Biomechanical differences associated with two different load carriage systems and their relationship to economy

Purpose. To explore relationships between load carriage economy and the kinematics and kinetics of load carriage using both a backpack (BP) and a double pack (DP). Basic procedures. Nine participants walked on a treadmill at gradients of between 27% downhill and 20% uphill, and over a force plate on level ground, at a speed of 3 km.h-1. Expired air was collected throughout the treadmill experiment and all experiments were filmed for subsequent biomechanical analysis. The relative economy of load carriage was expressed in terms of the Extra Load Index (ELI). Main findings. There was a tendency for the double pack system to be associated with better economy than the BP. The double pack system provoked significantly less forward lean than the backpack and the horizontal displacement of the CoM was also smaller for the double pack system and both of these factors were strongly related to economy. There was, however, a greater range of motion of the trunk in the DP condition and this was also associated with improved economy. Conclusions. The results suggest that the DP was associated with smaller perturbations in gait than the BP and that this represents an advantage in terms of economy. In particular freedom of movement of the trunk in the sagittal plane may be an important consideration in the efficiency of load carriage systems

Subjective perceptions of load carriage on the head and back in Xhosa women

The purpose of this study was to compare the subjective perceptual responses to both head-loading and back-loading in a group of Xhosa women. Thirty two women were divided into three groups based on their experience of head-loading and walked on a treadmill on two occasions, head-loading and back-loading, at a self selected walking speed for four minutes with a variety of loads until pain or discomfort caused the test to be terminated or a load of 70% body mass was successfully carried. After each workload there was a one minute rest period during which the women indicated feelings of pain or discomfort in particular areas of the body via visual analogue scales. At the end of each test the women were asked to complete further questionnaires relating to pain and discomfort and on completion of the second test were also asked to compare the two loading conditions. Finally the women were interviewed to establish their history of load carriage and associated pain and discomfort. The data indicate that whilst back-loading was generally associated with more areas of discomfort than head-loading, the pain and discomfort in the neck associated with head-loading was the predominant factor in the termination of tests and that this was independent of head-loading experience. This early termination meant that, on average, the women could carry greater loads on their backs than on their heads. The study suggests that further work needs to be carried out to establish viable alternatives to head-loading for rural dwellers in Africa

No 'free ride' for African women:a comparison of head-loading versus back-loading among Xhosa women

Although contrasting evidence exists in the literature as to the economy of head-loading, there is a notion that head-loading is an extremely economical method of load carriage. This has become known as the ‘free ride’ hypothesis and, although untested, it is widely accepted. The purpose of this study was to test the ‘free ride’ hypothesis for head-load carriage among African women by comparing the relative economy of head-loading and back-loading. Twenty-four Xhosa women walked on a level treadmill, attempting to carry loads of between 10% and 70% of their body mass (BM) using both a backpack and a head basket. All 24 women carried at least 25% of their BM in both conditions. The relative economy of load carriage was calculated for loads of 10% to 25% of BM. Results indicated that the ‘free ride’ was not a generalisable phenomenon, with both methods realising economy scores close to unity (1.04 ± 0.19 and 0.97 ± 0.15 for head-loading and back-loading, respectively). The results did, however, reveal significant individual differences in economy scores and it is suggested that analysis of such individual differences in future may well be instructive in understanding mechanisms associated with greater economy in load carriage

A kinetic comparison of back-loading and head-loading in Xhosa women

The purpose of this study was to compare the kinetic responses associated with ground reaction force measurements to both head-loading and back-loading in a group of Xhosa women. Altogether, 16 women were divided into two groups based on their experience of head-loading. They walked over a force plate in three conditions: unloaded or carrying 20 kg in either a backpack or on their head. The most striking finding was that there was no difference in kinetic response to head-loading as a consequence of previous experience. Considering the differences between the load carriage methods, most changes were consistent with increasing load. Head-loading was, however, associated with a shorter contact time, smaller thrust maximum and greater vertical force minimum than back-loading. Both loading conditions differed from unloaded walking for a number of temporal variables associated with the ground contact phase, e.g. vertical impact peak was delayed whilst vertical thrust maximum occurred earlier. Statement of Relevance: Consideration of the kinetics of head and back load carriage in African women is important from a health and safety perspective, providing an understanding of the mechanical adaptations associated with both forms of load carriage for a group of people for whom such load carriage is a daily necessity

BIOMECHANICAL SIMILARITIES OF LONGSWINGS AT VARYING SPEEDS AND THE KOVACS ACTION

INTRODUCTION: Low movement variability in the backward longswing has been partly attributed to external competitive constraints (Irwin & Kerwin, 2005). However, a number of high level skills (e.g. Gienger, Deff, Kovacs) derive from variations in the longswing action. If movement variability is viewed as having a functional role (Bartlett et al., 2007) then variability in the longswing might be beneficial to subsequent skill acquisition. This study aims to determine whether variations in the speed of longswing can induce movements thatare more similar to movements associated with a high level skill, in this case the Kovacs

The effects of postactivation potentiation on sprint and jump performance of male academy soccer players.

