Are Children’s Backpack Weight Limits Enough?: A Critical Review of the Relevant Literature
Back pain is a widespread musculoskeletal condition in the adult population. This problem has stretched into the younger age groups and has become a condition often experienced by adolescents. The overall lifetime prevalence of low back pain in children has been reported as high as 65%.1 Other lifetime prevalence reports in children (less than 18 years) are more modest and are reported between 30% and 50%. Although the debate continues, some researchers hypothesize that use of heavy backpacks may contribute to the high reports of back pain in children. This hypothesis has led to recommended load limits of 10% to 15% of a child’s body weight by many health professional associations. Although these weight limits have been recommended in several states or countries, controversy continues to exist in the literature about the effects of backpack weight on back pain in children. The purpose of this review is to evaluate the literature that has contributed to the recommended weight limits for children’s backpacks. The current use of backpacks by children, injuries related to backpack use, the physiologic and biomechanical findings that were used to develop the recommended weight limits, and the need for research into children’s backpack design will be discussed.
Studies indicate that the incidence of backpack use by school children in the developed world is at least 90%. The weight carried by students varies by region and day of the week. Although the average loads vary greatly between studies, the majority of reports indicate that the loads carried by students are greater than the recommended limits. A study found that the average daily load of Italian students over a week ranged from 22.0% body weight to 27.5% body weight with one student who carried 46.2% body weight. In this group, 34.8% carried more than 30% of their body weight. Other studies report more modest loads ranging from 10.3% body weight to 20% body weight. Although these values are less extreme, the majority of students surveyed are still above the recommended limits. Only two studies were found where the mean load was less than 10% body weight.
Different methods of wearing backpacks have been reported, but no studies were found that examined the negative consequences of these strategies. Fashion trends tend to influence the way in which children wear their backpacks and vary depending on the population studied. For example, 94.5% of Italian students reported carrying their backpack over two shoulders. The number of students using both straps was smaller in the Australian population; however, the majority (72%) still used a proper carrying technique. Conversely, in a study of American students, 73.2% of students carrying their backpack with only one strap. The trend of one-strap backpack carriage appears to be decreasing in the United States, as studies completed since 2002 have reported less than 20% of students using only one strap. This trend may be a result of the increased awareness of backpack use and backpack safety programs. Since packs have been designed to be worn over two shoulders to distribute the weight evenly about the spine and across the shoulders, health professional and lay critics have discouraged wearing a pack on one shoulder as it creates a torque around the spine. Two untested advantages of using one shoulder strap are that the pack can be alternated whenever the user is uncomfortable and it places the load mass closer to the body. Further research is needed to determine whether the one or two shoulder strap method of wearing the pack causes reduced postural deviation or fewer musculoskeletal problems.
An increase in sedentary lifestyle and more computer usage are raising concern over the risk of musculoskeletal disorders such as tendonitis in children, which traditionally have only been seen in adults. These musculoskeletal disorders are not likely limited to computer usage, and the correlative data between backpack use and injury indicate that backpack usage should also be suspected as a cause of pain and disability in children. Although a certain level of stress applied to the body will strengthen the musculoskeletal system, excessive stress on a child’s body may result in overuse injuries. Because of individual variation, it becomes difficult to determine what level of stress and how much exposure each child can tolerate before injuries will occur.
Overuse injuries have been demonstrated in pediatric athletes participating in intensive training programs. Although backpack use is not an intensive training program, the repetitive nature and loads do allow some comparisons to athletics. Children are at a higher risk of overuse injuries for two reasons. First, a child’s skeleton has great amounts of cartilage, especially in areas where growth occurs, as it is a predecessor to bone ossification. These cartilaginous regions include the articular cartilage, the epiphyses, and the apophyses. Each form of cartilage is susceptible to different types of injury. Articular cartilage is vulnerable to sheer stress as seen in osteochondritis dissecans, where the epiphysis and apophysis are more susceptible to repetitive microtrauma. In addition to the weakness of these cartilaginous areas, decreases in soft tissue flexibility or muscle imbalances can also lead to injury. Soft tissues elongation occurs as a response to longitudinal bone growth, resulting in a period during growth where muscles and tendons are taut and subsequently lose flexibility. The highest rate of growth for school aged children occurs during puberty (10-12 years of age for girls and 13-15 years of age for boys); therefore, proper backpack use should be emphasized during these years. Growth of the appendicular skeletal system ceases around 16 years of age for females and 18 years for males; however, secondary ossification of vertebrae is not completed until the mid-20s. Therefore, the spine may be susceptible to injury for a greater length of time.
