Cervico-Ocular and Vestibulo-Ocular Reflexes in Subclinical Neck Pain and Healthy Individuals: A Cross-Sectional Study.

Alterations in neck sensory input from recurrent neck pain (known as subclinical neck pain (SCNP)) result in disordered sensorimotor integration (SMI). The cervico-ocular (COR) and vestibulo-ocular (VOR) reflexes involve various neural substrates but are coordinated by the cerebellum and reliant upon proprioceptive feedback. Given that proprioception and cerebellar processing are impaired in SCNP, we sought to determine if COR or VOR gain is also altered. COR and VOR were assessed using an eye-tracking device in 20 SCNP (9 M and 11 F; 21.8 (SD = 2.35) years) and 17 control (7 M and 10 F; 22.40 (SD = 3.66) years) participants. COR gain (10 trials): A motorized chair rotated the trunk at a frequency of 0.04 Hz and an amplitude of 5° while participants gazed at a circular target that disappeared after three seconds. VOR gain (30 trials): Rapid bilateral head movements away from a disappearing circular target while eyes fixated on the last observed target. Independent t-tests on COR and VOR gain were performed. SCNP had a significantly larger COR gain (p = 0.006) and smaller VOR gain (p = 0.487) compared to healthy controls. The COR group differences suggest an association between proprioceptive feedback and SMI, indicating COR may be a sensitive marker of altered cerebellar processing.

Equations for estimating the static supportive torque provided by upper-limb exoskeletons.

Upper-limb exoskeletons are gaining traction in industrial work environments. However, other than advertised general specifications (e.g., peak support angle), the support torque provided throughout the reach envelope is largely unknown to end users. As such, this paper describes a methodology for measuring the specific supportive torque provided by upper-limb exoskeletons. The support of four commercially available passive upper-limb exoskeletons was quantified using an isokinetic dynamometer for all support ranges and levels (n = 68). Tests were repeated four times to determine between-session reliability. Intraclass correlation coefficients demonstrated 'Good' to 'Excellent' reliability, except for one condition. Polynomial regression equations were developed for each condition to predict exoskeleton support for any upper-limb elevation angle between 10° and 180°. These equations can be used to approximate upper-limb exoskeleton support in digital human modeling assessments, or to aid selection of exoskeleton settings specific to a worker's anthropometry and work task location.

Exploring Localized Muscle Fatigue Responses at Current Upper-Extremity Ergonomics Threshold Limit Values.

OBJECTIVE: The purpose of this study was to evaluate localized muscle fatigue responses at three upper-extremity ergonomics threshold limit value (TLV) duty cycles. BACKGROUND: Recently, a TLV equation was published to help mitigate excessive development of localized muscle fatigue in repetitive upper limb tasks. This equation predicts acceptable levels of maximal voluntary contraction (% MVC) for a given duty cycle (DC). Experimental validation of this TLV curve has not yet been reported, which can help guide utilization by practitioners. METHOD: Eighteen participants performed intermittent isometric elbow flexion efforts, in three separate counter-balanced sessions, at workloads defined by the American Conference of Governmental Industrial Hygenists' (ACGIH) TLV equation: low DC (20% DC, 29.6% MVC), medium DC (40% DC, 19.7% MVC), and high DC (60% DC, 13.9% MVC). Targeted localized muscle fatigue (LMF) of the biceps brachii was tracked across numerous response variables, including decline in strength (MVC), electromyography (EMG) amplitude and mean power frequency (MnPF), and several psychophysical ratings. RESULTS: At task completion, biceps MnPF and MVC (strength) were significantly different between each TLV workload, with the high DC condition eliciting the largest declines in MnPF and MVC. CONCLUSION: Findings demonstrate that working at different DCs along the ACGIH TLV curve may not be equivalent in preventing excessive LMF. Higher DC workloads elicited a greater LMF response across several response variables. APPLICATION: High DC work of the upper extremity should be avoided to mitigate excess LMF development. Current TLVs for repetitive upper-extremity work may overestimate acceptable relative contraction thresholds, particularly at higher duty cycles.

Higher body mass index and body fat percentage correlate to lower joint and functional strength in working age adults.

As the prevalence of obesity grows worldwide, it becomes an increasing concern in working populations. Ergonomists are faced with the challenge of accommodating workplace layouts to include this worker demographic. This study investigated the relationship between shoulder and low back isometric joint strengths across body mass index (BMI) groups. Additionally, relationships between body fat percentage (BF%), absolute strength, and strength normalized to body mass were examined. Ninety, healthy, working age participants performed 11 functional and isometric joint strength exertions. BMI group influenced normalized strength, as the obese 2+ (BMI >35.0) group had up to 63.1% lower joint strength than all other BMI groups (p < 0.05). Significant strong to moderate negative linear relationships existed between BF% and normalized strength for both males and females, and relationships were stronger for females. These strength deficits highlight the importance of considering body composition during ergonomics analyses and configuration of occupational tasks.

