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Kinematics, kinetics, and psychophysical perceptions in symmetric and twisting pushing and pulling tasks.

INTRODUCTION

The incidence of and costs associated with low-back pain have increased to staggering proportions. Manual materials handling injuries are a major source of worker absence and high compensation claims (National Institute for Occupational Safety and Health [NIOSH], 1981), costing 170 to 240 million work days and $4.6 billion per year in the United States (Khalil, 1991). The total direct costs of low-back pain reached $30 billion in 1985, and indirect costs possibly doubled that figure (Stephens, 1991).

According to Marras et al. (1993), repetitive lifting is a major contributor to injury of the lower back. Risk factors include high momentloads, awkward postures, and high frequencies of lifting. One way to reduce low-back injury in the workplace is to redesign jobs involving repetitive lifting by implementing material handling devices (MHDs). These devices, including hoists and articulated arms, are powered in the vertical direction, thus mechanizing the lifting components of industrial jobs. MHDs are also used by manufacturers who are attempting to comply with NIOSH lifting guidelines for reducing stresses on the back.

Despite the reduced lifting requirement when using MHDs, these devices do require the operator to horizontally transfer (push and pull) the load. This redesign fundamentally alters the forces that must be exerted by the operator and, thus, the biomechanical stresses on his or her body. The direction of the major resultant hand forces is changed from vertical to horizontal, and manipulation of the load now also includes the inertia of these load-assist devices and any frictional resistance effects.

BIOMECHANICAL METHODS

Early research found posture to be an important factor in determining the maximal forces that humans can exert in horizontal pushing and pulling tasks. Ayoub and McDaniel (1974) found mean peak static horizontal push forces of 620 N for young men and 335 N for young women with the hands at about 80% of shoulder height (about 100 cm from the floor).

When movements are dynamic, postures and forces can rapidly change during the course of an exertion. People cannot assume optimal postures for the duration of a task, and inertial components affect the forces that can be generated. Lee, Chaffin, Herrin, and Waikar (1991) reported that people assume postures in dynamic maximal push and pull exertions that are different from those assumed in static maximal push and pull exertions. In order to predict performance in dynamic tasks, dynamic hand force capabilities must be determined with realistic task requirements.

Lee et al. (1991) investigated the effects of handle height and hand force on predicted compression force at the L5/S1 spinal disc. Participants pushed and pulled a cart simulator at three handle heights and at three approximately constant load levels. Calculated L5/S1 compression forces were lowest for a handle height of 109 cm (about elbow height). Greater compression forces were calculated for faster cart velocities and greater peak hand force conditions. Although other components that may play a major role in back injury, such as L5/S1 shear force, were not included, this study first showed the importance of velocity, hand force, and posture in contributing to back stress in pushing and pulling tasks.

PSYCHOPHYSICAL METHODS

Psychophysical methods measure workers' exertion limits by examining their subjective perceptions of the exertion from a holistic perspective, rather than the biomechanical tolerance of specific tissues. This method's reliability depends on the fidelity of the scale in eliciting an appropriate response from the individual. Several psychophysical scaling techniques exist, but the physiological basis of the perceptions remains unknown. Some scales have been shown to be repeatable and to reliably predict individuals' abilities.

The most comprehensive psychophysical limits of manual exertion were compiled by Snook and colleagues (Snook, 1978; Shook & Ciriello, 1991; Snook & Irvine, 1967) with industrial workers who performed exertions with a broad variety of exertion frequencies, heights, and distances of movement in a laboratory. Acceptable initial mean peak hand push forces ranged from 180 N for frequent knuckle- and shoulder-height pushes to 530 N for infrequent elbow-height pushes. This maximum value is less than the static forces previously cited by Ayoub and McDaniel (1974), largely because of the more realistic postures in this study.

A commonly used scale in studies of repeated muscular effort is the CR-10 scale. According to Borg (1982) and Eastman Kodak (1983), the CR-10 units are one tenth of the individual's maximum muscular strength at the limb under study. Thus a rating of 5 would correspond to an exertion of 50% of the individual's maximum muscular strength. This would suggest that the CR-10 scale is appropriate for tasks with limitations arising from local fatigue at individual joints.

Borg (1990) reported that even in job situations involving short-term static work, the value of subjective estimations is evident. The association between the CR-10 and other scales with specific peak biomechanical stresses has been investigated, but with varied results. Using the CR-10 scale, Thompson (1993) found that biomechanical stresses in the back are not well perceived for infrequent exertions. Waikar, Lee, Aghazadeh, and Parks (1991) calculated the correlations between a rating scale such as the CR-10 from 0 to 100 and compression force at the L5/S1 spinal disc for participants who repeatedly lifted from floor to overreach positions.

