Human movement control is shaped by competing objectives, among which minimizing energy expenditure plays a central role, particularly in determining preferred walking patterns. Whether energetic cost similarly influences standing balance remains unclear because it has not been systematically quantified across a range of natural postures. Importantly, standing is the resting state from which most walking begins, suggesting that the optimization of posture may also reflect the energetic demands of initiating gait. In this study, we use a combination of indirect calorimetry and musculoskeletal simulations to characterize the energetic cost of standing and gait initiation across natural standing postures and investigate whether humans optimize energy expenditure under these conditions. In Experiment 1 (N = 13), we measured metabolic cost at preferred and six different prescribed whole-body lean angles. Energy expenditure was lowest at a slight anterior lean (1.15°) and increased monotonically with whole-body lean angle in either direction, rising twice as fast posteriorly compared to anteriorly. This asymmetry challenges the common modeling simplification that effort is symmetric and linear or quadratic with lean angle. Furthermore, participants preferred body angles (1.50 ± 0.73°) with similar energy expenditure to the minimum-cost lean but with significantly more postural variability, suggesting that strict postural regulation was not necessary for minimizing energetic cost. In Experiment 2 (N = 20), participants initiated forward and backward walking from preferred or prescribed lean angles. Participants did not alter their standing posture before expected gait initiations in the forward or backward direction, consistent with musculoskeletal simulations showing that leaning further in the anticipated direction did not significantly improve gait initiation time or energetic costs. Together, these findings suggest that postural strategies optimize energy efficiency when permitted by the demands of movement readiness. Our study quantifies the energetic cost landscape that governs human postural control, challenges widely used symmetric estimations of this cost, and offers an empirical foundation for developing more accurate simulations of posture and energy expenditure ... Read more

During prolonged standing, humans commonly shift their weight and spontaneously alternate between symmetric and highly asymmetric standing, bearing most of their body weight on one leg. Here, we tested the hypothesis that humans adopt asymmetric weight-bearing to reduce metabolic cost by exploiting the sublinear scaling of muscle activation maintenance costs predicted by metabolic models. We measured energy expenditure using indirect calorimetry while participants (n ¼ 20) completed three randomized standing conditions: 1) static symmetric standing, 2) symmetric standing with periodic lateral weight shifts, and 3) asymmetric standing with periodic switching of the supporting leg. Weight shifting every 30 s from an otherwise symmetric stance increased overall energy expenditure by 5.3% [95% CI: 2.4%, 8.3%] relative to static symmetric standing. Maintaining an asymmetric posture while shifting to the other leg every 30 s increased energy expenditure by a further 7.0% [3.5%, 10.4%], contrary to our hypothesis that asymmetry would reduce metabolic cost. We additionally measured postural variability and plantarflexor muscle activity to explore potential mechanisms underlying energetic differences. Asymmetric weight-bearing increased postural variability relative to symmetric standing, with the average mediolateral upper-body velocity increasing by 118% [90%, 147%], suggesting that an increased stabilization effort contributed to the cost. Muscle activity in the weight-bearing leg strongly increased for the medial gastrocnemius, whereas soleus activity increased less than predicted. These findings demonstrate that weight shifting and asymmetric weight-bearing, two common standing behaviors, may not reflect behavioral energetic optimization and instead originate from nonenergetic objectives such as alleviating discomfort or fatigue. NEW & NOTEWORTHY Natural human posture is often highly asymmetric, similar to the relaxed stance of Michelangelo’s David, and is characterized by frequent weight shifts between limbs. We experimentally tested whether such postures reduce the energetic cost of standing. Contrary to our hypothesis, participants expended more energy when standing in asymmetric postures and when shifting their weight periodically during symmetric standing, suggesting that these common postural behaviors come at an energetic expense. ... Read more

Traditional wheelchairs are pushed from behind the occupant, which hinders eye contact and communication. It was proposed that the wheelchair be pushed from the side using a push bar to place the caregiver beside the occupant. However, this method requires the caregiver to exert continuous effort to maintain a straight trajectory because the force at the lateral push location generates a moment. This study explores simple modifications to the front castor wheel of the wheelchair that allow pushing it from the side without additional effort. We used a three-dimensional dynamic model of the castor wheel to predict the effects of altering its dimensions and rake, cant, and bank angle and present a simplified steady-state solution. Experimental results support the model’s predictions, and a proof-of-concept experiment with a wheelchair showed that it is possible to push a wheelchair from the side without increasing forces or moments. The results also confirmed that the lateral ground reaction force generated on the castor wheel is proportional to the normal force and the cant angle, which can counter the moment caused by the lateral push location. The implications of this model extend beyond wheelchair design and can be applied to other applications that use castor wheels, such as robotics, trolleys, and walkers. ... Read more