Jim R. Potvin

Predicting maximum acceptable efforts for repetitive tasks: an equation based on duty cycle.

Objective: The objective was to develop an equation, for repetitive tasks, that uses frequency and/or duty cycle (DC) to predict maximum acceptable efforts (MAE) relative to maximum voluntary efforts (MVE). Background: Ergonomists must determine acceptable physical demands for a wide variety of tasks. Although a large database exists in the literature for maximum single-effort strength, far fewer repetitive tasks have psychophysical and/or physiological data available to guide the prediction of acceptable submaximal, repeated efforts. Method: DC represents the total effort duration divided by the cycle time. MAEs were calculated by dividing average psychophysics-based acceptable loads by corresponding single-effort maximum strength using 69 values from studies of the upper extremities. The author developed an equation to characterize the relationship between MAE and DC. Results: The resulting equation had DC taken to the exponent 0.24, and it predicted MAE very well (r2 = 0.87%, root mean square [RMS] difference = 7.2% of the maximum strength). At higher DC values, the equation also demonstrated good agreement with the published physiological data. Conclusion: The limited psychophysical database in the literature makes it difficult for ergonomists and engineers to recommend acceptable efforts for the large variety of repetitive tasks they evaluate. However, the proposed equation now allows for a correction of the large strength database to estimate acceptable force and torque limits for repetitive occupational tasks. Application: The proposed equation will have wide applications for ergonomic practitioners performing evaluations of repetitive tasks.

Effects of trunk muscle fatigue and load timing on spinal responses during sudden hand loading.

The purpose of this study was to investigate the responses of the spine during sudden loading in the presence of back and abdominal muscle fatigue, with a primary focus on the implications for spinal stability. Fifteen females were studied and each received sudden loads to the hands, at both known and unknown times. Participants received these loading trials (a) while rested, (b) with back muscle fatigue, and (c) with a combination of back and abdominal muscle fatigue. Measures were taken on the EMG activity of two trunk extensor and two abdominal muscles, and on the trunk angle and centre of pressure. A 3x2 Repeated Measures ANOVA was also performed. There were no preparations made prior to the perturbation even when it could be anticipated. However, the peak responses that followed were greater in the unexpected versus the expected condition. In addition, trunk muscle fatigue led to an increase in the baseline activity of the trunk muscles but no additional increase in activity just prior to loading. There was increased activation of both (opposing) muscle groups when only one muscle group was fatigued. Because the peak responses following the perturbation were enhanced in the unknown timing condition, preparations must have taken place prior to the anticipated perturbations, perhaps in other segments of the body that were not measured. Also, the load impact may not have been great enough to elicit large preparations. The heightened baseline activity with fatigue suggests that there may have been increased spinal stiffness whenever the spine was fatigued, and not just immediately prior to an impending perturbation. The increased activation of opposing muscle groups is evidence of increased cocontraction in response to fatigue, possibly to maintain stability with decreasing coordination.

The in vivo dynamic response of the human spine to rapid lateral bend perturbation: effects of preload and step input magnitude.

Study Design. A repeated measures design was used to determine the effects that combinations of two preloads and two added loads have on spine mechanics both before and during the response to the added load. Objective. To investigate the effects of varying initial isometric and added step input load magnitudes on mechanical and electromyographic responses of the trunk during sudden loading that causes lateral bending moments. Summary of Background Data. Cocontractions of the antagonistic and agonistic muscles of the trunk are required for stability during loading of the spine. In several in vivo studies, it was observed that trunk muscle cocontraction serves a functional role before the application of unexpected or sudden loads. The response of agonistic and antagonistic trunk muscles to rapid lateral bend moments would provide further insight into the dynamic stability mechanisms of the spine. Methods. In this study, 13 men maintained an upright standing posture while resisting the application of lateral bend moments produced by four different loading conditions comprising combinations of two preloads (5% or 15% of the maximum isometric lateral bend moment) and two added loads (20% or 30%). The preloading was used to develop different initial levels of trunk stiffness before the application of the added loads. The lateral bend moment and angular rotation of the trunk were measured, as well as the surface electromyogram amplitudes of the bilateral internal oblique, external oblique, rectus abdominus, lumbar erector spinae, and thoracic erector spinae muscles. Dependent measures were recorded during the steady state preload conditions, and peak values were recorded after the load was added. Results. Higher added loads resulted in higher peak lateral bend rotations, and higher preloads resulted in lower rotations. The patterns of response were similar for the peak lateral bend moments and the electromyogram amplitudes from four of the five agonistic muscles. The thoracic erector spinae excepted, each of the other four muscles demonstrated larger responses in the agonistic muscles. However, all of the antagonistic muscles showed some increase in electromyogram activity in response to the added load. The thoracic erector spinae appeared to have the role of counteracting the flexor moments created by the abdominal muscles and the maintenance of spine stability. The agonistic external obliques and lumbar erector spinae had the largest responses to the added load. A comparison of the 35% loading conditions showed an increased response of the trunk to the 5% + 30% condition (with lower initial trunk stiffness), as compared with the 15% + 20% condition. Conclusions. The findings from this study show that higher levels of preactivation can serve to increase spine compression and trunk muscle stiffness, thereby attenuating the lateral displacements caused by rapid loading. Furthermore, antagonistic muscles were observed to respond rapidly to such perturbations with large increases in activation when preactivation and spine stability were low. The trunk muscles monitored all were larger, multisegmental muscles. The results from this study lend support to previous studies suggesting that the larger multisegmental muscles make a significant contribution to spinal stability.

