Knapik load 2
- 1. KN1-1 Physiological, Biomechanical and Medical Aspects of Soldier Load Carriage Joseph Knapik, Sc.D., MAJ(ret) Directorate of Epidemiology and Disease Surveillance US Army Center for Health Promotion and Preventive Medicine Bldg E1570, 5158 Blackhawk Rd. Aberdeen Proving Ground (Edgewood Area), MD 21010-5403 United States of America Voice:(410)436-1328 FAX: (410)436-5449 e-mail: joseph.knapik@apg.amedd.army.mil INTRODUCTION Because of mission requirements or the limited transportation assets of some types of units (e.g., U.S. Army light infantry), soldiers are often required to move their own equipment using load carriage systems. These systems are important for soldier mobility and survivability and if properly designed can minimize performance decrements and fatigue. The carrying of loads by troops is an important aspect of military operations that can become critical in some situations. Overloading of troops and inadequate load-carriage systems can lead to excessive fatigue and impair the ability to fight. Military historians cite numerous examples where heavy loads directly or indirectly resulted in reduced performance, unnecessary deaths, and lost battles (14, 26, 94, 99, 100, 120). The purpose of this paper is to provide a broad overview of the published research on the historical, physiological, biomechanical, and medical aspects of soldier load carriage. A basic understanding of these topics can assist in the development of appropiate methods to improve soldier mobility.. Practical suggestions are offered for reducing the stress of loads on soldiers by equipment modifications, physical training, and prevention of load carriage-related injuries. Other reviews of similar topics are available (41, 80). HISTORICAL PERSPECTIVE Figure 1 shows loads carried by various military units, with emphasis on more recent times. Lothian (93) provides information on a greater number of more ancient military units. Until about the 18th century, troops carried loads that seldom exceeded 15 kg while they marched. Extra equipment and subsistence items were often moved by auxiliary transport including assistants, horses, carts, and camp followers. After the 18th century, auxiliary transport was de-emphasized and more disciplined armies required troops to carry their own loads. Modern soldiers often carried more equipment on the march and less in contact with hostile forces (94). There have been a number of recorded efforts to study and improve soldier mobility beginning with the British efforts after the Crimean War. These efforts generally focused on either 1) determining an acceptable soldier load based on soldier physical capability and/or operational necessity (1, 4, 15, 94, 120, 136, 137) or developing specialized load carriage systems (1, 72, 94, 120). In 1987, the U.S. Army Development and Employment Agency (1) proposed five approaches for improving soldier mobility. The first approach was to develop lighter weight components. However, technical developments were expected to reduce loads only by 6% overall (126). The second approach was the soldier load planning model. This was a computer program that aided commanders in tailoring loads through a risk Paper presented at the RTO HFM Specialists’ Meeting on “Soldier Mobility: Innovations in Load Carriage System Design and Evaluation”, held in Kingston, Canada, 27-29 June 2000, and published in RTO MP-056.
- 2. KN1-2 analysis based on the mission, enemy, terrain, troops and time (METT-T). The third approach was the development of specialized load-carrying equipment. This included such things as hand carts and all-terrain vehicles. The fourth approach was a reevaluation of current doctrine that might affect load carriage. An example of this was an increased emphasis on marksmanship to reduce ammunition loads. The fifth and final approach was the development of special physical training programs to condition soldiers to develop more physical capability for load carriage. Figure 1. Loads Carried by Various Infantry Units Through History. JRTC=Joint Readiness Training Center, Ft Chaffee, AR, USA (References: 25, 61, 69, 93, 117) Historical changes in soldier physical characteristics may be important (71, 95) because larger soldiers may be able to carry heavier loads by virtue of greater bone and muscle mass (85). It has been estimated that humans have increased their height about 10 cm since the Industrial Revolution, possibly because of better nutrition (29). Table 1 provides a summary of the heights and weights of various groups of soldiers and recruits derived from a variety of sources. Before the Crimean War, only minimum standards are available. U.S. samples show a progressive increase in height and weight since the Civil War, with the increase in weight primarily attributable to an estimated increase in fat-free mass (33). Table 1. Physical Characteristics of Various Groups of Soldiers and Recruits Height (cm) Body Mass (kg) Fat Free Mass (kg)a Body Fat (%)a FRENCH SAMPLES French (Crimean War)b 163 56 NA NA French (Post WWI)b 163 NA NA NA Table 1 continued on next page 0 10 20 30 40 50 EstimatedLoadMass(kg) AngloSaxonFreemen Wingate’sChinditsWWII GreekHoplites RomanLegions ByzantineInfantry EnglishPikemen NapoleonicWars USCivilWar BritishCrimeanWar BritishSommeWWI USMarinesKorea USVietnam RTC USDesertShield
