Katy Reynolds

Load carriage using packs: A review of physiological, biomechanical and medical aspects

This paper reviews the biomedical aspects of transporting loads in packs and offers suggestions for improving load-carriage capability. Locating the load mass as close as possible to the body center of gravity appears to result in the lowest energy cost when carrying a pack. Thus, the double pack (half the load on the front of the body and half the load on the back) has a lower energy cost than the backpack. However, backpacks provide greater versatility in most situations. The energy cost of walking with backpack loads increases progressively with increases in load mass, body mass, walking speed or grade; type of terrain also influences energy cost. Predictive equations have been developed for estimating the energy cost of carrying loads during locomotion but these may not be accurate for prolonged (>2 h) or downhill carriage. Training with loads can result in greater energy efficiency since walking with backpack loads over several weeks decreases energy cost. Load-carriage speed can be increased with physical training that involves regular running and resistance training. Erector spinae electrical activity (EMG) is lower during load carriage than in unloaded walking until loads exceed 30-40 kg, at which point erector spinae EMG activity is higher than during unloaded walking. EMGs of the quadriceps and gastrocnemius, but not the tibialis anterior or hamstrings, increase with load. Framed packs with hip belts reduce the electrical activity of the trapezius muscles, presumably by shifting forces from the shoulders to the hips. Increases in the backpack load mass result in increases in forces exerted on the grounds, amount of knee flexion and the forward inclination of the trunk. Compared to backpacks, double packs produce fewer deviations from normal walking. Common injuries associated with prolonged load carriage include foot blisters, stress fractures, back strains, metatarsalgia (foot pain), rucksack palsy (shoulder traction injury) and knee pain. Closed-cell neoprene insoles and use of an acrylic or nylon sock, combined with a wool sock, reduce blister incidence. A framed pack with a hip belt reduces the incidence of rucksack palsy. Backpack load carriage can be facilitated by lightening loads, optimizing equipment, improving load distribution and by preventive action aimed at reducing the incidence of injury.

Soldier Load Carriage: Historical, Physiological, Biomechanical, and Medical Aspects

This study reviews historical and biomedical aspects of soldier load carriage. Before the 18th century, foot soldiers seldom carried more than 15 kg while on the march, but loads have progressively risen since then. This load increase is presumably due to the weight of weapons and equipment that incorporate new technologies to increase protection, firepower, communications, and mobility. Research shows that locating the load center of mass as close as possible to the body center of mass results in the lowest energy cost and tends to keep the body in an upright position similar to unloaded walking. Loads carried on other parts of the body result in higher energy expenditures: each kilogram added to the foot increases energy expenditure 7% to 10%; each kilogram added to the thigh increases energy expenditure 4%. Hip belts on rucksacks should be used whenever possible as they reduce pressure on the shoulders and increase comfort. Low or mid-back load placement might be preferable on uneven terrain but high load placement may be best for even terrain. In some tactical situations, combat load carts can be used, and these can considerably reduce energy expenditure and improve performance. Physical training that includes aerobic exercise, resistance training targeted at specific muscle groups, and regular road marching can considerably improve road marching speed and efficiency. The energy cost of walking with backpack loads increases progressively with increases in weight carried, body mass, walking speed, or grade; type of terrain also influences energy cost. Predictive equations have been developed, but these may not be accurate for prolonged load carriage. Common injuries associated with prolonged load carriage include foot blisters, stress fractures, back strains, metatarsalgia, rucksack palsy, and knee pain. Load carriage can be facilitated by lightening loads, improving load distribution, optimizing load-carriage equipment, and taking preventive action to reduce the incidence of injury.

Physical Fitness, Age, and Injury Incidence in Infantry Soldiers

Injuries are a common occurrence in young, active civilian and military populations. This study examined injury incidence and the association of musculoskeletal injuries with age and physical fitness in soldiers. Subjects were a cohort of 298 male soldiers assigned to an infantry battalion in Alaska. The soldiers ages were obtained from the battalion records and their physical fitness was assessed from 2-mile run times, sit-ups, and push-ups. Injuries were documented from a retrospective review of the soldiers medical records for a 6-month period (October to March) before the fitness testing. Fifty-one percent of the soldiers suffered one or more injuries. The most common injury diagnosis was musculoskeletal pain, followed by strains, sprains, and cold-related injuries. Soldiers experienced a total of 212 separate injuries, which resulted in 1764 days of limited duty. The crude annualized injury rate was 142 injuries per 100 soldiers (one soldier could experience more than one type of injury). The proportion of soldiers injured decreased as age increased. Slower 2-mile run times and fewer sit-ups were associated with a higher incidence of musculoskeletal injuries. This study documents the injury incidence in infantry soldiers and identifies younger age and low physical fitness as potential risk factors for these injuries.

Friction blisters. Pathophysiology, prevention and treatment.

