Effects of team size on the maximum weight bar lifting strength of military personnel.
Teamwork can be used to reduce the load on an individual performing a heavy lifting task and is particularly useful when an object is bulky and mechanical aids are not available. Although there are guidelines for individual lifting, little information is available on lifting in teams. The Revised 1991 NIOSH Lifting Equation does not provide specific guidance on team lifting (Waters, Putz-Anderson, Garg, & Fine, 1993). Specific guidance is found in Military Standard Human Engineering Design Criteria for Military Systems, Equipment, and Facilities (commonly known as Military Standard 1472D, 1989). The recommended weight limits for an occasional lift from floor level to a height of 91 cm are 39.5 kg for men and 20 kg for women (Military Standard 1472D, 1989). For two-person lifts, the load is doubled, and a maximum of 75% of the one-person value can be added for each additional lifter beyond two.
Team lifting strength has been studied for both isometric and isokinetic lifting in teams of two and three men (Karwowski & Mital, 1986) and in teams of two and three women (Karwowski & Pongpatanasuegsa, 1988). Karwowski and Mital (1986) reported that the team lifting strength of men was less than the sum of individual lifting strengths and that this deficit increased when the number of men performing the lift increased from two to three. The team lifting strength of two women was less than the sum of individual strengths but showed little or no further decline with the addition of a third woman for isometric composite strength or isokinetic lifting strength (Karwowski & Pongpatanasuegsa, 1988).
Lifting tasks are not typically performed at a controlled velocity (isokinetic and isometric) but are isoinertial in nature. Using an isoinertial task, teams of two men or two women determined that the maximum load they could lift in a box was 89 cm (Karwowski, 1988). Pairs of men lifted 87.5% and pairs of women lifted 91.0% of the sum of their individually determined maximum box lifts. No mixed-gender teams were used, nor were any statistical comparisons between genders reported. Sharp, Rice, Nindl, and Williamson (1993) reported that three-person teams lifted between 74% and 91% of the sum of their individually determined maximum isoinertial lift, depending on the gender makeup of the team. The percentage tended to be greater for single-gender teams than for mixed-gender teams.
Given that 53% of all critical Army lifting tasks involve team lifting (Department of the Army, 1994), an understanding of the relationship between individual strength and team lifting strength is an important tactical issue for military commanders. In lieu of specific knowledge of individual lifting capabilities, knowledge of the mean lifting capacities of soldiers working in teams of various size and gender combinations provides a field-expedient method for a military commander to estimate the number of soldiers needed to safely complete a lifting task.
No data are available for isoinertial lifting strength in teams of three or four persons or for combined-gender teams of two or four persons. This study extends the current knowledge base and describes the effects of team size and team gender on isoinertial team weight bar lifting in two-,three-, and four-person teams. The objectives were (a) to examine the relationship between the sum of individual lifting strengths and isoinertial team lifting strength in two-, three-, and four-person teams, and (b) to make direct gender comparisons in team lifting ability to determine if gender differences typically observed for individual lifting strength carry over to team lifting.
Volunteers were 23 men and 17 women who were medically screened prior to participating in any testing procedure. Written informed consent was obtained from each participant following a detailed briefing. Volunteers were not screened for materials-handling experience.
During the first two weeks, descriptive measurements were made and participants were familiarized with the individual and team lifting tests. Individual lifting strength was determined during Week 3. Team lifting trials were conducted during Weeks 3 through 8. Individual lifting strength was reassessed at the beginning of Week 6.
Individual Lifting Strength
To measure individual lifting strength, a standard weight-lifting bar was lifted from a semisquat to a full standing position (deadlift; [ILLUSTRATION FOR FIGURE 1 OMITTED]). Volunteers placed the balls of their feet under the bar with their feet shoulder-width apart, bent over, and grasped the bar in a mixed grip (one palm facing forward, one backward). Volunteers then assumed a semisquat position with the head up, arms straight, back flat, shoulders over the bar, and feet flat on the floor. They lifted the bar to a full standing position by extending the knees and moving the hips forward and the shoulders back while maintaining a flat back and straight arms. The bar was kept close to the body (Baechle, Earle, & Allerheiligen, 1994). An unloaded bar on 20-cm-high blocks was lifted three times to warm up. Weights were placed on both ends of the bar in increments ranging from 1 to 20 kg. The load increment was rapid initially and reduced as participants approached their maximum. Volunteers were not required to hold the load, and technicians stepped in to help lower the load. Two to four trials beyond the 50% load were typically required to assess the one repetition maximum (1RM) with 3-5 min rest provided between near-maximal lifting attempts (Semenick, 1994; Walthen, 1994). Test-retest reliability for this measure was r = 0.97.