- 3. KN1-3 Table 1. Cont’d Height (cm) Body Mass (kg) Fat Free Mass (kg)a Body Fat (%)a BRITISH SAMPLES British (Post WWI)b 168 59 NA NA British Recruits (1978)c 175 70 NA NA British Infantry (1976)c 175 73 NA NA UNITED STATES SAMPLES U.S. Soldiers (1864)e 171 64 53 16.9 U.S. Soldiers (1919)e 172 66 55 15.7 U.S. Soldiers (1946)e 174 70 60 14.4 U.S. Male Soldiers (1984)e 174 76 63 17.3 U.S. Male Recruits (1986)f 175 71 59g 15.6g U.S. Male Soldiers 3 groups (1986)f 175-176 69-77 58-63g 15.9-19.5g NA=Not Available a Estimated from neck and waist girth (142), with exception of last 2 rows b Reference 94 c Reference 141 d Reference 40 e Reference 33 f Reference 143 g Estimated from skinfolds using equations of Durnin and Womersley (28) PHYSIOLOGICAL AND BIOMECHANICAL ASPECTS OF LOAD CARRIAGE Historical information indicates that the problems of load carriage have been with us for a considerable time. Physiological and biomechanical research conducted more recently has resulted in the development of general principles, but studies do not reveal a “best” way of carrying loads that applies to all situations. Improving load distribution across the body, use of combat load carts, and physical training have been demonstrated to improve soldier mobility. Load Distribution There are many ways to carry loads, and the technique the soldier will use depends on the characteristics of the load (size, shape, mass, etc.), how far the load may be carried, previous experience, and the equipment available to the soldier (89). Figure 2 illustrates techniques of carrying loads on the upper body that have been directly investigated (5, 8, 20-22, 88, 90, 97). Backpacks and Double packs. Where the load is carried on the body will affect both energy cost and body mechanics. Loads can be transported with the lowest energy cost (i.e., the most efficient way) when they are
- 4. KN1-4 carried on the head (55, 97). However, this method is impractical for military operations because it requires a very long training time to use effectively, is useful only in unobstructed horizontal terrain, and produces a high profile (greater body signature). Figure 2. Methods of Load Carriage A more practical choice for military operations is to carry a load as close as possible to the center of mass of the body (128, 148). In this regard, the backpack and double pack (half the load carried on the front of the body and half on the back) methods have been shown to have a lower energy cost than most other forms of load carriage in many (20, 21, 96, 118) but not all (89) studies. The double pack produces fewer deviations from normal walking than does a backpack, including less forward lean of the trunk (49, 73). With the double pack, increasing load produces a reduction in stride length and increase in stride frequency that is
- 5. KN1-5 more desirable because it may reduce stress on the bones of the foot. In contrast, stride length becomes longer as backpack loads increase, which could be potentially harmful (49). Double packs can be useful in some military situations (e.g., medics carrying their aid bags on the front of their bodies) but also imposes major limitations for many military situations. The double pack can inhibit movement and may limit the field of vision in front of the body, making it difficult to see obstructions and traps. They can be burdensome to don and doff; doffing can be very important in situations with sudden or unexpected enemy contact. The double pack can also induce ventilatory impairments (90) and greater heat stress symptoms (64) when compared to the backpack. The double pack may restrict tasks such as firing weapons and donning protective masks. Designers can take advantage of what has been learned from the double pack by distributing the load more evenly over the torso. Although it may be difficult or almost impossible to make the load equal on the front and back of the body, load carriage systems could allow a part of the load to be moved forward by the use of load-carrying vest and hip belts (see Figure 2). Soldering items included in the frontal load could optimally consist of equipment the soldier may need quickly or may need often. Moving a part of the load to the front would be expected to reduce energy cost, improve body posture, and reduce injuries. Pack frames and Hip Belts. Pack frames and hip belts reduce shoulder stress. The shoulder straps of a rucksack exert pressure on the skin, which can be measured with transducers under the straps. Shoulder pressure is considerably lower with a pack frame incorporating a wide hip belt compared to a pack frame without a hip belt. In one study, 10 kg carried in a frameless pack resulted in a peak pressure of 203 mm Hg; the same mass carried in a pack with a frame and wide hip belt resulted in a peak pressure of only 15 mm Hg. The pack with the frame and hip belt produced less electromyographic (EMG) activity in the trapezius muscle, also suggesting