1Calculated extra load index (ELI) values for published data relating to different forms of load carriage http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/153/283
While a holistic assessment of load carriage requires more than merely an assessment of energy expenditure and should consider biomechanical factors associated with injury risk, for instance, the aim of this study was to test the ‘free ride’ hypothesis in African women by making direct comparisons between the energy cost of head- and back-loading in two groups of women who differed only in their experience of head-load carriage. MethodsParticipantsTwenty-four Xhosa women, 13 with at least 10 years experience of head-load carriage (EXP) and 11 with no experience of head-load carriage (NON), were recruited to take part in the study. All participants gave informed consent for their participation in the study, which had received ethical approval through standard institutional review procedures at both the University of Abertay Dundee and the Cape Peninsula University of Technology. Participants were not paid for their participation in the study, but did receive compensation to cover travel costs. A summary of participant characteristics is presented in Table 2. Independent t-tests indicated no significant differences between the two groups for any of the parameters reported in the table. Experimental proceduresThe women each attended the Human Performance Laboratory at the Cape Peninsula University of Technology on three separate occasions. On the first occasion, participants were screened for any potential contraindications to exercise, stature and mass were assessed and questionnaires relating to load-carriage history were completed. The women were then habituated to the experimental protocol and the equipment to be used. A typical habituation session lasted between 20 min and 30 min and involved the women walking on the treadmill at various speeds both with and without a face mask. In addition, they also practised walking with the two load-carrying devices, a standard 45-L backpack (Karrimor, South Africa) for back-loading and a plastic crate for head-loading (the crate was placed either directly on the head or on a small piece of rolled cloth to provide some cushioning), with and without loads. At the end of the session, the women were asked to walk on the treadmill at a speed that they felt would be comfortable when carrying a heavy load. The chosen speeds, 3.15 ± 0.45 km.h-1 and 3.01 ± 0.30 km.h-1 for the EXP and NON groups respectively, were not significantly different (p = 0.401, independent t-test) and were similar to speeds employed in other similar studies.2,3 The chosen walking speed of each participant was noted and used for the subsequent experimental trials.
2Participant characteristics for the two groups, experienced head-loaders (EXP) and those without experience (NON) http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/153/284
On arrival at the laboratory at the next visit, each participant chose at random, via the picking of a suitably marked piece of paper from a hat, the loading method for the first experimental trial. This involved walking, at the previously determined speed, for 4 min unloaded after which, following a 1-min rest, a load of 10% of BM was added, which was carried for a further 4 min. After a further rest of 1 min, the load was increased to 15% and carried for 4 min. This pattern was repeated with loads of 20%, 25%, 30%, 40%, 50%, 60% and 70% of BM or until pain and discomfort led to voluntary cessation of the session. Workloads of 4-min duration were employed based on pilot work that showed that steady-state oxygen consumption was achieved within this time. This duration is consistent with previous studies in this field. 7The 1-min rest period was used to adjust the carried load and involved the participant standing still on the treadmill while the load was removed, adjusted and then replaced. The load was calculated based on the BM at the habituation session and was made up of the mass of the actual carrying device plus appropriate weightlifting plates (between 2.5 kg and 10 kg) and 100-g sandbags. This allowed the load to be adjusted to within 50 g of the required load. Each participant returned to the laboratory one week later to repeat the experiment with the other loading device. Data collection and analysisAll participants were fitted with a facemask in line with manufacturer guidelines to ensure that no leaks were present, and expired air was collected throughout the protocol by means of an on-line gas-analysis system (Quark b2, Cosmed, Rome, Italy). The system was calibrated prior to each test in accordance with manufacturer instructions using gases of known concentration and room air. Oxygen consumption was collected breath by breath and reported over 15-s intervals. It was subsequently averaged over the final minute of each workload and the associated ELI values calculated [Eqn 1]. The ELI values were subsequently analysed using an ANOVA (group × load × condition) with repeated measures (SPSS, version 16.0). Maximum load carried was recorded and compared between the groups and conditions by means of a further ANOVA (group × condition) with repeated measures. Pearson’s product moment correlation coefficients were calculated to establish relationships between loading conditions and between mean ELI values and anthropometric variables. Independent t-tests were used to assess differences in physical characteristics and walking speed between the two groups and a dependent t-test was used to compare unloaded oxygen consumption between the two trials. ResultsUnloaded