Steven F. Wiker

Shoulder posture and localized muscle fatigue and discomfort

In many industries workers perform manual assembly tasks with hands postured above the shoulders. Awkward shoulder and arm postures are often viewed as acceptable given costs of workplace modification, postural exertions which are in compliance with current design recommendations, ready availability of strong workers, and numbers of electromyographic studies which fail to detect significant signs of localized muscle fatigue (LMF). An experiment was conducted to: (a) study the onset and severity of (LMF) in the shoulder when performing a stylus-to-hole Fitts reciprocal movement task under a range of postures, hand loads, ratios of work-to-rest, and task durations, and (b) to evaluate the efficacy of three techniques (i.e., changes in EMG behaviour, postural tremor, and cross-modal matching estimates) in detecting and monitoring posturally-based LMF and discomfort in the shoulder complex. Experimental findings showed that posturing hands above shoulder level significantly increased the risk of LMF and postural discomfort even in light-weight manual assembly environs where postural exertions are small, and that cross-modal matching estimates and postural tremor were more sensitive metrics of LMF in the shoulder complex than EMG RMS voltage and mean spectral power frequency metrics. The basis for experimental findings, as well as potential application of LMF metrics in future postural stress investigations, are discussed. Recommendations for workplace posture are provided for job designers facing work height decisions in manual assembly environs.

Arm Posture and Human Movement Capability

Many workers perform manual assembly tasks or use hand tools while the hands are postured above the shoulders or to the side of the body. Experiments were conducted to study the sensitivity of speed-accuracy movement performance to a wide range of hand locations around the shoulder under various levels of hand loads, ratios of work to rest, and task durations. Subjects performed a spatially constrained stylus-to-hole Fitts reciprocal movement task designed to simulate high-incentive manual assembly operations while providing basic information regarding changes in human move and positioning capabilities. Significant decrements in movement performance occurred when hands were postured above shoulder level. Move and positioning times increased 15.3% and 26.5%, respectively, with elevation of the hand from −15 to 60 deg respective to shoulder level. Posturally based decrements in movement capability were unrelated to differences in subject strength capability found among test postures. The consequences of elevated arm postures on human move and positioning capability are presented along with workplace design and methods recommendations for job designers facing work-height decisions in manual assembly environments.

SHOULDER POSTURAL FATIGUE AND DISCOMFORT A Preliminary Finding of No Relationship with Isometric Strength Capability in a Light-Weight Manual Assembly Task

Does greater strength capacity in the shoulder-complex afford increased protection against regionalized fatigue and discomfort induced by sustained awkward arm postures in light-weight manual assembly environments? This question was addressed by testing the relationship between differences in shoulder complex strength capacity, produced by variations in arm posture within a subject, and among subjects assuming equivalent arm postures, and severity of fatigue and discomfort sensed during a low-exertion manual performance task. Experimental findings showed that: (a) awkward arm postures produced substantial and rapid onset of postural fatigue and discomfort during a light-weight manual performance task where strength demands were low (i.e., less than 15 percent of maximum voluntary contraction (MVC)), (b) variations in strength capability found among arm postures within an individual subject, or 3mong subjects assuming the same arm posture, did not affect onset of substantial fatigue or discomfort when hands are postured near or above shoulder level, and (c) postures which simply appeared to be awkward, or which compromised strength capacity (e.g., working with the arm to the side of the body, or aligned in the coronal plane), did not necessarily increase discomfort of fatigue. Our findings suggest caution against sole reliance upon population or individual worker upper-extremity strength capabilities as predictors of fatigue and discomfort in the shoulder complex when manual exertions are small (e.g., light-weight manual assembly activities involving small parts or small hand-tools) and hands are postured at or above shoulder level.

PERFORMANCE EVALUATION TESTS FOR ENVIRONMENTAL RESEARCH (PETER): CODE SUBSTITUTION TEST

The effects of repeated testing on code substitution performance were studied in three experiments to determine reliability and stability of task performance under laboratory and at-sea conditions. In Exp. 1, a single 2-min. testing trial per day was administered to a group of 19 subjects for 15 consecutive weekdays. In the second experiment a 4-min. per-day test was administered to 12 of the 19 original subjects for an additional 15 consecutive weekdays. In Exp. 3, six US Coast Guardsmen were tested hourly for four consecutive days, two days at dockside and two days at sea. In Exps. 1 and 2, means, variances, and cross-session correlations became stable after Day 8. Stabilized reliabilities were .75 for Exp. 1 and .80 for Exp. 2. The results of Exp. 3 were comparable to those obtained in the first two experiments with the exception that decrements in performance corresponding to high frequencies of motion sickness symptoms occurred during the at-sea trials. The Code Substitution Test is recommended for inclusion in the PETER battery.

Grasp force control in telemanipulation

This paper presents two experiments which focus upon the issue of grasp force control in telemanipulation. The first experiment examines the ability to control and stabilize master-controller grasp force during a 30-s compensatory tracking task under different levels of master controller digit mass, friction, and backlash. The second experiment explores the potential for substituting tactile feedback in lieu of direct force-feedback to gage and control remote grasp force. Results show that subjects were better able to control force when mass and friction levels were increased. Even when perceptual gains between tactile and direct force feedback displays were matched, force reflection produced better grasp control. The lack of backlash effects and improvements in performance with direct force reflection in comparison to tactile feedback are attributable to reflexive short-loop adjustment of grasp tension afforded by the muscles length-tension control system. The criterion of acceptable operator performance, dependent upon both the quality of the transmission of control commands and feedback, and the response of the remote device, is discussed.

Identification of human grasp dynamics in teleoperation

This paper presents a generic wrist-hand model for the human operator and a numerical algorithm for real-time identification of both human grasping using model reference adaptive approach. The impact of grasping postures, forces, and localized muscle fatigue upon grasp dynamics was studied. Results are given for experiments to determine human wrist-hand dynamics, to calibrate the identification algorithm, and to determine changes in hand/wrist grasp dynamics with onset of localized muscle fatigue.

Acceleration Exposures in Forward Seating Areas of Bowrider Recreational Boats

The objective of this study was to measure acceleration levels to which passengers would be subjected in the fonvard seating areas of two styles of bowrider recreational boats. Acceleration exposure data were collected aboard a monohull and a trihull boat as they crossed 0.48- and 0.76-m wakes at speeds of 24 and 40 km/h. Substantial increases in maximum peak accelerations and their rates of onset were found with increases in boat speed and wake height. Vertical acceleration levels and their onset rates were lowest aboard the monohull; however, the effects of boat speed and wake height upon vertical acceleration exposures were much greater than differences found between boats. Similar findings were obtained for longitudinal acceleration measures. Based upon presently available human acceleration tolerance information, the risk of spinal compression fracture aboard the boats examined appears remote due to the very short durations of the accelerations endured.

Habitatuation of Crew Performance, Stress and Mood Aboard a SWATH and Monohull Vessel

Crew performance and psychophysiological state were assessed aboard a SWATH and monohull vessel during a two day side-by-side continuous steaming period in low sea states. Crew performance was degraded and psychophysiological stress increased significantly aboard the monohull during the first steaming day. On the second day at sea all measures except mood changes and performance tasks perturbed directly by vessel motions habituated to near control levels. Performance and psychophysiological measures were relatively unaffected aboard the SWATH vessel.