less stress in the shoulder area (59). There is some suggestion that experienced individuals adjust their walking posture to reduce forces and force fluctuations in the shoulder straps (138). Subjective reports of discomfort vary, depending on the design of the pack system. For backpacks with and without frames, the majority of discomfort appears to be in the neck and shoulder region. For a backpack with a hip belt, discomfort is localized to the mid trunk and upper legs (90). Overall, when the load is carried primarily on the waist through use of a hip belt, there is less subjective discomfort compared to shoulder load carriage (60). Placement of the Load in the Backpack. Where the load is placed in the pack will affect both energy cost and body mechanics. Higher energy costs are associated with a load that is lower in the pack and farther away from the body. Lower energy costs are associated with loads placed higher in the pack and closer to the body. This is illustrated in Figure 3. The correlation between energy cost and an index that describes the vertical and horizontal position of the load is 0.85 (107). Figure 3. Effect of Placement of the Load in the Backpack on Energy Cost (Reference 107) Pack Frame Pack Higher Energy Cost = Center of Load Mass Lower Energy Cost
- 6. KN1-6 Both high and low load placements bring about forward body lean, but this effect is greater for low placements. This is because the lower load is closer to the ankles, requiring more forward body rotation to bring the pack center of mass over the feet (7). The additional forward body rotation tends to bring the body's center of mass over the front half of the foot, which could increase the likelihood of foot strain and injury. However, placement of the load high in the pack tends to destabilize posture to a greater extent than lower placements, especially among tall men, as measured by the amount of body sway while standing with the load (56). Dynamic moments are about 40% greater with the high-back placement, an affect attributed to the greater rotational inertia of the high load (10). A low or mid-back load placement might be preferable for stability on uneven terrain, particularly during unexpected stumbles where high-load placement can necessitate relatively high-muscle forces to maintain postural stability. The high load placement may be best for even terrain because it minimizes energy cost and keeps body posture with a load most similar to that without a load (7). Load Carriage on the Feet, Thighs and in the Hands. Loads can be carried in places other than the torso, although other body positions result in a higher energy expenditure. Loads carried on the feet result in an energy cost five to seven times higher than an equivalent load carried on the upper body (91, 128). For each kilogram added to the foot, the increase in energy expenditure is 7% to 10% (16, 68, 91, 128). This suggests that footwear should be as light as possible, compatible with durability requirements. Loads carried on the thigh result in energy costs lower than foot carriage but greater than torso carriage. For each kilogram added to the thighs (at about mid-thigh level) the increase in energy cost is about 4% (101, 139). Compared to the feet, less mechanical work is performed when load masses are carried on the thighs because of reduced inertia of the body segments; changes in gait with increasing thigh load are minimal (101). Carriage of loads in the hands also results in a higher energy cost than torso carriage (21, 96) and produces greater cardiovascular strain (92). Hand carriage is more efficient than foot carriage since the energy cost of carrying loads on the ankles exceeds that of carrying loads in the hands by five to six times if the hand load is carried close to the body (128). Strap Adjustments. Although not tested experimentally, it is reasonable to assume that shifting loads from one part of the body to another can improve soldier comfort and allow loads to be carried for longer periods of time. Load shifting is accomplished with some pack systems using various strap adjustments. Strap adjustments may redistribute the load to other muscles or other portions of previously loaded muscles. They also allow the skin to “recover” from the pressure of the load. Some rucksacks have “sternum straps” that are attached horizontally across both shoulder straps at mid-chest level. When the sternum strap is tightened, it pulls the shoulder straps toward the midline of the body and the load on shoulders is shifted in this direction. When the sternum strap is loosened, the shoulder straps move laterally and the load is shifted to more lateral portions of the shoulder. Most pack systems with hip belts and shoulder straps have adjustments that allow more of the load to be placed on the hips or shoulders. When the shoulder strap tension is reduced (straps loosened), more of the load is placed on the hips. With the shoulder straps tighter, more of the load is placed on the shoulders. Other strap adjustments that shift load pressures would further improve soldier mobility. Load Carriage Using Carts Military personnel seldom consider using carts to transport loads, but for some missions this may be an option. Positive and negative aspects emerged in a field trial of three combat load carts. On the positive