Their compressive forces were not well correlated with the psychophysical ratings. Ratings also were not correlated with participants' static back strength, leg strength, or grip strengths. They concluded that psychophysical methods may not be appropriate for judging tasks with high peak stress at the low back.

MATERIAL HANDLING DEVICE STUDIES

A limited number of MHD studies have been conducted. Woldstad (Woldstad, Langolf, & Chaffin, 1988; Chaffin, Woldstad, & Ali, 1989) performed a series of laboratory studies using an overhead cable hoist. In Woldstad et al. (1988), seven college students performed a Fitts task in which they were required to move the hoist back and forth between two horizontal targets as quickly and accurately as possible with loads ranging from 81.7 to 217.9 kg. The handle height was constrained at 109 cm (about elbow height), and the students were told to assume a pace that could be maintained for 4 h. Peak pushing forces ranged from 200 to 500 N, and peak pulling forces ranged from 150 to 300 N. Peak velocities were also measured, ranging from 1.0 to 1.8 m/s. The authors predicted compression forces around 2225 N in all conditions (well below the 3400-N limit for safety proposed by NIOSH).

Chaffin et al. (1989) performed case studies of several MHD jobs in industry involving an overhead hoist with loads of 15 and 150 kg and an articulated arm with loads of 15 kg. Peak push forces ranged from 130 N with the articulated arm to 280 N for the overhead hoist. Peak pull forces ranged from 125 N with the articulated arm to 400 N with the overhead hoist. They found that peak push and pull forces increased with heavier inertial loads.

Resnick and Chaffin (1995) studied the effects of inertial load using a free-moving cart. They found that inertial loads in the cart of up to 450 kg resulted in very high peak hand forces, reaching 500 N for stronger men. In addition, they calculated peak velocities of the cart. With inertial loads of 45 kg, the peak velocities approached 1.0 m/s, whereas with inertial loads of 450 kg, the peak velocities did not exceed 0.5 m/s.

ASYMMETRIC EXERTIONS

In many industrial jobs using MHDs such as articulated arms and hoists, tasks that involve pushing and pulling have a considerable amount of asymmetric loading. Workers often rotate their whole body 90 [degrees] to 180 [degrees] , with much of this rotation occurring at the torso. How this rotation affects workers' force-producing capabilities and the risk of back injury is critical for ensuring a productive and safe workforce. Little has been done to measure workers' horizontal hand strengths while rotating. The effects of torso rotation on muscle recruitment and the compression forces at the lumbar spine have been studied in vitro, though not in a situation requiring whole-body twisting while maneuvering a dynamic load.

Ladin, Murthy, and DeLuca (1989) found that the recruitment of muscles is different for asymmetric postures than it is for symmetric ones. When the torso is twisted, recruitment of the latissimus dorsi muscles increases. With different muscle recruitment patterns for torso rotation than for symmetric pushing and pulling, the strengths of workers performing these tasks will be different.

Andersson (1981), Hughes (1991), and others have found that occupational factors such as twisting of the torso contribute to the co-contraction of torso muscles, which increases the incidence of low-back disorders. Punnett, Fine, Keyserling, Herrin, and Chaffin (1991) found a strong relationship between occupational exposure to nonneutral trunk postures and musculoskeletal disorders of the back as a function of both duration and intensity.

Based on the results of these studies and observations in industry (Resnick, Chaffin, Foulke, & Woolley, 1991), several questions have been raised. How much force is required to manipulate MHDs under a wide range of job specifications? How psychophysically stressful are MHD exertions? Can psychophysical perceptions be used to estimate biomechanical stresses? How does axial twisting affect the performance and stresses of the task? These questions were addressed in the following studies.

EXPERIMENT 1: SAGITTALLY SYMMETRIC PUSH/PULL EXERTION

Objectives

The first objective of the study was to systematically measure hand force, hand velocity, and psychophysical variables that describe the kinematic and kinetic requirements of sagittally symmetric dynamic pushes and pulls of MHDs. These requirements were compared with those of previous pushing strength studies to ensure that load movement requirements can be completed within performance, strength, and psychophysical constraints.

The second objective was to determine the basis of psychophysical perceptions of exertion by calculating the correlations between kinematic variables and psychophysical ratings.

The final objective of the study was to investigate the movement strategies used by people to perform this type of exertion.

