The prevalence of overweight in adults of OECD countries was approximately 60 % in 2018 (Organisation for Economic Co-operation and Development, 2019). Excess body fat has been reported to be linked with an increased risk of musculoskeletal injuries or chronic diseases (Visser, 2011, Walsh et al., 2018), which could result in a lack of activity and lead to a vicious cycle of weight gain (Pietiläinen et al., 2008). Such a worrying trend has become an economic burden as the overweight population has doubled worldwide over the past 30 years.

Studies have reported that individuals with a high body mass index (BMI) (>25 kg/m2) exhibited different muscle activations or force productions during gait when compared to non-overweight individuals. Greater hip abductor forces were reported during walking in obese individuals (Lerner et al., 2014b), which could perturb sideway stability during gait (Arvin et al., 2016, Hahn et al., 2005, Kim and Chou, 2022). Obese women were also reported to exhibit a greater postural sway while standing compared to non-obese controls, suggesting that obesity may be a potential risk factor for loss of balance (Dutil et al., 2013). Hence, changes in muscle activations (force or timing) due to being overweight can lead to abnormal joint loading that can be attributed to joint disease or gait imbalance (Lewek et al., 2004, Zajac et al., 2003). Therefore, identifying differences in muscle activations between overweight and non-overweight individuals during walking may enhance our understanding of how body size is linked to mobility impairment.

One way to evaluate the contribution of individual muscle forces, including both intrinsic and extrinsic muscles, to the whole-body center of mass (COM) motion during walking is by applying the induced acceleration analysis with musculoskeletal simulation (Anderson and Pandy, 2003). The COM position mechanically represents the centroid of the whole-body during movement, and any changes to its three-dimensional trajectory could indicate balance responses to gait perturbations (Chou et al., 2001). Recent studies quantified the contribution of individual muscles to the COM accelerations in the vertical, anteroposterior, and mediolateral directions during level walking (Pandy et al., 2010), stair negotiation (Lin et al., 2015), pathological gait (Steele et al., 2010), or balance-perturbed stepping (Graham et al., 2017). While one study (Graham et al., 2017) revealed a similar muscular control in regulating the whole-body COM motion across age groups, it is yet to know whether the body size affects muscle activations that are required to maintain gait balance. We, therefore, performed a preliminary study (Kim and Chou, 2021) to demonstrate the feasibility and found that overweight individuals exhibited greater gastrocnemius contributions to the mediolateral COM accelerations during walking. However, this study was limited by the small sample size and did not examine gait kinetics and kinematics. Therefore, a comprehensive investigation with an adequate sample size is needed to provide a robust understanding of muscle coordination strategies during walking in individuals with different body sizes. Such knowledge will provide insights into how overweight individuals recruit lower extremity muscles to modulate whole-body COM during walking.

Therefore, the aim of this study was to investigate the contribution of lower extremity muscle forces to the whole-body COM accelerations during walking in overweight and non-overweight adults. It was hypothesized that 1) body size would affect the COM motion during walking, and the frontal plane COM motion would be greater in the overweight group compared to the non-overweight group; and 2) the overweight group would exhibit greater muscular contribution to COM acceleration while walking compared to the non-overweight group.