- 7. KN1-7 side, the tested carts were generally durable, and were effectively used in flat terrain, in barrier construction, and in resupply. On the negative side, the carts created problems in rugged terrain: they were noisy in brush or rocky areas, thus reducing tactical surprise; equipment could get caught in the wheels of some carts (140). A combat load cart appropriate for military operations should have a low center of gravity, a wide wheel base, and a large wheel size (42, 43). Compared to body carriage, energy cost was reduced by 88% when a 50-kg load was pushed in a cart on a smooth surface (43). Pulled carts (rather than pushed) appear to be easier to control on uneven terrain and also result in considerable energy cost savings (42). A specially designed combat load cart that was pulled by soldiers using a hip belt resulted in faster march speeds than moving the same loads with a rucksack. Over mixed terrain (paved road, dirt road, field, and rough trail), 34-kg and 61-kg loads were moved 22% and 44% faster over a 3.2-km distance (48). This combat load cart, specifically developed for military operations, is available in the US Army. Physical Training and Load Carriage Appropriately designed physical training is another method of increasing soldier mobility. Walking with backpack loads over a period of weeks results in a decrease in the energy cost of carrying the load (134). Australian military recruits with high initial aerobic capacity (predicted VO2max=51 ml.kg-1.min-1) further improved their aerobic fitness by engaging in regular backpack load carriage. Loads were progressively increased during an 11-week basic training program, and improvements in aerobic capacity were similar to those of a control group performing the traditional recruit training program involving running (124). Twelve-week physical training programs involving a combination of aerobic training (running) and resistance training (weight lifting), improved the speed at which military men completed a 3.2-km distance carrying 46 kg (87) and military women completed a 5-km distance carrying 19 kg (78) even when these load carriage tasks were not included in the training program. Interestingly, neither running nor resistance training alone improved march speed (87), suggesting that both aerobic capacity and muscle strength must be trained to improve road marching capability. When regular road marching with loads (at least twice a month) was included in a program that also involved running and resistance training, soldiers marched faster than if march training was not included (74). Substantial improvements in load carrying performance were found when civilian women were trained with a combination of resistance training, running, and load carrying (50). Gender Differences Compared to men, women walk with shorter stride length and greater stride frequency. As loads increase, the women’s stride length decreases while that of the men does not show significant change. With increasing load, women also show a more pronounced linear increase in the time both feet are on the ground (double support time) than do men. Difference between men and women persist even when differences in body size and body composition are taken into account (103). When men and women were asked to complete a 10-km road march as quickly as possible carrying loads of 18 kg, 27 kg, and 36 kg, men were about 21% faster, regardless of load. On questionnaires, women commented more often than the men that the pack straps were uncomfortable, hip belts ill fitting, and rucksacks unstable. An independent predictor of march time (when gender was included in the equation) was acromial breath (shoulder breadth). Since pack systems have been designed primarily based on the anthropometry of men, these data suggest that if consideration is given to the anthropometry of women in military pack systems, the time gap between men and women may decrease (52, 53). Factors Involved in the Energy Cost of Load Carriage Studies conducted on treadmills for short periods of time show that energy cost increases in a systematic manner with increases in body mass, load mass, velocity, and/or grade (9, 11, 39, 130). Type of terrain also