Methods

Participants. Seven young men and three young women were selected to participate in the experiment. They reported no musculoskeletal disorders, were healthy at the time of testing, and were paid for their participation. Their informed consent was obtained prior to each session. Gross anthropometric data are given in Table 1.
TABLE 1

Gross Anthropometric Characteristics of the 10 Subjects

Measure            Mean     SD    Max: Percentile   Min: Percentile
Age (years)    m   26        5        NA                NA
               f   24        3

Height (m)     m   1.78     0.08       97                16
               f   1.70     0.03       98                25

Weight (kgs)   m   72       11         99                12
               f   54        3         50                13

Note: m = male, f = female.


Apparatus. An articulated arm MHD designed by Ergomatic Systems (Pontiac, MI) was used [ILLUSTRATION FOR FIGURE 1 OMITTED]. This articulated arm has a 2.5-m high base and two linear links of 1.4 m. Vertical movement is assisted through a pneumatic cylinder located at the joint of the jib and boom links. The arm is manipulated using a 3.8-cm diameter cylindrical handle centered at the end of the jib link. Friction in the arm joints was regulated by a pair of disc brakes, one at each joint, which created an average 25-N static horizontal hand force in both push and pull directions.

Global positional information of the MHD was determined through a set of potentiometers on each joint of the arm, which provided accuracies of 0.005 m in the horizontal plane and 0.002 m in the vertical plane. An unpublished validation study (Raschke & Resnick, 1992) verified that this system provided a measure of hand velocity with a maximum error of 4%.

Hand forces were measured using a triaxial force transducer, calibrated at 0- and 100-N loads, producing errors of 2.5 N.

Loads of 0, 23, 45, and 68 kg were placed on a 2.5-cm diameter mandrel oriented perpendicular to the handle facing away from the operator. These loads represent the range reported in Marras et al. (1993), which are typically lifted manually but are large enough to require MHD assistance. The mandrel was designed to fit into a circular hole target in an aluminum plate located at the end of a 1.5-m (5-foot) movement zone and mounted on a unistrut base. The plate had two holes 3.8 cm and 5 cm in diameter, establishing two levels of positioning difficulty for the task, corresponding to Fitts' Indices of Difficulty of 6.3 and 5.9, respectively. These diameters were used to set moderate levels of difficulty for inexperienced users based on preliminary findings.

In order to complete the experimental task, participants were not required to change the vertical position of the mandrel, and thus only horizontal push and pull (start and stop) hand forces were needed.

Experimental design. Each participant manipulated the arm under 16 conditions corresponding to levels of handled load (0, 23, 45, and 68 kg), target size (3.8 and 5.0 cm), and added joint friction (0 and 25 N added). Load handled and level of positioning difficulty were randomized within blocks that had constant joint friction. Each combination was repeated five consecutive times.

Dependent variables measured were the motions of each arm joint, triaxial hand forces exerted at the force handle as a function of time, the postures assumed during the exertion, and Borg CR-10 ratings obtained immediately after each set of five repetitions. MHD handle velocities were calculated from the position data as described in the previous section.

Procedure. The required tasks were explained in detail to each participant at the beginning of the session. They were instructed to work at a comfortable rate that could be maintained for 8 h and to begin with the MHD mandrel facing the target. The initial locations of their feet and the height of the MHD handle were constrained to control the starting position. The participants walked forward, pushing the handle ahead of them until they reached the target and inserted the mandrel. They then walked backward, pulling the handle along with them, until they reached the starting position. The experimental layout is shown in Figure 1. This was performed at a rate of once every 15 s, five times for each load/brake/target size combination.

Participants were given several practice trials prior to data collection to become familiar with the dynamics of the procedure. They were not informed of the load magnitudes. At the beginning of each trial a tone was sounded, at which time the participant was instructed to proceed.

Immediately after the completion of each condition, the participants were asked to rate the exertion on a Borg CR-10 rating scale (see Table 2) and to report what general area of the body (arms, legs, or back) was the most stressed during the trial. If a participant reported fatigue, extra rest would be provided before the next trial to avoid a cumulative fatigue effect confounding the CR-10 exertion ratings. The weight was changed while the participant rested; this break lasted at least 60 s. After half the trials had been completed, the participant was given an additional 15-min rest to minimize any cumulative fatigue effects. The entire experimental procedure lasted between 2 and 3 h.
TABLE 2

The Borg CR-10 Scale

0     Nothing at all
0.5   Extremely weak (just noticeable)
1     Very weak
2     Weak (light)
3     Moderate
4
5     Strong (heavy)
6
7     Very strong
8
9
10     Extremely strong (almost max.)

