Soldier posture and body mechanics are critical factors for performance during load carriage. Proper posture and body mechanics allow Soldiers to move efficiently, create great force and absorb heavy resistance. Posture is the position in which the body resides. Though posture often is thought of as a stationary position, control of moving postures is important while conducting dismounted movement.
Body Mechanics (Posture in Motion)
C-1. Body mechanics (posture in motion) can be defined as the ability to control body movement. Many discussions of posture are limited to static positions such as sitting and standing. Good posture during walking and other movements is imperative for efficiency and injury control. The primary goals of all the biomechanical responses to the addition of a load are to control the load as best as possible and minimize the energy cost of carrying the load.
Management of the Load to Conserve Energy
C-2. Walking can be described as a series of falls, as can be observed from by the rise and fall of the hips whether someone is walking with or without a load. When the foot strikes the ground, muscles activate eccentrically to control and stabilize the load as the body accepts the weight of the gear and moves forward. Later in the same step, the muscles act concentrically to propel the body forward and prepare for the next step. The ultimate objectives of biomechanical changes observed during load carriage are to manage the load and to conserve energy. The load is managed by maintaining the load plus body center of mass over the base of support (feet), and energy cost is optimized by minimizing the vertical excursion of the pack center of mass (walking as smoothly as possible). When the Soldier dons a load, the addition of the pack shifts the center of mass of the Soldier-pack system higher on the back and away from the body relative to the center of mass of the Soldier without the load. This weight shift becomes more pronounced as the load gets heavier.
Body Alignment and Movement
C-3. When body segments are aligned properly, movement is efficient, and injury risk is minimized. When body segments are not aligned properly, movement is less efficient and risk of injury increases. Consider Soldiers attempting to lift heavy loads from the ground with their legs straight and trunk twisted. Not only does the load seem heavier than if their knees are bent and the back is straight, but the risk of injury increases. Back injuries occurring during an improper lift are an obvious example of the relationship between posture, body mechanics, performance, and health. Less obvious, but just as damaging, is the cumulative toll on the body when faulty.
C-4. It is important to walk as smoothly as possible to reduce the high impacts and accelerations the body is subjected to by heavy loads, especially at the feet and legs. If possible, it is important to distribute the load on the body as evenly as possible while still meeting mission requirements; this allows the Soldier to walk using more natural mechanics and reduces the energy requirements to walk with a given load.
C-5. Head and trunk checkpoints (CPs) for standing apply to marching. Allow arms to swing naturally, though crossing midline of the body is excessive. Allow hips to naturally rotate forward with each stride. Do not allow knees to lock at any point in the walking cycle. Stride naturally, landing on the heel and pushing off with most of weight toward the big toe. The feet remain directed generally forward, but Soldiers should not strain to keep feet directed forward, since variations in skeletal alignment prevent some Soldiers from assuming a feet-forward position.
Body Adjustments to Load
C-6. Foot marching with a load on the back will require body adjustments to load, specifically some forward lean of the trunk. Typical body adjustments to load include trunk lean, gait changes, and increased stress on lower extremities. However, Soldiers must not allow their trunk and shoulders to round forward.
Trunk Lean
C-7. The most visible response to adding pack-borne load is that the Soldier leans forward while walking (see figure C-1). This happens with pack loads as light as 15 to 20 pounds. Mechanically, this forward lean response serves to center load as closely over the base of support (feet) as possible and serves to stabilize the load. Physically, this forward, or trunk lean serves to counterbalance the torque that the load causes at the pelvis and hips. When walking, excessive forward lean places undue stress on the lower back and abdominal musculature.
C-8. When the pack loads are heavier, the Soldier also involves the neck and head to act as an additional counterweight. This can result in increased muscular activity, head accelerations, and force transmission at the head as load increases which could lead to risk for injury. Increased movement at the head can have a negative effect on body mechanics and decrease situational awareness while marching.
Gait Changes
C-9. The gait cycle (walking, marching) can be divided into two primary phases: the stance phase, during which the foot is on the ground and propelling the body forward, and the swing phase, during which the foot is off the ground and moving forward to take the next step. As loads get heavier, Soldiers walk slower and keep their feet on the ground longer to stabilize the load and conserve energy. When walking speed is held constant, individuals respond to heavier loads (typically greater than 66 pounds) by taking shorter, faster steps.