- 8. KN1-8 influences energy cost, as shown in Figure 4 (42, 110, 129). Pandolf et al. (109) expanded on the work of Givoni and Goldman (38) to develop an equation to predict the energy cost of load carriage: Mw=1.5*W+2.0*(W+L)*(L/W)2+T*(W+L)*(1.5*V2+0.35*V*G) Where: Mw = Metabolic Cost of Walking (Watts) W = Body Mass (kg) L = Load Mass (kg) T = Terrain Factor (1.0 = Black Top Road; 1.1 = Dirt Road; 1.2 = Light Brush; 1.5 = Heavy Brush; 1.8 = Swampy Bog; 2.1 = Loose Sand; Snow, dependent on depth of depression (T=1.30+0.082*D, where D=depression depth in cm)(110) V = Velocity or Walk Rate (m/sec) G = Slope or Grade (%) Figure 4. Influence of Terrain on the Estimated Energy Cost of Backpack Load Carriage (References: 109, 110, 129). Numbers after the Snow Estimates are the Depression of the Footwear in the Snow. The Pandolf equation has been independently validated using a range of loads and body masses (27). However, the equation has several limitations. First, it does not accurately predict the energy cost of downhill walking (115, 116). Downhill walking energy cost approximates a U-shape when plotted against grade: it initially decreases, then begins to increase (98, 145). The lowest energy cost appears to occur between -6% to -15%, depending on individual gait characteristics (145). A second limitation of the Pandolf equation may be the fact that it may not account for increases in energy cost over time. In studies used to develop the equation, energy cost was examined for short periods, usually less than 30 minutes. Some studies (31, 113) have shown that the energy cost of prolonged (>2 hours) load carriage at a constant speed increased over time at higher loads and/or speeds. Another study did not find an increase in energy cost after about 4 hours of walking (125). There were differences in the type of backpacks used in these studies. The studies showing the increase in energy cost used a pack that place loads primarily on the shoulder; the study not finding the increase in energy cost used a pack with a hip belt that placed much of the load on the hips. Whether energy expenditure increases over time is important because the individual carrying the load may become more easily fatigued if energy cost does increase. PavedRoad DirtRoad LightBrush HeavyBrush 0 10 20 30 40 Treadmill Swamp Sand Snow(4") Snow(8") 70 kg soldier, carrying 33-kg load, walking at 4km/h, no grade VO2(ml/kg/min) KN1-8
- 9. KN1-9 MEDICAL PROBLEMS ASSOCIATED WITH LOAD CARRIAGE Injuries associated with load carriage, while generally minor, can adversely affect an individual's mobility and thus reduce the effectiveness of an entire unit. Table 2 shows the results of two studies that recorded acute injuries during military road marching operations (76, 123). Foot blisters, back problems, and metatarsalgia were the most common march-related injuries. Table 3 provides a summary of these and other common load carriage related injuries with their signs, symptoms, and prevention measures. Table 2. Injuries Experienced During Military Road Marches in Two Studies Table 2a. Injuries Among 335 Infantry Soldier During a 20-Km Maximal Effort Road March (76) During Marcha TotalsInjury Continued March (n) Did Not Continue March (n) 1-12 Days Post- March (n)b N % Foot Blisters 16 0 19 35 38 Back Pain/Strain 5 7 9 21 23 Metatarsalgia 1 1 9 11 12 Leg Strain/Pain 0 0 7 7 8 Sprains 1 1 4 6 7 Knee Pain 0 0 4 4 4 Foot Contusion 0 1 1 2 2 Other 1 2 2 5 5 Total 24 12 55 91 100 a From medics and physician during the march b From medical records after the march Table 2b. Injuries Among 218 Infantry Soldiers During a 5-Day, 161-Km Road March (123) During Marcha TotalsInjury Continued March (n) Did Not Continue March (n) 1-15 Days Post- March (n)b N % Foot Blisters 43 3 3 49 48 Metatarsalgia 8 2 9 19 19 Back Pain/Strain 4 1 1 6 6 Sprains 2 3 0 5 5 Knee Pain 3 1 3 7 7 Ingrown Toenail 0 3 0 3 3 Stress Fracture 0 1 0 1 1 Other 8 3 1 12 12 Total 68 17 17 102 100 a From physician’s assistances at fixed medical sites along the march b From medical records after the march
- 10. KN1-10 Table 3. Summary of Common Load Carriage-Related Injuries With Prevention Stategies (see text for full descriptions) INJURY SIGNS AND SYMPTOMS PREVENTION Foot Blisters Elevated area, lighter in color than surrounding skin, filled with fluid. Pain, burning, warmth, erythema 1. Acrylic, nylon, or polyester inner sock; thick, snug, dense weave outer sock with inner sock 2. Spenco insoles 3. Antiperspirants 4. Load distribution more evenly around body center of mass 5. Reduce load mass 6. Pre-condition feet through physical training and road march practice 7. Improve aerobic fitness 8. Smoking/tobacco cessation Low-Back Pain Pain, muscle spasm, neurological symptoms 1. Load distribution more evenly around body center of mass 2. Reduce load mass 3. Trunk and abdominal strengthening and stretching Metatarsalgia Pain, swelling on sole of foot 1. Pre-condition feet through physical training and road march practice 2. Reduce load mass Stress Fractures Persistent, boney pain 1. Smoking/tobacco cessation 2. Pre-condition feet and legs through progressive physical training and road march practice Knee Pain Pain, swelling, crepitus, instability Lower extremity strengthening and stretching Rucksack Palsy Upper extremity numbness, paralysis, cramping; scapular winging Framed rucksack with use of hip belt on rucksack Foot Blisters Foot blisters are the most common load carriage-related injury (17, 76, 104, 123). Blister can occur when slight movements of the foot in the footwear produce shear forces on the skin. Some portions of the footwear exert more pressure on the skin than other portions. If the foot movements produce enough shear cycles at these pressure points, and if the pressure is great enough, a blister will result (84). Blisters can cause extreme discomfort, may prevent soldiers from