All subjects experienced the same split-belt paradigm illustrated in Figure 1A. Only the inclination at which subjects walked was altered across groups to determine the role of anterior–posterior ground reaction forces on the adaptation and after-effects of kinetic and kinematic gait features. We specifically tested subjects at three inclinations: flat (0°), incline (8.5°), or decline (-8.5°). These slopes were selected based on previous studies investigating walking on inclined surfaces (e.g., Lay et al., 2006) and because greater inclinations were too strenuous for split-belt walking, as assessed in pilot studies. Subjects from each group walked at the specified inclination throughout the experiment, including baseline and post-adaptation. Baseline: Subjects first experienced a baseline epoch to characterize their gait at the specific inclination at which they were going to walk throughout the study. During the baseline epoch, subjects walked with the belts moving at the same speeds (i.e., tied condition). Belts moved either at a slow (0.5 m/s), fast (1.5 m/s), and medium (1 m/s) speeds for at least 50 strides. Strides were counted in real-time using raw kinetic data. A stride was defined as the period between two consecutive heel strikes (i.e., foot landing) of the same leg. Adaptation: The adaptation epoch was used to assess subjects’ ability to adjust their locomotor pattern in response to a split-belt perturbation. During this period, one leg moved three times faster than the other (0.5 and 1.5 m/s) for 600 strides (∼10 min), except for one subject in the decline group who experienced the adaptation condition for 907 strides (due to technical difficulties in heel strike detection). This subject was not excluded from the analysis given that this participant’s behavior did not differ from that of other subjects and its inclusion or exclusion did not alter our conclusions. The leg walking fast was the dominant leg, which was determined as the self-reported leg used to kick a ball. Post-adaptation: This epoch was used to assess the after-effects when the split-belt condition was removed. Subjects experienced tied walking at the medium speed (1 m/s) for 600 strides at their group-specific slope. The speeds for the adaptation and post-adaptation periods were chosen based on other studies in young subjects (e.g., Reisman et al., 2005) so that the fast speed in the 3:1 belt ratio was not at the walk-to-run speed transition for any participant. The duration of the adaptation and post-adaptation period was comparable to studies in healthy and clinical populations (e.g., Reisman et al., 2009) to ensure that all individuals had enough exposure to the split condition and could complete the entire protocol at all inclinations. In addition, subjects in all groups where given resting breaks every 200 strides during the adaptation and post-adaptation epochs to prevent fatigue.

Kinematic and kinetic data were used to characterize subjects’ ability to adapt their gait during adaptation, and maintain the learned motor pattern during post-adaptation. Kinematic Data: Kinematic data were collected with a passive motion analysis system at 100 Hz (Vicon Motion Systems, Oxford, United Kingdom). Subjects’ behavior was characterized with passive reflective markers placed symmetrically on the ankles (i.e., lateral malleolus) and hips (i.e., greater trochanter) and asymmetrically on shanks and thighs (to differentiate the legs). The origin of the kinematic data was rotated with the treadmill in the incline and decline conditions such that the z-axis (‘vertical’ in the flat condition) was always orthogonal to the surface of the treadmill (Figure 1B). Gaps in raw kinematic data were filled with a quintic spline interpolation (Woltring; Vicon Nexus Software, Oxford, United Kingdom). Kinetic Data: Kinetic data were collected with an instrumented split-belt treadmill at 1,000 Hz (Bertec, Columbus, OH, United States). Force plates were zeroed prior to each testing session so that each force plate’s weight did not affect the kinetic measurements. In addition, the reference frame was rotated at the inclination of each specific experiment such that the anterior–posterior forces were aligned with the surface on which the subject walked. A heel strike was identified in real time when the raw normal force under each foot reached a threshold of 30 Newtons. This threshold was chosen to ensure accurate identification of foot landing at all sloped conditions. On the other hand, we used a threshold of 10 Newtons on median filtered data (with a 5 ms window) to detect the timing of heel strikes more precisely for data processing.

