Women's Load Carriage Performance Using Modular Lightweight Load-Carrying Equipment

Military Medicine, Nov 2004 by Ling, Wen, Houston, Vern, Tsai, Yung-Sheng, Chui, Kevin, Kirk, John

The purposes of this study were to evaluate how Modular Lightweight Load-Carrying Equipment (MOLLE) fits women while walking on level surfaces with different loads, to examine women's load carriage performance before and after a simulated march using five load levels, and to examine the relationship between shoulder and leg muscle strength and load carriage performance of women while carrying loads using MOLLE. Seven physically active women carried five levels of load (no load, 20, 30, 40, and 50 pounds) using MOLLE. With increased loads, women showed increased double-limb support time, decreased single-limb support time, increased trunk forward inclination excursion, decreased knee excursion, decreased medial-lateral excursion of center of gravity (COG), and increased vertical excursion of COG. Hip abductor strength was a strong predictor of COG vertical excursion. Some women required modification of the padded hip belt to ensure weight distributed evenly around the pelvis.

Introduction

The percentage of women in the United States Armed Forces increases each year. Since the lifting of the combat exclusion rule in 1993, many military positions were open to women. According to the 2003 U.S. Census Bureau, more than 210,000 women serve on active duty, representing 15% of all active duty personnel in the U. S. Armed Services.1 However, women are injured at a much higher rate than men during basic combat training. After studying more than 4,000 recruits, Macleod et al.2 found that 10.9% of female recruits sustained stress fractures, shin splints, and covert fractures as compared with less than 3% of male recruits. Knapik et al.3 report that women have over twice the injury rate of men during basic combat training. The literature indicates that a number of factors contribute to this higher incidence of injury observed among female recruits during basic training, including lack of upper body strength4-6 and poorly fitting equipment attributable to a general design based on men's physical characteristics.7 For example, combat boots are designed for the larger male midfoot and heel dimensions and therefore do not fit female soldiers well and leads to a high incidence of ankle and foot injuries among female recruits during basic training.7 Similar principles have been applied to the design of personal load-carrying equipment.

In 1999, the U.S. Marine Corps adopted the Modular Light-weight Load-Carrying Equipment (MOLLE) system as standard equipment for the individual Marine. Mer additional testing, the U.S. Army adopted the modified MOLLE in 2001. MOLLE is a backpack system to replace the All-Purpose Lightweight Individual Carrying Equipment (ALICE) system that had been introduced in 1973. ALICE is an external frame backpack and is carried by the soldier using two shoulder straps. There is a small padded waist belt on ALICE that provides stability but does not transfer load to the hips well. Soldiers often report shoulder discomfort while carrying heavy loads with ALICE.8 MOLLE consists of two main components: a weight-bearing vest and a modular backpack. The weight-bearing vest has removable pockets to accommodate different mission needs. The modular backpack has an external molded frame, adjustable padded shoulder straps, a padded hip belt, a main rucksack, multiple removable pouches, and a sleeping bag compartment. MOLLE is quite versatile and can be configured in several ways to accommodate the needs of soldiers for specific missions. The advantage of MOLLE over ALICE is that the anatomically contoured external frame and heavily padded hip belt allow the soldier to shift the load from shoulders to hips to lessen shoulder discomfort and to facilitate more upright posture to prevent low back problems.9,10 In fact, according to LaFiandra et al.,11 male soldiers could shift 30% of carried weight from shoulders to hips (pelvis) and legs with MOLLE. In addition, male soldiers were able to maintain an upright posture and reported less shoulder and overall discomfort while carrying heavy loads with MOLLE,9 allowing for improved mobility and a decrease in the incidence of injury.12

A number of studies were conducted to examine men's load carriage performance using framed backpacks. When walking with increased loads over level surfaces, men showed consistent changes in gait, including increased trunk forward flexion (inclination),11,13 decreased stride length,11 increased stride frequency,8,11 increased double-limb support time,8 increased knee flexion,8'14 and a lower body center of gravity (COG) location.8 In addition, when walking on a treadmill with increased loads, men showed less trunk rotation while maintaining constant vertical excursion of the COG when compared to men walking on a treadmill without a load.15'16 These observed changes in gait may adversely affect a soldier's ability to fight by causing low back injuries, foot blister, foot pain, knee pain, stress fractures, and rucksack palsy.17

There are very few studies on women's load carriage performance. Researchers have shown that because of lower upper body and torso absolute strength, women experience greater difficulty and typically do not perform as well as men when carrying heavy loads.18 Women prefer to carry heavy loads on their hips to be as close to their COG as possible19 as opposed to men who prefer to carry the load high on the back.20 Harman et al.9,10 examined load carriage performance among active duty male and female soldiers and reported that female soldiers experienced a higher incidence of discomfort than male soldiers when carrying heavy loads, with the highest percentage reporting discomfort in their shoulders. With MOLLE's ability to shift load from the shoulders to the hips, women can carry heavy loads using their strong leg muscles, thus experiencing less discomfort and demonstrating less gait changes. Unfortunately, MOLLE was designed based on male physical characteristics. For example, with a wider pelvis, the padded hip belt of MOLLE may not fit women properly around the waist and therefore may not transfer sufficient weight from shoulders to hips. Harman et al.10 observed excessive up/down movements of load when the padded hip belt was not clinched tightly around the pelvis of female soldiers, causing hip/pelvis discomfort. On the other hand, if MOLLE shifts the load to women's hips to allow women to use their leg muscles, women's load-carrying performance should be relatively constant with different load levels. Thus, there is a need to further examine female load carriage performance using MOLLE. We postulate that women will show similar biomechanical changes in load carriage performance to men when properly fitted with a load carriage system.