completing marches, and can lead to many days of limited activity (2, 76, 104, 119, 123). Especially in field conditions, if blisters are not properly managed, they can progress to more serious problems such as cellulitis or sepsis (2, 58). Heavy loads have been shown to increase blister incidence (52, 75, 122), possibly by increasing pressure on the skin and causing more movement of the foot inside the boot through higher propulsive and breaking forces (73). Other blister risk factors include tobacco use, low aerobic fitness, and ethnicity (82, 83, 123). When loads are very heavy (61 kg), the double pack has been shown to result in less likelihood of blisters than the backpack (77), suggesting that better load distribution can reduce blisters. Spenco shoe insoles have also been shown to reduce foot blister incidence, possibly because they absorb some frictional forces in anteroposterior and mediolateral directions (127, 131, 132). Regular physical training with load carriage may induce skin adaptations that reduce the probability of blisters (84). Blisters may thus be less of a
- 11. KN1-11 problem in units that march regularly; however, sudden increases in march intensity or distance will probably make blisters more likely, regardless of training regularity. Moist skin increases frictional forces and probably increases blister incidence (2, 84, 105). Acrylic socks decrease the number and size of blisters among runners (57), possibly by conducting sweat away from the foot (32). A nylon sock worn inside a wool sock reduces the incidence of blisters among soldiers who are road marching (3, 146). A polyester sock worn inside a very thick wool-polypropylene sock reduced blister incidence during Marine recruit training (79). It is reasonable to assume that changing wet socks for dry ones may also reduce foot blisters. Antiperspirants also reduce foot sweating (19, 70). A 20% solution of aluminum chloride hexahydrate in an anhydrous ethyl alcohol base is effective in reducing the likelihood of march-related blisters if the preparation is applied to the entire foot for at least three nights before a march (81). Once the antiperspirant effect has been achieved, it may be maintained with applications once per week (12). However, many individuals report irritant dermatitis using this preparation (81), which may require the application of a topical steroid. Possible ways or reducing irritant dermatitis include using a lower concentration preparation, changing the treatment schedule (same number of applications but over a longer period of time), or discontinuing use. Antiperspirants in emollient bases are not effective in reducing blisters, presumably because emollients interfere with the antiperspirant effect (121). Low Back Injuries Low back injuries can pose a significant problem during load carriage. Low back injuries are difficult to define because the pain may result from trauma to a variety of structures including spinal discs, the ligaments connecting the vertebral bodies, nerve roots, or supporting musculature (67). In one study (76), 50% of the soldiers who were unable to complete a strenuous 20-km walk reported problems associated with the back. Dalen et al. (18) reported frequent problems with back strains during a 20 to 26-km walk. However, Reynolds et al. reported only a 3% low-back injury incidence and few associated days of limited duty after a 161-km road march. Heavy loads may be a risk factor for back injuries (122). This could be because heavier loads lead to changes in trunk angle muscles (44, 47, 106, 111) that can stress back, or because heavier loads do not move in synchrony with the trunk (106, 114) causing cyclic stress of the back muscles, ligaments, and the spine (47, 106). It has been suggested that the double pack may help reduce the incidence of back problems because it results in a more normal posture and eliminates prolonged bending of the back (73). Thus, better load distribution could reduce back injuries. Also, a general overall strengthening and warm-up program involving the back, abdomen, hamstrings, and hip muscles may assist in prevention of back injuries (67). Metatarsalgia Metatarsalgia is a descriptive term for nonspecific painful overuse injury of the foot. The usual symptom is localized tenderness on the sole of the foot under the second or third metatarsal head. Sutton (133) reported a 20% incidence of metatarsalgia during a strenuous 7-month Airborne Ranger physical training program that included regular load carriage. One study (76) reported a 3% incidence after a single strenuous 20-km walk with soldiers carrying 45 kg. Another study reported a 9% incidence following a 5-day, 161-km road march with soldiers carrying an average (SD) 47±5 kg (123). Metatarsalgia is sometimes associated with foot strain caused by rapid changes in the intensity of weight- bearing activity (67). Walking with heavy loads may be a predisposing factor for metatarsalgia since this may cause the foot to rotate anterio-posteriorly around the distal ends of the metatarsal bones for more prolonged periods of time, resulting in more mechanical stress in this area (73).