Increased Stress on Lower Extremities
C-10. An increase in loads on the back translates to an increase in loads on the muscles at the hip, knee, and ankle. The stress on all lower extremity joints increases as load gets heavier; however, the knee is the primary joint that controls the lowering of the load immediately after the foot contacts the ground, especially at lighter loads. Care should be taken to ensure adequate rest and recovery between bouts of heavy load carriage whenever possible.
Note. Under heavy weights, Soldiers should take shorter, faster strides to maximize efficiency. During a prolonged foot march, halt briefly after the first mile to retie boots and adjust equipment. This is important-it increases blood flow to the feet and calves and decreases pressure around the lower leg. This method helps to alleviate shin splints due to pressure build up.
Energy Expenditure
C-11. Most recommendations on safe loads to carry are based on energy expenditure or the metabolic cost associated with carrying a load. The Soldier needs to carry the appropriate supplies to complete the movement and execute the mission. Several factors increase the energy expenditure associated with load carriage. These factors include load distribution, terrain, weather, gait/posture, and fitness level.
C-12. Energy expenditure of load carriage increases in a systematic manner as the load carried increases, and with increases in walking velocity, grade, terrain, and weather or a combination of these conditions. For example, the same load weight, walking through swamp or on sand essentially doubles the energy cost of walking on a paved road, and walking in snow without snowshoes can increase this cost by four to six times.
C-13. Increasing load weight can substantially increase Soldier energy expenditure during typical foot march conditions. When carrying loads less than or equal to 30 percent of Soldier’s bodyweight, energy expenditure remains constant; however, when Soldier’s load increases above 30 percent of bodyweight the rate of energy expenditure increases throughout the march.
C-14. The addition of an external load increases energy cost. The distribution of the heavier items within the load within a pack can affect the energy expenditure during load carriage, as well as the body mechanics of how the load is carried. Concentrating the heavier items higher in the pack and closer to the body can reduce energy cost of marching by as much as 25 percent as compared to a load that is placed low in the pack and away from the body (see figure C-2 on page 98).
Note. For reduced energy expenditure, heavy items should be packed high and close to the body area of the pack rather than low and away from the body.
Physiological Differences
C-15. Compared to males, females walk with shorter stride length and greater stride frequency. As loads increase, a female’s stride length decreases, whereas a male’s stride length does not show significant change. With increasing loads, females show a more pronounced linear increase in time when both feet are on the ground than males. To bring center of load mass over the feet, females tend to hyperextend their necks and bring their shoulders farther forward than do males, possibly to compensate for less upper body strength. Many of these differences between males and females persist even when differences in body size and composition are considered.
Note. Female Soldiers should avoid foot march posture that involves hyperextending the neck forward.
Prepare and Pack the Load
C-16. Everything a Soldier wears or carries during an operation should be relevant to accomplishing the mission. While there are numerous items a Soldier could carry during a mission, it is up to leaders to decide what they should carry. By thoroughly reviewing the packing list, assessing the value of every item, and weighing the threat the enemy presents against the need for mobility and survivability, leaders can ensure their Soldiers carry the necessary equipment during the operation while maintaining a protective posture capable of preserving combat power and saving lives.
C-17. Leaders can establish the correct procedures to pack a rucksack based on the Soldier’s duty position. A correctly packed ruck will allow Soldiers to sustain themselves for extended periods. Equipment should be optimally configured to allow for easy access to critical items, stowing lesser-used items near the bottom of the rucksack, and ensuring the load is balanced and comfortable when carried.
Packing the Ruck
C-18. Ultimately, it is up to the commander to dictate what goes into the ruck for the mission. Whether the item is accessible, balanced, compressed, and streamlined may assist in determining pack procedures and should be used as guidelines.
C-19. Accessible mission-critical items should be readily retrievable at any given time in a mission. The location of these items should be standard across the unit. Keep similar items together.
Balanced
C-20. A lopsided pack is a miserable experience. Ounces make pounds. Subordinate leaders supervise Soldiers in packing to reduce nonessential and redundant equipment. Strive to use multifunction equipment as much as possible. Note that waterproofing is necessary. A drenching rain can more than double the weight of a rucksack. Measures to maintain balance include—
- Placing heavy items close to the body and midway up the back.
- Evenly distributing mid-weight items side to side.
