Effect of body lean on stopping a voluntary forward sway

Introduction: The ability to inhibit a planned action or stop an ongoing one is fundamental to postural control and fall prevention. Motor inhibition has been studied mostly in upper limb reaching and aiming tasks (Hannah & Aron, 2021). Fewer studies investigating stopping in whole-body postural actions such as a balance recovery step (Bolton & Richardson, 2022; Rowley et al., 2022) or voluntary step initiation (Fiori et al., 2025; Kwag et al., 2024) have shown that the latency of the stop relative to the go signal critically determines whether the action would be inhibited (no movement at all) or suppressed at a later stage (stop an ongoing movement).In the present study, we asked whether the ability to stop a voluntary forward sway depends on the degree of body incline at the time of the stop, expressed in center of pressure (CoP) distance away from the neutral stance position. The stop command was introduced at two positions; at 30 % (short) and 45 % (long) of the maximum individual CoP leaning distance. It was hypothesized that success rate, time and distance to stop depends on the CoP position relative to the foot at the time of the stop command. These predictions were tested under two task conditions: a) a single (discrete) forward voluntary sway; b) continuous (periodic) voluntary sway.Methods: The study was jointly conducted at King’s College London in the Biomechanics Laboratory (ethics MRA-24/25-48273) and Aristotle University of Thessaloniki in the Motor Performance and Adaptive Physical Activity Laboratory (ethics 306/2025). Twenty-four (24) healthy young adults (14 males, 10 females; mean ± SD age: 30.0 ± 9.80 years; height: 1.728 ± 0.82 m; body mass: 66.8 ± 13.03 kg; foot length: 0.256 ± 0.017 m) stood barefooted on a force platform facing a monitor (1.5 m) displaying a GO (green target) and END (red target) signal. CoP data were sampled (Vicon Nexus, version 2.12, Vicon Motion Systems Ltd., Oxford, UK) and the CoP signal was used in real time to display the targets and trigger the events using specialized software written in LabView. At the onset of the GO signal, participants performed 2 sets of 50 forward sway trials until the CoP reached a target corresponding to 60 % of maximum leaning CoP displacement. At random trials (28 %), the GO target disappeared signaling a STOP, a command to halt forward sway and maintain this posture for 3 s. CoP data were low-pass filtered (6 Hz cut-off) and further processed in MATLAB. A STOP was considered successful if CoP velocity decreased and maintained at baseline level (± 10 % of maximum) for at least 1000 ms after the STOP command. In this case, the distance traveled by CoP (Stop Distance, SD) and the time to the CoP velocity reaching the baseline level (stabilization time, ST) were calculated. Outcome measures were compared between the two STOP positions (30 % and 45 %) and two sway tasks (discrete vs. continuous) using Generalized Linear Mixed Models (GLMM).Results: Stopping at a long (45 %) stop position (successful stops: 29.6 %) was less successful (χ²(1) = 4.51, p = .034) compared to stopping at a short (30 %) stop position (successful stops: 36.1 %). Random intercept estimates indicated substantial inter-individual variability in baseline stopping success (ranging between 3.5 % and 95.7 %). Successful stopping was accompanied by a shorter CoP travel distance at the long stop position (45 %: 94.34 ± 4.59 mm) compared to stopping at the short (30 %: 105.5 ± 4.56 mm) stop position (χ²(1) = 25.35, p <.001). Stabilization time (45 % position: 1.664 ± 0.083 ms; 30 % position: 1.706 ± 0.082 ms) was not different between the two stop positions. In addition, success rate did not differ between the two tasks (continuous: 32 %; discrete: 33 %), but stopping required less time (continuous: 1.635 ± 0.083 ms; discrete: 1.734 ± 0.083 ms; χ²(1) = 3.81, p=0.05), and a shorter travel distance (continuous: 96.78 ± 4.57 mm; discrete: 103.3 ± 4.59; χ²(1) = 8.086, p=0.004) in the continuous than the discrete sway task.Conclusions: Results confirm that the ability to stop a voluntary forward sway action does not only depend on the time of the stop but also on the position of the CoP away from the neutral stance; the further the distance and presumably the greater the degree of body lean, the more difficult is to stop. Moreover, successful stopping is accomplished in a shorter time and by a smaller CoP distance when the stop command is introduced further away from the neutral stance position, at 45% compared to 30% of maximum CoP leaning position. These findings agree with literature evidence stressing the importance of the latency of the stop relative to the go command during a balance recovery step (Rowley et al., 2022) and step initiation (Fiori et al., 2025). In addition, they support the idea that successful stopping is also determined by biomechanical constraints defined by the center of pressure location relative to the limits of the base of support.Acknowledgements: The study received funding from The Royal Society (ref: IES\R2\242223 - International Exchanges 2024).