographic signals were detected from the vastus lateralis muscle of the dominant limb. MFCV decreased with power output, though with a somewhat different time course, and the two parameters were strongly correlated (r = 0.87; P < 0.001). A modest decrease in MFCV (17.7 ± 4.8%), probably due to the endurance characteristics of the subjects, may help explain why a relatively high power output (79 ± 8% of the peak power output of the incremental test; 60 ± 14% of the difference between this peak value and the gas exchange threshold) was still maintained at the end of the test. These findings suggest that muscle fatigue substantially affects performance in the 3-min all-out test, expanding on the traditional bioenergetic explanation that performance is limited by rate and capacity of energy supply.