The effect of neck muscle fatigue on shoulder humeral rotation joint position sense.

INTRODUCTION: Cervical extensor muscle (CEM) fatigue causes decrements in upper limb proprioceptive accuracy during constrained single-joint tasks. This study used a novel humeral rotation joint position sense (JPS) measurement device to compare JPS accuracy in participants who received acute CEM fatigue vs. non-fatigued controls. METHODS: Participants had vision occluded and were passively guided into postures of internal humeral rotation from a baseline posture before and after a CEM fatigue or control protocol. Mixed model repeated measures ANOVAs were used to verify fatigue and compared absolute, constant, and variable JPS error between groups. RESULTS: CEM fatigue was verified via pre-post reduction in CEM strength, and myoelectric indicators of fatigue. However, between-group comparisons of absolute, constant, and variable JPS error were not statistically significant, despite having large effect sizes. DISCUSSION: Contrary to prevailing literature, unconstrained humeral rotation JPS did not appear to be affected by CEM fatigue in this study. However, between-group differences in JPS error were dwarfed by inter-trial variability, which likely arose due to the unconstrained nature of this task, conflating chances for a Type II error. Future research should perform a kinematic analysis of task constraints to highlight potential compensatory mechanisms obscuring significant findings in this otherwise robust effect.

A comparison between measured female linear arm strengths and estimates from the 3D Static Strength Prediction Program (3DSSPP).

This study performed a direct comparison between empirically measured female linear arm strengths and those estimated with the 3D Static Strength Prediction Program (3DSSPP). Linear arm strengths were collected from 15 female participants, at four hand locations and six primary directions (n = 360), and then estimated with 3DSSPP incorporating each participant's own segment lengths, body masses and joint strengths, and the measured arm postures from each trial to optimize the accuracy of 3DSSPP. In spite of this, the errors in 3DSSPP's estimated arm strength values were very high (RMS error = 56.0 N and 40.4%) and poorly correlated (r2 = 29.2%) with measured strengths. These results seriously question the accuracy of 3DSSPP to estimate female linear arm strengths and percent capable values, for the range of conditions tested, likely due to the overly simplified assumptions made to estimate triaxial shoulder strength.

Nine Degree-of-Freedom Kinematic Modeling of the Upper-Limb Complex for Constrained Workspace Evaluation.

The design of rehabilitation devices for patients experiencing musculoskeletal disorders (MSDs) requires a great deal of attention. This article aims to develop a comprehensive model of the upper-limb complex to guide the design of robotic rehabilitation devices that prioritize patient safety, while targeting effective rehabilitative treatment. A 9 degree-of-freedom kinematic model of the upper-limb complex is derived to assess the workspace of a constrained arm as an evaluation method of such devices. Through a novel differential inverse kinematic method accounting for constraints on all joints1820, the model determines the workspaces in which a patient is able to perform rehabilitative tasks and those regions where the patient needs assistance due to joint range limitations resulting from an MSD. Constraints are imposed on each joint by mapping the joint angles to saturation functions, whose joint-space derivative near the physical limitation angles approaches zero. The model Jacobian is reevaluated based on the nonlinearly mapped joint angles, providing a means of compensating for redundancy while guaranteeing feasible inverse kinematic solutions. The method is validated in three scenarios with different constraints on the elbow and palm orientations. By measuring the lengths of arm segments and the range of motion for each joint, the total workspace of a patient experiencing an upper-limb MSD can be compared to a preinjured state. This method determines the locations in which a rehabilitation device must provide assistance to facilitate movement within reachable space that is limited by any joint restrictions resulting from MSDs.

Determining best practices for manual pill crushing through evaluation of upper extremity muscle exposures.

Nurses in long-term care homes often crush pills into a fine powder using a manual pill crushing device. This study provides novel quantitative data on muscle loading experienced during pill crushing. The influence of surface height, number of pills and device orientation were studied in twelve muscles of the upper extremity. Variations in the work surface height and number of pills crushed resulted in static shoulder and forearm muscle activations that exceeded recommended static limits. In most cases, working at approximately a 50th percentile female's hip height (87 cm) reduced the level of muscle activity, often to below the EMG-based exposure limit, compared to higher heights. A perpendicularly oriented device required substantially lower muscle activity in some shoulder muscles, with marginal differences occurring in muscles of the elbow and wrist. These data can inform practical design and work practice recommendations to reduce muscular activity while performing this important healthcare task.