Step length symmetry was not recovered in the sloped split-belt conditions. All groups were perturbed by split-belt walking and subsequently adapted (Figure 2A). However, each group reached a different step length asymmetry by Late Adaptation (p < 0.001) such that the flat group plateaued near values that were not different from zero (p = 0.08) (i.e., symmetric step lengths), whereas the decline group undershot step length symmetry (p < 0.001) and the incline group overshot step length symmetry (p < 0.001) (Figure 2B). Interestingly, while both sloped groups were perturbed more than the flat group (Figure 2C; EarlyA: group main effect p = 0.003; incline vs. flat p = 0.036; decline vs. flat p = 0.002), only the incline group adapted more than the other two groups (Figure 2D; ΔAdapt: group main effects p < 0.001; incline vs. flat p < 0.001; incline vs. decline p = 0.002) and faster (Figure 2A; dots above the adaptation epoch time courses indicate the time constants with 95% confidence intervals). Importantly, groups were not only distinct in their adaptation, but also in their after-effects (Figure 2E; p = 0.002) such that the incline group had greater after-effect than the decline (p = 0.03) and flat groups (p = 0.002). Therefore, contrary to our hypothesis, the slope condition augmenting propulsion (i.e., incline walking) rather than braking led to more and faster adaptation and greater after-effects. It was also unexpectedly found that the sloped groups plateaued at values that were distinct from their baseline step length symmetry, begging the questions of whether inclination had a different effect across individual step lengths (addressed in Figure 3) and why this would happen (addressed in Figure 4).

While incline walking augmented the adaptation and after-effects of step length asymmetry, it had a unilateral effect on individual step lengths: it predominantly increased the adaptation of one leg and the after-effects of the other leg. Figure 3 illustrates the time courses for each step length at each slope. While both legs adapted and had after-effects in all groups, the adaptation of the fast leg was greater than that of the slow leg, whereas the opposite was observed during post-adaptation. In other words, the leg that was modified the most switched between the fast and slow legs depending on the epoch, which was substantiated by the significant interaction between leg and epoch on the changes of step length (Figure 3B; leg#epoch p < 0.001). In addition, the sloped conditions augmented this switching between legs, as indicated by the significant interaction between leg, epoch, and slope (leg#epoch#slope p < 0.001). Once more, the incline group drove this effect by exhibiting the largest changes on the fast side during adaptation (ΔAdapt: incline vs. flat p < 0.001; ΔAdapt: incline vs. decline p < 0.001; ΔAdapt: flat vs. decline p = 0.65) and on the slow side during post-adaptation (ΔPost: incline vs. flat p = 0.003; ΔPost: incline vs. decline p = 0.02; ΔPost: flat vs. decline p = 0.70). The impact of inclination on the changes of step length was further substantiated by the significant effect of slope (p < 0.001) in contrast with the non-significant effect of leg (p = 0.13) or epoch (p = 0.89). In sum, incline walking exaggerated the split-belt phenomenon of predominantly adapting the step length of the fast leg and observing after-effects on the other leg during post-adaptation.

