LA CADENA LÁCTEA EN COLOMBIA.

There is a need for the development of interventions aimed at reducing students' exposure to the risk factors present in a school environment (Trevelyan and Legg 2004). Because there is evidence to suggest that schoolbag weight may be associated with back pain in students (Negrini et al. 1 998, Viry et al. 1 999, Grimmer and Williams 2000, Sheir-Neiss et al. 2003, Korovessis et al. 2004, Siambanes et al. 2004), it would be prudent to include schoolbag carriage guidelines within wider risk management interventions.

It could be argued that it is inappropriate to recommend a maximum schoolbag weight as it has been demonstrated that schoolbag design, adjustment and carriage duration also affect the demands placed on students. However, it would be impractical not to recommend a maximum schoolbag weight as this would offer no

guidance to students. Instead, a recommended maximum schoolbag weight should be supplemented with recommendations for schoolbag design, adj ustment and carriage duration. Likewise, it would be impractical to recommend a very low maximum schoolbag weight (2.8 kg) in accordance with what was acceptable to 95% of participants in chapter 7. A more practical recommendation would be the 5 0th percentile load of 1 0% BW, supplemented with advice recommending that this load is highly subjective and that there are many other factors such as physical fitness, previous injury and other schoolbag factors which might mean that an individual' s recommended load might be lower or higher than 1 0% of BW.

There are some practicalities associated with schoolbag carriage that mean that students are required to carry a given load regardless of their size. For example, most students will require their lunch, sports gear, text books, stationery and possibly a computer or musical instrument throughout the day, and the weight of these items are unlikely to be significantly affected by a student' s size. This would mean that there may be some merit in recommending an absolute load that students should carry (for example 5 kg) based on a practical representation of their typical schoolbag contents. However, this would be a re-active approach to determining weight limits. A more pro-active approach would be to maintain a recommended MASW of 1 0% BW as it would require students, parents and schools to make planning changes which would lead to the reduction of loads in some cases, rather than forcing smaller students to carry relatively large loads.

Sometimes backpacks are slung over one shoulder when carried by students. Although this might represent a risk to students carrying their belongings due to the asymmetrical nature of the loads they are carrying, it has been shown that most students carry their backpacks over two shoulders. Whitfield et al (200 1 ) showed that 70% of New Zealand school students in their study carried their backpack over two shoulders. Likewise, Grimmer and Williams (2000) found the over two thirds of students carried their schoolbag over two shoulders. Despite these positive findings, it is clear that a minority of students carries their schoolbag over one shoulder, and advice against this practice should be included in load carriage recommendations. However, although intuitively asymmetric load carriage may seem undisireable, and

Chapter 7 -Discussion

single handed schoolbag carriage has been associated with increased MSD (Viry et al. 1 999), more studies have shown no relationship between schoolbag carriage mode and reported MSD (Korovessis et al. 2004, Pucktree et al. 2004, Siambanes et al. 2004).

In addition, it might be appropriate to recommend that a more comprehensive suspension system be used in schoolbag designs if loads greater than 1 0% BW are to be carried. Most students do not use properly designed hip-belts when carrying their schoolbags and therefore hip-belts were not used by participants in the present studies to provide evidence for an upper school bag weight. I f a well designed backpack with sufficient padding and a good hip-belt were used, then students may be able to carry a little more than 1 0% B W without causing undue strain or fatigue.

The effects of shoulder strap adjustment and schoolbag carriage duration on posture, RPE, muscular strain and perceived ability to walk and balance were inconclusive in the present studies. Previously, shoulder strap tightness has been shown to affect posture (Grimmer et al. 2002) and schoolbag carriage duration has been shown to affect reported MSD (Grimmer and Williams 2000, Negrini and Carabalona 2002, Jones et al. 2003, Sheir-Neiss et al. 2003). Therefore, more research should be carried out in these areas before recommendations about them can be made.

Individual ' s fitness, or history of previous injury may also affect students' schoolbag carriage capability and MSD but have not been addressed in the present series of studies. These factors should also be considered when developing guidelines for school bag carriage. It might be that more physically conditioned school students or students who are practiced in carrying their schoolbags are more capable of carrying a given load compared with less physically conditioned students. Furthermore, a previous back i njury might predispose students to further MSD and prevent them from carrying their schoolbag. Alternatively, appropriate schoolbag carriage might provide necessary strengthening for a previously weak and injury prone back. Further work addressing the implications of previous injuries for school bag carriage would provide evidence for recommendations in this area.

