Validation of pitchAITM markerless motion capture using marker-based 3D motion capture.

This study sought to compare and validate baseball pitching mechanics, including joint angles and spatiotemporal parameters, from a single camera markerless motion capture solution with a 3D optical marker-based system. Ten healthy pitchers threw 2-3 maximum effort fastballs while concurrently using marker-based optical capture and pitchAITM (markerless) motion capture. Time-series measures were compared using R-squared (r2), and root mean square error (RMSE). Discrete kinematic measures at foot plant, maximal shoulder external rotation, and ball release, plus four spatiotemporal parameters were evaluated using descriptive statistics, Bland-Altman analyses, Pearson's correlation coefficients, p-values, r2, and RMSE. For time-series angles, r2 ranged from 0.69 (glove arm shoulder external rotation) to 0.98 (trunk and pelvis rotation), and RMSE ranged from 4.37° (trunk lateral tilt) to 20.78° (glove arm shoulder external rotation). Bias for individual joint angle and spatiotemporal parameters ranged from -11.31 (glove arm shoulder horizontal abduction; MER) to 12.01 (ball visible). RMSE was 3.62 m/s for arm speed, 5.75% height for stride length and 21.75 ms for the ball visible metric. pitchAITM can be recommended as a markerless alternative to marker-based motion capture for quantifying pitching kinematics. A database of pitchAITM ranges should be established for comparison between systems.

Assessment of Joint Angle and Reach Envelope Demands Using a Video-Based Physical Demands Description Tool.

BACKGROUND: Current methods for describing physical work demands often lack detail and format standardization, require technical training and expertise, and are time-consuming to complete. A video-based physical demands description (PDD) tool may improve time and accuracy concerns associated with current methods. METHODS: Ten simulated occupational tasks were synchronously recorded using a motion capture system and digital video. The tasks included a variety of industrial tasks from lifting to drilling to overhead upper extremity tasks of different cycle times. The digital video was processed with a novel video-based assessment tool to produce 3D joint trajectories (PDAi), and joint angle and reach envelope measures were calculated and compared between both data sources. RESULTS: Root mean squared error between video-based and motion capture posture estimated ranged from 89.0 mm to 118.6 mm for hand height and reach distance measures, and from 13.5° to 21.6° for trunk, shoulder, and elbow angle metrics. Continuous data were reduced to time-weighted bins, and video-based posture estimates showed 75% overall agreement and quadratic-weight Cohen's kappa scores ranging from 0.29 to 1.0 compared to motion capture data across all posture metrics. CONCLUSION AND APPLICATION: The substantial level of agreement between time-weighted bins for video-based and motion capture measures suggest that video-based job task assessment may be a viable approach to improve accuracy and standardization of field physical demands descriptions and minimize error in joint posture and reach envelope estimates compared to traditional pen-and-paper methods.

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review.

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure.

KEY POINTS: Performing resistance exercise with heavier loads is often proposed to be necessary for the recruitment of larger motor units and activation of type II muscle fibres, leading to type II fibre hypertrophy. Indirect measures [surface electromyography (EMG)] have been used to support this thesis, although we propose that lighter loads lifted to task failure (i.e. volitional fatigue) result in the similar activation of type II fibres. In the present study, participants performed resistance exercise to task failure with heavier and lighter loads with both a normal and longer repetition duration (i.e. time under tension). Type I and type II muscle fibre glycogen depletion was determined by neither load, nor repetition duration during resistance exercise performed to task failure. Surface EMG amplitude was not related to muscle fibre glycogen depletion or anabolic signalling; however, muscle fibre glycogen depletion and anabolic signalling were related. Performing resistance exercise to task failure, regardless of load lifted or repetition duration, necessitates the activation of type II muscle fibres. ABSTRACT: Heavier loads (>60% of maximal strength) are considered to be necessary during resistance exercise (RE) to activate and stimulate hypertrophy of type II fibres. Support for this proposition comes from observation of higher surface electromyography (EMG) amplitudes during RE when lifting heavier vs. lighter loads. We aimed to determine the effect of RE, to task failure, with heavier vs. lighter loads and shorter or longer repetition durations on: EMG-derived variables, muscle fibre activation, and anabolic signalling. Ten recreationally-trained young men performed four unilateral RE conditions randomly on two occasions (two conditions, one per leg per visit). Muscle biopsies were taken from the vastus lateralis before and one hour after RE. Broadly, total time under load, number of repetitions, exercise volume, EMG amplitude (at the beginning and end of each set) and total EMG activity were significantly different between conditions (P < 0.05); however, neither glycogen depletion (in both type I and type II fibres), nor phosphorylation of relevant signalling proteins showed any difference between conditions. We conclude that muscle fibre activation and subsequent anabolic signalling are independent of load, repetition duration and surface EMG amplitude when RE is performed to task failure. The results of the present study provide evidence indicating that type I and type II fibres are activated when heavier and lighter loads are lifted to task failure. We propose that our results explain why RE training with higher or lower loads, when loads are lifted to task failure, leads to equivalent muscle hypertrophy and occurs in both type I and type II fibres.