Weight-lifting belts and gloves were available, and volunteers were taught how to use them. Volunteers determined their equipment preference for using a belt and/or gloves during the initial training period. Men were more likely than women to use a weight belt, and women were more likely than men to use gloves. Although there is some disagreement in the literature regarding the effectiveness of weight-lifting belts to increase intra-abdominal pressure, there is no evidence that they increase lifting strength, which is the current measure of interest (Sherman & Woldstad, 1995). To accurately compare the relationship between individual lifting and team lifting, each volunteer used the same equipment during the individual deadlift and team lifting trials.
Individual lifting strength was also measured using a weight stack machine (liftest). This device is in use by the Air Force as a pre-enlistment screening tool (McDaniel, Skandis, & Madole, 1983). Participants grasped the handles of the weight carriage from a starting position of 40 cm and lifted to a handle height of 152 cm. Several practice trials with the unloaded carriage were conducted, and attention was given to lifting technique. The unloaded carriage weighed 18 kg. The load was increased by 2.3-9.1 kg with each successful lift until the lift could not be completed. Initial load increments were 9.1 kg for men and 4.5 kg for women. The increment was decreased as participants began to experience difficulty with the lift. Participants were allowed a 2-min rest between near-maximal trials.
Team Lifting Strength
Two- and four-person team lifting strength was measured using a square weight-lifting device, which weighed 68.6 kg [ILLUSTRATION FOR FIGURE 2A OMITTED]. A triangular device weighing 64.3 kg was used for three-person lifting [ILLUSTRATION FOR FIGURE 2B OMITTED]. The grip surface and diameter were identical to the standard weightlifting bar used to measure individual deadlift strength. Both devices had reinforcing center bars and extensions beyond the triangular or square shape to hold weight plates.
Volunteers were assigned to teams on a random basis for two- and three-person lifting. Four-person teams consisted of two teams of previously selected pairs that were randomly assigned to lift together. Four-person mixed-gender teams contained two men and two women.
The lifting technique was identical to that for individual deadlift testing [ILLUSTRATION FOR FIGURE 1 OMITTED], except that it was performed simultaneously by multiple lifters in response to an oral cadence. On the command "feet ready," volunteers placed the balls of their feet under the bar. On the command "hands ready," volunteers bent from the waist to grasp the bar in a mixed grip. On the command "down," volunteers assumed a semisquat position with their heads up, backs and arms straight, and feet flat on the ground. On the command "and lift," volunteers lifted the bar from floor level to the full standing position of the shortest team member using a smooth, continuous motion. The team was not required to hold the load. Technicians immediately stepped in to assist the team in lowering the load. If a team member was unable to safely complete the lift, technicians assumed the load and lowered it to the ground.
Each team lifted the unloaded bar from wooden blocks three times to warm up. Weight was then added to the device with each lift in increments of 15-80 kg until the team was unable to complete the lift. Initial load increments were large but were reduced as the team approached its estimated maximum load. When a team was unable to complete a lift, the load was reduced in small increments (3-10 kg) until a lift was completed or until the last load lifted successfully (prior to first failure) was reached. Two to four trials beyond the 50% 1RM load were typically required to assess maximum team lifting strength. Adequate rest (3-5 min) was provided between lifts as the maximum load was approached (Kraemer & Fleck, 1982; Semenick, 1994; Wathen, 1994). If proper form was not adhered to, the lift was not accepted. One determination of maximum lifting strength was conducted per day with a minimum of 48 hr rest between testing sessions. The test-retest reliability for team lifting strength was r = 0.98 (p [less than] .01).
To examine the relationship between the sum of individual deadlift strength of team members and team lifting strength, the percentage of the sum of individual deadlift represented by the teamlift (%sum) was calculated.
%sum = (team lift(kg)/sum individual deadlifts(kg)) x 100.
Two-way analysis of variance was used to examine the effects of team size (two-, three-, and four-person teams) and gender (all men, all women, and mixed-gender teams) on team lifting (kg and %sum). The three-person mixed-gender teams (one man with two women, n = 18 teams, and two men with one woman, n = 18 teams) were combined for this analysis (for a discussion of the four three-person gender groupings, see Sharp et al., 1993). Tukey's Honestly Significant Difference Tests were performed to identify significant differences between means.
