Energetic cost of weight shifting and asymmetric weight-bearing during standing in humans

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 = 20) completed three randomized standing conditions: (i) static symmetric standing, (ii) symmetric standing with periodic lateral weight shifts, and (iii) 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, while 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 non-energetic objectives such as alleviating discomfort or fatigue.