Exploring the role of task constraints on motor variability and assessing consistency in individual responses during repetitive lifting using linear variability of kinematics

Exploring Optimal Objective Function Weightings to Predict Lifting Postures Under Unfatigued and Fatigued States

OBJECTIVE

To explore whether the optimal objective function weightings change when using a digital human model (DHM) to predict origin and destination lifting postures under unfatigued and fatigued states.

BACKGROUND

The ability to predict human postures can depend on state-based influences (e.g., fatigue). Altering objective function weightings within a predictive DHM could improve the ability to predict tasks specific lifting postures under unique fatigue states.

METHOD

A multi-objective optimization-based DHM was used to predict origin and destination lifting postures for ten anthropometrically scaled avatars by using different objective functions weighting combinations. Predicted and measured postures were compared to determine the root mean squared error. A response surface methodology was used to identify the optimal objective function weightings, which was found by generating the posture that minimized error between measured and predicted lifting postures. The resultant weightings were compared to determine if the optimal objective function weightings changed for different lifting postures or fatigue states.

RESULTS

Discomfort and total joint torque weightings were affected by posture (origin/destination) and fatigue state (unfatigued/fatigued); however, post-hoc differences between fatigue states and lifting postures were not sufficiently large to be detected. Weighting the discomfort objective function alone tended to predict postures that generalized well to both postures and fatigue states.

CONCLUSION

Lift postures were optimal predicted using the minimization of discomfort objective function regardless of fatigue state.

APPLICATION

Weighting the discomfort objective can predict unfatigued postures, but more research is needed to understand the optimal objective function weightings to predict postures during a fatigued state.

Exploring the role of task on kinematic variability and assessing consistency in individual responses across repetitive manual tasks

To gain a greater understanding of motor variability (MV) as an individual trait, the effect of task type on MV and individual consistency in MV across three tasks was investigated. Twenty participants performed repetitive carrying, lifting, and simulated sawing tasks. MV was assessed using the linear measure of mean point-by-point standard deviation in three-dimensional upper body joint angles. Task type affected MV, where carrying showed higher MV compared to sawing (23-29%) and lifting (12-19%). Furthermore, MV was higher in lifting compared to sawing (12-25%). Poor to moderate individual consistency (ICC = 0.42-0.63) was found across tasks. Task type determined MV and only some support for MV as an individual trait across tasks was found. Based on this work, differences in degrees of freedom afforded by the task influence the opportunity to exploit MV, and possibly individual consistency in MV magnitude is specific to the degrees of freedom afforded by the task. Practitioner summary: In repetitive tasks, movement variability has been proposed as an individual characteristic independent of task characteristics, where repeaters show consistently low variability, while replacers show consistently high variability. In the current study, only moderate support was demonstrated for variability as a consistent individual characteristic across different manual tasks.AbbreviationMV: Motor variability; WRMSDs: Work-related musculoskeletal disorders; DOF: Degrees of freedom; meanSD: Mean standard deviation; SD: Standard deviation; H: Handle (of simulated sawing setup); T: Track (of simulated sawing setup); F: Frame (of simulated sawing setup); ICC: Intraclass correlation; UE: Upper extremity; MMH: Manual material handling; EMG: Electromyography.

Exploring the relationship between kinematic variability and fatigue development during repetitive lifting

To investigate the variability-fatigue and repeaters-replacers hypotheses, motor variability (MV) and indicators of fatigue were assessed during repetitive lifting. Eighteen participants performed sequential repetitive bouts of lifting divided into a short bout, and three phases of a prolonged bout until volitional fatigue (or until a 1-h time limit). Whole-body kinematics were collected to calculate variability in three-dimensional joint angles and in continuous relative phase (CRP) of sagittal joint angle couplings, which were summed for the upper and lower body, and whole-body. Excellent individual consistency (ICC = 0.95-0.97) was demonstrated across lifting bouts as fatigue developed. Therefore, strong evidence was obtained for MV as an individual trait in support of the repeaters-replacers hypothesis. Associations were found for endurance and baseline effort with lower body variability, while no associations were found for rate of fatigue. Thus, some support was found for the variability-fatigue hypothesis which suggests that repeaters are less fatigue-resistant than replacers.