Since antiquity, human locomotion has allowed individuals to meet others, to find better food and to survive a fight-or-flight situation. As pointed out by Rodolfo Margaria, ‘The study of human locomotion first requires the determination of the energy cost of this exercise and secondly a detailed analysis of the mechanical work performed’. Indeed, to characterize a given type of locomotion, the cost of transport (i.e. the metabolic energy expended to cover a unit distance) and the total mechanical work performed at the whole-body level must be computed. The latter can be considered as the sum of two components: external work (the work done to raise and accelerate the body centre of mass within the environment) and internal work (the work associated with the acceleration of the body segments with respect to the centre of mass) (e.g. Cavagna & Kaneko, 1977). A third parameter, locomotion efficiency, can then be computed and used to describe the interaction between energy expended and work produced. As suggested by Alexander (1991), measurement of locomotion efficiency can help in understanding whether mechanical work is ‘recycled’ via storage and release of elastic energy, providing important information on the locomotor capability of a subject. Indeed, locomotion efficiency (i.e. work done as a proportion of metabolic cost) approximates muscle efficiency values (∼0.25) in the forms of locomotion where elastic recoil is negligible (e.g. cycling; Minetti, Pinkerton, & Zamparo, 2001 or swimming; Zamparo et al., 2002) but can reach far larger values in walking and running (e.g. up to 0.4 and 0.7, respectively; Cavagna & Kaneko, 1977). In a pathological gait, locomotion (walking) efficiency could thus be impaired when elastic recoil is compromised. In this issue of Experimental Physiology, Oliveira et al. (2020) try to understand how the larger body weight in obese children could affect their mechanics and energetics when walking at different speeds, in comparison to healthy children of the same age. Total mechanical work, energy cost and locomotion efficiency were determined using the research approach summarized above, based on whole-body kinematics and by using gas analysis. Advancements in the field often create more questions than answers, and the study of Oliveira et al. (2020) is no exception. In fact, they observed that mechanical (internal and external work) and physiological (energy cost and locomotor efficiency) variables showed no difference between healthy and obese children but that the optimal walking speed (i.e. the speed that minimized the energy cost of walking) was slower in obese children. As discussed by the authors, the differences in the optimal walking speed could be explained by other physiological and/or psychological factors (such as the higher fatigability of obese subjects), and the higher body mass in obese children is, probably, not sizeable enough to alter the pendulum-like patter of human walking. This hypothesis is reinforced by the study by Maloiy, Heglund, Prager, Cavagna, and Taylor (1986) that investigated the effects of an added mass on the energy cost of walking. Studying the women of the Luo tribe (situated in East Africa), who are able to carry loads equivalent to 70% of their body mass balanced on the top of their heads, the authors showed that these women could carry loads up to ∼20% of their body weight without increasing their energy expenditure. Taken together, these results suggest the presence of a ‘breaking point’ in fat mass (in obese people) below which no negative effects on locomotor capability could be observed. The study by Oliveira et al. (2020) thus provides new insights into the biomechanics and energetics of walking in obese children, and future studies should focus on the mechanisms underpinning the lack of differences in the physiomechanical responses compared with healthy children and on how these parameters evolve as a function of age. Useful tools to investigate the ‘breaking point’ between normal and abnormal walking capacity in obese people could involve the assessment of joint load, joint stress and muscle–tendon behaviour (e.g. tendon elasticity). These analyses could provide important information about the musculoskeletal capacity to generate and absorb mechanical work during locomotion (in obese children and adults), thus providing important information for doctors and physiotherapists. Thus, as we look to the future, there is lots of work to do! The study by Oliveira et al. (2020) represents a pretty good starting point. None declared.
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Oct 8, 2024
Sieglinde Bogaert + 2
Open Access