A biomechanical evaluation of different footrest heights during standing computer work.

Working at a standing desk is a popular strategy to help reduce low back pain development during prolonged computer work. The purpose of this study was to examine how muscle activity, joint kinematics, weight distribution, balance and low back discomfort were affected by utilising footrests at different heights while working at a standing desk. Sixteen individuals performed a computer task at a standing workstation under four conditions: flat ground stance, and standing with one leg elevated on a low (10 cm), medium (20 cm), or high (30 cm) footrest. Footrest usage altered lumbo-pelvic and bilateral hip joint angles, muscle activity, weight distribution, and range of sway in the elevated limb. Additionally, footrest height altered lumbo-pelvic and hip joint position in the elevated limb. Discomfort increased with time across all conditions. Results suggest that intermittent utilisation of a footrest should be considered to promote changes in posture and muscle activity during prolonged computer use. Practitioner summary: This laboratory study showed that utilising a footrest between the heights of 10-30 cm during standing computer work may be beneficial to promote changes in posture and muscle activity over time. However, we recommend exercising caution while maintaining any standing position beyond 10 min of consecutive use. Abbreviations: LBP: low back pain; PD: pain developer; NPD: non-pain developer; GMe: gluteus medius; TFL: tensor fascia lata; LES: lumbar erector spinae; COP: centre of pressure; NDI: northern digital incorporated; CV: coefficient of variation; WHQ: waterloo handedness questionnaire; WFQ: waterloo footedness questionnaire; VAS: visual analogue scale; OBDI: Oswestry back disability index; IBS: International Society of Biomechanics; sEMG: surface electromyography; MVIC: maximum voluntary isometric contraction; RMS: root mean square; A/P: anterior/posterior; M/L: medial/lateral; % MVE: percentage of maximum voluntary excitation; ROM: range of motion; MCID: minimum clinically important difference.

The influence of hand location and handle orientation on female manual arm strength.

Accurate estimations of manual arm strength (MAS) are crucial in the evaluation of occupational force demands relative to population capacity. Most current strength predictions assume force application with a vertically oriented handle, but it is unknown how uni-manual force capability changes as a function of handle orientation and hand location. This study evaluated the effect of handle orientation on MAS throughout the reach envelope. Fifteen female participants exerted maximum forces in six directions (i.e. superior, inferior, anterior, posterior, medial, lateral), at five different hand locations, and MAS was measured with the handle oriented at 0° (i.e. horizontal), 45°, 90° (i.e. vertical) and 135°. Handle orientation affected MAS in all but the anterior exertion direction, with significant interactions between hand location and grip orientation existing for the superior and inferior directions. These results suggest that handle orientation is important to consider in future predictive models of manual arm strength.

An evaluation of off-axis manual forces and upper extremity joint moments during unilateral pushing and pulling exertions.

This study quantified changes in off-axis manual force production and upper extremity joint moments during sub-maximal one-handed push and pull tasks. Off-axis forces in the up/down and left/right directions were quantified in the presence or absence of constraints placed upon the direction of manual force application and/or arm posture. Resultant off-axis forces of 13.1% and 9.4% were produced for pulls and pushes, respectively. Off-axis forces during pulling were oriented downwards and to the right and were associated with a decreased should flexion moment when posture was constrained. Off-axis forces in the up/down direction were minimized with increased on-axis force level. Off-axis forces during pushing tended to be oriented to the left and were associated with increased elbow flexion moment when off-axis forces were allowed. By not accounting for these off-axis forces, we may not be accurately reflecting actionable muscle- and joint-level loading characteristics derived from biomechanically-based proactive ergonomics assessment approaches. Practitioner Summary: Constrained arm postures and directions of manual force application influence the production of off-axis forces. As inaccurate estimation of true manual forces can markedly influence actionable outcomes of proactive ergonomic assessments, this study suggests that simplification of these estimates is insufficient and potentially misleading.

Algorithmically detectable directional changes in upper extremity motion indicate substantial myoelectric shoulder muscle fatigue during a repetitive manual task.

