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heel strike. It consists of three parts: initial swing, mid-swing, and terminal swing. he stance phase (62-68% of the gait cycle) occurs when the foot is planted irmly on the ground. It consists of ive parts: initial contact, loading response, mid-stance, terminal stance, and pre-swing. he period when both feet are touching the ground is called the “double limb support time,” and the period when only one foot is on the ground called the “single limb support time.” Other terms used to describe gait are cadence, stride length, and step length. Cadence is the number of steps per minute, stride length is the distance between heel strikes of the same foot, and step length is the distance between the heel strike of one foot and the heel strike of the other [4].

Anatomical Reference SystemMost clinicians and researchers use a standard anatom-

ical reference system. A person is bisected into right and left halves by the sagittal plane, front and back halves by the coronal plane, and top and bottom halves by the trans-verse plane. Abduction of a limb segment refers to moving it away from the body, and adduction means the opposite. Flexion refers to bending a joint, whereas extension refers to extending the joint.

IntroductionMost high school students carry huge backpacks that

heavily inluence their body posture and gait by adding a lot of additional load onto the student while he or she walks, and the general consensus is that the gait of a back-pack-wearing student difers substantially from the gait of an unburdened student [1]. Researchers studied the gait of people of all ages, with or without backpacks, by using force plates and camera systems to record data, although it is possible to use other types of measuring devices [2]. However, few take into account factors such as height and physical itness [3], and virtually none have studied stu-dents between the ages of 15 and 18.

Review

Overview of Gait

Gait Cyclehe gait cycle represents the events between successive

points of contact of a single foot. here are two sets of terms used to describe the gait cycle, shown in igure 1, but most researchers use the more recent set of terms created by the Rancho Los Amigos Hospital [4]. It consists of the “heel strike” and the “toe-of.” he gait cycle has two basic components: the swing phase and the stance phase. he swing phase (32%-38% of the gait cycle) occurs when the foot is completely of the ground, between a toe-of and

Effect of Backpack Load on Gait Parameters

Figure 1. he gait cycle and gait terms [4].

Alice Li

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distributed to the rearfoot, or the heel. During the heel-strike, the heel bears almost all of the force exerted by the rest of the leg. Figure 2 shows how the center of pressure upon the sole of the foot changes during the stance phase of a stride.

he ankle has a range of motion in the sagittal plane ranging from 10 degrees during dorsilexion, or pointing the toes upward, to 12 degrees during plantarlexion, the inverse to dorsilexion [6].

Normal GaitIn 1990 at Helen Hayes Hospital in New York, Kadaba

et al. measured spatiotemporal parameters and joint angles of 40 healthy young adults, 28 male and 12 female. He found that the average stride length of his subjects was about 1.35 +/- 0.12 m, an average cadence of 113 +/- 9 steps/min, and a double limb support time of 61% of the gait cycle.

About a decade later in Korea, Cho et al.. performed a similar experiment but discovered diferent results which may be attributed to racial diferences between Americans and Koreans, or the fact that Cho et al.’s experiment in-volved barefoot individuals, while Kadaba did not mention whether or not his subjects wore shoes.

In Cho et al.’s data, the average stride length is about 20 centimeters less than Kadaba’s average stride length for both males and females, which can be attributed to height diferences between the two sets of subjects. here is also a 4 steps/min diference between the male cadences. How-ever, their data on stance phase agrees around 61%, and both got similar results regarding female cadences.

In Kadaba’s data, the bold line represents the mean values and the dotted lines represent the deviation, while in Cho et al.’s, the dotted lines are the graphs of the fe-males, and the bold lines are the graphs of the males. heir kinematic data seems to match fairly well regarding hip kinematics. As shown in igure 3, their experiments got very similar data in hip lexion and extension as well as hip adduction and abduction, with peak angles of about 40 ° and 6 ° respectively. here seems to be some discrepancy in transverse joint motion, or hip rotation. However, Kadaba’s bold line is an average, while Cho et al. did not average

Muscles and Joints

he Musculoskeletal Systemhe human body consists of over 200 joints, 206 bones,

and about 640 diferent muscles. A possible description for the musculoskeletal system is a machine capable of apply-ing forces to other objects. Muscles create forces; the more muscles used to create the force, the greater the force. All the muscles of the human body working simultaneously in the same direction could move about 22 tons. How-ever, the muscles are arranged so that they work in pairs: one muscle the agonist, the other the antagonist. Working against each other prevents either from overstretching [5].

here are two types of contraction: isotonic and iso-metric. Isometric contraction occurs when the muscle is activated, but the length of the muscle does not change. Isotonic contraction occurs when the length of the muscle changes, and can be further divided into two types: con-centric and eccentric. Concentric contraction, the focus of many studies occurs when the muscle shortens, decreasing the tension upon it. Eccentric contraction occurs when the muscle lengthens, increasing tension [2].