       Maximal


Results

Hand forces and kinematics. The results of an analysis of variance (ANOVA) for the peak push [TABULAR DATA FOR TABLE 3 OMITTED] (acceleration) hand forces, pull hand forces, and velocities are reported in Table 3. The p values for all independent variables and significant interactions are shown. Table 4 shows the mean and standard deviations for all hand force and kinematic variables. A discussion of the velocity profiles is available in Resnick (1993). Average velocities can be calculated from the total movement times and distances, which are also discussed in Resnick (1993). Because of the high correlations between the average and peak velocities, only the peak velocities will be presented here.

The hand force values reported here are the aggregate forces calculated by summing the individual x, y, and z force components using the vector sum [[([x.sup.2] + [y.sup.2] + [z.sup.2]).sup.1/2]]. Figure 2 depicts the mean peak push force, pull force, and velocity. As predicted, greater handled loads produced higher peak hand forces in all cases. When the brake was activated, participants exerted the largest peak hand forces, both for pushes and pulls. The significant Subject x Load interaction indicates that subjects use different strategies in determining how much force to exert in response to higher handled loads. The magnitudes of the peak push forces ranged from 43 to 72 N without the brake and from 67 to 85 N with the brake. The magnitudes of the peak pull forces were greater than the peak push forces, averaging about 20 N greater without the brake and 30 N greater with the brake.

Psychophysics. Participants were asked to give psychophysical ratings on the Borg CR-10 scale for each condition in the study. The p values from the ANOVA also are shown in Table 3. The average ratings for each condition are shown in Figure 3. The psychophysical ratings were significantly higher when the brake was activated. Recall that when the brake was activated, it added 25 N of static hand force. Ratings were also higher in general for higher-handled loads, though this factor did not reach statistical significance because of the large between-subjects variance.
TABLE 4

Means and Standard Deviations for the Force and Kinematic
Variables Averaged across all Independent Variables in Experiment
1

Variable                   Mean      SD         Range

Peak push force (N)         65       17         23-312
Peak pull force (N)         88       32         37-299
Peak velocity (m/s)          1.2    0.3       0.72-2.03
Peak acceleration (g)        0.8    0.2       0.25-1.48
Peak deceleration (g)        0.7    0.3       0.37-1.56


CR-10 ratings without the brake ranged from 0.0 to 2.0 with no-handled load and 0.5 to 4.0 with a 68-kg handled load. With the brake activated, the CR-10 ratings ranged from 2 to 6 with no-handled load and from 2 to 7 with a 68-kg handled load. Four people reported that their arms were the most stressed in all cases, and five reported that their arms were most stressed when the brake was activated and their legs were most stressed when it was not. One participant was inconsistent. None of the participants reported that their back was the most-stressed body area.

Correlations between the CR-10 ratings and the kinematic variables of peak push hand force, pull hand force, and velocity were calculated individually for all participants. The minimum, maximum, and mean for each factor are reported in Table 5. These correlations are an attempt to discover the basis for the psychophysical ratings. The correlations for handled load and peak velocity are low, indicating that the participants did not base their ratings on these measures. The correlations for peak push and peak pull hand forces are higher (mean r = .65, .68, p [less than] .05). The peak hand forces seem to be the most critical parameter in the formation of perceptions of pushing and pulling exertion limits.
TABLE 5

Correlation Coefficients between Force and Velocity
Variables and CR-10 Ratings in Experiment 1

                  Minimum r     Maximum r     Mean r

Handled load       0.01           0.84        0.46
Peak push force    0.18           0.93        0.68
Peak pull force    0.01           0.93        0.65
Peak velocity      0.13           0.66        0.43


EXPERIMENT 2: ASYMMETRIC TWISTING DURING PUSH/PULL EXERTIONS

Objectives

As mentioned earlier, axial torso rotation can increase the stresses of an exertion. The objective of the second study was to measure the differences in performance and stress when torso twisting is introduced to the experimental task. The results are contrasted with those of Experiment 1, in which similar pushing exertions were constrained to sagittal plane movements.

Method

Participants, apparatus, experimental design. The same 10 people participated in Experiment 2 approximately one week after Experiment 1. They signed an updated consent form and were paid for their participation. The apparatus used in Experiment 1 was again used; however, an additional target was located 1.5 m from the first target, as shown in Figure 4. The second target altered the task so that the participants were required to rotate their whole body 180 [degrees]. The same levels of each independent variable were manipulated in this study as in Experiment 1. Videotape recordings of the trials were taken to provide postural data for biomechanical analysis.