The mean and standard deviation of descriptive measurements for men and women, respectively, were as follows: age (yrs) 20.3 [+ or -] 1.7 and 26.7 [+ or -] 6.4, t(39) = -4.59, p [less than] .0001; height (cm) 177.9 [+ or -] 6.4 and 163.3 [+ or -] 4.2, t(39) = 8.49, p [less than] .0001; weight (kg) 76.3 [+ or -] 12.2 and 61.1 [+ or -] 7.8, t(39) = 4.71, p [less than] .0001. The women were of average weight and height for an Army population; however, the men represented the 60th percentlie (Gordon et al., 1988).
The lifting strength of men was greater than that of women for the liftest, 76.4 [+ or -] 13.4 kg versus 39.3 [+ or -] 6.7 kg, t(39) = 10.72, p [less than] .0001; and for individual deadlift, 137.0 [+ or -] 22.1 kg versus 84.7 [+ or -] 14.2 kg, F(1, 41) = 97.54, p [less than] .0001. There was a 5% increase in deadlift strength pre- to midstudy, F(1, 41) = 17.23, p [less than] .0002. Calculations involving [TABULAR DATA FOR TABLE 1 OMITTED] individual deadlift used the most current deadlift measurement at the time the team lifting trial was conducted.
Team Lifting Strength
Table 1 contains the maximum team lifting strength values (kg) for team sizes by gender. Differences were found in the load lifted for the main effects team size, F(2, 198) = 555.2, p [less than] .0001, team gender, F(2, 198) = 240.4, p [less than] .0001; and for the interaction, F(4, 198) = 9.17, p [less than] .0001. Post hoc tests revealed differences between all team sizes (two, three, and four, p [less than] .01), between all gender groupings (male, female, and mixed, p [less than] .01), and between most Gender x Team Size combinations (p [less than] .01, noted in Table 1).
Table 2 contains the %sum by team size and gender. There was a gender effect for %sum, F(2, 198) = 27.3, p [less than] .0001, but no team size or interaction effects were found. Differences between the means for all-men teams (87.3%) and all-women teams (91.1%, p [less than] .05), and between single-gender teams (all-men or all-women) and mixed-gender teams (80.2%, p [less than] .01) were found post hoc.
An increase in the number of lifters resulted in a heavier load lifted, but the number of lifters beyond two did not affect the amount lifted as a percentage of the sum of their individual lifts. This indicates that the ability to generate lifting force was not affected by an increase in the number of persons lifting from two to three [TABULAR DATA FOR TABLE 2 OMITTED] or four. This conflicts with the findings of Karwowski and Mital (1986), who reported a decrease in the %sum with an increase (from two to three) in the number of men performing an isokinetic or isometric lift, but it supports the parallel study in women (Karwowski & Pongpatanasuegsa, 1988). The %sums from Karwowski and colleagues (Karwowski, 1988; Karwowski & Mital, 1986; Karwowski & Pongpatanasuegsa, 1988) for isometric and isoinertial lifting are similar to those of the present study for comparable team sizes and genders, with the exception of two-woman isometric lifting. This inconsistency is difficult to explain. Two-woman isometric lifting %sum (79.1%) was lower than both comparable isoinertial measures: two-woman weight bar lifting in the present study (91.9%) and two-woman box lifting (91.0%) reported by Karwowski (1988).
The %sums for isokinetic lifting (Karwowski & Mital, 1986; Karwowski & Pongpatanasuegsa, 1988) were 18-25 percentage points less than those for isoinertial lifting (present study; Karwowski, 1988), and are likely attributable to differences between isokinetic and isoinertial lifting. Isokinetic lifting is less familiar than isoinertial lifting; thus the ability to exert maximum force may degrade more readily as the number of team members is increased. In addition, isoinertial lifting may accommodate greater differences in technique and in the timing and angle of force application compared with isokinetic lifting.