Repetitive workplace tasks are associated with fatigue-induced changes to shoulder muscular strategies, potentially altering kinematics and elevating susceptibility to tissue overexposures. Accessible and reliable methods to detect shoulder muscle fatigue in the workplace are therefore valuable. Detectable changes in joint motion may provide a plausible fatigue identification method. In this investigation, the onset of the first kinematic changes, as identified by a symbolic motion representation (SMSR) algorithm, and the onset of substantial surface electromyography (sEMG) mean power frequency (MPF) fatigue were not significantly different, both occurring around 10% of task duration. This highlights the potential utility of SMSR identified directional changes in joint motion during repetitive tasks as a cue of substantial muscle fatigue, enabling ergonomics responses that can mitigate shoulder muscular fatigue accumulation and its associated deleterious physical effects. Practitioner Summary: The onset of substantial muscle fatigue during a repetitive dynamic task was assessed using kinematics and myoelectric-based techniques. Algorithmically detectable directional changes in upper extremity joint motion occurred with the onset of substantial muscle fatigue, highlighting the potential of this as a useful approach for workplace fatigue identification.

A musculoskeletal model to estimate the relative changes in wrist strength due to interacting wrist and forearm postures.

Wrist rotations about one wrist axis (e.g. flexion/extension) can affect the strength about another wrist axis (e.g. radial/ulnar deviation). This study used a musculoskeletal model of the distal upper extremity, and an optimization approach, to quantify the interaction effects of wrist flexion/extension (FE), radial/ulnar deviation (RUD) and forearm pronation/supination (PS) on wrist strength. Regression equations were developed to predict the relative changes in strength from the neutral posture, so that the changes in strength, due to complex and interacting wrist and forearm rotation postures, can be incorporated within future ergonomics assessments of wrist strength.

The biomechanical demands of manual scaling on the shoulders & neck of dental hygienists.

The purpose of this study was to evaluate the postural and muscular demands placed on the shoulders and neck of dental hygienists when performing a simulated manual scaling task. Nineteen healthy female dental hygienists performed 30-min of simulated manual scaling on a manikin head in a laboratory setting. Surface electromyography was used to monitor muscle activity from several neck and shoulder muscles, and neck and arm elevation kinematics were evaluated using motion capture. The simulated scaling task resulted in a large range of neck and arm elevation angles and excessive low-level muscular demands in the neck extensor and scapular stabilising muscles. The physical demands varied depending on the working position of the hygienists relative to the manikin head. These findings are valuable in guiding future ergonomics interventions aimed at reducing the physical exposures of dental hygiene work. Practitioner Summary: Given that this study evaluates the physical demands of manual scaling, a procedure that is fundamental to dental hygiene work, the findings are valuable to identify ergonomics interventions to reduce the prevalence of work-related injuries, disability and the potential for early retirement among this occupational group.

The 'Arm Force Field' method to predict manual arm strength based on only hand location and force direction.

This paper describes the development of a novel method (termed the 'Arm Force Field' or 'AFF') to predict manual arm strength (MAS) for a wide range of body orientations, hand locations and any force direction. This method used an artificial neural network (ANN) to predict the effects of hand location and force direction on MAS, and included a method to estimate the contribution of the arm's weight to the predicted strength. The AFF method predicted the MAS values very well (r2 = 0.97, RMSD = 5.2 N, n = 456) and maintained good generalizability with external test data (r2 = 0.842, RMSD = 13.1 N, n = 80). The AFF can be readily integrated within any DHM ergonomics software, and appears to be a more robust, reliable and valid method of estimating the strength capabilities of the arm, when compared to current approaches.

Multidirectional manual arm strength and its relationship with resultant shoulder moment and arm posture.

Previous work has quantified manual force capabilities for ergonomics design, but the number of studies and range of conditions tested are limited in scope. Therefore, the aims of this study were to collect seated manual arm strength (MAS) data from 24 females in several unique exertion directions (n = 26) and hand locations relative to the shoulder (n = 8), and to investigate the associations between MAS and shoulder/elbow moments. MAS was generally highest when the direction of force application was oriented parallel to the vector from the shoulder to knuckle, and weakest when oriented orthogonal to that vector. Moderate correlations were found between MAS and: (1) resultant shoulder moment (r = 0.34), (2) resultant moment arms (r = -0.545) and (3) elbow flexion/extension moment (r = 0.481). Our strength data will be used in the development of a comprehensive MAS predictive method, so that strength capabilities can be predicted to help design acceptable tasks in the workplace. Practitioner Summary: This study sought to enhance our understanding of one-handed manual arm strength capabilities for ergonomics task evaluations. Our findings provide researchers and practitioners with manual strength data for off-axis force directions, as well as hand locations not previously measured. These data will contribute to future methods for predicting strength capabilities.

Predicting manual arm strength: A direct comparison between artificial neural network and multiple regression approaches.