The Three-Dimensional Static Strength Prediction Program (3DSSPP[TM]) was used to calculate torques and obtain population strength norms for the shoulder, torso, and hip postures observed in the study (Chaffin & Andersson, 1991). The validity of this model for predicting strengths at various joints has been discussed in Chaffin and Erig (1991) and Chaffin, Freivalds, and Evans (1987).

Procedure. Participants were instructed to begin with the handle at the first target, to pull the load/mandrel from the first target, to move it horizontally to the second target behind them, to insert (push) the mandrel into the second target, and then to return it to the first target. The trial ended when the mandrel came to rest in the second target, as shown in Figure 4. This was performed at a rate of once every 15 s, five times for each load/brake/target size combination. Participants were allowed several practice trials prior to data collection to become familiar with the dynamics of the procedure. At the beginning of each trial a tone sounded, and they were instructed to proceed. This tone was recorded on videotape and was used to synchronize the force and location data with the postural data on the videotape.

Immediately after the completion of each condition, participants were asked to rate the exertion on a Borg CR-10 rating scale and to identify the general area of the body (arms, legs, or back) that was most stressed during the trial. Again, fatigue was monitored and, if it was reported, additional rest times were used. After the first block of trials was completed, participants were given an additional 15-min rest. The entire experimental procedure lasted between 2 and 3 h.

Results

Hand forces and kinematics. The results of an ANOVA of the peak push hand forces, pull hand forces, handle velocities, and Borg CR-10 ratings are reported in Table 6; p values from the ANOVA are listed for all independent variables and significant interactions. Table 7 shows the means and standard deviations for all kinematic variables. The high standard deviations of the hand force variables illustrate the wide range of strengths among the 10 participants and the difference in required pull force for different load magnitudes.
TABLE 6

P-values from the Analysis of Variance (ANOVA) for all Independent
Measures and Significant Interactions in Experiment 2

                     Borg CR-10   Peak Push   Peak Pull      Peak
Factor                 Rating       Force       Force      Velocity

Handled load           0.12        0.94         0.75        0.30
Brake                  0.03(*)     0.01(*)      0.06        0.02(*)
Target size            0.03(*)     0.86         0.83        0.01(*)
Subject                0.02(*)     0.03(*)      0.43        0.29
Subject x Brake        0.01(*)     0.00(*)      0.01(*)
Subject x Load                     0.05(*)

* Significant at the p [less than] .05 level.


Unlike in the first experiment, participants pulled the load/mandrel to accelerate it toward each target. Thus peak pull hand forces occurred at the beginning of the exertion. Peak push hand forces occur in the middle of the exertion, when additional momentum is necessary to complete the movement. When the brake is activated, this extra momentum is considerable; however, when the brake is not activated, little extra momentum is required.

The mean peak push force, pull force, and velocity exerted for each task parameter are depicted in Figure 5. Contrary to expectations from the first experiment, peak push hand force did not increase with larger masses being moved. Mean peak push forces were only 5% higher for the 68-kg handled load than for no-handled load. When 25 N of handle friction force was added to the arm through the use of the dynamic brakes, participants increased both their peak push and pull hand forces. The magnitudes of the peak forces ranged from 20 N to 80 N in the no-brake condition and 60 to 300 N in the brake condition. The 20-N exertion was measured for the no-brake condition for several participants, whereas the 300-N exertion was measured with the brake on for only one. For eight participants, no forces greater than 110 N were measured at any time.

The increase in peak pull force with the inclusion of the brake may seem counterintuitive, as the brake should have made the MHD arm easier to stop. However, the peak pull forces do not occur during deceleration but rather at the beginning of the exertion during acceleration, when both inertia and friction add to the hand force. Peak pull forces were 30% lower when the MHD was not loaded than with a 68-kg handled load. The magnitudes of the peak pull forces ranged from 40 N to 150 N with no brake and 50 N to 220 N with the brake activated.
TABLE 7

Means, Standard Deviations, and Ranges for all Kinematic and Hand
Force Variables in Experiment 2

Variable                 Mean         SD             Range

Peak push force          72 N         53 N          22 N-103 N
Peak pull force          97 N         51 N          32 N-167 N
Peak velocity             1.4 m/s      0.3 m/s      0.56-1.86 m/s
Peak acceleration         0.9 g        0.3 g      0.13 g-1.29 g
Peak deceleration         0.9 g        0.4 g      0.04 g-1.18 g


There was no significant decrease in peak velocity attributable to an increase in handled load when the brake was activated, but when it was not, velocities decreased by 15%. The activation of the brake decreased peak velocities by 20%. The magnitudes of the peak velocities ranged from 0.8 m/s to 2.0 m/s with no brake and from 0.8 m/s to 1.75 m/s with the brake activated.

