Anthropometric,muscle strength,and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting

The rating of acceptable load (RAL) attained with a standard test (RAL_st) and a work-simulating test (RAL_w) for postal parcel sorting was related to anthropometric, muscle strength, and spinal mobility characteristics of 18 male sorters. The subjects comprised a subsample of 103 experienced male sorters who carried out the RAL tests at postal sorting centres. The dynamic hand-grip endurance correlated significantly (p=0·036) to the RAL_st results. Correspondingly, there was a significant correlation ( p=0·044) between the ratio of maximal isometric strength of trunk extension to body weight and the RAL_w. The dynamic hand-grip endurance predicted 26% of the variation in the RAL_st; in the RAL_w the maximal isometric strength of trunk flexion to body weight ratio predicted 24%. The subjects who rated heavier weights for RAL_st, tended to have a better trunk mobility. The dynamic endurance of hand-grip muscles, trunk strength, and spinal flexibility seemed to be the most powerful predictors for the psychophysically assessed ‘acceptable loads’ in experienced workers performing manual materials handling tasks.     

Physical work and strain involved in manual sorting of postal parcels

A field study was conducted to assess cardiorespiratory and musculoskeletal stress and strain and work output during manual sorting of postal parcels, and to detect the effects of parcel sorting on the maximal muscle strength and endurance. The volunteer subjects comprised 32 healthy male sorters with mean (+/- s.d.) age of 34 +/- 7 years at five different sorting sites. Each subject was studied during one evening work shift. During the shift of 391 +/- 46 min the subjects manually sorted 1173 +/- 630 parcels and walked 4.7 +/- 2.3 km with and without the load. While sorting, heart rate was 101 +/- 18 beats min-1. In the heaviest tasks the oxygen consumption was 1.2 +/- 0.41 min-1, and no elevated blood lactate concentrations were found. Work postures in which the back was bent forward averaged 24% of the time for sorting. The overall cardiorespiratory rating and local ratings of perceived exertion for arms, back, and legs did not exceed the 'somewhat strong' level during the work shift. The maximal static strength both for the right and left hand-grip muscles was, on average, 3% lower (p less than 0.05) after the work shift than before the shift. No significant differences were found in the static or dynamic endurance times for the hand-grip muscles when the results obtained after the work shift were compared to the baseline values. At sorting centres the stress and strain on the cardiorespiratory and musculoskeletal system was evaluated to remain within acceptable limits for healthy male sorters.     

Rating of acceptable loads in manual sorting of postal parcels

The psychophysical test, the rating of acceptable load (RAL) were used to assess acceptable weights for dynamic lifting in postal workers engaged in sorting parcels. The standard test (RAL_St) and a work-simulating test (RAL_W) were administered to 103 volunteers: all experienced male sorters. In the RAL_St, subjects selected the weight which would be acceptable for lifting in a box with handles from table to floor and back to the table once every 5 min for the working day. For the RAL_w, the box was without handles and the weight was chosen to be acceptable for transfer 4-6 times/min from a table to the parcel container and back to the table. Both tests were made during normal working hours at postal sorting centres. The overall means for RAL_St and RAL_w were 16·4 kg and 9·4 kg respectively (p < 0·001): both being substantially higher than the average parcel weight of 4 kg. The RAL_St and RAL_w tests proved to be repetitive and sensitive for differentiating the effects of load and task variables in actual manual materials handling. Thus they appear to be applicable to the evaluation of manual materials handling problems.     

Ratings of acceptable load and maximal isometric lifting strengths: the effects of repetition

The effects of repetition on psychophysically acceptable loads and maximal isometric lifting strengths were studied in two groups of subjects. In both groups, subjects selected acceptable loads for dynamic lifting between table and floor and were tested for maximal and acceptable lifting strength isometrically at knee and waist levels.

In series I, 33 subjects (15 males, 18 females) were tested 4 times with a minimal interval of 5 days between tests. In series II, 12 subjects (8 males, 4 females) were tested daily from Monday to Friday on 2 consecutive weeks.

Differences in acceptable isometric lifting strength between the two groups appeared to arise from minor differences in the instructions given; but in neither series was there a significant change in acceptable lifting strength, either dynamic or isometric. In series I, no change was noted in maximal isometric lifting strength. But in series II there was a gain in maximal lifting strength at knee level of 25%. Also in series II, the acceptable isometric lifting strength at waist level was consistently found to be 60% of the acceptable dynamic lifting strength.     