Although determining causation is important, there are many limiting factors in finding specific cause-and-effect relationships between backpack use and musculoskeletal disorders. Almost all research studies have treated backpack problems as amusculoskeletal disorderand not an acute injury. For amusculoskeletal disorderto occur, the amount of time required to demonstrate injury from overuse is large. In addition, the association between cause and effect may be affected by multiple factors such as physical activity, child’s growth and development, and spinal posture in addition to backpack related variables such as load carried, duration of backpack use, backpack design, and fit of the pack. Controlling all of these factors for extended periods of time has not yet been completed. Therefore, correlations between backpack use and pain or injury are the best indications, to date, of what backpack use is doing to children.
The types of backpack injuries reported in children include back pain, rucksack palsy, shoulder pain, and muscle soreness. A correlation between backpack use and back pain has been documented by numerous authors with children ranging from 9 to 18 years of age. Although this correlation exists, the association between the actual load and back pain is not consistent. The Italian Backpack Study found that backpack load was not a good predictor of back pain; however, reports of fatigue were highly correlated with back pain reports. The odds ratio calculated for back pain when reporting fatigue while carrying a backpack was 3.8 when wearing the backpack and 5.6 for previous back pain. Because fatigue, not weight, was found to be the predicting factor in this study, limiting only weight may not decrease the prevalence of injury, as fatigue is associated with physical fitness, pack design, time of carriage, and the load. Some cross-sectional studies link backpack weight with back pain. However, because of the cross-sectional nature of these studies, a selection bias may be confounding the results. To further investigate the association between backpack weight and back pain, a prospective study is required. At the time of publication, no prospective studies have been reported on backpacks and children.
In addition to back pain, subjects reported tingling and numbness in their arms and shoulder pain, which are symptoms consistent with rucksack palsy. Rucksack palsy is caused when the shoulder straps place direct pressure on the top of the brachial plexus structure and push the nerves against the underlying rib cage, creating a crush type injury. Primary symptoms of this injury include weakness in muscles supplied by the suprascapular and axillary nerves and sensory impairment within the C5 and C6 distribution. Other reports of this condition are limited to case studies, generally from camping trips with boys who carry greater loads for longer periods of time than would be carried on a daily basis by schoolchildren. Recovery from these episodes occurred between 3.5 weeks and 3 months. It is unknown what the actual incidence of neurologic symptoms with backpack use by school children is because the symptoms tend to subside when the backpack is removed; therefore, early signs of the condition, such as tingling, are not likely reported to the medical community.
The injuries of concern with backpack use, especially for long periods of time or with heavy loads, are the repetitive stresses on the growing body. Acute backpack injuries from emergency room reports noted that the danger of backpacks with children in causing injuries that required emergency treatment were caused by improper use, such as swinging the backpack at another child or improper storage, which resulted in a tripping hazard. Although these issues should be addressed, this report should not compromise the efforts to understand safer backpack weight limits and designs. It is only through the improvement of daily loading and the use of properly designed backpacks that the excessive loads on a child’s body can be decreased to safe long-term levels.
Blood pressure was found to be dependent on the load carried. A significant increase in systolic and diastolic blood pressure was measured after 20 minutes of walking for the 20% body weight conditions compared to the unloaded condition in 10-year-old boys. Diastolic blood pressure recovery was significantly faster for the lighter loads (0% and 10% body weight) than the loads of 15% and 20% body weight.