Hip Jointhe hip joint consists of the head of the femur and the

acetabulum of the pelvis, as shown in igure 3. It can move 140 degrees forward and 15 degrees back, 30 degrees out-ward and 25 degrees inward, and rotate 90 degrees out-ward and 70 degrees inward [6]. When both feet are on the ground, no muscle contraction is needed to maintain that posture. However, in a single leg stance, such as dur-ing the single limb support phase of walking, the abductor muscles must exert torque upon the hip joint to counteract the torque exerted by the entire body’s weight [5].

Knee Complexhe knee complex receives very high loads during dy-

namic weight bearing, which is why it is one of the most common sports injuries. he knee complex consists of two separate joints: the tibiofemoral joint and the patel-lofemoral joint. An important part of the knee is the me-nisci, which act as shock absorbers for the joint and brings about normal movement between the two bones. he knee also uses two ligaments, the anterior cruciate ligament and posterior cruciate ligament, to stabilize the knee during tibia axial rotation [6].

Ankle and Foot Jointhe foot consists of 28 diferent bones, and the ankle,

3 bones. he ankle is a hinge joint, and consists of the tib-iotalar, ibulotalar, and the tibioibular joints. In addition to the ankle joint, the foot also consists of ive metatar-sophalangeal joints, where the toes connect to the rest of the foot. his joint is heavily employed during walking, especially during the toe-of phase [6].

During walking, most of the weight the foot bears is

Figure 2. Progression of the center of pressure upon the foot during normal gait [6].

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between the studies. he average pelvic tilt from Kadaba’s study is at about 15°, whereas the average pelvic tilt from Cho et al.’s study is approximately 10°. However, as the deviation is the same for both, and this diference could be attributed to the way the two studies deined their axes of reference. he other graphs look fairly similar, with both maximum pelvic obliquity and maximum pelvic rotation at 5°.

Gait Under Load

Double Strap BackpackWearing a backpack increases the double limb support

time of the gait and decreases swing time. Wang et al. dis-covered this trend in 2001 in his experiment with college-aged students [7]. In the next few years, several other re-searchers reported the same results with younger students, from age 9 to age 15 [3,8,9]. Connolly et al. discovered the double support time of middle-school students increased from about 19% of the gait cycle when the students walked without a backpack to 21% of the gait cycle when the stu-dents walked while wearing a backpack.

Similarly, Chow et al. reported that the mean double limb support time of adolescent girls increased from about 11.1% to 12.4% when the load increased from 0% of the wearer’s body weight (BW) to 15% BW as indicated in igure 6.

Although these changes in percentage are quite small, they do indicate that there is a small diference between the gait of a person wearing a backpack and the gait of a person not wearing a backpack.

here is also discrepancy over the spatiotemporal pa-rameters, especially stride length. Pascoe et al. claimed that wearing any kind of bag decreased the stride length

the data from both genders. If the two were averaged, the graphs from both studies would look more similar.

As with the kinematics of the hip, the kinematics of the knee between the two studies are also very similar, with peak knee lexion angles of approximately 60° and peak knee varus angles of around 5°. Like with igure 4, Cho et al.’s graph of knee varus and valgus angles look dissimilar because he did not average the data from males with the data from females.

he kinematics of the pelvis also show some diferences

Figure 3. Comparison of hip kinematics between studies. Cho’s data is on the left, and Kadaba’s on the right .

Figure 4. Comparison of knee kinematics between studies. Cho’s data is on the left, and Kadaba’s on the right.

Figure 5. Comparison of pelvis kinematics between studies. Cho’s data is on the left, and Kadaba’s on the right.

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Chow et al. claimed that the critical limit is between 10-12.5% of the wearer’s body weight using several pa-rameters such as peak knee lexion and peak hip rotation moment.

Figure 7 shows the dramatic change in parameter 43, knee extension moment, as soon as the load is increased from 10% BW to 12.5% BW, illustrating the critical value at which the knee begins to react diferently to the load.

However, even though the parameters indicate major changes in gait pattern, these changes may not necessar-ily be detrimental, especially when the rest of the body’s movements are taken into consideration [8].

In a diferent experiment, Hong had his 9 to 10 year old subjects walk four diferent distances in increasing order while wearing the backpacks of diferent weight and re-corded their trunk inclination angles for each distance and weight, as shown in igure 8.