Isoinertial tests to predict lifting performance

Previous research has found isoinertial strength testing to be superior to isometric and isokinetic strength testing for prediction of task performance. The purpose of this study was to investigate tests on an isoinertial lifting machine (ILM) and their ability to predict performance on actual lifting tasks. Sixteen male subjects performed two lifting tasks: maximum box lift to truck-bed height of 1.35 m; and 'speed lifts' of 60 concrete blocks (each 22.7 kg) to the same height. These performance tests were compared to three ILM tests: a maximal lift to 1.83 m, a second maximal lift to 1.52 m, and an endurance test that entailed 60 timed ILM lifts of 22.7 kg to 1.83 m. Pearson product-moment correlations between ILM tests and performance tasks varied from r = 0.55 to 0.71. Therefore, the isoinertial test protocols employed in this study were able to account for only 30% to 50% of the variance in the performance of maximal lifting and endurance tasks. In was concluded that prediction of maximal lifting ability or endurance ability using an ILM might be enhanced by closer approximation of specific task variables, or by inclusion of dynamic parameters to measure technique.     

The role of the abdominal musculature in the elevation of the intra-abdominal pressure during specified tasks

Abstract

A series of standardized tasks, isometric trunk flexion and extension and maximal Valsalva manoeuvres, were used to evaluate the role of the abdominal musculature in developing an increased intra-abdominal pressure (IAP). Seven male subjects were measured for IAP, myoelectric activity of rectus abdominis (RA), obliquus externus and intemus (OE and OI respectively), erector spinae (ES) and isometric trunk torque. IAPs in all experimental conditions were markedly greater than those that occurred while relaxed. In isometric trunk flexion, IAPs were increased with accompanying high levels of activity from the abdominal muscles. In contrast, little activity from the abdominal muscles occurred during isometric trunk extension, although levels of IAP were similar to those found in the isometric flexion condition. With maximal voluntary pressurization (Valsalva manoeuvre) slightly higher levels of IAP than those found in torque conditions were recorded, this pressure being produced with abdominal activities (OE and OI) less than one fourth their recorded maximum. When isometric torque tasks were added to the Valsalva manoeuvre, patterns of muscle activity (RA, OE, OI and ES) were significantly altered. For Valsalva with isometric trunk extension, activity from OE and OI was reduced while IAPs remained fairly constant. These findings indicate that in tasks where an IAP extension moment is warranted, abdominal pressure can be increased without the development of a large counter-moment produced by the dual action of the trunk flexors. Activation of other muscles such as the diaphragm and transversus abdominis is suggested as helping provide control over the level of IAP during controlled trunk tasks.     

The role of the abdominal musculature in the elevation of the intra-abdominal pressure during specified tasks

A series of standardized tasks, isometric trunk flexion and extension and maximal Valsalva manoeuvres, were used to evaluate the role of the abdominal musculature in developing an increased intra-abdominal pressure (IAP). Seven male subjects were measured for IAP, myoelectric activity of rectus abdominis (RA), obliquus externus and internus (OE and OI respectively), erector spinae (ES) and isometric trunk torque. IAPs in all experimental conditions were markedly greater than those that occurred while relaxed. In isometric trunk flexion, IAPs were increased with accompanying high levels of activity from the abdominal muscles. In contrast, little activity from the abdominal muscles occurred during isometric trunk extension, although levels of IAP were similar to those found in the isometric flexion condition. With maximal voluntary pressurization (Valsalva manoeuvre) slightly higher levels of IAP than those found in torque conditions were recorded, this pressure being produced with abdominal activities (OE and OI) less than one fourth their recorded maximum. When isometric torque tasks were added to the Valsalva manoeuvre, patterns of muscle activity (RA, OE, OI and ES) were significantly altered. For Valsalva with isometric trunk extension, activity from OE and OI was reduced while IAPs remained fairly constant. These findings indicate that in tasks where an IAP extension moment is warranted, abdominal pressure can be increased without the development of a large counter-moment produced by the dual action of the trunk flexors. Activation of other muscles such as the diaphragm and transversus abdominis is suggested as helping provide control over the level of IAP during controlled trunk tasks.     