Some of the physiologic consequences of load carriage have been attributed to the changes in posture required to compensate for the load. Increased trunk lean and decreases in trunk angular range of motion may affect chest and abdominal respiratory muscle movement, therefore limiting the methods of increasing oxygen uptake to compensate for increased metabolic costs, thus leading to an increased respiration rate. Additionally, when a load is carried on the trunk, the chest is compressed, causing lung mechanics to be altered. Vital capacity was significantly reduced in children, mean age 9.6 years, when the backpack load was greater than 20% or a kyphotic standing posture was present.
By adding weight to the back of the body through the use of a backpack, the center of gravity is shifted toward the rear of the base of support. This elicits modifications in posture, in both stationary and dynamic situations, to maintain balance and control over movement. The combination of increased weight and postural changes can also alter gait patterns. The degree of alteration in posture and gait is dependent on the load being carried, but it does not follow a dose-response curve. This may be explained by the increase in backpack load being not equivalent to the increase in the forces felt by the body. For example, a backpack load of 15% body weight resulted in a 26.7% increase in lumbosacral force, while a backpack load of 30% body weight increased the same forces by 64%. Thus, if linear increases in load do not produce linear increases in force, it is logical that postural adaptations also do not follow linear responses. This may be more problematic for children between the ages of 10 and 15 years when the greatest rate of growth is occurring.
The majority of biomechanical studies with children’s backpacks have examined the effect of different loads on three main parameters: trunk forward lean, craniovertebral angle, and gait. trunk forward lean is defined as the angle between a vertical line and a line created by connecting points at the greater trochanter and the acromion process. This postural adaptation has been associated with increased forces at the junction of L5 and S1, which may contribute to low back injuries. Increased forces through the vertebral column may be more hazardous for children and adolescents because secondary ossification is not completed until well into the 20s. craniovertebral angle is a measure of head on neck posture. It is defined as the angle between a horizontal line and a line between the tragus of the ear and the spinous process of C7. Gait analyses performed in backpack studies examine stride length, stride frequency, and double support time.
Loads used in biomechanical studies in children range from 0% to 25% body weight. As the load increases, the degree of trunk forward lean also increases. Statistically significant differences in trunk forward lean occur with loads of 15% body weight, 20% body weight, and 25% body weight when compared to the 0% and 10% load conditions. A significant increase in trunk forward lean at 17% body weight has also been observed.
Trunk forward lean can be affected by both pack weight and demands of the task being performed, such as standing compared to walking or running. In a pilot study with two subjects, changes in posture were found to be greater as the load or the demand increased. Trunk forward lean increases naturally as one increases the pace of locomotion from walking to running; thus, it was expected that, under the same loads, trunk forward lean would be greater in a running task than a walking task. This occurred with loads less than 17% body weight; however, no differences were seen in trunk forward lean between walking and running with 17% body weight load. The authors suggested that a ceiling effect existed for trunk forward lean, but this interpretation must await a larger sample size before the effects of task demand on biomechanical parameters can be evaluated.
A second method of counterbalancing the posterior load of backpack carriage is to decrease the craniovertebral angle by poking the head forward. This mechanism was found in children between the ages of 11 and 18 with 15% body weight and self-selected weights. More than 20% of students in each age group had a mean change of 5° in craniovertebral angle, a change that has been previously associated with headaches in young women. Conversely, observations also indicate a change in head orientation angle and concluded that children counterbalanced the weight of the backpack through trunk lean, not neck extension. One explanation for these differences may be due to the differences in the ages and therefore the growth and development differences of children studied, as subjects were much younger (6 years old) than those in studies, which found this compensatory response. Additionally, the occurrence of altered craniovertebral angle was lower in the older age groups. Combining these results, it is possible that this compensatory mechanism is only present in younger and perhaps middle aged children where they are undergoing the greatest amount of growth and have immature bony ossification, thus relying on muscular contraction to sustain certain postures.
