Michael W. Sonne

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 3CMGMU). In nine experiments of submaximal efforts, the 3CMGMU 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 3CMGMU model performed poorly for endurance tasks. The 3CMGMU 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.

The Rapid Office Strain Assessment (ROSA): Validity of online worker self-assessments and the relationship to worker discomfort

The purpose of this study was to determine if office workers were capable of using an online version of the Rapid Office Strain Assessment (ROSA) tool to accurately assess musculoskeletal disorder risk factors in their own offices, and see if online training can reduce worker-reported discomfort. Fifty-five participants completed a four week program where they assessed their own office simultaneously with a trained observer, and either received or did not receive feedback on their performance. Significant differences were found between worker- and observer-reported ROSA final scores, and for the mouse and keyboard section, with workers underestimating these risk factors on average, compared to the trained observer. Worker and observer assessments of the chair, monitor and telephone were not significantly different but were significantly correlated (R values of 0.60 and 0.48). There were a greater number of significant correlations between worker-reported ROSA final scores and total body discomfort (3 instances) compared to observer-reported relationships (1 instance). Feedback appeared to have a detrimental effect on worker-assessment accuracy, and the relationship between discomfort and ROSA scores. Mean discomfort decreased across the four weeks of the study (up to a 51.6% decrease), as did ROSA final scores (3.9 to 3.5). Additional work is required to improve the validity of worker-reported scores in all sections of ROSA, but self-assessments of office workstations using the current ROSA online application do show promise in terms of assisting workers to decrease risk factors related to musculoskeletal disorders, and decrease discomfort levels.

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.

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. Practitioner Summary: The maximum acceptable effort (MAE) equation can be used by ergonomists to estimate acceptable forces and torques where both duty cycle (DC) is known and maximum strength data are available. Our psychophysical study provides evidence to validate the MAE equation for high DC tasks (DC ≥ 0.50). In fact, the relationship between the equation and existing data is improved with the inclusion of our data.

Optimized maximum voluntary exertion protocol for normalizing shoulder muscle activity

Abstract Muscle activity is typically normalized to maximal activation from isometric maximum voluntary exertions (MVE) in posture and direction specific exertions for each muscle. This is challenging for the shoulder complex due to the large number of muscles. The objective of this investigation was to compare maximum shoulder muscle activity elicited from a multi-muscle MVE test protocol versus individual muscle MVE tests and determine their reliability. Ten healthy males had muscle activity recorded from 12 trunk and upper extremity muscles while performing 3 repetitions of 12 individual and 4 multi-muscle MVEs. Peak surface EMG (sEMG) amplitudes were compared using paired sample t-tests between the two protocols for each muscle. Individual muscle test maximum sEMG amplitudes differed significantly from the multi-muscle test protocol in 3 of 12 muscles (p < 0.05). In muscles that did not attain statistical significance, maximum amplitude differences of 6–15% were found. There was high reliability (Interclass Correlation Coefficient, ICC = 0.831–0.986) and no significant differences between the second and third repetitions of the protocol. Since differences of 6–15% could have functional significance, 8 MVE tests (3 multi-muscle, 5 individual muscle) were selected for future use. Using two repetitions of the reduced MVE protocol will reduce time, risk of pain and injury during experiments.

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

ABSTRACT 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.