oxygen consumption was not different between the successive measurements in each of the two conditions (8.3 ± 2.1 mL·kg-1·min-1 vs 7.7 ± 1.8 mL·kg-1·min-1 for head- and back-loading respectively, p = 0.261). Figure 1 shows the ELI values for each group in each of the conditions for loads of up to 25% of BM (the maximum load completed by all 24 participants in both conditions). Statistical analysis revealed no significant difference (p = 0.206) in the relative economy of head-loading and back-loading across all loads (ELI = 1.04 ± 0.19, 0.97 ± 0.15 respectively). Similarly, there was no significant difference between the two groups across all loads (p = 0.186, ELI = 0.98 ± 0.17 for EXP and 1.04 ± 0.18 for NON) or between the different loads (p = 0.891, ELI = 1.00 ± 0.16, 1.00 ± 0.17, 1.01 ± 0.18 and 1.00 ± 0.20 for 10%, 15%, 20% and 25% loads, respectively). There was, however, a significant interaction between carrying method and load (p = 0.053), implying a difference in ELI value associated with each of the loading conditions for at least one of the loads. Post-hoc analysis revealed that this difference was at the 10% load. The mean ELI for back-loading at the 10% load was significantly lower than that for head-loading (95% confidence interval, 0.897–0.986 vs. 0.991–1.15 for back- and head-loading, respectively). There was a significant difference (p = 0.015) between the maximum load carried in each condition, with the average maximum load associated with head-carriage being 42.1 ± 14% of BM versus 51.5 ± 15.8% of BM for back-load carriage. These equate to absolute loads of 27.1 ± 8.3 kg and 33.4 ± 9.6 kg respectively. There was no difference between the loads carried by the two groups (p = 0.382, EXP = 48.8 ± 16.9% of BM, NON = 44.3 ± 13.7% of BM) and no interaction between the two groups and load-carriage method (p = 0.965). Only two women, both from the EXP group, managed to carry 70% of their BM on their head, while seven women (five from the EXP and two from the NON group) managed to carry 70% of their BM on their backs. Figure 2 shows the results for the whole group. The diminishing number of women completing both conditions at the higher loads made interpretation difficult. One notable result was the high degree of intra- and inter-subject variability in economy both between loading methods and loads (Figure 3). There was no apparent relationship between the conditions for mean ELI value across the 10% – 25% of BM loads (r = 0.147, p = 0.430). Overall, 9 out of 24 participants had lower average ELI values for head-loading than back-loading. This was independent of previous head-loading experience, with 38.5% of the experienced head-loaders exhibiting better economy in head-loading than back-loading and 36.4% of the NON group exhibiting the same tendency. The magnitude of the standard deviations in Figure 3 gives an indication of the variability in ELI value across the different loads. DiscussionThe data presented here for back-loading are broadly consistent with previous studies, with ELI values ranging between 0.94 and 0.99 across all loads. 4,11,12,13,14The data are also consistent with previous data for head-load carriage, 8,17with ELI values ranging between 1.03 and 1.07 across all loads. The mean data do not, however, support either the ‘free ride’ hypothesis, or the view that head-load carriage allows heavy loads to be carried with ease. Indeed, the present study indicates that, on average, the relative economy of head-load carriage in these African women is much less than previously reported; there appears to be no physiological advantage to head-load carrying over back-loading. Even though back-loading shows some tendency to be more economical than head-loading, very few women could carry very heavy loads on their heads, while greater loads could be carried on the back than on the head. However, closer examination of individual results reveals that it would be possible to select a subset of women who did achieve remarkable levels of economy, in line with the previously reported data. Given the small sample sizes in most of the previous studies on head-loading, this is not altogether unexpected, but lends support to the notion that the ‘free ride’ hypothesis is not a generalisable finding, when tested with larger, more representative samples of African women. The average ELIs of seven of the women were less than 0.9 for back-loading (all of the women had experience of carrying loads on their backs, as this is the traditional African method of carrying babies and small children), while four women achieved this for head-loading. Remarkably, three of the four most economical head-loaders were women with no experience of head-load carriage. This finding would seem to indicate that structural changes to the spine associated with early and prolonged exposure to head-loading are unlikely to provide explanations for such efficiency in individuals, as previously speculated. 