Whereas the %sum reported here is similar to that for isoinertial box lifting reported by Karwowski (1988), the quantities lifted were 50% greater in the present study. This may be attributable to the participant sample, the type of object lifted, or the lifting task. Soldiers are required to pass a physical fitness test and a weight-for-height screen twice annually, and therefore they may be stronger and more physically fit than the student population used by Karwowski (1988). The liftest strength of participants in the present study was similar to recent samples of U.S. Army men and women who performed it with a 2-min rest between near-maximum lifts. These samples were from prebasic-training women (40.4 [+ or -] 8.8 kg, n = 124; see Sharp, Nindl, Westphal, & Friedl, 1994) and male soldiers applying for Army Ranger training (77.4 [+ or -] 9.6 kg, n = 55; see Johnson, Friedl, Frykman, & Moore, 1994). This is greater than the liftest strength measured with no rest between load increments previously reported for both military (McDaniel et al, 1983; Sharp & Vogel, 1992) and student populations (Jacobs, Bell, & Pope, 1988; Jiang & Ayoub, 1987; Stevenson, Andrew, Bryant, Greenhorn, & Thomson, 1989) of approximately 60 kg for men and 30 kg for women. The current participant sample might be stronger than the civilian population, so the absolute team lifting strength values should not be considered normative data for use in industrial task design.
A second difference between the work of Karwowski (1988) and the present study is the type of object lifted. The team weight-lifting devices used in the current study were nearly identical to weight-lifting bars. Weight-lifting bars used by athletes are designed to maximize lifting potential by providing room for participants to assume optimal foot and body position. Although more realistic, a box (as used by Karwowski, 1988) is not likely to permit the same body and foot positioning as a weight-lifting bar. The lifting tasks differed between the two studies as well (Karwowski, 1988; present study). The present lifting task was from floor to knuckle height, so little upper-body strength was involved. However, Karwowski's (1988) participants lifted to a waist-high platform, which would involve greater use of the upper-body musculature.
Although the use of strong military participants and unusual lifting devices may appear to limit the use of these data in an industrial setting, the %sum relationship was similar to that for two-person box lifting as determined by Karwowski (1988). Given that the two-person %sum was similar for different populations performing different lifting tasks, using different equipment (Karwowski, 1988; present study), %sum might be used to estimate industrial team lifting capacities from industrial individual lifting data. Before this can be applied, the %sum relationship needs to be substantiated for three- and four-person teams, other populations (industrial), and other types of lifts (waist to shoulder, etc.).
The NIOSH guidelines (Waters et al., 1993) do not address team lifting. However, one might apply the revised equation to each lifter and divide the load by the number of lifters to obtain an approximation of the lifting index (personal communication, Thomas R. Waters, January 15, 1995). Military Standard 1472D (1989) appears to offer reasonable recommendations for team lifting design limits, but the recommendations are the same for mixed-gender and all-woman teams. This study has shown that mixed-gender teams lift significantly more than do teams composed of all women. A second problem is that Military Standard 1472D does not result in male, female, and mixed-gender teams lifting the same percentage of their maximal team lifting strength as established in this study. For an occasional lift to 91 cm, the male standard represents approximately 30% of the load that the men were capable of lifting, whereas the female and mixed-gender standards are about 25% and 20% of the load they were capable of lifting, respectively.
Although the current sample size is small, these results are also supported by Karwowski's data (1988) for two-person box lifting. Military Standard 1472D represents 75% of the maximum box load lifted by two men and 52% of the maximum box load lifted by two women (Karwowski, 1988). As the U.S. military moves toward a totally gender-integrated force, the standards might be adjusted to reflect a given percentage of the maximum lifting strength for the total military population, such as the lowest 5th percentlie.
When lifting in teams of up to four persons, no evidence of decreasing effectiveness was found with the addition of a lifter beyond two persons. Knowledge of the %sum relationship for lifting in teams of various size and gender enables military commanders to most effectively allocate human resources for heavy lifting tasks. The absolute values for team lifting strength should be applied with caution because the volunteers were strong, young soldiers and the testing methods used were designed to yield maximal loads under ideal conditions. Further examination of team lifting strength with equipment typically found in industrial and military situations and of a larger, more diverse population is warranted.
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Marilyn A. Sharp received a M.Sc. from the University of Massachusetts in 1981. She has worked as a research health exercise scientist at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts, since 1981.
Valerie J. Rice received a Ph.D. from Virginia Polytechnic Institute in 1990. LTC Rice (U.S. Army) is currently serving as the director of the U.S. Army Occupational Therapy Assistant School in San Antonio, Texas.
Bradley C. Nindl received an M.S. in exercise physiology from Springfield College, Massachusetts, in 1993. He is pursuing a doctoral degree in applied physiology from Pennsylvania State University.
Tania L. Williamson received an M.S. in exercise science from Kent State University in 1989. She is currently pursuing other interests in Colorado springs, Colorado.
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|Author:||Sharp, Marilyn A.; Rice, Valerie J.; Nindle, Bradley C.; Williamson, Tania L.|
|Date:||Sep 1, 1997|
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