Below is an additional discussion of the 'backpack vs. flexible poles' project results:
This experiment provided an excellent opportunity for the researchers to gain experience designing and preparing an experiment using complex equipment such as the instrumented treadmill, and metabolic measuring cart along with the force transducer, goniometer, and machine weight equipment to study the biomechanical parameters of human subjects running in both an unloaded an loaded condition. Of special interest has been the difference in load carrying between a rigidly attached backpack and three lengths of compliant poles. Ideally, an experiment using human subjects running on a treadmill should involve a number of familiarization sessions to ensure that each subject has adapted to running while breathing through the oxygen consumption measuring equipment and with the various loads. A familiarization process involving four or five repetitions of each loading condition would ensure a much better assessment of steady state conditions and allow for repeatable performance values. The lengthy familiarization process would have allowed subjects to go through the learning curve and reach a level of performance governed by a normal distribution of metrics. Complete standardization of footwear could also have been an improvement to the methodology.
Of course, despite the methodological weakness of a short familiarization process and the fact that only ten seconds of force profile data was gathered for each subject during each experimental trial, the researchers feel confident that the results presented provide good insight into the biomechanical differences between backpack vs. pole load carrying techniques. This confidence results from the observation that each subject was in reasonably good physical condition (S5 and S6 were in excellent physical condition) and the fact that all subjects were familiar (to some degree) with treadmill running. Another factor promoting general confidence in the results is that the activity performed (running) involved steady state performance of a repetitious task video analysis suggests that each subject quickly found, and then maintained, a steady state gait pattern. A couple of slight deviations to the steady state nature of the gait pattern were observed, but these were temporary and typically due to the compliant poles shifting on the shoulders (S5 harness did not fit tightly enough).
Therefore, acknowledging the above methodological limitations, here are the important results from this study:
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Energy consumption (as implied from oxygen consumption), did increase in a manner that supports the hypothesis that energy expenditure increases in direct proportion to the load carried. In this case, as the load increased by 15% of body mass, the oxygen consumption values increased from 11-20% depending upon the subject. Also, the greater overall VO2 values and VO2 per body mass values for subjects 5 and 6 indicate that they are better able to deliver oxygen to working muscles they are shown to be in better aerobic shape than subjects 1 and 2.
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Peak shoulder forces were not definitively shown to be higher for the backpack load carrying than the compliant pole carrying methods (as implied from maximum vertical force profiles) as was suggested by previous research in which the back pack was assumed to be rigidly attached to the subject (Kram 1991) . It is the impression by the subjects that peak shoulder forces were higher when using the backpack method, but it is also noted that the complex movement of the backpack and the way in which the pack was supported by both shoulder straps and a waist belt enabled impact forces to be distributed in non-vertical directions. Of course, improved collection of tension data at the point of contact between the backpack straps and the shoulder (or gathering kinematic data) would have been ideal for resolving the peak force issue.
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There were noticeable differences in preference between load carrying methods and these preferences (from subjective data, pg. 42) correspond well to the quantitative measurements made of the average vertical forces applied to the treadmill belt. Subjects 1, 5 and 6 all reflected this relationship well while subject 2 showed the originally expected relationship of nearly constant average vertical forces for all of the loading trials. It may be said that if a much larger data set was gathered (>> than 10 seconds), then all of the averages may have looked very similar, but the rest of the data suggests that these ten second profiles were an accurate reflection of the steady state locomotion dynamics (see figures19 and 21). Subjects 1 and 2 preferred the short poles while subjects 5 and 6 preferred the medium poles. It may be true that the difference in these preferences had to do with differences between the natural frequencies of subjects 1,2 and 5,6. Physical differences in musculature may help explain effects on the persons natural frequency, but that is highly speculative at this point.
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In general, the stride length went down (cadence went up) as subjects moved from the backpack condition to the pole conditions. This was expected due to the even loading (anterior-posterior) and greater stability created by the lagged movement of the suspended weights (providing more constant vertical force to the shoulders). Interestingly, individual differences due not appear to be related to stature height, leg length, or flexibility. It is still very much open to debate whether muscular conditioning or certain strategic parameters played a significant role in the stride length differences. From table one, it may be seen that the comfortable walking speed for subject one is noticeably higher than those for subjects 2,5 and 6. Could this mean that subject one is accustomed to taking larger strides? It is hard to tell, kinematic analysis may have helped with speculation regarding this point. One interesting additional finding regarding stride length is that there might be a connection between body mass and stride length. Looking at page 27 you can see that there is roughly a seven percent difference between stride lengths of S1 and S6. There is also an eight percent difference between S1 and S6 in body mass (note that overall stature and leg length are nearly identical). Subjects 2 and 5 do not show such a clear relationship of course there are more confounding factors (different leg lengths, and possibly different mass composition). Does this imply that stride length corresponds closely with lean body mass? More research is needed to investigate this possibility.
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Weight acceptance rates generally increased as the stride length decreased when the subjects moved from backpack load carrying to pole carrying (figure 22). This is an expected result as it takes less time for the body to fully load the supporting leg.
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Push-off rates were much higher for subjects 2,5 and 6 than for subject 1. It is suspected that this may be due to greater gastrocnemius and soleus strength in S2, S5, and S6 per body mass, of course calf strength measurements were not taken further research should investigate this possibility.
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Regardless of the noticeable differences in running effort that were reported by subjects between long, medium, and short pole running all of the subjects preferred the shorter pole lengths in terms of energy expenditure and ability to control movement of the loads. Of course, subjects 1 and 2 apparently did not like the control characteristics of the medium poles so that implies that the short poles would be the ideal compliant pole length for people who decide that carrying loads with flexible poles suits the tasks to be performed. An additional detailed analysis of how to fit the poles to the subject should be conducted by examining the frequency characteristics of the poles, taking a closer look at anthropometric characteristics and continuing to use subjective assessment techniques. VO2 data would not be necessary for this new experiment. At this point, it appears that the timing of the forcing function (vertical movement of the person) and the magnitude of the suspended loads are critical for establishing the proper compliant pole length. The authors suspect that pole characteristics should be individual and task specific.
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General subjective comments and observations revealed that even when the long poles were deemed comfortable for carrying loads with regard to average force values they were not deemed easy to control with regard to the suspended load. Therefore, when designing/selecting compliant poles for load carrying, it is highly recommended that pole length be in the short to medium ranges shown here. If the pole length becomes to short, there is a risk of impact between the load and the person if horizontal perturbations occur. Also, the pole carrying method is especially recommended in situations where load levels are high, terrain elevation does not change rapidly (suspended loads would hit the ground) and the person will not be traveling down hallways or natural constrictions which prevent turning the person pole system around. Of course, lowering the loads and reversing the direction of the person could partially circumvent the hallway problem. Three other important factors to consider regarding the choice between carrying methods include:
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Fragility of the load (newborn baby, glass items, etc.) the backpack method provides less chance of loosing control of load position.
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Flexibility required (military soldier on patrol, pedestrian trying to navigate crowded streets, etc.) the small size and secure connection of the backpack to the body allows for rapid change of direction, orientation, speed and also protection if the person falls to the ground on their back.
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Conditioning subject 5 in this study was very much accustomed to running and cycling with a backpack, therefore she felt much more comfortable and efficient moving with a backpack load. This further emphasizes the task specific nature of load carrying techniques and the need to gradually be introduced or conditioned to use the compliant pole method if the functional aspects of the task encourage it.
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