Falls in older adults cause injuries and deaths, restricted independence, fear of falling, and economic burdens (Burns and Kakara, 2018, Hartholt et al., 2019). Young- and middle-aged adults are not immune from falls (Kool, 2012, Verma et al., 2016). It is pressing to reduce the fall risk among various age segments to mitigate falls’ negative impact.

One essential prerequisite of fall prevention is to identify and modify fall risk factors (Morsch et al., 2016). Previous research recognized footwear as a fall risk factor. It was suggested that footwear has detrimental effects on the body’s balance and elevates the fall risk compared to barefoot conditions (Aboutorabi et al., 2016, Tencer et al., 2004). Other studies found that adults are less stable while shod compared to unshod during static and dynamic balance tests (Brenton-Rule et al., 2011, Lord, 1996).

Among the shoes’ mechanical characteristics, such as heel height, collar height, and sole flaring, the sole is a major contributor to falls, given that the sole adds a layer of impedance between the foot and the supporting surface and obstructs their direct interactions (Aboutorabi et al., 2016). The sole’s interference could reduce the accuracy of the somatosensory afferent feedback of the environment from the plantar cutaneous sensation to the central nervous system, which impairs body balance and elevates fall risk (Romer et al., 2019). To avoid a fall after a postural perturbation, prompt perturbation detection and subsequent quick reactive movements should be implemented (Marigold and Patla, 2002) as a fall could occur rapidly (Qiao and Yang, 2020). The sole’s impedance in detecting a balance loss could lead to a failure to regain balance and consequently raise the falling probability.

Limited studies examined whether the sole affects the human body’s balance control. It was documented that a thick sole (27 mm at the heel and 16 mm at the metatarsophalangeal (MP) joint) increased the balance loss frequency and reduced the body’s stability during walking compared to a thin sole (13 mm at the heel and 6.5 mm at the MP joint) among older adults (Robbins et al., 1994). Another study measured the alterations in joint position awareness between different sole conditions (Sekizawa et al., 2001). Participants with the thick sole exhibited a higher error in estimating the ankle joint angle than the barefoot condition, which confirms the weakened proprioception with the sole.

However, those previous studies had limitations. First, the potential impacts from footwear characteristics besides the sole were not eliminated, which could contaminate the results. As aforementioned, each footwear property influences the body’s balance differently (Aboutorabi et al., 2016). The footwear type used in previous studies included athletic shoes (Brenton-Rule et al., 2011), standard low- and high-heeled shoes (Lord, 1996), minimalist style shoes (Romer et al., 2019), experimentally designed running shoes (Robbins et al., 1994), and standard commercial shoes (Sekizawa et al., 2001). Therefore, it would be nearly impossible to pinpoint the effects of the sole on the fall risk.

Second, prior studies primarily focused on footwear’s impact on balance control during volitional motor tasks, such as standing or walking (Brenton-Rule et al., 2011, Robbins et al., 1994) without external perturbations, although up to 59 % of falls are induced by external perturbations (slips/trips) (Simpkins and Yang, 2022, Simpkins et al., 2022). Thus, a novel study is needed to target the sole’s effects on fall risk using a standardized and valid external perturbation. The ActiveStep treadmill (Simbex, NH) is a specialized platform to generate precisely controlled postural perturbations. This treadmill has been broadly adopted to produce slips during various movements such as walking (Pigman et al., 2019, Yang et al., 2022) and standing (Akinlosotu et al., 2020, Simpkins et al., 2022, Simpkins and Yang, 2022, Yang et al., 2018). Balance assessment based on standardized slips would allow for a unique way to investigate the sole’s effects on human body stability control in a well-controlled condition.

This pilot study examined how the shoe sole affects the human body’s balance control following a standing-slip among young adults. Specifically, we focused on the direct effects of the sole, as a layer of impedance between barefoot and the ground, on the body’s stability without contamination from other shoe mechanical characteristics. Dynamic stability ( HYPERLINK "SPS:refid::app1" Appendix) and leg muscle activations were analyzed to investigate the body’s reactions to a standing-slip. Spatiotemporal parameters, including the slip distance and recovery step length/latency, were also investigated. We hypothesized that, relative to the barefoot condition, young adults with soles would 1) be more unstable and 2) exhibit longer EMG latency responding to the standing-slip. The findings would enrich our understanding of the shoe sole’s influence on the body’s balance control.