Note that there is a signiicant increase in trunk angle between 15% BW and 20% BW, suggesting that 15% BW is the critical load, rather than the 10% BW given by Chow’s experiment. he diference between Chow’s experiment and Hong’s experiment can be attributed to gender diferences, as Hong used 9-10 year old boys while Chow used 10-15 year old girls. In addition, Chow came to his conclusion by analyzing gait parameters, while Hong observed the trunk’s angle of inclination.

Despite this evidence regarding the critical limit, it does not determine what the critical limit is exactly, or if it even exists. No studies have been done on whether consistently going over the critical limit causes any sort of injury, other than a few surveys of students which give somewhat vague results [1].

ConclusionMany studies have been done on how wearing a back-

pack afects the person’s gait, all of which have slightly conlicting results. Although many of these diferences can be attributed to demographic diferences in the sub-ject, there is still much research to be done. For instance, few researchers have looked into the diferences between wearing a backpack on only one shoulder instead of on both shoulders. In addition, former studies have not taken into consideration the height and physical condition of their subjects, which may be a factor in a gait’s response to loading [3]. hese factors may be especially signiicant in children and adolescents, as their bodies have not fully

Figure 6. Efect of load weight on double support time.

Figure 7. Efect of backpack load on peak knee extension moment .

Figure 8. Efect of load on trunk inclination [9].

of 11-13 year olds; in contrast, Connolly did not ind any signiicant diference between loaded and unloaded stride lengths.

Both Chow et al. and Hong et al. also got results similar to Connolly’s. Hong believes that it is diicult to compare their results to Pascoe et al., as Pascoe did not make any mention of the walking distance or walking velocity of his subjects [10].

Critical LimitAnother topic of debate among the pediatric medical

community is the “critical limit,” or the weight at which a backpack becomes too heavy and causes dramatic changes in gait, potentially contributing to injury or back pain [8].

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matured yet. here has also been no research done on 16-18 year olds, with most researchers focusing on children around the age of 10-12 or fully grown adults. here is also no consensus on the “critical limit” weight, as diferent researcher’s claims range from 10% BW to 20% BW to no critical limit at all [8,9].

In the United States, the average backpack weight for an elementary school student is 17% BW, with some car-rying up to 30% BW or 40% BW. Meanwhile, about 50% of adolescents complain of back pain, reaching more than 60% by adulthood. Back pain is mostly likely related to backpack usage, as wearing a backpack forces the back muscles to lex in response to the torque applied to the body by the backpack, so inding this “critical limit” may be key to preventing musculoskeletal harm [1].

References

[1] Cho, S. H., Park, J. M., & Kwon, O. Y. (2004). Gender diferences in three dimensional gait analysis data from 98 healthy Korean adults. Clinical Biomechanics 19(2), 145–152 [2] Cottalorda, J., Bourelle, S., & Gautheron, V. (2004). Efects of backpack carrying in children. Orthopedics 27(11), 1172-1175[3] Winter, D. A. (2004). Biomechanics and motor control of human movement. (3rd ed.). New York, NY: Wiley.[4] Connolly, B. H., Cook, B., Hunter, S., Laughter, M., Mills, A., Nordtvedt, N., & Bush, A. (2008). Efect of backpack carriage on gait parameters in children. Pediatric herapy 20(4), 347-355[5] Cuccurullo, S. (2004). Physical medicine and reha-bilitation board review. New York, NY: Demos Medical Publishing. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK27235/ [6] Watkins, J. (1999). Structure and function of the mus-culoskeletal system. Champaign, IL: Human Kinetics. [7] Nordin, M., & Frankel, V. H. (2001). Basic biome-chanics of the musculoskeletal system. (3rd ed.). Philadel-phia, PA: Lippincott Williams & Wilkins. [8] Wang, Y., Pascoe, D. D., & Weimar, W. (2001). Evalu-ation of book backpack load during walking, Ergonomics 44(9), 858-869[9] Chow, D., Kwok, M., Au-Yang, A., Holmes, A., Cheng, J., Yao, F., & Wong, M.S. (2005). he efect of backpack load on the gait of normal adolescent girls. Ergonomics 48:6, 642-656[10] Hong, Y. & Cheung, C. (2003). Gait and posture re-sponses to backpack load during level walking in children, Gait and Posture 17(1), 28-33[11] Pascoe, D. D., Pascoe, D. E., Wang, Y. T., Shim, D. M., & Kim, C. K. (1997). Inluence of carrying book bags on gait cycle and posture of youths, Ergonomics 40(6), 631-641[12] Kadaba, M. P., Ramakrishnan, H. K., & Wootten, M. E. (1990). Measurement of Lower Extremity Kinematics During Level Walking, Journal of Orthopaedic Research 8(3), 383-392