Finally, it must be considered that schoolbag carriage may be an important component of school students' physical activity, especially as childhood inactivity and obesity are becoming increasingly problematic. School students should not be discouraged from carrying their schoolbags and recommendations for schoolbag carriage should emphasize the health benefits of schoolbag c arriage. A focus on distinguishing between 'unsafe' and 'healthy' schoolbag carriage might be more appropriate than simply outlining the possible dangers associated with schoolbag carriage. Furthermore, schoolbag carriage guidelines could promote a minimum duration of schoolbag carriage per week in order to promote physical activity among students. It appears that no studies have examined the effects of schoolbag load on students' daily energy expenditure or physical well-being, despite the fact that schoolbag carriage is an integral part of most student's lives. Most studies of children's daily physical activity and energy expenditure levels have focused on the temporal patterns of physical activity (Janz 1 994, Epstein et al. 1 996, Puyau et al. 2002) without considering the intensity of students' physical activity . The contribution of school bag load to daily physical activity intensity levels would be a useful area of research as it would address an important, yet neglected, factor in measuring school students' physical activity.

Commuting to and from school is potentially a source of students' daily physical activity (Ziviani et al. 2004) and has often been neglected in studies of children' s physical activity (Tudor-Locke e t al. 2002, 2003). In chapter 4 ( The temporal patterns of schoolbag carriage, page 1 07), it was reported that students who walk to school spent approximately 25 minutes walking to school and 32 minutes walking home. This means that some students may spend approximately one hour walking each day while carrying their schoolbag. For a given walking pace, carrymg a schoolbag would be associated with an increased exercise intensity and could possibly proffer health benefits.

Commuting by foot to and from school with an appropriately adjusted and weighted backpack would have the dual benefit of providing valuable physical activity for school students and reducing road congestion. Intervention studies that aim to increase the number of school students walking or cycling to and from school could

Chapter 7 -Discussion

significantly contribute to the improvement of quality of life for both students and adults.

Schoolbag carriage guidelines should also contain advice regarding other modes of transporting school students' belongings. In addition to possibly contributing to MSD, cycling while carrying a schoolbag may affect students' balance (Legg et al. 2003) and increase the risk of accidental injury. Trolley-bags are commonly used by urban travelers; however they have only recently been used to transport school belongings. Pulling ones belongings behind them on wheels removes the burden of load carriage. However, other challenges such as lifting it with one hand up stairs may predispose a user to MSD or acute injury. It appears that the usability of trolley­ bags or their relationship with MSD or injury have not been studied in adults or children. Further research in this area would provide guidance for trolley-bag use.

Conclusion

I t i s concluded that the effects o f schoolbag design, adjustment carriage duration and weight on students' responses to carrying schoolbags were as follows:

• School students' preferred attributes in a backpack changed over short periods of experiential use (20 minutes) from ' style and image' to 'function and fit' . Also, backpacks preferred by students were associated with less muscular strain in the back and pressure in the shoulders.

• Load weight, hip-belt use and shoulder strap length had significant effects on shoulder strap tension forces and shoulder interface pressure during simulated schoolbag carriage.

.. Posture, RPE, muscular strain and perceived ability to walk and balance were significantly affected when students' schoolbag load reached 1 0% BW, but were not affected by the duration of carriage over a school day for loads of up to 1 5% BW, or by shoulder strap tightness.

• The magnitude of self-reported muscular strain and MSD as a result of

carrying 1 5% BW, suggests that carrying 1 5% BW may be excessive.

• Using a psychophysical approach, a maximum acceptable schoolbag weight

of 1 0.4% (SD 3.8) BW was determined.

The objective evidence obtained in this thesis suggests that an upper schoolbag weight limit should be 1 0% of a school student's body weight. However, in addition to a recommended upper schoolbag weight, guidelines for schoolbag carriage should include recommendations on schoolbag design and adjustment. Schoolbag carriage guidelines may also address the duration of schoolbag carriage, although more research is required in this area. Based on postural, self-reported and psychophysical measures, the findings of this thesis support a recommended upper schoolbag weight of l 0% BW.

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