Muscle fatigue and contraction intensity modulates the complexity of surface electromyography

Nonlinear dynamical techniques offer a powerful approach for the investigation of physiological time series. Multiscale entropy analyses have shown that pathological and aging systems are less complex than healthy systems and this finding has been attributed to degraded physiological control processes. A similar phenomenon may arise during fatiguing muscle contractions where surface electromyography signals undergo temporal and spectral changes that arise from the impaired regulation of muscle force production. Here we examine the affect of fatigue and contraction intensity on the short and long-term complexity of biceps brachii surface electromyography. To investigate, we used an isometric muscle fatigue protocol (parsed into three windows) and three contraction intensities (% of maximal elbow joint moment: 40%, 70% and 100%). We found that fatigue reduced the short-term complexity of biceps brachii activity during the last third of the fatiguing contraction. We also found that the complexity of surface electromyography is dependent on contraction intensity. Our results show that multiscale entropy is sensitive to muscle fatigue and contraction intensity and we argue it is imperative that both factors be considered when evaluating the complexity of surface electromyography signals. Our data contribute to a converging body of evidence showing that multiscale entropy can quantify subtle information content in physiological time series.

Biomechanical changes at the knee after lower limb fatigue in healthy young women

BACKGROUND The purpose of this study was to identify changes in knee kinematics, kinetics and stiffness that occur during gait due to lower limb neuromuscular fatigue. METHODS Kinematic, kinetic and electromyographic measures of gait were collected on healthy, young women (n=20) before and after two bouts of fatigue. After baseline gait analysis, two bouts of fatiguing contractions were completed. Fatigue was induced using sets of 50 isotonic knee extensions and flexions at 50% of the peak torque during a maximum voluntary isometric contraction. Fatigue was defined as a drop in knee extension or flexion maximum voluntary isometric torques of at least 25% from baseline. Gait analyses were completed after each bout of fatigue. Dynamic knee stiffness was calculated as the change in knee flexion moment divided by the change in knee flexion angle from 3 to 15% of the gait cycle. Co-activations of the biceps femoris and rectus femoris muscles were calculated from 3 to 15% and 40 to 52% of gait. Repeated measures analyses of variance assessed differences in discrete gait measures, knee torques, and electromyography amplitudes between baseline and after each bout of fatigue. FINDINGS Fatigue decreased peak isometric torque. Fatigue did not alter knee adduction moments, knee flexion angles, dynamic knee stiffness, or muscle co-activation. Fatigue reduced the peak knee extension moment. INTERPRETATION While neuromuscular fatigue of the knee musculature alters the sagittal plane knee moment in healthy, young women during walking, high intensity fatigue is not consistent with known mechanical environments implicated in knee pathologies or injuries.

Occupational spine biomechanics: A journey to the spinal frontier

This paper provides a brief introduction to the variety of research areas focusing on spine biomechanics as it pertains to understanding and preventing low back injuries in the workplace. While certainly not a comprehensive review of the literature, some of the earliest, pioneering studies are presented from the following areas: (1) spine tissue testing, (2) estimating spine tissue loading, (3) manual materials handling studies, (4) prolonged or repetitive spine loading, (5) ergonomic assessment tools, (6) sudden/unexpected loading and (7) spine stability. Where possible, some of our own research contributions are integrated into the relevant sections. This paper concludes with a suggestion of some future research directions to continue and enhance the important impact of occupational spine biomechanics.

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.

Trunk muscle contributions of to L4-5 joint rotational stiffness following sudden trunk lateral bend perturbations

The purpose of this research was to investigate the contributions of individual muscles to joint rotational stiffness and total joint rotational stiffness about the lumbar spines L(4-5) joint prior to, and following, sudden dynamic lateral perturbations to the trunk. Kinematic and surface EMG data were collected while subjects maintained a kneeling posture on a robotic platform, while restrained so that motions caused by the perturbation were transferred to the pelvis, causing motion of the trunk and head. The robotic platform caused sudden inertial trunk lateral perturbations to the right or left, with or without timing and direction knowledge. An EMG-driven model of the lumbar spine was used to calculate the muscle forces and contributions to joint rotational stiffness during the perturbations. Data showed 95% and 106% increases in total joint rotational stiffness, about the lateral bend and axial twist axes, when subjects had knowledge of the timing of the perturbation. Also, the contralateral muscles exhibited a significantly larger total joint rotational stiffness about the lateral bend axis, and earlier surface EMG responses, than the ipsilateral muscles. The results indicate that, when the timing of the perturbation was unknown, subjects relied more on delayed muscle forces following the perturbation to stiffen the L(4-5) joint.

An equation to predict maximum acceptable loads for repetitive tasks based on duty cycle: evaluation with lifting and lowering tasks

Recently, an equation was developed to predict maximal acceptable effort (MAE) for repetitive tasks based on the product of task frequency and effort duration (ie. duty cycle). This equation has been shown to closely match data from psychophysical studies of the upper extremities. In the current paper, the applicability of this equation was tested on lifting and lowering data from Snook and Ciriello (1991) and was found to fit closely, even at very low duty cycles.

Strength limitations to proper child safety seat installation: Implications for child safety

A majority of child safety restraints are misused in some manner, often leading to an increased risk of serious injury or death. It is possible that at least some instances of misuse are the result of biomechanical limitations during the installation process. Twenty-seven adult participants were trained and then monitored in three stages of child safety seat installation. All installations were done with an identical restraint system in the rear bench seat of a mocked-up minivan. EMG of 10 muscles, as well as trunk, shoulder, and wrist postures were analyzed. Peak maximum efforts were often required of the trunk extensor, forearm, and anterior shoulder muscles during the installation process. Routing and tightening of the seatbelt, as well as placing and securing the child into the seat were observed to be particularly difficult tasks. Many portions of the child safety seat installation process were found to be very physically demanding; some individuals may not be capable of performing these tasks correctly, thereby putting the child at greater risk in the motor vehicle.