The force-velocity relation and intra-abdominal pressure during lifting activities

An experiment was performed to test the static and dynamic lifting capabilities of the back. Twenty healthy male and female subjects exerted maximal force with the back under isometric and isokinetic lifting positions. Torque about the lumbosacral junction and intra-abdominal pressure (IAP) production were monitored under the experimental conditions. Torque production capability was found to increase as trunk angle increased and decreased as angular velocity increased. IAP was found to be primarily a funciton of angle and a weak indicator of torque. A significant IAP-torque onset delay was identified as was a physiological source of IAP.     

Three dimensional dynamic motor performance of the normal trunk

Traditionally, human strength has been described in terms of the maximum force one could exert under isometric conditions. However, in the past decade ergonomists have attempted to find strength measures that better represent the dynamic aspects of occupational tasks. Recent efforts have focussed upon dynamic motor performance capabilities of the human body that go beyond the traditional measures of isometric strength. The objective of this study was to document the three dimensional dynamic motor performance capabilities of the normal human trunk as subjects flexed and extended their trunks as fast as they could under 'sagittaly symmetric and asymmetric “back lifting” conditions. The results indicated that the range of motion, trunk angular velocity and trunk angular acceleration decreased in the sagittal plane as the trunk became more asymmetric. Dynamic motor performance characteristics increased in the frontal and transverse planes as the trunk became more asymmetric, however, these differences were also dependent upon the device used to measure trunk motor performance.     

The effect of handgrip span on isometric exercise performance

Fourteen male and eight female volunteers served as subjects in these experiments lo determine the effect of hand tool dimensions on isometric strength, endurance, the surface EMG above the active muscle, and the cardiovascular responses to isometric exercise. As reported by others, we found that for each individual, there existed one handgrip size at which he or she could exert the greatest isometric strength. Endurance was the same at any work load relative to the maximum strength for a given grip dimension. The EMG and blood pressure responses to isometric exercise were the same at any given grip span: however, the heart rate response was lowest when subjects worked with their muscles at the optimal grip span.     

Maximal manual stretcher carriage: performance and recovery of male and female ambulance workers

The effects of a maximal duration stretcher carriage on heart rate (HR), lactate concentration, hand steadiness and hand-grip strength were studied up to 72 h post-exercise in 17 male and 15 female military ambulance personnel. Using both hands for transport, the participants walked on a treadmill ergometer at a speed of 4.5 km/h. Force measurements at the handlebars yielded mean loads of 245 N (25 kg) on each side. Each step on the treadmill induced additional force oscillations with peak forces up to 470 N corresponding to 130% (women) and 98% (men) of maximal voluntary contraction (MVC). In the males the maximal transport time was about twice the time in women (mean +/- SD: 184 +/- 51 s vs. 98 +/- 34 s). These differences had no significant effect on HR and lactate values. The same applies to hand steadiness, which showed only a transient deterioration immediately after exercise. In contrast to these parameters, substantial differences were seen in hand-grip strength recovery. Immediately after exercise, maximal hand-grip strength decreased by 150 N (25% MVC) in the males vs. 50 N (14%) in the females. Irrespective of gender, individuals with larger hand-grip strength and longer carriage durations (range 120 s-280 s) showed the slowest strength recoveries (up to 72 h) as compared to 1 h of recovery in participants with short transport durations (range 27 s-120 s). These findings suggest that the increasing number of eccentric strains during uninterrupted stretcher carriage induces cumulative muscle damages that may require some days for complete recovery.     

Maximal manual stretcher carriage: performance and recovery of male and female ambulance workers