The purpose of this investigation was to evaluate the postactivation potentiation (PAP) effects of both dynamic and isometric maximum voluntary contractions (MVCs) on sprint and jump performance and establish whether PAP methods could be used effectively in warm up protocols for soccer players. Twelve male soccer players performed 4 warm up protocols in a cross-over, randomized, and counterbalanced design. In addition to a control warm up, subjects performed deadlift (5 repetitions at 5 repetitions maximum), tuck jump (5 repetitions), and isometric MVC knee extensions (3 repetitions for 3 s) as PAP treatments in an otherwise identical warm up protocol. After each treatment, the subjects underwent 3 10 m and 20 m sprints 4, 5, and 6 minutes post-warm up and 3 vertical jumps (VJ) at 7, 8, and 9 minutes post-warm up. Repeated measures analysis of variance showed no significant differences in the first 10 m (p = 0.258) and 20 m (p = 0.253) sprint and VJ (p = 0.703) performance and the average 10 m (p = 0.215), 20 m (p = 0.388), and VJ (p = 0.529) performance between conditions. There were also no significant differences in performance responses between the strongest and weakest subjects, but large variations in individual responses were found between the subjects. The findings suggest that there was no significant group PAP effect on sprint and jump performance after dynamic and isometric MVCs compared with a control warm up protocol. However, the large variation in individual responses (-7.1% to +8.2%) suggests PAP should be considered on an individual basis. Factors such as method, volume, load, recovery, and interindividual variability of PAP must be considered in the practical application of PAP and the rigorous research design of future studies to evaluate the potential for performance enhancement

A comparison of the physiological consequences of head-loading and back-loading for African and European women

The aim is to quantify the physiological cost of head-load carriage and to examine the ‘free ride’ hypothesis for head-load carriage in groups of women differing in their experience of head-loading. Twenty-four Xhosa women [13 experienced head-loaders (EXP), 11 with no experience of head-loading (NON)] attempted to carry loads of up to 70% of body mass on both their heads and backs whilst walking on a treadmill at a self-selected walking speed. Expired air was collected throughout. In a second study nine women, members of the British Territorial Army, carried similar loads, again at a self-selected speed. Maximum load carried was greater for the back than the head (54.7 ± 15.1 vs. 40.8 ± 13.2% BM, P <0.0005). Considering study one, head-loading required a greater oxygen rate than back-loading (10.1 ± 2.6 vs. 8.8 ± 2.3 ml kg bodymass−1 min−1, P = 0.043, for loads 10–25% BM) regardless of previous head-loading experience (P = 0.333). Percentage changes in oxygen consumption associated with head-loading were greater than the proportional load added in both studies but were smaller than the added load for the lighter loads carried on the back in study 1. All other physiological variables were consistent with changes in oxygen consumption. The data provides no support for the ‘free ride’ hypothesis for head-loading although there is some evidence of energy saving mechanisms for back-loading at low speed/load combinations. Investigating the large individual variation in response may help in identifying combinations of factors that contribute to improved economy

The Extra Load Index as a method for comparing the relative economy of load carriage systems

The Extra Load Index (ELI) has been proposed as a suitable method of assessing the relative economy of load carriage systems. The purpose of this study was to determine, based on empirical evidence, that the ELI can accommodate variations in both body composition and added load. In total, 30 women walked carrying loads of up to 70% body mass at self-selected walking speeds whilst expired air was collected. In addition, each of the women had body composition assessed via dual energy X-ray absorptiometry. Results show that the ELI is independent of body composition variables, the magnitude of additional loads and the speed of progression. Consequently, it is suggested that it represents an appropriate method of comparing load carriage systems in both scientific and commercial arenas. Statement of Relevance:This paper demonstrates that ELI is independent of body composition, added load and speed and is therefore an appropriate method to generalise comparisons of load carriage systems. It has the advantage of being easily understood by manufacturers and consumers whilst retaining appropriate scientific precision

PERFORMANCE RELATED TECHNIQUE FACTORS IN OLYMPIC SPRINT KAYAKING

A sprint kayaking specific deterministic model was used to identify key performance related technique factors using data from 12 international-level kayakers. There was large variability in the strength of the between-factor relationships across the group. The pull phase was split into 3 components with the 1st phase contributing the most to increases in boat velocity and the 3rd phase causing a decrease in velocity. The propulsive impulse had the largest influence on velocity, but the magnitude of the impact was moderated by blade slip. Large propulsive impulses in the 3rd phase of the pull were associated with larger decreases in velocity. The results show that the model can be used to identify key technique factors on an individual level, although the use of the model should be confirmed on additional kayakers before being used in an applied setting by practitioners

INSTEP SOCCER KICK TECHNIQUE AND PERFORMANCE THROUGH AN ANALYSIS OF MOVEMENT VARlABILlN

The purpose of this study was to investigate the structure of movement variability (MV) for hit and missed instep kicks using the uncontrolled manifold (UCM) analysis. University soccer players (n=1 1 ) performed a maximal instep kick at a target 11 metres (m) away. Hit and missed kicks were analysed using the UCM method. Dependent measures for UCM analysis were UCM variability measures and strength of synergy across six performance variables. Clear differences were noted between hit and missed kicks based on the UCM analysis. The UCM analysis provided a powerful tool for showing clear differences between hit and missed kicks and suggest that accurate kicking performance could be dependent on the structure of MV within the human motor system