Psychophysics. Participants were asked to give psychophysical ratings on the Borg CR-10 scale for each condition in the study. The results of the ANOVA are shown in Table 7. The ratings for each condition are shown in Figure 6. The psychophysical ratings were significantly higher when the brake was activated. They also were higher in general for higher-handled loads, though this factor did not reach significance. The magnitude of the CR-10 ratings without the brake ranged from 0 to 2 when the MHD was not loaded and from 1 to 3 with a 68-kg handled load. With the brake activated, the CR-10 ratings ranged from 1 to 3 with no-handled load and from 3 to 6 with a 68-kg handled load.

Six participants reported that their arms were the most stressed throughout the study, and two reported that their legs were the most stressed. One reported being more stressed in the arms when the brake was activated and in the legs when it was not, and another reported the reverse. The arms were the most stressed on the average, and few participants reported a perception of leg stress when the brake was not activated. No participants reported the torso as being the most stressed, supporting the conclusions of Thompson (1993) that people do not perceive low-back stress as well as they perceive stress in other body segments, such as the arms and legs. The lack of any reports of fatigue supports the use of Borg CR-10 ratings as measures of local muscle exertion levels.

Correlations were calculated between the CR-10 ratings and the kinematic variables of handled load, peak push hand force, peak pull hand force, and peak velocity for all participants. The minimum, maximum, and mean for each factor is reported in Table 8. The correlations for the peak push and pull hand forces are significant at the p [less than] 0.05 level, though they are lower than the correlations calculated in Experiment 1.

Biomechanical effects. In order to estimate the strength required around the shoulder and torso joints, the moments around the shoulder, torso, and hip joints were calculated using a three-dimensional biomechanical model (3DSSPP[TM]). Using peak forces and the corresponding video-measured postures at which the peak forces were exerted [ILLUSTRATION FOR FIGURE 7 OMITTED!, three-dimensional moments at the shoulder, torso, and hip were calculated (see Table 9). Although dynamic strengths will be lower at high-contraction velocities than the strengths predicted by the static model, the velocities averaged only 1.2 m/s and rarely exceeded 2 m/s. Nevertheless, caution is needed for the following interpretations.
TABLE 8

Correlation Coefficients between Borg CR-10 Ratings and Kinematic
and Hand Force Variables in Experiment 2

                        Minimum r     Maximum r     Mean r

Handled load               0.12          0.89         0.56
Peak push force            0.05          0.83         0.52
Peak pull force            0.09          0.93         0.60
Peak velocity              0.10          0.59         0.31


The magnitude of the moments are not high compared with those often reported when manually lifting loads in industry. Comparing the joint moments depicted in Table 9 with the strength moment norms in the 3DSSPP[TM] model indicates that no strength limitation should prevent the general population from being able to complete this task at comparable movement speeds. However, many researchers (Marras et al., 1993; Pope, Andersson, Broman, Svensson, & Zetterberg, 1986; Shirazi-Adl, 1994) predict that high torso moments during twisting can increase the risk of low-back injury.

DISCUSSION

Force and Velocity

The manipulation of several task parameters had significant effects on the participants' performance on the articulated arm used in this study. Greater loads handled and the activation of the brake increased the peak hand forces exerted. The peak push and pull forces of around 100 N measured for most participants were lower than those found in Woldstad et al. (1988) with an industrial hoist and in Resnick, Chaffin, and Erig (1991) with a cart. In both studies, peak forces of 300 to 500 N were measured. However, both Woldstad et al. (1988) and Resnick et al. (1991) used much greater loads (up to 450 kg). Furthermore, in Resnick et al. (1991), peak forces were measured while participants worked at maximal performance, whereas the present participants were instructed to work at a comfortable rate that could be maintained for 8 h.

