Split-belt treadmills that move the legs at different speeds are thought to update internal representations of the environment, such that this novel condition generates a new locomotor pattern with distinct spatio-temporal features compared to those of regular walking. It is unclear the degree to which such recalibration of movements in the spatial and temporal domains is interdependent. In this study, we explicitly altered subjects' limb motion in either space or time during split-belt walking to determine its impact on the adaptation of the other domain. Interestingly, we observed that motor adaptation in the spatial domain was susceptible to altering the temporal domain, whereas motor adaptation in the temporal domain was resilient to modifying the spatial domain. This non-reciprocal relation suggests a hierarchical organization such that the control of timing in locomotion has an effect on the control of limb position. This is of translational interest because clinical populations often have a greater deficit in one domain compared to the other. Our results suggest that explicit changes to temporal deficits cannot occur without modifying the spatial control of the limb.

2Split-belt treadmills that move the legs at different speeds are thought to update internal 3 representations of the environment, such that this novel condition generates a new locomotor 4 pattern with distinct spatio-temporal features compared to those of regular walking. It is unclear 5 the degree to which such recalibration of movements in the spatial and temporal domains 6 is interdependent. In this study, we explicitly altered subjects' limb motion in either space or 7 time during split-belt walking to determine its impact on the adaptation of the other domain. 8 Interestingly, we observed that motor adaptation in the spatial domain was susceptible to altering 9 the temporal domain, whereas motor adaptation in the temporal domain was resilient to modifying 10 the spatial domain. This nonreciprocal relation suggests a hierarchical organization such that the 11 control of timing in locomotion has an effect on the control of limb position. This is of translational 12 interest because clinical populations often have a greater deficit in one domain compared to 13 the other. Our results suggest that explicit changes to temporal deficits cannot occur without 14 modifying the spatial control of the limb. 15 16 21representations of the treadmill for the control of the limb in space and time (Malone et al., 2012). There is 22 a clinical interest in understanding the interdependence in the control of these two aspects of movement 23 because pathological gait often has a greater deficiency in one domain compared to the other (Finley et al., 24 2015; Malone and Bastian, 2014). Thus, there is a translational interest to determine if spatial and temporal 25 asymmetries in clinical populations can be targeted and treated independently. 26 Ample evidence supports that the adaptation, and hence control, of spatial and temporal gait features 27 is dissociable. Notably, studies have shown that inter-limb measures such as step timing (temporal) and 28 step position (spatial) adapt at different rates (Sombric et al., 2017; Malone and Bastian, 2010), they 29 exhibit different generalization patterns (Torres-Oviedo and Bastian, 2010), and follow distinct adaptation 30 dynamics throughout development (Vasudevan et al., 2011; Patrick et al., 2014) or healthy aging (Sombric 31 et al., 2017). In addition, several behavioral studies show that subjects' adjustment of spatial metrics 32 can be altered (Malone and Bastian, 2010; Malone et al., 2012; Long et al., 2016) without modifying 33 the adaptation of temporal gait features. However, the opposite has not been demonstrated. For example, 34 altering intra-limb measures (i.e., characterizing single leg motion) of timing such as stance time duration 35 (Afzal et al., 2015; Krishnan et al., 2016) also leads to changes in intra-limb spatial features such as stride 36 lengths. In sum, the spatial and temporal control of the limb is thought to be dissociable, but it remains 37 unclear if the adaptation of internal representations of timing can be altered and what...