Compressed
C-21. Certain items like mission equipment, food, and/or sleeping gear make up the bulk of the load. Other items, like clothes, make perfect buffers to squeeze into the cracks of bulky items. Continue this process of layering, keeping in mind the access need for each item. Continue to push down and compress each layer. A tightly packed ruck carries much easier and provides more room to pack. Utilize the compression straps to keep the internal load from shifting around. This keeps the load closer to the body. Once the ruck is fully compressed, there should be no flexing if pressing down on the top.
Streamlined
C-22. Hanging equipment on the outside of the pack is an extremely poor practice. Heavier items can be felt swinging with each step. Heavy items, such as hydration bladders and assault packs, will act like a pendulum the further away they get from the body, increasing the felt weight. Place these items directly on top or inside, close to the body. Items will snag on brush, make excessive noise, and may be broken or lost in a tumble. When climbing rocky terrain, they can catch on vegetation and in cracks and cause a loss of balance or a fall. Try to keep all items inside the pack until needed.
Additional Techniques
C-23. Some additional techniques that align with accessible, balanced, compressed, and streamlined are as follows:
- Accessible—
- Confirming the top zone is used for essential, immediate-use items Soldiers will require on the mission.
- Confirming the accessory pockets for urgent essentials.
- Balanced—
- Waterproofing of all items.
- Determining the three zones and peripheral storage for Soldiers’ rucksacks.
- Confirming rows, not columns, are used when packing rucks.
- Confirming Soldiers fill nooks and crannies until they have a solid, stable, and equally balanced load on both sides of the pack.
- Confirming the bottom zone is used for the Soldiers’ bulky gear and items not needed until later in the mission.
- Confirming the core zone is used for the Soldiers’ denser, heavier items.
- Breaking down rations; do not carry unnecessary food items.
- Working at lightening the load by working as a team-not everyone needs to carry cooking equipment, cleaning kits, sleeping bags, and so forth.
- Compressed—
- Evaluating rucksacks when they are fully loaded to ensure efficiency and balance.
- Verifying Soldiers wrap soft items around bulky gear to prevent shifting.
- Streamlined—
- Placing all items in medium-sized dry bags, allowing for quick retrieval.
- Removing sleeping bags only when it is tactically sound to do so; consider putting it in the bottom of the pack.
C-24. Packing bulky, soft, light weight, or less-essential gear at the bottom of the ruck offers an additional advantage. It creates an internal shock-absorption system for the Soldiers’ back and the ruck. Roll items as small and tight as possible. Packing heavy items in the core zone creates a stable center of gravity and directs the load downward rather than backward.
C-25. A good rule of thumb is to pack least-used items in the bottom of the ruck, place more frequently used items in the top of the ruck, and fill nooks with soft items. Rucks differ in that they provide front pockets, side pockets, and so forth. The pockets may be perfect for a compass, Global Positioning System, maps, or other frequently used items.
Forms of Load Carriage
C-26. Where loads are carried on the body affects energy expenditure and gait mechanics. While it is not the most efficient method (from an energy expenditure perspective) to carry heavy loads on the back, the requirement to do so while navigating complex terrain and keeping the load from protruding excessively beyond a Soldier’s frame necessitates carrying in less efficient locations such as on the back.
Rucksacks and Double Packs
C-27. A practical choice in load carriage is to carry loads as close as possible to center mass of the body. For this reason, rucksacks and double pack methods use less energy than other forms of load carriage. (See figure C-3.) Even so, rucksacks place most of the load on the back, pushing the trunk and head forward relative to the load.
C-28. Although this forward lean keeps the weight over the feet (base of support), the downside is that it causes repetitive contractions and stress to low-back muscles. Even just standing still with a backpack on increases postural sway (anterior-posterior, medial-lateral center of pressure excursions) in a linear manner as the load increases.
C-29. On the other hand, a double pack produces fewer deviations from normal walking than does a backpack, including less forward lean. Also, increasing the load reduces stride length and increases stride frequency. This is desirable because it can reduce stress on the bones of the foot. Alternatively, increasing the load when using a regular backpack lengthens the stride, with potentially harmful effects.
C-30. Double packs can be especially useful in some military situations, for example, they allow medics to carry aid bags on the fronts of their bodies. However, backpacks generally provide more versatility in military situations because double packs can inhibit movement and limit field of vision. Double packs are hard to get on and off, and that can be a problem-Soldiers need to be able to drop their stuff in a hurry when sudden enemy contact occurs. The double pack can induce ventilator impairments and greater heat stress symptoms, compared with the backpack. The double pack can restrict tasks, such as firing weapons and donning protective masks.