Leg orientation mediated by inclination and walking speed caused the distinct step length asymmetries across sloped conditions. More specifically, subjects oriented each leg to prioritize speed- and slope-specific demands on AP forces at the expense of step length symmetry during adaptation. This is shown in Figure 4, which illustrates the top-down view of ankle positions relative to the body (Figure 4A) and the step lengths to which these positions contribute (Figure 4B) at baseline walking (slow and fast speeds) and late adaptation under each sloped condition (Figure 4C) for the sloped groups. Note that in slow baseline walking (Figure 4C; 1st column) subjects shifted their ankle position backward in the incline condition (i.e., |X| > |α|) relative to the body (Figure 4C and green symbols in Figure 4D). The opposite effect (i.e., |X| < |α|) was observed for the decline group (Figure 4C and red symbols in Figure 4D). Leg orientation was additionally regulated by walking speed, as shown by the distinct leg orientations between slow (Figure 4C; 1st column) and fast baseline walking (Figure 4C; 2nd column). The demands on each leg to walk at the specific speed and inclination persisted during split-belt walking. Consistently, subjects recovered the speed-specific baseline leg orientation during late adaptation (Figure 4C; 3rd column: black lines with colored squares indicate leg orientations at the speed-specific baselines). The similarity between leg orientations across the baseline and late adaptation epochs in all sloped conditions was quantified by the regression shown in Figure 4D (y = 0.93^∗x; R² = 0.98, p < 0.001). The idealized case, in which baseline and late adaptation values are exactly the same, is also displayed in Figure 4D as a reference (dashed gray line). These leg orientations consequently led to step lengths that were only the same at the end of adaptation in the flat group, but not in the two sloped conditions (e.g., step length in Figure 2, 4C). Lastly, the leading legs’ orientation (α_slow or α_fast) were similar before and after removal of the split perturbation, whereas the trailing legs’ orientation (X_slow or X_fast) were swapped between legs. In other words, the slow leg’s trailing orientation during post-adaptation (X_slow in Figure 4E) was similar to the fast leg’s trailing orientation (X_fast) during late adaptation (dotted red line in Figure 4E) and vice versa for the other leg. This switching of the trailing legs’ orientation contributed to the reduced step lengths of the slow leg during post-adaptation (Figure 4E; blue solid line), which was also presented in Figure 3. The ipsilateral similarity between α and the contralateral relationship between X from late adaptation to early post-adaptation in all sloped conditions was quantified by the regression in Figure 4F (y = 0.99^∗x; R² = 0.96, p < 0.001). Of note, neither the leading nor the trailing legs’ orientation in post-adaptation matched the respective leg orientations at medium baseline walking (Figure 4E; black lines with black squares), which is the speed at which subjects walked during post-adaptation. Thus, leg orientation exhibited after-effects. In sum, all groups recovered their baseline leg orientation at the expense of step length symmetry.

We assessed the adaptation of forces under the distinct inclination conditions before investigating the relation between kinetic and kinematic measures during and after split-belt walking. We found that braking and propulsion forces were predominantly modified during adaptation and post-adaptation in the sloped condition that naturally prioritized them (Figure 5A,B). In other words, braking was mostly modulated in the decline group, whereas propulsion was primarily modulated in the incline group. This preferential regulation of braking and propulsion forces was indicated by the significant group effect during early adaptation (Figure 5C) for braking forces on both legs (B_slow EarlyA: p = 0.001; B_fast EarlyA: p < 0.001) and propulsion forces of the slow leg (P_slow EarlyA: p = 0.007) but not the fast leg (P_fast EarlyA: p = 0.15). More specifically, the braking and propulsion forces were, respectively, more perturbed in the decline and incline groups compared to the flat group (B_slow EarlyA: decline vs. flat p = 0.014; B_fast EarlyA: decline vs. flat p < 0.001; P_slow EarlyA: incline vs. flat p = 0.028). Despite the group differences during early adaptation, only the forces associated with the step lengths of the fast leg (i.e., fast braking and slow propulsion) were adapted differently across groups (Figure 5D; B_fast ΔAdapt: p < 0.001; P_slow ΔAdapt: p < 0.001), whereas the other forces were adapted similarly across sloped conditions (B_slow ΔAdapt: p = 0.10; P_fast ΔAdapt: p = 0.35). Once again, fast braking was preferentially adapted in the decline group compared to the flat group (p = 0.001) and slow propulsion was preferentially adapted in the incline group compared to the flat group (p < 0.001). Group analyses on early post-adaptation (Figure 5E) indicated that sloped condition had a significant impact on the after-effects of slow braking (p < 0.001), slow propulsion (p = 0.031), and fast propulsion (p = 0.007) and a trending effect on fast braking (p = 0.076). However, only the forces contributing to after-effects of the slow step length (i.e., slow braking and fast propulsion) exhibited greater after-effects than those regularly observed after flat split-belt (B_slow ΔPost: decline vs. flat p < 0.001; P_fast ΔPost: incline vs. flat p = 0.005 in contrast with P_slow ΔPost: incline vs. flat p = 0.98). In sum, inclination conditions predominantly modified the perturbation, adaptation, and after-effects of braking and propulsion forces in a preferential manner: incline walking mostly modified the adaptation of propulsion forces, whereas decline walking mostly altered the adaptation of braking forces.