The peak velocities observed in this study range from 0.6 to 1.9 m/s. These are slightly higher than the 0.6 to 1.2 m/s reported by Resnick and Chaffin (1995) for similar loads and postures in cart pushing. The articulated arm can move in all three directions, whereas the cart in Resnick and Chaffin was unidirectional. Forces in both the lateral and vertical directions will contribute to total velocity, whereas with the cart of Resnick and Chaffin, forces exerted in these directions did not contribute to the sagittal plane velocity of the cart. For this reason, using cart studies to predict MHD performance may underestimate the dynamics of MHD use. However, accounting for differences in load and degrees of freedom, the performance of the participants [TABULAR DATA FOR TABLE 9 OMITTED] is comparable, suggesting that they approach these paradigms similarly. The establishment of benchmarks for hand force and velocity levels expected for a given set of job parameters would be very useful for job designers when implementing MHDs. These results suggest that such benchmarks are possible.

The differences in peak push and pull forces were 24 and 23 N, respectively, between the strongest and weakest individuals. Because these participants were selected randomly, they should reflect a wide range of strengths. In Resnick and Chaffin (1995), strong participants exerted double the peak forces of the weak ones in a cart-pushing task. In that study, however, participants were exerting near-maximum forces. When people are exerting lower forces, as in this study, it seems that strength does not play as significant a role in the magnitude of the peak hand forces that are exerted.

The group used two strategies to compensate for higher inertial loads [ILLUSTRATION FOR FIGURE 8 OMITTED]. One strategy, used by seven participants, was to push with greater force to counteract the higher inertia. This enabled them to reach a similar peak velocity in each trial. Another strategy, used by the other three participants, was to ignore the higher inertia and exert approximately the same force. This required them to slow down for the heavier conditions.

Psychophysical Ratings

It was expected that a large part of the CR-10 ratings would reflect the peak push and pull forces required in the task. The correlation coefficients for the push and pull forces were 0.68 and 0.65, respectively, thus accounting for 46% and 42% of the variance. Because the peak push and pull forces are themselves correlated, the total amount of variance accounted for by these two measures is less than the sum. Additionally, the correlation of these factors varied considerably among the participants, suggesting that this relationship is not universal. For one participant the peak push hand forces and Borg CR-10 ratings had a correlation of 0.93, whereas for another the correlation was only 0.18.

These results do not provide a sufficient relationship to predict the perceptions of exertion with any degree of accuracy based on the task parameters investigated here. Clearly, more research is needed to determine the basis for psychophysical perceptions of exertions. Perhaps an investigation of loads as a percentage of individuals' capacities would reveal a more consistent relationship.

The significant effect of target size on participants' Borg CR-10 ratings suggests that positioning accuracy was also a component of the perception of the exertion. Though the smaller target did not cause them to use smaller peak hand forces, it did cause them to reduce the peak velocities used to transfer the mandrel from one target to the other. The participants were instructed to base their ratings only on the difficulty of the muscular requirements, but they may have been unable to differentiate their perceptions completely. This is a critical factor in interpreting the ratings and attributing task parameters to increased perceptions of exertion.

Participants' ranking of body parts was also an important result. They all reported their arms or legs as the most stressed body part, and none reported high perceptions of general fatigue. This supports the hypothesis that the perceptions of the exertion are based on feelings of local fatigue in the limbs. Thus if psychophysical ratings can be focused on appropriate physiological stresses, they may be a useful tool for evaluating materials handling jobs in which the complexity of movement is too great for current biomechanical models. However, the lack of reports of torso stress suggests that when the back is the chief source of stress, psychophysical ratings may not be appropriate.

The basis of psychophysical ratings for the exertions involved in complex tasks such as MHD use does not seem to be based solely on a small set of kinematic or hand force variables. More of the variance must be explained through known parameters before the ratings can be used to predict performance or safety. An understanding of how perceptions of exertions are generated can be complete only through evaluating an array of biomechanical, physiological, and psychological factors.

However, the results do suggest that Borg CR-10 ratings are appropriate for measuring localized muscle limitations in the arms and legs. Because these ratings are easy to use and inexpensive, their applications in other areas should be further investigated. Furthermore, to address the problem of back stresses not being well perceived, additional psychophysical methods and scales should be studied.

Though there was no significant effect of the magnitude of the load being handled on psychophysical ratings, there was an increase in the average rating from 1.5 for no load to 2.5 for a 68-kg load. The results of the psychophysical rating analysis indicate that what makes up the perception of an exertion is a complicated combination of many factors. Participants' perceptions of the difficulty of an exertion are not generally dependent solely on biomechanical factors as specified by the 3DSSPP model. Fatigue, for instance, has been shown to increase ratings on the Borg CR-10 scale.

