Minimizing fatigue during repetitive jobs: optimal work-rest schedules.

Twenty women were asked to generate forces using a dynamometer that were consistent with one of three different work-rest schedules (a low-, medium-, and high-force schedule). Each work-rest schedule consisted of 6 identical blocks of 10 work-rest cycles. Each of the 10 work-rest cycles lasted 1 min. The first work-rest cycle in each block consisted of a 6-s maximal voluntary contraction and a 54-s rest. The remaining 9 work-rest cycles in each block consisted of a submaximal contraction and a rest period. The desired force of the submaximal contraction, the length of this contraction, and the duration of the rest period remained constant within schedules but varied across schedules. The amount of physiological work was kept constant among schedules. The fatigue that developed in the medium-force schedule was significantly lower than that developed in either the low- or high-force schedule. A model was developed that predicted the amount of fatiguable strength at the beginning and end of each contraction of a work-rest cycle. When fit to the results from the experiment, the model explained 94% of the variance. The model can be used to predict the work-rest schedule that minimizes fatigue in a given repetitive job, thereby potentially increasing productivity and reducing the incidence of cumulative trauma disorders.

Repetitive motion disorders: the design of optimal rate-rest profiles.

Repetitive motion disorders of the upper extremities are the primary cause of lost time in hand-intensive industries. Typical remedies include the introduction of job aids and the redesign of the job. An alternative approach is considered here. Procedures are developed that can be used to determine the parameters of a job (e.g., the rate of work and the number and duration of the rest breaks) that minimize repetitive motion disorders and maximize productivity. It is shown that in theory the job parameters can be set so that one can achieve not only a decrease in repetitive motion disorders but also an increase in productivity. The application of the procedures to an actual manufacturing job requiring high-repetitive, high-force hand motions is discussed in detail.

Predicting the minimal energy costs of human walking.

Preferred stride frequency (PSF) of human walking has been shown to be predictable as the resonant frequency of a force-drive harmonic oscillator (FDHO). The purpose of this study was to determine whether walking at the PSF and FDHO leads to minimal metabolic and mechanical costs. Subjects walked on a level treadmill at the PSF, FDHO, and frequencies above and below. Effects of stride length (SL) and speed (S) were assessed by two conditions, one in which SL was constant and the other in which S was constant. The predictability of PSF from resonance was replicated. Walking at the PSF and FDHO frequencies resulted in metabolic costs which were not significantly different (P greater than 0.05). A U-shaped oxygen consumption curve was observed with the minimum at the PSF and FDHO conditions when S was constant. A two-component curve in which a breakpoint was observed was found in the SL constant condition. A significant increase in metabolic cost was observed above the PSF/FDHO (P less than 0.01). Internal work (power) values were not significantly different between walking frequencies for the S constant condition. In the SL constant condition, internal work values showed linear increases as frequency increased. It was concluded that PSF of walking arises from the interface of the resonance properties of the limbs as oscillators and the tendency of biological systems to self-optimize.

Validation of a biodynamic model of pushing and pulling.

Pushing and pulling during manual material handling can increase the compressive forces on the lumbar disc region while creating high shear forces at the shoe-floor interface. A sagittal plane dynamic model derived from previous biomechanical models was developed to predict L5/S1 compressive force and required coefficients of friction during dynamic cart pushing and pulling. Before these predictions could be interpreted, however, it was necessary to validate model predictions against independently measured values of comparable quantities. This experiment used subjects of disparate stature and body mass, while task factors such as cart resistance and walking speed were varied. Predicted ground reaction forces were compared with those measured by a force platform, with correlations up to 0.67. Predicted erector spinae and rectus abdominus muscle forces were compared with muscle forces derived from RMS-EMGs of the respective muscle groups, using a static force build-up regression relationship to transform the dynamic RMS-EMGs to trunk muscle forces. Although correlations were low, this was attributed in part to the use of surface EMG on subjects of widely varied body mass. The biodynamic model holds promise as a tool for analysis of actual industrial pushing and pulling tasks, when carefully applied.

Prosthetic alignment effects on gait symmetry: a case study.

The loss of a significant portion of a lower extremity causes changes in the usual pattern of human ambulation. These changes have been documented kinematically, kinetically and metabolically, giving insight into the costs of limb amputation relative to ambulatory efforts. The role of the prosthetist is to provide a limb substitute to achieve the best gait performance, while assuring maximum comfort for the patient. This case study examined the effects of antero-posterior alignment of a below-knee prosthesis on sagittal plane gait kinematics by comparing the anatomical side with the prosthetic side. The greatest changes due to variations of alignment were found during the prosthetic stance phase; knee angles showed the greatest asymmetry between anatomical and prosthetic sides. The stance phase on the prosthetic side was reduced with anterior socket displacement due to early knee flexion and toe-off. Posterior socket displacement caused a greater maximum centre of gravity height, but anterior socket displacement caused greater knee flexion which decreased the maximum centre of gravity height. Asymmetries in temporal and other kinematic parameters were not always minimal at the optimal alignment subjectively selected by a certified prosthetist. Comparisons of asymmetry ratios with prosthetic side data revealed the subclinical sensitivity of this amputee to antero-posterior alignment discrepancies.

Exercise moderation of foot function during walking with a re-usable semirigid ankle orthosis.

A study was conducted to investigate the effects of a re-usable semirigid ankle orthosis on the support phase of the walking stride in both pre- and post-exercise conditions. Ten young, adult males were required to complete ten trials in each of four orthosis/no orthosis and pre-/post-exercise conditions. Data were collected via a force platform and a high speed camera. The analysis consisted of the evaluation of selected ground reaction force parameters and kinematic parameters describing rearfoot motion. The exercise regimen consisted of 70 maximal eccentric actions of the ankle everters, with 15 s between each action. The results revealed significant differences between the orthosis/no orthosis conditions for the variables describing the mediolateral action of the ankle during walking. Only the rearfoot touchdown angle was affected by the exercise regimen. The data indicated that the semirigid orthosis moderated ankle joint mechanics, although the measured values were within normal bounds.

Designing a better postural measurement system.

For more than a century the phenomenon of postural sway has attracted the attention of neurologists, control engineers, and otoneurologists, because body sway provides a global assessment of the sensorimotor systems involved in postural control. This article presents an overview of the history of body sway studies, highlighting some of the shortcomings of past techniques. Spurred on by these shortcomings, we specified design criteria for a better postural measurement system. The system has been implemented, and some of the preliminary applications are detailed that demonstrate the utility of our approach.