Dynamic joint stiffness, also referred to as quasi-stiffness (Davis and DeLuca, 1996), quantifies the simultaneous changes in intersegmental joint angle and moment throughout the progression of a task. During human movement, quasi-stiffness differs from passive joint stiffness in that quasi-stiffness includes the effect of active muscle force generation, which can generate positive or negative work, in addition to passive soft tissue energy storage and return (Rouse et al., 2013).

Sagittal-plane ankle quasi-stiffness has been used to develop control strategies for powered ankle prostheses, exoskeletons and orthotic devices to provide assistance during various phases of the gait cycle (e.g., Au and Herr, 2008, Caputo and Collins, 2013, Rouse et al., 2014). These devices aim to improve various aspects of walking performance such as walking speed as well as reduce the metabolic cost of walking and/or compensations needed from the unaffected limb (e.g., Hedrick et al., 2019, Herr and Grabowski, 2011). However, improving balance control and fall risk are rarely considered when developing quasi-stiffness-based controllers. Although current assistive devices primarily focus on emulating sagittal-plane dynamics, frontal-plane quasi-stiffness remains largely unexplored but has important implications given that maintaining frontal-plane balance requires more active control than the sagittal plane (Bauby and Kuo, 2000).

Clinical populations, such as those with lower-limb prostheses or neuromuscular impairments, have poorer frontal-plane balance control compared to healthy adults (e.g., Nolasco et al., 2021, Pickle et al., 2014, Silverman and Neptune, 2011) and are at an increased risk of falling. These populations also often walk with wider step widths (Hof et al., 2007, Kurz et al., 2012, Roerdink et al., 2007), which are associated with poorer balance control in healthy young (Molina et al., 2023) and older (Vistamehr and Neptune, 2021) adults. At wider step widths, a larger moment arm between the body center-of-mass (COM) and center-of-pressure (COP) leads to a greater destabilizing moment due to gravity (MacKinnon and Winter, 1993). Thus, walking with wider steps likely requires greater neuromuscular control to maintain balance and prevent a fall.

Commonly used strategies for maintaining balance during walking include altered foot placement, lateral ankle, ankle push-off and hip strategies (Reimann et al., 2018). Consistent with these strategies, the ankle plantarflexors and hip abductors are primary contributors to mediolateral COM acceleration (Pandy et al., 2010) and frontal-plane whole-body angular momentum (Neptune and McGowan, 2016), which is a mechanics-based measure of dynamic balance that is correlated with clinical balance measures (Nott et al., 2014, Vistamehr et al., 2016). Previous studies investigating balance control found that healthy young adults rely more on a lateral ankle strategy than a hip strategy to maintain balance even at wider step widths (Molina et al., 2023) whereas older adults rely more on a hip strategy (Vistamehr and Neptune, 2021). These studies evaluated joint moments to determine which balance control strategies were used to counteract the increasing destabilizing moment about the body COM at wider step widths. However, how quasi-stiffness at the hip and ankle joints change with step width to maintain balance is more complex as it is influenced by both the joint moment and angle.

The objective of this study was to characterize frontal-plane hip and ankle quasi-stiffness in healthy young adults as well as determine how quasi-stiffness in both the sagittal and frontal planes varies across a range of step widths. We hypothesized that when healthy young adults walk at wider step widths, quasi-stiffness would be primarily modulated at the ankle joint to maintain balance. We expected that ankle quasi-stiffness in both the sagittal and frontal planes would increase at wider step widths and that hip quasi-stiffness would not change in either plane at wider step widths. Understanding how hip and ankle quasi-stiffness relate to specific balance control strategies would provide important insight into the concurrent joint moment and angle responses as well as provide the foundation for future studies seeking to identify differences between quasi-stiffness in healthy individuals and clinical populations. In addition, the characterization of quasi-stiffness in response to altered step widths would help inform the development of biomimetic controllers for assistive devices to improve balance control and decrease fall risk.