While braking and propulsion forces were both altered with inclination during and after split-belt walking, only disruptions to the slow leg’s propulsion force during adaptation was indicative of step length asymmetry after-effects during post-adaptation. Namely, only the slow propulsion force during early adaptation was associated to step length asymmetry after-effects (Figure 6A: P_slow: p = 0.001, R² = 0.63; data not shown: P_fast: p = 0.74, B_slow: p = 0.49, B_fast: p = 0.49). This relation can be explained by the impact of the slow leg’s propulsion force on the slow step length’s after-effects (results summarized in Figure 6B). Specifically, large perturbations of the slow leg’s propulsion force (P_slow EarlyA) led to large adaptation of this parameter (P_slow ΔAdapt) (Figure 6C; p = 0.012, R² = 0.72). In addition, the adaptation of the slow leg’s propulsion force (P_slow ΔAdapt) was positively associated to After-Effects on the fast leg’s propulsion force (P_fast After-Effects) (Figure 6D; p < 0.001, R² = 0.53). This is consistent with the results in Figure 3 indicating that adaptation of step length on one side led to after-effects on the other side. Importantly, there was a strong association between the fast leg’s propulsion after-effects (P_fast After-Effects) and those of the fast leg’s trailing orientation (X_fast After-Effects) (Figure 6E; p < 0.001, R² = 0.86), which have a direct impact on the after-effects of the slow leg’s step length (SL_slow After-Effects) (Figure 6F; p < 0.001, R² = 0.84). Lastly, the after-effects on the slow leg’s step length (SL_slow After-Effects), and not the fast one (data not shown), were the ones driving the after-effects in step length asymmetry (Figure 6G; p < 0.001, R² = 0.90). It is worth pointing out that group was a factor in all these regressions since it was either a significant categorical predictor in the multiple regressions (Figure 6A: p_group = 0.012; Figure 6C: p_group < 0.001; Figure 6F: p_group = 0.015) or significantly correlated to the continuous variables (Figure 6D: Pearson p < 0.001; Figure 6E: Pearson p = 0.043; Figure 6G: Pearson p = 0.024). Thus, these associations depend on the large inter-subject variability facilitated by the different sloped conditions. Taken together, the after-effects in step length asymmetry were unilaterally due to after-effects in the slow leg’s step length, which were facilitated by the large perturbation and adaptation of the slow leg’s propulsion force.

All groups recovered their baseline leg orientation at the expense of step length symmetry, which indicated that speed-specific leg orientation was prioritized over symmetric step lengths. This has two key implications. First, kinetic demands strongly shape our movements. Notably, inverted pendulum models of walking suggest that subjects orient their legs to maintain a constant speed by equalizing positive and negative work over the gait cycle (Kajita et al., 2001; Donelan et al., 2002; Kuo, 2002; Kuo et al., 2005). Consistently, we observed that subjects oriented their legs in the incline and decline groups to generate the forces counteracting the distinct effect of gravity on the center of mass at the different slopes (Lay et al., 2006). Thus, we believe that subjects in each sloped condition were equally proficient at adapting their movements, but they reached different step lengths during late adaptation because they had distinct kinetic demands, rather than because subjects in the decline or incline group were worse at recovering symmetric steps than those in the flat group. In sum, leg orientation is closely regulated in order to walk at the distinct speeds and inclination (Leroux et al., 2002; Orendurff et al., 2008; Dewolf et al., 2018) imposed on each group. We particularly observed a strong effect of slope on leg orientation when walking slow, but this is also observed at fast speeds when using a coordinate frame aligned with gravity (Leroux et al., 2002), rather than aligned with walking surface as done in this study. It is worth pointing out that there are other factors in addition to leg orientation that are modulated to meet kinetic demands at each slope, such as muscle coordination (Wall-Scheffler et al., 2010), body posture (Leroux et al., 2002), and joint angles (Hsiao et al., 2016). Therefore, although we focused our analysis on leg orientation to explain the asymmetric step lengths across groups, there are other factors that could also influence the differences in locomotor adaptation that we observed across sloped conditions.