- 12. KN1-12 Stress Fractures Lower extremity stress fractures are common in military recruits (13, 30, 36, 37, 65, 66) and have also been reported in trained soldiers (66). During the Central Burma campaign in WWII, 60 stress fracture cases were reported in one infantry unit during a 483-km road march (24). Stress fractures are attributable to repetitive overloading of bones during activities such as road marching. The most common areas of involvement are the metatarsals of the feet (24), although many other lower extremity sites can be involved (66). When the metatarsals are involved, tenderness is generally localized on the dorsal side of the metatarsal shafts, which distinguishes the pain from metatarsalgia. Other common stress fracture areas include the tibia (46) and fibula of the leg (45). Under similar training conditions, here may be gender differences in the anatomic location of stress fractures, with men experiencing proportionally more stress fractures in the foot and women experiencing more in the hips, pelvis and legs (Unpublished data, Ft Jackson, SC). Demonstrated risk factors for stress fractures include female gender (13, 66), white ethnicity, older age (13), taller body stature (37), prior physical inactivity (35, 37), and smoking (34). Other factors that may increase risk include load-carriage distance (62, 66) and walking style (37, 108). Knee Pain Knee pain is another condition that has been associated with load carriage. Dalen et al. (18) reported a 15% incidence (17 cases of 114) of knee pain during their load-carriage study. Knapik et al. (76) reported only a 1% incidence of knee pain (2 cases from 335 soldiers) following a 20-km march, but the two cases resulted in a total of 14 days of disability. Reynolds et al. (123) found a 3% incidence of knee pain (7 cases from 218 soldiers) following a 161-km march. Various disorders that may be involved in knee pain include patellofemoral pain syndrome, patellar tendinitis, bursitis, and ligamentous strain. These conditions can arise from an abrupt increase in road marching mileage or intensity or from climbing hills if soldiers have not been conditioned for this. Quadriceps and hamstring strengthening and stretching exercises, along with heel cord stretching, are important for the prevention of recurrence (144). Rucksack Palsy Rucksack palsy is a disabling injury and has been widely reported in association with load carriage (6, 23, 54, 63, 133, 147). It is hypothesized that the shoulder straps of a backpack can cause a traction injury of the C5 and C6 nerve roots of the upper brachial plexus. In minor cases, compression results in entrapment of the long thoracic nerve. Symptoms include numbness, paralysis, cramping and minor pain in the shoulder girdle, elbow flexors, and wrist extensors. Long thoracic nerve injuries usually present with "scapular winging" because of weakness of the serratus anterior muscle. Sensorimotor deficits from rucksack palsy injuries are usually temporary but in some cases, may result in a chronic condition. Nerve conduction studies and EMG studies may be necessary to document this condition (6, 147). Use of a frame and hip belt has been demonstrated to reduce the incidence of rucksack palsy (6), presumably by reducing pressure on the shoulders (59). Hypothetical risk factors for rucksack palsy include heavy loads, load distribution, and longer carriage distances (6, 122). LOAD CARRIAGE & PERFORMANCE OF OTHER TASKS A significant consideration from a military perspective is how well soldiers are able to perform military tasks during load carriage. Load mass, load volume, and load distribution appear to be important variables. As the mass increases, there are systematic decrements in the performance of tasks such as longer runs, short sprints, agility runs, ladder climbs, and obstacle courses (51, 60, 102). The decrement in performance of
- 13. KN1-13 some tasks is estimated at about 1% per kilogram load (60). Loads of greater volume inhibit movement under obstacles. The distribution of the load within the backpack can also influence performance of specific tasks (60). Gender differences in load carriage are also apparent with women having more difficulty than men in climbing walls (presumably due to shorter stature and lower muscle strength) and women less accurate than men in throwing grenades (51). In some operations, soldiers are required to walk long distances and perform critical military tasks at the completion of the march. Very strenuous marches (maximal speed with loads of 34 to 61 kg over 10- to 20- km distances) lead to post-march decrements in marksmanship and grenade throw distance (52, 75, 77, 86). The decrements in marksmanship are presumably attributable to small movements of the rifle resulting from fatigue of the upper body muscle groups, fatigue-induced tremors, or elevated heart rate or respiration (75, 86, 135). Marksmanship decrements last for only short periods of time (77). The decrements in grenade throw distance may be due to a nerve entrapment syndrome (6, 147) or pain in the shoulder area, both resulting from the pressure of the rucksack straps. Lower body muscular power (as measured by the vertical jump and Wingate test) and muscle strength do not appear to be adversely affected by prolonged pack load carriage (52, 75, 86, 112). CONCLUSIONS: IMPROVING SOLDIER MOBILITY This review suggests many ways of facilitating soldier mobility including load reductions, load redistribution, equipment modifications, and physical training. Load reductions can be accomplished by tailoring the load to the specific operations and by using special load-handling devices. Commanders must make realistic risk analyses and take only the equipment necessary for the mission. Special combat load carts are available that could be useful in special situations such as marches on unobstructed terrain or in close resupply operations (1, 48). Reducing loads will lower energy cost, increase