A third mechanism of compensating for the load is by altering spinal curvature into hyperlordosis to balance the pack weight with the mass of the abdomen. This mechanism has not been studied in children; however, in a study of young women, a significant hyperlordotic curve was noted when the women were fatigued. Measurement of spinal curvature is difficult with a backpack because the spine is obscured by the pack; however, a devised a measurement tool using 15 spring-loaded rods connected to displacement potentiometers protruding from the back of the pack to determine the horizontal displacement of each rod, thereby giving an approximation of the spinal curvature. Head flexion was also found to be extreme with fatigue, while trunk flexion did not vary between the fatigued and nonfatigued conditions. This points out the importance of accounting for all three adaptations that could occur in backpack loading, namely, trunk forward lean, craniovertebral angle, and spinal curvature.
Although biomechanical study recommendations have been a major contributor to the weight limits adopted by many health professionals, the measurements have been relatively crude. It is expected that muscle fatigue is a major contributor to the pain felt by students while wearing their backpack; however, no studies, to the author’s knowledge, have been completed that examine specific muscle activity using electromyography in children. This information may identify key areas of the back or shoulder region that would benefit from improved backpack design. Additionally, the examination of changes in spinal curvature while wearing the pack may provide more specific information about the anatomic structures that undergo strain and provide a more sensitive measure of changes in posture than trunk forward lean and craniovertebral angle.
Based on biomechanics, one group recommends loads of less than 10% body weight, two recommend less than 15% body weight, and two recommend less than 20% body weight. The biomechanical studies have examined a wider age range (6-18 years) compared to the physiologic studies; however, most of the studies have had small sample sizes.
The first study that looked at load carriage in children studied the most efficient method of carrying school bags for children. Although, in theory, carrying loads on the head is the most efficient, this is a difficult task to manage; thus, it was concluded that the backpack is the most realistic method of load carriage for children. Pascoe et al concluded that a backpack produced fewer changes in the gait and posture of children than single-strap bags. Therefore, it seems reasonable, both from the popularity of backpacks and the recommended use of the backpack for load carriage, that an optimal design to minimize changes in posture should be pursued. Although design studies completed in adults demonstrate that the design of a pack has a great impact on the relative motion between the pack and person, skin contact pressures and forces and moments experienced by the body, limited research has been completed to determine the best design for children’s backpacks.
Using a load carriage simulator, 32 combinations of gait speed, backpack weight, load distribution, shoulder strap length, and use of a hip-belt during load carriage with a school bag were examined. It was shown that backpack weight was the greatest influence on shoulder strap tension and shoulder pressure, but hip-belt use and shoulder strap adjustment were also significantly related to strap tension and shoulder pressure. Looser straps decreased shoulder pressure and strap tension. Gait speed was inversely related to shoulder pressure and not related to strap tension. Data from human trials on the effects of strap tension, or hip-belt tension could not be found. Research on the effects of compression straps, sternum straps, shoulder strap design, or frame sheets could also not be found for children’s packs, although these features have been shown to significantly alter the perceived load and the forces on the shoulders and back by transferring the weight of the pack to the pelvis to be distributed to the lower body.
The only published paper that examines any aspect of design in children’s backpacks on human subjects examined the effects of different loads and load placements of children’s backpacks on standing posture in the sagittal plane. Horizontal movement was minimized when the load was placed at L3, indicating that less postural adjustment is required to maintain the body’s position in space. This conclusion contradicts traditional instructions of high load placement in adults, indicating that findings from adult studies cannot directly transfer to children. It has been found that lower load placements produced less lumbar and shoulder forces but created a significant change in head angle. Therefore, lower forces were applied but posture was adversely affected. Additional work in load placement with children during walking is required before optimum load location can be recommended to pack designers.
Based on the current literature, the value of 10% to 15% body weight is a justified weight limit based on epidemiologic, physiologic, and biomechanical approaches; however, limiting backpack load may not be enough to prevent musculoskeletal disorder tissue injuries or back pain. The distance the load is to be carried, design of the backpack, a child’s physical fitness, and physical maturation are also important factors that should be considered in recommending a pack weight. Further study is required to examine how these different factors affect postural adaptations of the child. By understanding these changes, safer backpack practices can be taught and, hopefully, backpack related musculoskeletal injuries can be better controlled.