Chronic limb loading results in remarkable load carriage economy in growing fowl

Energy minimization is regarded to be a fundamental characteristic of legged locomotion, yet the influence of life history on the energy cost of locomotion has not been widely studied. We tested the hypothesis that growth-period limb loading minimizes the energy cost of walking in mature animals. We found that limb-loaded animals that chronically carried approximately 4% of their body mass on their distal right limb for 14 weeks (LL group) moved this mass with much less energy (22% less) than control animals (CON group) (p < 0.05). Furthermore, the majority of LL animals had a lower cost of walking with the limb load compared to walking without it. On average, LL animals did not use more energy walking with the limb load compared to CON animals walking unloaded. This remarkable limb-load carrying economy is, to our knowledge, the lowest reported for legged animal locomotion. Notably, we also found that CON and LL animals used the same amount of energy during unloaded walking, despite this being a novel condition for the LL group. These data suggest that energy minimization is critical not only across evolutionary and acute behavioural time frames, but also drives adaptation during the growth span.