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.

Evaluation of a virtual reality head mounted display as a tool for posture assessment in digital human modelling software.

The purpose of this work was to assess the feasibility of using a head mounted display with a motion capture system to simulate real world occupational tasks. Participants performed a pointing task under 3 conditions: (1) real environment (REA), (2) virtual environment with auditory stimulus (VEA) and (3) virtual environment with visual stimulus (VEV). End point error, movement time and peak fingertip velocity were calculated for each discrete point event. Upper extremity joint angles were calculated at the end-state for each point and did not significantly differ between real and virtual conditions. There was significantly greater target error in virtual conditions, compared to the real condition. Peak pointing velocity was slower and movement time was longer during virtual conditions. The similarity of joint angles between real and virtual conditions suggests future use of posture-based ergonomic assessments for use with virtual reality task simulations using Oculus Rift and Siemens Jack.

Grip Type Alters Maximal Pinch Forces in Syringe Use.

OBJECTIVE: The purpose of this study was to determine maximum forces during syringe use for different grips found in the field. BACKGROUND: Prolonged syringe use in chemotherapy drug delivery is associated with pain and injury in nurses and technicians. METHOD: Twenty healthy female hospital workers generated isometric maximum voluntary force using a 30 cc syringe with four pinch grips (chuck, chuck variation, thenar, two-handed). Both dominant and nondominant hands were used with the syringe plunger fixed in wide (8.3 cm) and narrow (2.5 cm) grip spans. Participants were encouraged to position the apparatus in the most comfortable position and exert a maximal effort for 5 seconds. RESULTS: Significant interaction effects were found: Grip Span × Pinch Type, Hand × Pinch Type, and Grip Span × Hand × Pinch Type ( p < .05). The results demonstrated that the thenar (103.6 ± 22.9 N) and two-handed (104.7 ± 17.1 N) pinches produced the highest forces. CONCLUSION: Thenar and two-handed pinch grips may be the preferred pinch type to lower the relative efforts required to use a syringe and may be one strategy to assist with reduction of musculoskeletal disorder risk associated with syringe use. APPLICATION: Determining maximal syringe press forces allows workers and ergonomists to develop better strategies for managing the cumulative loads during drug delivery and mixing.

Physiological responses to incremental, interval, and continuous counterweighted single-leg and double-leg cycling at the same relative intensities.

PURPOSE: We compared physiological responses to incremental, interval, and continuous counterweighted single-leg and double-leg cycling at the same relative intensities. The primary hypothesis was that the counterweight method would elicit greater normalized power (i.e., power/active leg), greater electromyography (EMG) responses, and lower cardiorespiratory demand. METHODS: Graded-exercise tests performed by 12 men (age: 21 ± 2 years; BMI: 24 ± 3 kg/m2) initially established that peak oxygen uptake ([Formula: see text]; 76 ± 8.4%), expired ventilation ([Formula: see text]; 71 ± 6.8%), carbon dioxide production ([Formula: see text]; 71 ± 6.8%), heart rate (HRpeak; 91 ± 5.3%), and power output (PPO; 56 ± 3.6%) were lower during single-leg compared to double-leg cycling (main effect of mode; p < 0.05). On separate days, participants performed four experimental trials, which involved 30-min bouts of either continuous (50% PPO) or interval exercise [4 × (5-min 65% PPO + 2.5 min 20% PPO)] in a single- or double-leg manner. RESULTS: Double-leg interval and continuous cycling were performed at greater absolute power outputs but lower normalized power outputs compared to single-leg cycling (p < 0.001). The average EMG responses from the vastus lateralis and vastus medialis were similar across modes (p > 0.05), but semitendinosus was activated to a greater extent for single-leg cycling (p = 0.005). Single-leg interval and continuous cycling elicited lower mean [Formula: see text], [Formula: see text], [Formula: see text], HR and ratings of perceived exertion compared to double-leg cycling (p < 0.05). CONCLUSIONS: Counterweighted single-leg cycling elicits lower cardiorespiratory and perceptual responses than double-leg cycling at greater normalized power outputs.

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.

A modified version of the three-compartment model to predict fatigue during submaximal tasks with complex force-time histories.

The three-compartment model (3CM) was validated previously for prediction of endurance times by modifying its fatigue and recovery rates. However, endurance times do not typically represent work demands, and it is unknown if the current version of the 3CM is applicable for ergonomics analysis of all occupational tasks. The purpose of this study was to add biological fidelity to the 3CM, and validate the model against a series of submaximal force plateaus. The fatigue and recovery rates were modified to represent graded physiological motor unit characteristics (termed 3CM(GMU)). In nine experiments of submaximal efforts, the 3CM(GMU) produced a root-mean squared difference (RMSD) of 4.1 ± 0.5% MVC over experiments with an average strength loss (i.e., fatigue) of 31.0 ± 1.1% MVC. The 3CM(GMU) model performed poorly for endurance tasks. The 3CM(GMU) model is an improvement for evaluating submaximal force patterns consisting of intermittent muscle contractions of the hand and forearm. PRACTITIONER SUMMARY: We modified an existing fatigue model using known physiological properties in order to predict fatigue during nine different submaximal force profiles; consistent with efforts seen in industrial work. We expect this model to be included in digital human modelling software, for the assessment of repetitive work and muscle fatigue in repetitive tasks. SOCIAL MEDIA Summary: The proposed model has applications for estimating task fatigue in proactive ergonomic analyses of complex force patterns using digital human models.

