How locomotor mechanics and energetics are affected by long-term alterations to external loading during growth remains largely unknown. Here we test the hypothesis that a reduction in mechanical limb work contributes to a lower cost of carrying externally-applied limb mass after chronic loading. To test this, we applied a mass equal to 3.75% body mass unilaterally to the lower limb of a group of guinea fowl continuously from 1-16 wks of age (EXP group; n = 6). We raised a second group of birds in the same conditions but with no external limb loading (CON group; n = 6). At 16 wks of age we measured the metabolic cost of walking (CoT) on a treadmill at 0.5 m/s in both unilaterally-loaded and non-loaded conditions. We used an inverse dynamic procedure to compute pelvic limb joint powers and work during the swing phase of walking from motion capture (100 Hz). The mass-specific CoT of unilateral limb loading was 34% greater in the CON group compared to the EXP group (p = 0.03). Surprisingly, the CoT in the habitual condition (limb loaded in EXP; unloaded in CON) was the same for both groups, despite the EXP group moving substantially more limb mass. Preliminary data indicate that animals subjected to increased limb loading across their growth period exhibit lower mass-specific mechanical joint work during limb swing. However, it is unlikely that mechanical work alone can explain the markedly lower cost of carrying external limb mass in the EXP group. We aim to further understand the determinants of the lower energy cost of carrying mass in EXP including the extent by which this is achieved by locomotor kinematic plasticity, adaptations to musculoskeletal morphology and/or physiology.