A second implication from our results is that step length symmetry is a gait feature that may not be as valued by the motor system as previously considered. Specifically, step length symmetry is thought to be tightly controlled because subjects self-select symmetric step lengths even in asymmetric environments (e.g., Reisman et al., 2005; Savin et al., 2014) and those with more symmetric step lengths in these environments are also those exerting less metabolic energy (Finley et al., 2013). Thus, it was unexpected that subjects in the incline and decline split-belt groups overshot or undershot step length symmetry to recover baseline leg orientations. Given that humans have least-effort tendencies (Margaria, 1976; Alexander, 1989; Bertram and Ruina, 2001; Bertram, 2005; Kuo et al., 2005; Selinger et al., 2015) we think that step length symmetry is not necessarily energetically optimal when walking in sloped surfaces, and perhaps even in flat split-belt environments (Sánchez et al., 2017; Leech et al., 2018). Instead, subjects might self-select leg orientations to optimally generate the mechanical work (Selgrade et al., 2017) for walking at the speed and inclination imposed on each leg, but further studies are needed to determine if this is the case.

The self-selected leg orientation also led to a contralateral relation between adaptation and post-adaptation. In other words, the disruption to forces and movements of individual legs led to more ipsilateral adaptation, but it did not result in larger ipsilateral after-effects. In fact, more adaptation of one leg changed the gait of the other leg as indicated by after-effects in kinetic and kinematic measures (e.g., propulsion forces, step length, and trailing leg orientation). This is in contrast to sensorimotor adaptation of other motor behaviors such as reaching, in which adaptation and de-adaptation effects are mostly constrained to a single effector (Nozaki et al., 2006; Yokoi et al., 2011, 2017; Wang et al., 2013). The contralateral relation between adaptation and post-adaptation might be exclusive to locomotion; possibly because, unlike bimanual tasks (Ahmed et al., 2008; Yokoi et al., 2014), legs share a common goal in walking (i.e., keep the body on the treadmill) while experiencing opposite perturbations (i.e., one leg moves fast and the other leg moves slow) (Choi and Bastian, 2007). This contralateral relation between legs and unilateral effects reported on each group are, of course, occluded when using symmetry measures to characterize locomotor adaptation. Consequently, it is important to characterize each leg’s adjustments in tasks inducing locomotor adaptation (e.g., Reisman et al., 2005), particularly when targeting unilateral deficits on hemiparetic gait (Bowden et al., 2006; Balasubramanian et al., 2007). Another consistent observation in the analysis of individual limbs was that all groups converged to the same step length value for the leg walking slow during adaptation. This is exclusively observed during split and not tied conditions, indicating that this might be a task-constraint of split-belt walking, which will be the subject of future work. In sum, split-belt walking at different slopes predominantly altered the adaptation of one leg and led to subsequent after-effects on the other leg, highlighting the bilateral nature of sensorimotor adaptation in walking.

Our results suggest distinct control between the leading and trailing legs’ orientation when taking a step because they transition differently upon sudden changes in the walking environment. More specifically, the leading leg’s orientation at foot landing (α) exhibits smooth and continuous changes when transitioning from the split to tied situations in all our inclination conditions and other perturbation magnitudes (Malone et al., 2012), whereas the trailing leg’s orientation (X) is discontinuous. This suggests that the leading leg’s orientation at foot landing is controlled in a feedforward manner –it is planned before the movement is executed based on a slowly updated internal representation of the environment. This is supported by the fact that sensory information about leg orientation at foot landing is sent to cerebellar structures (Bosco and Poppele, 2001), housing the feedforward control of movements (Herzfeld et al., 2015) and the fact that spinalized cats cannot adapt the orientation of the leading leg during split-belt walking (Frigon et al., 2017). On the other hand, the trailing leg’s orientation when taking a step could be determined by a combination of feedback and feedforward control. Consider that feedback mechanisms adjust our movements by transforming delayed sensory information into actions in real-time (Jordan and Rumelhart, 1992; Bhushan and Shadmehr, 1999). Accordingly, the trailing leg’s orientation is immediately regulated upon manipulations to ipsilateral sensory information from spindles in hip muscles, load sensors, and cutaneous information (Grillner and Rossignol, 1978; Duysens and Pearson, 1980; Duysens et al., 2000; Pang and Yang, 2000; Rossignol, 2006). In addition, the trailing leg’s orientation is determined by the step time (i.e., period between ipsilateral and contralateral heel-strikes) (Finley et al., 2015), which is controlled in a feedforward manner as suggested by behavioral split-belt studies (Malone et al., 2012; Finley et al., 2015) and Purkinjie cells tracking heel-strikes (Apps et al., 1995; Bosco and Poppele, 2001). In particular, the feedforward control of step timing and hence that of the trailing leg’s orientation might be based on subjects’ expectation of the speed at which the belt is moving. In sum, our results suggest distinct control mechanisms of the leading and trailing legs’ orientation when taking a step: the leading position is mostly mediated by feedforward mechanisms, whereas the trailing position is mediated by a combination of feedforward and feedback mechanisms.