Fatigue accumulation and twitch potentiation during complex MVC-relative profiles.

Fatigue accumulation can be significantly influenced by post-activation potentiation (PAP). This phenomenon leads to increased force generating capacity after a series of conditioning efforts. The purpose of our study was to examine how the order of force demands impacted muscle fatigue accumulation, and how fatigue was affected by potentiation. We had 33 participants complete one of four different force orders, consisting of 5 cycles of 12-s submaximal isometric "task plateaus". Every order consisted of 8 force plateaus of the different intensity, arranged in a different ways. A subset of 18 participants then received a stimulated muscle twitch, and all participants performed a brief MVC. Each task plateau was followed by a 12s long 10% MVC "reference plateau" also followed by twitch and MVC. Overall, the order of the force presentation only had an impact on fatigue levels and twitch potentiation during the first cycle. Maximum voluntary forces decreased, and twitch forces increased, at different rates between orders during the first cycle. At the end of each cycle, there was no difference between any of the orders in terms of twitch potentiation or decrease in MVC force. In a task that features identical force patterns, arranged in different orders, the order did not affect the final fatigue accumulation or potentiation level.

Force time-history affects fatigue accumulation during repetitive handgrip tasks.

Muscle fatigue is associated with a higher risk of workplace injury, in particular during repetitive tasks. This study aimed to identify the effect of a complex force-time history (a task with multiple different submaximal effort levels) on fatigue accumulation and recovery during a handgrip task. We measured surface electromyography of the brachioradialis (BRD) and flexor carpi ulnaris (FCU) of ten right hand dominant females with no history of upper limb injury while they performed a complex submaximal visually targeted gripping task. The task consisted of 15%, 30%, 45%, 30%, and 15% maximum voluntary contraction (MVC) plateaus. Each plateau was held for 15s, followed by a 3s MVC and 3s of rest. The "pyramid" was repeated until fatigue criteria were met. Grip force, average EMG and mean power frequency (MnPF) for first cycle and fatigued last cycle, were compared. Post-plateau peak grip force was on average 20.5% MVC lower during the last cycle (p<0.01). Post-plateau grip forces decreased on average by 5.1% MVC after the first 15% MVC plateau (from baseline), by 5.3% MVC after the 30% MVC plateau and 6.8% MVC after the 45% MVC plateau. Further accumulation of fatigue after the second 30% MVC plateau however was minimal, only decreasing by 1.6% MVC. Recovery appeared to occur during the last 15% MVC plateau with an increase in post plateau grip force of 1.6% MVC. Interestingly, MnPF parameters confirmed significant fatigue accumulation during the back end of a force pyramid. We conclude that in a pattern of contractions with ascending, then descending force intensity, voluntary force recovery was present when the preceding force was of a lower intensity. These findings indicate preceding demands play a role in fatigue accumulation during complex tasks.

A psychophysical study to determine maximum acceptable efforts for a thumb abduction task with high duty cycles.

Potvin (2012, 'Predicting Maximum Acceptable Efforts for Repetitive Tasks: An Equation Based on Duty Cycle', Human Factors: The Journal of the Human Factors and Ergonomics Society, 54 (2), 175-188) developed an equation using psychophysical data to estimate maximum acceptable efforts (MAEs) as a function of duty cycle (DC). However, only ∼6% of the data featured DCs ≥ 0.50. The purpose of this study was to evaluate the MAE equation in the high DC range. We tested a repetitive thumb adduction task with DCs of 0.50, 0.70 and 0.90, at frequencies of both 2 and 6 per minute (n = 6 conditions). Participants were trained for 2 hours and tested for 1 hour on each condition. The MAE decreased with increasing DC, and MAEs at 2/min were higher than those at 6/min. When these current six means were added to the original psychophysical studies, the root-mean squared difference of the MAE equation decreased from 7.23% to 7.05% maximum voluntary contraction. The values from our study are also consistent with those demonstrating physiological evidence of fatigue during both continuous isotonic and high DC tasks.

It wears me out just imagining it! Mental imagery leads to muscle fatigue and diminished performance of isometric exercise.

The purpose of this study was to investigate the aftereffects of self-generated mental imagery of an effortful task on physical self-control endurance and muscle fatigue. Participants performed two isometric handgrip endurance trials (50% of maximum contraction) separated by either an imagery manipulation or a quiet rest period. The imagery group showed greater negative changes in endurance performance from trial 1 to trial 2 (p=.003, d=0.87) and increased muscle activation at baseline (p=.01, d=0.73) and at 25% (p=.03, d=0.61) of the second endurance trial compared to controls. We conclude that imagined performance of an effortful task depletes self-control strength and contributes to muscle fatigue.