The effects of a maximal duration stretcher carriage on heart rate (HR), lactate concentration, hand steadiness and hand-grip strength were studied up to 72 h post-exercise in 17 male and 15 female military ambulance personnel. Using both hands for transport, the participants walked on a treadmill ergometer at a speed of 4.5 km/h. Force measurements at the handlebars yielded mean loads of 245 N (25 kg) on each side. Each step on the treadmill induced additional force oscillations with peak forces up to 470 N corresponding to 130% (women) and 98% (men) of maximal voluntary contraction (MVC). In the males the maximal transport time was about twice the time in women (mean ± SD: 184 ± 51 s vs. 98 ± 34 s). These differences had no significant effect on HR and lactate values. The same applies to hand steadiness, which showed only a transient deterioration immediately after exercise. In contrast to these parameters, substantial differences were seen in hand-grip strength recovery. Immediately after exercise, maximal hand-grip strength decreased by 150 N (25% MVC) in the males vs. 50 N (14%) in the females. Irrespective of gender, individuals with larger hand-grip strength and longer carriage durations (range 120 s–280 s) showed the slowest strength recoveries (up to 72 h) as compared to 1 h of recovery in participants with short transport durations (range 27 s–120 s). These findings suggest that the increasing number of eccentric strains during uninterrupted stretcher carriage induces cumulative muscle damages that may require some days for complete recovery.     

Review of:“Sex Differences in Human PerformanceEdited by M. A. BAKER (Chichester,England: John Wiley, 1987.) [Pp. xvi + 202.] £22·00. ISBN 0-471-91119-4.

An experiment was conducted to examine the role that maximal lifting power has in predicting maximum acceptable weight of lift (MAWL) for a frequency of one lift per 8 h. The secondary aim of the study was to compare the ability of power to predict MAWL to previously used measures of capacity including two measures of isometric strength, five measures of isokinetic strength, and isoinertial capacity on an incremental lifting test. Twenty-five male subjects volunteered to participate in the experiment. The isometric tests involved maximum voluntary contractions for composite lifting strength at vertical heights of 15 and 75 cm. Peak isokinetic strength was measured at velocities of 0.1, 0.2, 0.4, 0.6 and 0.8 m s^?1 using a modified CYBEX® II isokinetic dynamometer. Isoinertial lifting capactity was measured on the X-factor incremental lifting machine and peak power was measured on the incremental lifting machine by having subjects lift a 25 kg load as quickly as possible. The results indicate that peak isoinertial power is significantly correlated with MAWL, and this correlation was higher than any of the correlations between the other predictor variables and MAWL. The relationships between the isokinetic strength measures and MAWL were stronger than the relationships between the isometric measures and MAWL. Overall, the results suggest that tests used to predict MAWL should be dynamic rather than static.     

Endurance time of grip-force as a function of grip-span, posture and anthropometric variables

The time to volitional exhaustion (endurance time) for sustained contractions is considered as a valid parameter to quantify fatigue and to determine the required rest pauses between two successive contractions. In this study, the effects of grip-span, shoulder posture and anthropometric characteristics on endurance time of grip-force during sustained 30% of maximal voluntary grip-force were investigated. Both subjective and objective measures of fatigue were used in determining the endurance times. Twelve male subjects performed sustained isometric handgrip contractions using a handgrip dynamometer at the combinations of three different grip span settings and two shoulder postures. The investigated three grip spans were the optimal, 2 cm narrower than the optimal, and 2 cm wider than the optimal. The investigated two shoulder postures were neutral and 25° flexion. The outcome measures were: endurance time, surface electromyography of related forearm muscles, heart rate, blood pressure, and ratings of perceived discomfort/pain. The results indicate that the endurance time decreases significantly as the grip span deviates from the optimal in both directions. On the other hand, the considered shoulder postures did not have a significant effect on the endurance time. Further analysis indicated a significant negative correlation between endurance time and rest pause and a marginal positive correlation between maximum voluntary grip-force and rest pause. Body mass index, and volume of forearm and hand had also significant negative correlation with endurance time. The comparisons are made with a number of existing endurance models and the impact of findings are discussed.

Relevance to industry

In accurate establishment of the time standards, muscular fatigue allowances need to be taken into account. The endurance time for sustained isometric contractions is correlated with the required rest allowances (pauses) for intermittent static contractions; and therefore, required muscular fatigue allowances can be estimated from the endurance times.     

