This study aimed to investigate the interactions between anticipation and direction of surface translations applied during standing on distal leg muscle excitations measured via surface electromyography in the context of resultant changes in CoM displacements. Taken together, our results provide compelling support for our hypothesis. For anterior perturbations, the TA acts as the primary agonist, whereas the MG and SOL take on this role for posterior perturbations. Accordingly, the TA showed greater EMG following anterior surface translations while the MG and SOL showed greater EMG following posterior perturbations. Moreover, our results support the interpretation that anticipated balance challenges elicited greater proactive EMG, evidenced by greater MG, SOL, and TA iEMG pre-perturbation. Simultaneously, these neuromechanical adjustments, particularly for anterior surface translations that would precipitate a backward fall, appeared protective and effective based on resultant patterns of CoM displacements. Specifically, anticipation led to increased CoM displacements pre-perturbation, reflecting anticipatory postural adjustments. Anticipation of anterior translations was accompanied by larger CoM displacements early post-perturbation, likely playing a protective role in preventing a backward fall. We also found one example that supported our hypothesis that unanticipated perturbations would elicit greater reactive EMG; specifically, during the late-post perturbation period, unanticipated anterior translations elicited greater TA iEMG and larger CoM displacements than when anticipated. Though not something that generalized to posterior translations, those results imply that additional corrective strategies such as RPAs may be necessary to mitigate instability. These data can now be used as a reference for understanding how aging and disease impact proactive and reactive postural control, especially for populations who may have difficulty with both the planning and execution of corrective neuromuscular adjustments.

In agreement with existing literature and our hypothesis, participants employed proactive control strategies to maintain their balance in preparation for anticipated perturbations via increased agonist activation (Chambers and Cham 2007). Regardless of direction, anticipation led to higher MG, SOL, and TA proactive activation in comparison to the unanticipated condition, suggesting that the nervous system increased muscle activity before the perturbation occurred. For the riskier anterior surface translations, those changes precipitated or were at least accompanied by protective anterior displacements of the body’s CoM– an opportunity to mitigate the pending and anticipated posterior displacements. Higher activity likely reflects a strategy of antagonist coactivation, which serves to stiffen the body in preparation for pending perturbations (Milner et al. 1995; Finley et al. 2012; Thompson et al. 2018). By engaging both agonist and antagonist muscles, participants can better stabilize their posture, a tactic that may be especially relevant for individuals who are more fearful of instability. Such proactive adjustments can reduce the necessity for reactive adjustments (Marigold and Patla 2002; Pavol et al. 2004; Bastian 2006; Chambers and Cham 2007; Chvatal and Ting 2012; Eichenlaub et al. 2023).

Immediately after a perturbation, the body relies on localized stretch reflexes to maintain stability and prevent falls. According to Henry et al., posterior perturbations cause an initial stretch reflex in the MG and SOL while anterior perturbations elicit an initial stretch reflex in the TA (1998). Here, we found direction-specific reactive muscle activations, as only the TA exhibited greater reactive activation in response to unanticipated anterior surface translations during the last 750-ms epoch. Prior work has shown that when confronted with unanticipated perturbations, participants do not adjust their posture to counteract the effects of a pending disturbance, likely requiring larger reactive neuromuscular corrections (Horak 2006; Smith and Fisher 2018; Duarte et al. 2022). Our results are consistent with this premise. Moreover, we find evidence that the proactive neuromuscular adjustments deployed in anticipation of anterior surface translations were protective; proactive anterior CoM shifts prior to perturbation onset prevented the larger CoM displacement evident when those perturbations were unanticipated.

While early reactive responses rely on short latency reflexes, late reactive adjustments involve more complex neural pathways that integrate sensory feedback and higher-level processing (Christensen et al. 2000; Morton and Bastian 2004; Bastian 2006; Finley et al. 2012). Over time, the body integrates ongoing sensory feedback to adjust its response, allowing for more refined motor control (Finley et al. 2012). The cerebellum plays a crucial role in this process by processing sensory information and projecting to the motor cortical areas, influencing feedforward control and motor planning (Morton and Bastian 2004; Bastian 2006). During the late post-perturbation period (i.e., 750–1500 ms), unanticipated anterior translations led to increased activation of the agonist muscle (i.e., TA), further emphasizing that reactive responses are intensified in the absence of APAs. Notably, young adults responded to anterior perturbations (i.e., those that would compel a backward fall) by increasing activation of their plantarflexor muscles (i.e., MG and SOL). This response may require greater SOL-TA muscle coactivation to increase ankle joint stiffness, as this has been shown to be a strategy to maintain postural stability (Milner et al. 1995; Finley et al. 2012; Thompson et al. 2018). Finally, accompanying CoM displacements during the late post-perturbation period continue to demonstrate the efficacy of the proactive neuromuscular adjustments deployed by younger adults when perturbations were anticipated. Specifically, muscle actions that induced or at least were accompanied by protective anterior CoM displacements prior to perturbation onset ultimately lessened the resultant displacements observed during the late post-perturbation period. Thus, the patterns of proactive adjustments deployed by our healthy younger adults provide a benchmark for protective and effective strategies to mitigate instability.

To gain a better understanding of how anticipation and perturbation direction influence responses to surface translations and thus inform strategies to mitigate instability and minimize the risk of falls, we suggest reproducing this study in older adults as well as in individuals affected by neurological conditions, such as those with cognitive decline. Age-related changes in postural control, which contribute significantly to fall risk, can be identified by comparing the proactive and reactive postural adjustments of older adults with those of younger adults (Smith and Fisher 2018; Duarte et al. 2022). We anticipate that older adults may exhibit less effective proactive adjustments and greater reliance on reactive strategies, potentially leading to increased instability. We hypothesize that, in healthy older adults, reactive balance will be impaired, and that older adults with cognitive decline will experience greater impairments in both proactive and reactive balance, likely due to compromised executive function. Additionally, we recommend incorporating ultrasound technology to assess muscle dynamics, as muscle excitation is not necessarily a surrogate for muscle force. This approach would allow for a more comprehensive evaluation of the mechanical state (i.e., length and velocity) of the muscles involved, providing deeper insights into how these factors contribute to balance and stability.

This study has several limitations. One limitation was that participants were always aware that they could be perturbed at any moment and thus we cannot exclude the potential for postural adjustments even for unanticipated perturbations. We randomized the delivery of anticipated and unanticipated perturbations, as well as their direction, to minimize this effect. The absence of CoM displacements during the pre-perturbation period for unanticipated perturbations should mitigate concerns of this limitation. In addition, we averaged three trials for each experimental condition. Repeated exposure may have an impact on muscle activations and patterns of CoM displacement (Wang et al. 2022; Eichenlaub et al. 2023). Quantifying adaptation to perturbation exposure was outside the scope of the present study. Though, we suspect that the effects of this methodological decision are likely to only lessen the magnitude of changes we would expect in the absence of any adaptation. Another limitation is that we averaged the absolute values of iEMG across males and females. However, a post-hoc exploratory analysis revealed no sex-related main effects across any experimental condition– an outcome that builds confidence in our methodological decision.