soldier comfort, and may reduce some types of injuries, especially blisters, back problems, and metatarsalgia. Equipment modifications should first focus on redistributing the load about the center of mass of the body. Many military carriage systems have vests and belts that have pockets and attachment points useful for moving some items from the rucksack to the front of the body. Items carried on the front of the body should be those likely to be needed suddenly or needed often. Pack frames and well-padded hip belts provide several benefits including reducing loads on the shoulder, great comfort for the shoulder, and a possible reduction of some types of injuries. Frames and hip belts may improve soldiers’ performance on tasks requiring the use of the upper body. Load shifting through the use of belts and buckles (e.g., sternum straps to move loads to different points on the shoulder) may also be helpful. The optimal distribution of the center of mass of load within the rucksack may depend on the type of terrain. On roads or well graded paths, placement of heavy items high in the pack is preferable to lower energy cost, maintain a more upright body posture, and possibly reduce low back problems. On uneven terrain a more even distribution of the load within the pack may be more helpful to maintain stability. Regular physical training that includes aerobic exercise, resistance exercise, and road marching can improve load carriage performance. Road marching should be conducted at least twice a month with loads that soldiers are expected to carry in unit operations. Loads and distances should be increased gradually over sessions until a maintenance level has been achieved. New unit members should be given time to adapt through the same gradual program. Regular physical training has been shown to increase march performance and may reduce some types of injuries. It is desirable to reduce load-carriage-related injuries that impair performance, cause discomfort and disability, and result in a loss of manpower. Blister incidence can be reduced by keeping the feet dry and this may be accomplished by 1) the use of an polyester inner socks combined with wool or wool-polypropylene outer socks, 2) antiperspirants applied at least three days prior to a march and reapplied at least once a week, and 3) frequent sock changes. Spenco insoles and distributing the load more evenly around the torso may also reduce blister incidence. Aerobic physical training combined with regular load carriage marches may
- 14. KN1-14 reduce the incidence of stress fractures and blisters. The use of frames and hip belts can reduce the incidence of rucksack palsy. Soldier mobility can be improved by lightening loads, improving equipment and load distribution, appropriate physical training, and specific techniques directed at injury prevention. Suitable changes will allow soldiers to complete missions at lower energy costs, with more comfort, and with fewer injuries. DISCLAIMERS The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as official Department of the Army position, policy or decision, unless so designated by other official documentation. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of this organization. Approved for public release; distribution is unlimited. REFERENCES 1. Army Development and Employment Agency (ADEA). Report on the ADEA soldier's load initiative. Ft Lewis, WA: Army Development and Employment Agency 1987. 2. Akers, W.A. and M.B. Sulzberger. The friction blister. Mil. Med. 137:1-7, 1972. 3. Allan, J.R. and A.L. Macmillan. The immediate effects of heat on unacclimatized paratroops. Exercise "Tiger Brew II". United Kingdom: Army Operational Research Establishment Research Memorandum No. 16/62, 1963. 4. Bailey, T.L. and W.M. McDermott. Review of research on load carrying. Natick MA: United States Army Quartermaster Research and Development Center Tentage and Equipage Series Report 9, 1952. 5. Balogun, J.A. Ergonomic comparison of three modes of load carriage. Int. Arch. Occup. Environ. Health 58:35-46, 1986. 6. Bessen, R.J., V.W. Belcher and R.J. Franklin. Rucksack paralysis with and without rucksack frames. Mil. Med. 152:372-375, 1987. 7. Bloom, D. and A.P. Woodhull-McNeal. Postural adjustments while standing with two types of loaded backpacks. Ergonomics 30:1425-1430, 1987. 8. Bloswick, D.S., A. Gerber, D. Sebesta, S. Johnson and W. Mecham. Effect of mailbag design on musculoskeletal fatigue and metabolic load. Hum. Factors 36:210-218, 1994. 9. Bobbert, A.C. Energy expenditure in level and grade walking. J. Appl. Physiol. 15:1015-1021, 1960. 10. Bobet, J. and R.W. Norman. Effects of load placement on back muscle activity in load carriage. Eur. J. Appl. Physiol. 53:71-75, 1984. 11. Borghols, E.A.M. Influence of heavy weight carrying on the cardiorespiratory system during exercise. Eur. J. Appl. Physiol. 38:161-169, 1978. 12. Brandrup, F. and P.O. Larsen. Axillary hyperhidrosis: local treatment with aluminium chloride hexahydrate 25% in absolute ethanol. Arch Dermatoven. 58:461-465, 1978. 13. Brudvig, T.G.S., T.D. Gudger and L. Obermeyer. Stress fractures in 295 trainees: a one-year study of incidence as related to age, sex, and race. Mil. Med. 148:666-667, 1983. 14. Carre, E.T. Historical review of the load of the foot-soldier. United States Army Quartermaster Research and Development Center, Natick, MA Tentage and Equipage Series Report 8, 1952. 15. Cathcart, E.P., D.T. Richardson and W. Campbell. Army Hygiene Advisory Committee Report No. 3. On the maximal load to be carried by the soldier. J. R. Army Med. Corp 41:12-24, 1923. 16. Catlin, M.J. and R.H. Dressendorfer. Effect of shoe weight on the energy cost of running. Medicine and Science in Sports 11:80, 1979. 17. Cooper, D.S. Research into foot lesions among Canadian field forces. 13th Commonwealth Defence Conference on Operational Clothing and Combat Equipment, Malaysia, CDA-11, 1981.
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