2It has also been argued that body composition influences load-carriage economy 29and that the explanation for the remarkable economy observed in some head-load carriers is a consequence of their low body fat, 24with the extent of the ‘free ride’ being determined by the combination of fat and external load up to 140% of fat-free mass (FFM). While this argument is helpful in untangling some of the issues relating to the ‘free ride’ hypothesis, it does not provide support for the hypothesis and would only provide an explanation if all extremely economical load carriers are relatively lean. In the present study, the body mass index (BMI, mean ± s.d.) for the 11 women with average ELIs below 0.9 for either load-carriage method was 26.0 ± 4.1 kg.m-2, implying that these women were, if anything, slightly overweight. It might also be expected that if the size of the load relative to FFM is the determinant of economy, there would be a strong relationship between economy across different load-carriage methods. However, in the present study it was apparent that economy in one method of load carrying was not an indicator of economy in the other method. An exploration of relationships between economy and basic anthropometric measurements (mass, stature and BMI) in the present study revealed significant relationships only in relation to head-loading. Both BMI and stature were significantly related to the mean ELI value across 10% – 25% of BM loads. In the case of BMI, this was a moderately positive relationship (r = 0.482, p = 0.017), which suggests that as BMI increases, the economy of head-load carriage decreases. The relationship between BMI and economy for back-loading was weak and not significant (r = 0.308, p = 0.143). In the case of stature, there was a significant negative correlation with mean ELI value across loads of 10% – 25% of BM (r = -0.551, p = 0.005) for head-loading, but not for back-loading (r = 0.162, p = 0.450). This implies that, for head-loading, taller individuals exhibited better economy. Interestingly, the relationship between stature and ELI value became stronger as loads increased up to 20% of BM with significant r-values of -0.485, -0.556 and -0.663 for loads of 10%, 15% and 20% of BM respectively and then diminished at 25% of BM (r = -0.326, p = 0.121). The lack of association between economy and load-carriage method, depicted in Figure 3, in addition to the lack of consistency in relationships between anthropometric variables and economy in each of the loading methods, is an important finding. It suggests that cause and effect relationships between economy and efficiency of load carriage are not likely to be explained by a common set of factors for different forms of load carriage in the same individuals, whether or not they are experienced in either or both forms of load carriage under investigation. This suggests that future work, and evaluations of previously completed studies in load carriage, should focus on an evaluation of the mechanisms responsible for the economy of individuals rather than expecting one or more mechanism to explain the observed variation within a particular method. This shift in focus, with a view to understanding how particular individuals carry loads more efficiently than others and why particular methods are more efficient for some individuals than others, may provide a better understanding of the interactive effects of factors related to different forms of load carriage. Thus, some of the proposed explanations for the greater economy of head-loading, such as improved energy transfer between gravitational potential energy and kinetic energy 25,26(based on the five participants of the original study 2), balancing the loaded segment above the hip 22(based on five experienced head-loaders carrying loads uphill but with no direct measurement of unloaded walking) and for back-loading at some low-speed–light-load conditions, the interaction of rotative torque and the burden on the lower limbs, 27,28may be best examined on a case-by-case basis, attempting to account for individual difference rather than seeking general explanations. This may have implications for both military and recreational applications, as it is likely to be the case that either the optimum load-carriage system may be specific to an individual or that particular carrying methods require different techniques. It is clear that individualisation of load-carriage strategies may be impossible for most applications, although in some particularly sensitive cases it may be worthwhile. Nevertheless, an understanding of the factors that lead to improved economy in particular individuals, rather than pooled results, may well provide a useful way forward in the design and customisation of mass-market products. The present findings, in conjunction with other data that suggest that head-loading is associated with significant and chronic neck pain, 30suggest that there is a need to not only reconsider the appropriateness of head-loading as a means of transporting heavy loads but also to establish viable alternative methods. Such investigations will need to examine not only the energy expenditure associated with different loading methods, as was the case here, but also biomechanical measures. This will allow for consideration of potential injury and health risks. ConclusionThis study sought to test the ‘free ride’ hypothesis in African women. The mean data presented provide no support for such a phenomenon, suggesting that, on the whole, both head-loading and back-loading are associated with ELIs close to unity. It was, however, also apparent that there was a significant degree of individual difference in response, with certain individuals achieving something close to a ‘free ride’ in one or other of the conditions, but not both. It is therefore concluded that, (1) the interactions between load-carriage systems and individuals are complex and future work should be focused on this subject–load–loading system interaction with a view to elucidating the key factors associated with greater economy in load carriage, with the potential to incorporate these findings into systems that can be more readily customised to individual needs and (2) there is a need to explore alternatives to head-loading as a means of load carriage in rural Africa. 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