Intelligence,Work Performance,and Inspection Time

The additivity of strengths for teams of two and three untrained female subjects in eam-work was evaluated in static (isometric) and dynamic (isokinetic) terms. Eight healthy college students were tested under laboratory conditions. Four standard values were used to evaluate isometric strengths: arm, leg, stooped back, and composite measures. The isokinetic strength was tested by means of dynamic lift strength and dynamic back extension. Following individual measurement of the subjects, they were tested in two-member and three-member teams. Two-female teams w«re evaluated in terms of 28 combinations for each of the six measures; the three-female teams were tested in 56 combinations among the subjects. With the exception of isometric arm strength, the actual team strengths were significantly lower then the corresponding sums of the team-members' individual strengths. On average, the isometric back, leg and composite strengths were approximately 83·3% for the two-female teams, and 83·9% for the three-female teams. The isokinetic strengths for two-female and three-female teams accounted for about 68·0% and 68·4% of the sums, respectively. These results indicate that lifting strength of females in team-work is generally not additive and depends upon the muscle group in use, and suggests that lifting capacity in team-work will be reduced as the number of team members increases.     

Back muscle strength, lifting, and stooped working postures

When lifting loads and working in a forward stooped position, the muscles of the back rather than the ligaments and bony structures of the spine should overcome the gravitational forces. Formulae, based on measurements of back muscle strength, for prediction of maximal loads to be lifted, and for the ability to sustain work in a stooped position, have been worked out and tested in practical situations. From tests with 50 male and female subjects the simplest prediction formulae for maximum loads were: max. load = 1.10 x isometric back muscle strength for men; and max. load = 0.95 x isometric back muscle strength - 8 kg for women. Some standard values for maximum lifts and permissible single and repeated lifts have been calculated for men and women separately and are given in Table 1. From tests with 65 rehabilitees it was found that the maximum isometric strength of the back muscles measured at shoulder height should exceed 2/3 of the body weight, if fatigue and/or pain in the back muscles is to be avoided during work in a standing stooped position.     

Some considerations in the use of isometric,isoinertial and isokinetic strength models for predicting lifting capability

In order to predict lifting capacity of individuals, various mathematical models have been developed. These models have been based on several strength measurements of individuals including isometric, isokinetic and isoinertial strengths. A study was conducted to review and compare prediction models based on their performance. The results indicate that models based on dynamic (isokinetic and isoinertial) strengths are superior to models based on static (isometric) strength. Furthermore, different types of equipment used in measurement of strengths are discussed, and the advantages and disadvantages of each are cited. Their safety, accuracy, price and mobility are compared.     

Endurance limits in different modes of load holding

This study examines the influence of different modes of load holding on the isometric strength exertion and endurance capacity. Six healthy males underwent longitudinal testing with the trunk, knee and elbow in 10 different (straight and bent) postures. The load heights (LH) of 320, 440, 850 and 1250 mm from the floor level with subject-load-distance (S-L-D) of 120 and 320 mm were used. It was noted that the level and duration of force exertion were higher when the load was placed closer to the body. The endurance times were consistently high with the increase in LH, highest being recorded in Position 9 - i e, with the load placed at the shoulder level and S-L-D of 120 mm. As evident from MVC and endurance times, the back straight/knee bent postures may not be preferred to back bent postures.     

Measures of localized spinal muscle fatigue

The objective of this study was to determine the best variable, if any, to indicate the level of localized muscle fatigue. Six male and six female subjects were studied while they exerted their maximal voluntary contraction and 40% of maximal voluntary contraction of spinal extensors in an isometric lifting activity. The electromyography (EMG) of erectores spinae at thoracic and lumbar levels was measured bilaterally. Also, the muscle bed blood volume, level of blood oxygenation to erector spinae at L3 level and heart rate were measured. The initial and final values of subjective feelings of fatigue through visual analogue score, rate of perceived exertions and body part discomfort rating were recorded. The mean maximal voluntary contraction forces for males and females were 899 (238) N and 431 (135) N respectively. The mean durations of hold in maximal voluntary fatiguing contraction were 68.1 (39.9) s and 72.3 (37.0) s for men and women respectively. By the end of the hold the force declined to 52% for males and 62% for females. The EMG amplitudes and median frequencies also progressively declined (p < 0.01). ANOVA revealed that the task percentile values of all variables were significantly different (p < 0.01). Gender had a significant main effect (p < 0.01). The correlation coefficients between force and other individual variables were weak to modest, but significant (p < 0.001). None of the single variables predicted fatigue reliably for either gender and levels of contraction. The regression equations developed were highly significant (p < 0.01) and they explained 96 to 98% of variance in both genders and contractions.