The CR-10 scale may not be appropriate when workers of radically different strengths perform the same job because strong and weak workers may rate the same task differently. Resnick (1995) has shown that for static lifting tasks, ratings on the CR-10 scale reflect the percentage of a person's strength being exerted. For most jobs there will be a variety of perceptions of how difficult they are, depending on the worker. Design based on workers' psychophysical effort ratings will be difficult if there are multiple workers at a workstation.

Torso Twisting

Participants' peak push hand forces were 20 to 30 N greater when the experimental task was altered to introduce torso rotation. It was expected that peak handle forces would decrease when torso rotation was introduced because of the asymmetry of the exertion and the increase in the risk of back injury. This was not the case. The peak handle forces increased, as did the moments around the torso and shoulder.

The higher peak hand forces measured in Experiment 2 as compared with those in Experiment 1 indicate a trade-off between safety and performance. Several researchers have reported that twisting the torso is a risk factor for back injury (Andersson, 1981; Bigos et al., 1986; Frymoyer et al., 1980; Hughes, 1991; Marras et al., 1993). However, higher movement velocities in this torso-twisting task suggest that torso twisting can also provide an improvement in movement speed, attributable in part to the strength of the torso musculature. When loads are low and movement heights are constrained to elbow height, such as in this study, the performance benefit does not seem to put workers at an excessive risk of back injury, given our current understanding of these injuries. As we learn more about the negative effects of low-back shear force and axial torsion, however, this conclusion may change. If higher loads are manipulated and bent postures are required, the increase in back stress attributable to torso twisting may increase the risk of injury to excessive levels. Spinal stresses do need to be analyzed, but this would require a fully dynamic torso prediction model, which currently does not exist.

Implementation of MHDS

One of the original objectives of this research was to determine whether MHD use could be completed within the strength, psychophysical, and safety limits of a broad worker population. The average peak push and pull hand forces exerted by participants in both studies did not exceed 150 N and rarely exceeded 100 N. Though strength is inherently posture dependent, these forces are not excessive for the largely upright postures observed. Psychophysical ratings varied from very weak to moderate without the brake, albeit occasionally reaching strong with the brake.

Increasing the handled load to 68 kg caused participants to increase the peak push forces they exerted by 30% compared with the no-load condition. This increase in peak hand force allowed participants to maintain the peak velocities achieved without the added load. On average, the psychophysical ratings were about 50% higher for the 68-kg load versus no load, increasing from a rating of weak for no-load handled to moderate for the 68-kg load. This indicates that heavier loads make the job perceptually more difficult, possibly because of the increased energy that must be expended to exert this extra force. In order to reduce the level of effort required to perform the job, the load handled should be kept to a minimum, including specifying an MHD with low mass in its moving parts. The load limit will depend on the combined mass of the MHD and the load being moved.

The two targets used had diameters 50% and 100% larger than the mandrel, giving the individual twice as much clearance in the large target condition. The large target enabled participants to increase their peak velocity by an average of 5% to 10%, indicating that productivity is at least partially dependent on positioning accuracy in an insertion task. Thus there is a potential gain in productivity available if a job can be redesigned to allow workers additional clearance. Though this is well known through methods time measurement analysis, designers of MHD jobs do not always take advantage of this knowledge - for instance, through the inclusion of a guidance system of mechanical stops and shock absorbers to assist a worker in positioning the end effector of an MHD. These additions would result in a further improvement in productivity.

Because NIOSH has focused largely on lifting stresses in its guidelines, it is a natural inclination for job designers to focus on removing stressful lifting tasks from their workplaces. It is important to remember, however, that the current lack of specific guidelines for pushing and twisting tasks is not an indication that these exertions are safer. Simply replacing lifting tasks with MHD pushing and pulling tasks is not a comprehensive solution for reducing musculoskeletal injuries.

ACKNOWLEDGMENTS

We thank Jim Foulke and Chuck Woolley for their assistance in the design of the equipment used to conduct this study, and Ulrich Raschke for his help programming the data acquisition software for the MHD and also running the participants. This research was sponsored by Ford Motor Company under contract NP 8800-55169.

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Marc L. Resnick is an assistant professor of industrial engineering at Florida International University and director of the Human Factors and Ergonomics Laboratory. He received a M.S. degree from the University of Michigan in industrial engineering and completed his Ph.D. at the University of Michigan in 1993, also in industrial engineering.

Don B. Chaffin is the G. Lawton and Louise G. Johnson Professor of Industrial and Operations Engineering and Occupational Health and director of the Center for Ergonomics at the University of Michigan. He received his M.S. and Ph.D. degrees in industrial engineering, the latter from the University of Michigan in 1967.
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