C-31. Soldiers can take advantage of what has been learned from the double pack by distributing loads evenly over the torso. Although it is difficult to make the load equal on the front and back of the body, modular systems allow part of the load to be moved forward onto the load-carrying vest. Doing this might be expected to reduce energy expenditure, improve body posture, and reduce injuries.
Pack Frames and Hip Belts
C-32. Pack frames and hip belts reduce shoulder stress. Shoulder straps 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 properly fitting hip belt, compared with a pack frame without a hip belt. Packs with frame and hip belt produce less stress in the trapezius muscle and in the shoulder area.
C-33. When a pack frame and hip belt are used for loads between 31 and 90 pounds, the proportion of the load is supported on the hips and lower back is 30 percent and the load on the shoulders is 70 percent regardless of load mass. A consistent anterior force exerted on the lower back increases stress in this area. Suggestions indicate that experienced Soldiers adjust their walking posture to reduce forces and force fluctuations in the shoulder straps. Rigid rods attached to both sides of the pack and extending into the hip belt transfer about 14 percent of the vertical load from the upper torso to the pelvis.
C-34. Internal frame packs have supporting structures inside the fabric of the pack and keep the pack closer to center of mass of the body. External frame packs have supporting structure on the outside of the pack, and the pack is usually farther away from center of mass of the body. Conflicting information is ambiguous regarding whether the internal frame pack has lower energy expenditure than the external frame pack. There is no difference in the perceived exertion between external and internal frame packs when walking on level, even terrain. However, perceived exertion over rough terrain is lower with the internal frame pack.
C-35. For backpacks with or without frames, majority of discomfort appears to be in the neck and shoulder region, although foot discomfort can be substantial, presumably because of the development of hot spots and blisters. For backpacks with hip belts, discomfort is localized to the mid-trunk and upper legs. Overall, when portions of the load are carried on the waist through use of a hip belt, less subjective discomfort occurs than with shoulder load carriage. When walking uphill, Soldiers give higher ratings for balance and ease of gait for packs with hip belts which pivot in the sagittal plane.
Strap Adjustments
C-36. It is reasonable to assume shifting loads from one part of the body to another during marches can improve Soldier comfort and allow loads to be carried for longer periods. Load shifting is accomplished with some pack systems using various strap adjustments. Strap adjustments can redistribute the load to other muscles or other portions of previously loaded muscles. Portions of the body subjected to high-load pressures for long periods of time can suffer discomfort, circulatory occlusion, and paresthesia.
C-37. Some rucksacks have sternum straps attached horizontally across both shoulder straps at mid-chest level. When the sternum strap is tightened, it pulls shoulder straps toward the midline of the body, so pressure is shifted medially. When the sternum strap is loosened, shoulder straps move laterally, and the load is shifted laterally.
C-38. Most pack systems with hip belts and shoulder straps have adjustments presumably allowing more of the load to be placed on the hips or shoulders. When shoulder strap tension is reduced, more of the load is placed on the hips. With shoulder straps tighter, more of the load is placed on the shoulders.
C-39. Some pack systems have load-lifter straps attaching top of the shoulder straps to the pack frame. When the strap is tightened, the top of the load is pulled anterior over the base of support; however, when the strap is loosened, the top of the load drops posterior. Other strap adjustments shift load pressures, center the pack, and improve lumbar support that can further improve Soldier mobility and comfort.
Note. Pull shoulder straps forward while walking uphill, this shifts the center of mass of the load higher on the back which reduces trunk lean, reduces energy cost, and assists hip mechanics. Loosen shoulder straps while walking down hill, this makes the load more stable.
Rifle Carriage
C-40. Rifles almost are always carried in dismounted military operations. Rifle carriage restricts arm swing, adds weight, and moves center of mass anteriorly. During rapid walking, a rifle has a small, but significant, effects on human gait. The rifle carriage increases forces produced at heel strike (ground impact forces, about 5 percent), results in forces to decelerate the body (maximum breaking forces, about 1 percent), and increases side-to-side forces (mediolateral impulse, about 12 percent). Many of these changes are less because of the mass of the rifle and due to restrictions of arm movement, which increases movement of the body center of mass.
Body Armor Carriage
C-41. Wearing body armor as part of the total Soldier load increases exertion due to increased heat retention and chest wall restriction. These factors are considerations when determining personal protective equipment requirements during foot march planning. (See paragraph 3-19 for more information.)
Next Page : Appendix D – Training for the March