Effects of Team Size on the Maximum Weight Bar Lifting Strength of Military Personnel
Sharp, Marilyn A., Rice, Valerie J., Nindle, Bradley C., Williamson, Tania L., Human Factors
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. …