In ergonomics, strength prediction has typically been accomplished using linked-segment biomechanical models, and independent estimates of strength about each axis of the wrist, elbow and shoulder joints. It has recently been shown that multiple regression approaches, using the simple task-relevant inputs of hand location and force direction, may be a better method for predicting manual arm strength (MAS) capabilities. Artificial neural networks (ANNs) also serve as a powerful data fitting approach, but their application to occupational biomechanics and ergonomics is limited. Therefore, the purpose of this study was to perform a direct comparison between ANN and regression models, by evaluating their ability to predict MAS with identical sets of development and validation MAS data. Multi-directional MAS data were obtained from 95 healthy female participants at 36 hand locations within the reach envelope. ANN and regression models were developed using a random, but identical, sample of 85% of the MAS data (n=456). The remaining 15% of the data (n=80) were used to validate the two approaches. When compared to the development data, the ANN predictions had a much higher explained variance (90.2% vs. 66.5%) and much lower RMSD (9.3N vs. 17.2N), vs. the regression model. The ANN also performed better with the independent validation data (r(2)=78.6%, RMSD=15.1) compared to the regression approach (r(2)=65.3%, RMSD=18.6N). These results suggest that ANNs provide a more accurate and robust alternative to regression approaches, and should be considered more often in biomechanics and ergonomics evaluations.

Testing the assumption in ergonomics software that overall shoulder strength can be accurately calculated by treating orthopedic axes as independent.

To predict shoulder strength, most current ergonomics software assume independence of the strengths about each of the orthopedic axes. Using this independent axis approach (IAA), the shoulder can be predicted to have strengths as high as the resultant of the maximum moment about any two or three axes. We propose that shoulder strength is not independent between axes, and propose an approach that calculates the weighted average (WAA) between the strengths of the axes involved in the demand. Fifteen female participants performed maximum isometric shoulder exertions with their right arm placed in a rigid adjustable brace affixed to a tri-axial load cell. Maximum exertions were performed in 24 directions, including four primary directions, horizontal flexion-extension, abduction-adduction, and at 15° increments in between those axes. Moments were computed and comparisons made between the experimentally collected strengths and those predicted by the IAA and WAA methods. The IAA over-predicted strength in 14 of 20 non-primary exertions directions, while the WAA underpredicted strength in only 2 of these directions. Therefore, it is not valid to assume that shoulder axes are independent when predicting shoulder strengths between two orthopedic axes, and the WAA is an improvement over current methods for the posture tested.

Working position influences the biomechanical demands on the lower back during dental hygiene.

This investigation monitored the biomechanical demands on the lower back during simulated dental hygiene work. A total of 19 female, registered dental hygienists performed 30 continuous minutes of manual scaling (plaque removal) of a manikin's teeth while seated. We monitored the working location and orientation of the dental hygienists, with respect to the manikin, along with their spine kinematics, spine extensor muscle activities and seat pressure, throughout the 30 min. A clock representation was used to express the working location. The location significantly influenced the dental hygienists' pelvic orientation with respect to the manikin, spine posture, erector muscle activity and pressure distribution. Findings from this study suggest that the prevalence of lower back pain amongst dental hygienists may be directly related to low-level tonic activity of the spine's extensor musculature, and the combined flexed and axially rotated spine postures. Practitioner Summary: Low back pain (LBP) is prevalent in dental hygienists, yet occupational demand on the low back has not been investigated. Posture, muscle activity and seat pressure were monitored. Combined spine rotation and flexion, and tonic activity of the extensor musculature may be related to LBP in dental hygienists.

Equations to predict female manual arm strength based on hand location relative to the shoulder.

The purpose of this study was to develop regression equations to predict manual arm strength for a wide variety of hand locations within the reach envelope. Maximum voluntary manual arm strength was determined from 71 female participants in six exertion directions (superior, inferior, anterior, posterior, medial and lateral), in a total of 28 hand locations. Forces ranged from 51.3 to 164.4 N, and had a pooled coefficient of variation of 29.9%. Across all 168 combinations of hand locations and exertion directions, the multivariate regression equations explained 92.5% of the variance and had a root mean square error (RMSE) of only 6.4 N, using only the anterior, lateral and vertical location of the hand relative to the active shoulder joint as inputs. These equations provide a proof-of-principle for our novel regression approach, and represent a first step towards a more comprehensive equation to estimate maximum acceptable forces for occupational tasks. PRACTITIONER SUMMARY: The equations presented here demonstrate a first step towards a novel and improved method to predict manual arm strength. Although a more comprehensive equation is still needed, these equations can be confidently used in the field by ergonomists to estimate the maximum acceptable forces in the six primary force directions.