Obesity is a condition of adipose mass excess, associated with increased mortality and many health problems; given the high prevalence and health burden, it is critical to improve its prevention and management. In a recent issue of The Journal of Physiology, Malatesta and colleagues (2022) investigated the impact of bariatric surgery and the subsequent large body mass loss on the energetics and mechanics of walking in obese participants, evaluated before and after surgery. Walking is the most common and practical means to promote physical activity and reduce sedentary behaviour. However, obese people have a higher absolute energy cost of walking – the energy above resting demands needed to walk for a unit distance (in J m−1) – than normal-weight people (for recent review see Peyré-Tartaruga et al. 2021). When energy cost is normalised to the unit of body mass (C; J kg−1 m−1), this difference persists, hinting that mass-independent factors are involved (e.g. gait pattern, muscle contractions at disadvantageous lengths or velocities; Malatesta et al. 2022). Such greater C leads obese people to increased effort during walking and favours sedentary behaviour. Knowing the causes of the greater C can bring insight into solutions that facilitate exercise in obese adults, leading to better health outcomes. In the work of Malatesta and colleagues (2022), nine obese participants underwent bariatric surgery, losing ∼26% of body mass after 6 months and ∼31% after 1 year, mostly from adipose tissue. The absolute cost of walking dropped by ∼25% after 6 months and ∼35% after 1 year. Differently, C was reduced only 1 year post-surgery, reaching values comparable to those of normal-weight people (Fig. 1A). A, cost of walking (J kg−1 m−1) vs. speed. Obese participants pre- (black circles and line) and 6 months post-surgery (blue triangles and line) had a higher cost than normal-weight people (black dashed line). Such cost lowered to values of normal-weight people 1-year post-surgery (red squares and line). Older adults (black dotted line), as obese participants, have a more energy-demanding gait (Mian et al. 2006). The descending dashed grey curves are iso-metabolic power lines, which show combinations of speed and cost demanding the same metabolic power (given by the product cost × speed). B, cost of locomotion (J kg−1 m−1) in different animal species (grey points) as a function of body mass; adapted from Biancardi et al. (2011). The lower the body mass, the higher the cost of locomotion across species; on a logarithmic scale, observations lie on a straight (grey) line with a negative slope. Black, red and blue symbols: minimum C of walking in obese participants (as in A), from Malatesta et al. (2022). C, percentage change in the minimum cost of walking (J kg−1 m−1) vs. percentage change in body weight. Plus signs represent data from participants carrying loads (Bastien et al. 2005), while crosses represent data in hypogravity (i.e. lower body weight) on Mars and Moon, at 0.38 and 0.17 g, respectively (from Pavei et al. (2015) in Peyré-Tartaruga et al. 2021). Changes in the minima of cost are related to changes in body weight with a quadratic equation. Six months post-bariatric surgery obese participants had a higher than predicted cost, while 1 year post-surgery the cost was close to the regression line, which may indicate that behaviour-related adaptations were effective. The direct benefits of this drop for cardiometabolic health and exercise capacity are straightforward. With a given walking distance, the lower the cost, the lower the energy expenditure; for a given speed, the lower the metabolic power (iso-power curves in Fig. 1A). As metabolic power is a limiting factor for exercise intensity and time-to-exhaustion and a determinant of perceived exertion, reducing the metabolic power needed to move increases the walking speed that can be maintained for a significant span of time or, alternatively, the timespan during which people can sustain walking and avoid sedentary time. This is crucial for obese people, who are at higher risk for cardiac and respiratory conditions which can severely reduce their maximum attainable metabolic power. Experiments on animals from different species, ranging from small arthropods to large mammals, show that across all the terrestrial species the lower the body mass, the higher the C (J kg−1 m−1) (Fig. 1B; Biancardi et al. 2011). In contrast, C diminished in obese participants when losing mass after bariatric surgery (black, blue and red symbols in Fig. 1A–C) (Malatesta et al. 2022). This latter trend has also been observed in other human models: when walking while carrying loads, C increases (Bastien et al. 2005); when gravity is reduced (i.e. lowered body weight), C decreases (for a recent review see Peyré-Tartaruga et al. 2021). In Fig. 1C, data from these studies have been pooled to obtain the relationship between the percentage changes in body weight and C. The lower the body weight, the lower C; the relationship is not directly proportional, as it approximately follows a quadratic equation. The large body weight loss post-surgery can be compared to the two above-mentioned models of load carrying and hypogravity by superimposing the data from Malatesta et al. in Fig. 1C. Six months post-surgery, participants lost ∼26% of their body weight, but their minimum C did not change, moving away from the predicted quadratic relationship. In contrast, 1 year post-surgery, the minimum C declined by ∼8%, with a ∼31% reduction in body weight from baseline, following the regression trend. In other words, once participants had enough time to re-optimise their walking pattern through behaviour-driven adaptations, the percentage change in the minimum C followed the quadratic relationship. The underlying causes have been nicely discussed by Malatesta and colleagues (2022). Studying the mechanics of locomotion can explain the metabolic demands of walking and the impact of mass variations. In locomotion, the work needed to raise and accelerate the body centre of mass (BCoM) is called external work (Wext). Additionally, work is needed to accelerate the limbs relative to the BCoM (internal kinematic work, Wint,k) and overcome the friction within and between the swinging limbs (internal frictional work, Wint,f). The total mechanical work (Wtot) is the sum of Wext and Wint (for a review see Peyré-Tartaruga et al. 2021). Six months and 1 year post-bariatric surgery, Wint,k declined by 9–10% (average across both sessions). Such reduction is likely due to the different gait kinematics post-surgery. In fact, as predicted in Minetti's model equation, Wint,k decreases when reducing the stride frequency and the fraction of the stride during which the foot is on the ground (Minetti, 1998). This is what was observed in people after bariatric surgery (Malatesta et al. 2022). The estimate of Wint,k also depends on the mass distribution within the limbs, which was separately addressed for each participant by Malatesta and colleagues (2022). Notably, also Wint,f could impact on Wtot in obese participants: its role and interplay with Wint,k still have to be quantified. The efficiency of locomotion describes how well metabolic energy is converted into mechanical work. Throughout the follow-up, Wtot remained constant while C decreased only 1 year post-surgery. Hence, the efficiency of locomotion increased (∼15%) only after 1 year; the causes may reside in behavioural adaptations, which need time to develop. Even if Wtot was similar at the three time points, obese participants post-surgery changed the strategy to generate it, turning their walking pattern into one more similar to normal-weight people. The authors found changes in hip, knee and ankle kinematics 6 months and 1 year post-surgery, indicating that such modifications were triggered by the body mass loss. These kinematic changes may have led muscles to contract at more favourable lengths and speeds or to diminish co-contraction. For instance, it is known that elderly people have higher C than their young counterparts (Mian et al. 2006; Fig. 1A), with similar Wtot; higher muscular co-contractions are thought to enhance joint stability and balance, at the price of higher C. By analogy, obese may start with high levels of co-contractions and then require several months post-surgery in order to reorganize their walking pattern into a less costly one. In conclusion, the study of walking in obese people represents a useful model to check and gain knowledge on the biomechanics and bioenergetics of locomotion. As shown by Malatesta and colleagues, mechanical and behavioural features of locomotion can dynamically change; hence, it is crucial to study their interplay in order to explain the energy demands of locomotion. After enough time for behavioural adaptations to occur, changes in metabolic cost with bariatric surgery-induced body mass loss are similar to those occurring when carrying loads and in hypogravity. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. The authors declare no competing interests. All the authors conceived and wrote the final version of the manuscript. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. The author received no specific funding for this work.