SummaryBlisters occur frequently, especially in vigorously active populations. Studies using repetitive rubbing techniques show that blisters result from frictional forces that mechanically separate epidermal cells at the level of the stratum spinosum. Hydrostatic pressure causes the area of the separation to fill with a fluid that is similar in composition to plasma but has a lower protein level. About 6 hours after formation of the blister, cells in the blister base begin to take up amino acids and nucleosides; at 24 hours, there is high mitotic activity in the basal cells; at 48 and 120 hours, new stratum granulosum and stratum corneum, respectively, can be seen. The magnitude of frictional forces (Ff) and the number of times that an object cycles across the skin determine the probability of blister development — the higher the Ff, the fewer the cycles necessary to produce a blister. Moist skin increases Ff, but very dry or very wet skin decreases Ff. Blisters are more likely in skin areas that have a thick horny layer held tightly to underlying structures (e.g. palms of the hands or soles of the feet). More vigorous activity and the carrying of heavy loads during locomotion both appear to increase the likelihood of foot blisters.Antiperspirants with emollients and drying powders applied to the foot do not appear to decrease the probability of friction blisters. There is some evidence that foot blister incidence can be reduced by closed cell neoprene insoles. Wearing foot socks composed of acrylic results in fewer foot blisters in runners. A thin polyester sock, combined with a thick wool or polypropylene sock that maintains its bulk when exposed to sweat and compression, reduces blister incidence in Marine recruits. Recent exposure of the skin to repeated low intensity Ff results in a number of adaptations including cellular proliferation and epidermal thickening, which may reduce the likelihood of blisters. More well-designed studies are necessary to determine which prevention strategies actually decrease blister probability.Clinical experience suggests draining intact blisters and maintaining the blister roof results in the least patient discomfort and may reduce the possibility of secondary infection. Treating deroofed blisters with hydrocolloid dressings provides pain relief and may allow patients to continue physical activity if necessary. There is no evidence that antibiotics influence blister healing. Clinical trials are needed to determine the efficacy of various blister treatment methods.Considering the pervasive nature of friction blisters, there is a substantial amount of basic and applied research that remains to be performed, especially in the areas of prevention and treatment.

Influence of an antiperspirant on foot blister incidence during cross-country hiking

BACKGROUNDnRubbing moist skin results in higher frictional forces than rubbing very dry skin. As friction increases, the probability of activity-related blisters also increases. Therefore reducing moisture may reduce blister incidence during physical activity.nnnOBJECTIVEnWe examined whether an antiperspirant can reduce foot blisters during hiking.nnnMETHODSnIn a double-blind study, cadets attending the US Military Academy were separated into two groups that used either an antiperspirant (20% aluminum chloride hexahydrate in anhydrous ethyl alcohol) or placebo (anhydrous ethyl alcohol) preparation. Cadets were told to apply preparations to their feet for 5 consecutive nights. On day 6, cadets completed a 21-km hike, and their feet were examined for blisters before and after.nnnRESULTSnBecause of dropouts, the final sample size was 667 cadets with 328 in the antiperspirant group and 339 in the placebo group. There was a high rate of noncompliance with the treatment schedule: Cadets used the preparations from 0 to 5 nights before the hike. For cadets using the preparations at least 3 nights before the hike (n=269), the incidence of foot blisters was 21% for the antiperspirant group and 48% for the placebo group (P < 0.01). However, reports of skin irritation were 57% for the antiperspirant group and 6% for the placebo group (P < 0.01).nnnCONCLUSIONnA 20% solution of aluminum chloride hexahydrate in anhydrous ethyl alcohol may be effective in reducing foot blisters during hiking; however, the side effect of skin irritation should be considered and preventive measures studied to reduce this irritation.

Effects of an antiperspirant with emollients on foot-sweat accumulation and blister formation while walking in the heat

BACKGROUNDnFriction blisters are a common injury in sports activities and military operations. Blisters can compromise performance, so it is important to devise preventive strategies to reduce these injuries.nnnOBJECTIVEnThis study investigated the influence of an antiperspirant with emollient additives on frequency and severity of friction blisters, hot spots, and irritant dermatitis.nnnMETHODSnTwenty-three healthy men walked on a treadmill (1.39 m/sec, 1% grade) in a warm environment (28 degrees C, 25% relative humidity) carrying a total mass of 21 +/- 1 kg. For 4 consecutive days before the walk, the subjects feet were treated with either (1) an antiperspirant (20% aluminum zirconium tetrachlorohydrex glycine concentration plus water) with emollient additives, (2) emollient additives alone (placebo control), or (3) nothing (nontreated). In two separate trials (1 month apart) each participant received the antiperspirant treatment and both control treatments (emollient [placebo] and no treatment).nnnRESULTSnNo differences were seen among treatment conditions for sweat accumulation (p = 0.86), blister incidence (p = 0.36), hot spot incidence (p = 0.83), or blister severity (p = 0.31). Irritant dermatitis was not reported in any of the treatment conditions.nnnCONCLUSIONnThe use of an antiperspirant with emollients reduces irritant dermatitis but does not reduce total foot-sweat accumulation, blister or hot spot incidence, or blister severity.