Physiological and Environmental Factors

Body and Load Weight

Ideally, it is best to select the load-bearing weight as a function of body weight rather than selecting an absolute load-weight per man. You may not have the flexibility to tailor the load weight per man due to mission requirements. Therefore, an upper weight limit for load-bearing should be calculated so the load will not create a higher physical demand (energy cost) for individuals with smaller stature and lower lean body weight.

Determining the maximum weight of the load is a more important factor affecting load-bearing than the type of load carriage design. Numerous studies have examined the optimal load-bearing weight you should maximally carry before there is a disproportional increase in the rate of energy expenditure. This is calculated as a percentage of body weight. Although those who are physically fit according to Physical Fitness Test Scores can typically carry 45% of their body weight for 8 hours at a pace of 4 mph, a lighter load is considered optimal.

"Rule of Thumb": A maximal load should be 40% of body weight. Example: 40% of 170 lbs. is 68 lbs.

The energy cost of the task rises steeply beyond this percentage. It has been reported that the energy cost of humping with a load increases proportionally with the weight carried. For loads more than 40% of body weight, the energy expenditure rises disproportionately and fatigue occurs sooner. The key is to carry your load as close as possible to your body's center of gravity. This will result in the lowest energy cost when you carry a load, which is evenly distributed, on your back.

Biomechanics of Load-Bearing

The duration of time your foot is on the ground during a normal gait does not increase until pack loads are greater than 50% of your body weight. However, the length of time your foot is in the air during the swing phase of your gait decreases with increasing load-bearing weight. This response occurs to increase the length of time both feet are on the ground for double support. As the load weight increases, your feet increase the ground force downward, forward, rearward and in lateral directions. Shorter stride lengths will increase stride frequency and help maintain normal walking patterns during moderate to heavy load-bearing.

Shorter strides may minimize strain and possibly reduce lower back and lower extremity injuries.

Load-Bearing and Walking Pace

Self-pacing during load-bearing results in a lower energy cost than a forced-pace. However, not all missions allow self-pacing. When the pace is forced, a self-selected decrease in walking pace will be observed as the load-bearing weight increases. The self-selected walking pace or exercise intensity during load-bearing depends on many factors. Some of the factors affecting the self-selected pace include:

Aerobic fitness level (maximal aerobic capacity)

♦ Total distance walked

You will normally self-pace at 30% - 35% of your maximal aerobic capacity when carrying a moderate load of 50 pounds.

With lighter loads, self-pacing normally results in an exercise intensity of around 45% of your maximal aerobic capacity. However, an exercise intensity of 60% or greater can be achieved when the load is light and the distance short. Remember: walking pace and load weight carried during an infiltration or exfiltration contribute to the rate of exhaustion. Figure 10-1 presents estimates of when exhaustion might occur at various walking speeds and loads. High rates of energy expenditure {900 to 1000 calories per hour) can be sustained for only 6 to 10 minutes. To be able to move quickly at this intensity, load weight must be light.

Carrying a load at a walking pace that requires an energy expenditure in excess of 300 kcal per hour can compromise the energy reserves you will need at the desired destination. Figure 10-2 shows the speeds that you can expect to sustain with given loads at the designated paces: note these all result in an energy expenditure of 300 kcal per hour.

Figure 10-2. Effects of Walking Pace by Pack Weight and Terrain q.

CO cl

Load (lbs)

Figure 10-2. Effects of Walking Pace by Pack Weight and Terrain

Sand • Heavy Brush I Easy Terrain q.

CO cl

Load (lbs)

Sand • Heavy Brush I Easy Terrain

Hydration Status

Appropriate water/fluid discipline must be observed at all times during load-bearing. Exercise without a load is severely compromised when dehydration occurs, and when exercise is coupled to load-bearing, dehydration potentially becomes an even greater threat. This is true regardless of the environmental conditions. The Navy SEAL Nutrition Guide provides information on optimal fluid ingestion, but in brief, you should ingest at least one to two cups of fluid every 30 minutes during a hump.

Fluid requirements are higher during exercise with load-bearing than during exercise without load-bearing.

Environmental Stress

♦ Hot and dry or hot and humid environments will increase the heat strain of individuals during load-bearing. Thermal heat load will be increased due to body heat production from load-bearing activities, and from the heat load from the sun and other heat sources in the environment. ALICE packs and double packs can restrict the body's ability to dissipate heat. Load-hearing vests with a nylon mesh body will trap less heat than backpack designs.

♦ Cold environments increase the energy cost of walking because the clothing worn is usually heavy and cumbersome. This can result in overheating and sweating, especially on the back where the load is carried. Hypothermia (drop in body temperature below 95"C) can also occur rapidly in cold environments if the inner layers of clothing are wet when you stop exercising.

♦ Moderate to high altitudes decrease exercise capacity since the available oxygen in blood decreases as altitude increases due to lower barometric pressure.

♦ Sandy terrain significantly increases energy expenditure up to 80% or greater as compared to walking on a firm surface. The energy demand increases with the difficulty of the terrain.

Sleep Loss

Lack of sleep can compromise your mental capacities, including attention, logical reasoning, and mental processing. In contrast, physically demanding skills seem little affected by up to 72 to 96 hours of sleep loss.

Protective Clothing

The type of clothing worn during load-bearing is an important consideration for dissipation of body heat. Mission Oriented Personnel Protection (MOPP) gear for nuclear, biological and chemical environments (NBC) is a very heat-restrictive garment.

Guidelines with estimates for work/rest cycles and hydration guidelines when in MOPP gear arc available. With load-bearing activities in MOPP gear, work/rest guidelines should be adjusted downward and hydration requirements adjusted upward, since load-bearing activities have not been accounted for. The use of MOPP gear will be equivalent to adding 10° Wet, Bulb, Globe, Temperature fWBGT) to the existing ambient temperature. Therefore, the medical threat, of heat illness will increase substantially in MOPP clothing.

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