While braking and propulsion forces were adapted in all groups, augmenting propulsion forces facilitated motor adaptation. This was indicated by the greater adaptation and after-effects of the incline group (larger propulsion demands), whereas the kinematic adaptation was reduced and the after-effects were unchanged in the decline group (larger braking demands). The preferential impact of propulsion on locomotor adaptation was unexpected given prior studies reporting minimal (Roemmich et al., 2012) or absent adaptation of propulsion forces (Ogawa et al., 2014) and subsequently lacking propulsion after-effects (Roemmich et al., 2012; Ogawa et al., 2014) in contrast with the adaptation and after-effects of braking forces (Ogawa et al., 2014). Our findings are at odds with these reports possibly because our participants experienced greater speed differences and were adapted at a more naturalistic walking speed (i.e., mean speed across legs of 1 m/s vs. 0.75 m/s). We additionally found that the incline group adapted faster, indicating that incline walking also augmented the saliency (i.e., easier to detect) and/or sensitivity (i.e., quicker to respond) to the split-belt perturbation. The saliency of the speed difference between legs may be augmented in the incline condition because of reduced cadence (Kawamura et al., 1991; Sun et al., 1996; McIntosh et al., 2006; Phan et al., 2013) and hence longer stance times (i.e., period in direct contact with the environment), which increase subjects’ ability to perceive speed differences (Hoogkamer et al., 2015). In addition, sensory inputs encoding walking speed are stimulated more when walking incline (Sinkjaer et al., 2000; Lamont and Zehr, 2006; Klint et al., 2008; Tillakaratne et al., 2014; Choi et al., 2016) also increasing the saliency of speed differences between the legs in the split situation. On the other hand, subjects in the incline group might have adapted faster because they were more sensitive to increments in energetic cost due to step length asymmetry (Finley et al., 2013; Bhounsule et al., 2014), because incline walking has large energetic demands in and of itself (Johnson et al., 2002). Therefore, larger propulsion demands experienced by the incline group resulted in more and faster gait changes, indicating that altering propulsion forces during split-belt walking facilitated locomotor adaptation.

The relevance of propulsion forces was also evident by the association between individual propulsion forces during early adaptation and subject-specific after-effects. The positive relation between large movement disruptions in early adaptation and after-effects is well documented in error-based learning (Körding and Wolpert, 2004; Wei and Körding, 2009; Green et al., 2010). However, the lack of correlations between the braking force or the fast leg propulsion force strongly suggest that slow leg propulsion forces during adaptation regulate after-effects. Of note, we observed a large range of propulsion forces during early adaptation across individuals, which might be needed to identify the reported association between propulsion forces and after-effects. In sum, our findings indicate that augmenting propulsion demands during split-belt walking facilitates gait adaptation during and after split-belt walking, suggesting that altering propulsion forces could be used as a training stimulus for gait rehabilitation.

Our results might have an impact on the rehabilitation of hemiparetic gait because error-augmentation protocols, like the one presented here, can induce gait improvements in stroke survivors (Reisman et al., 2007; Savin et al., 2014; Reissman et al., 2018) that persist with repeated exposure (Reisman et al., 2013; Lewek et al., 2018). Moreover, we show that increasing propulsion demands during split-belt walking facilitates the adaptation of gait in healthy individuals, as we previously observed in stroke survivors (Sombric et al., 2015). Thus, incline split-belt walking might be beneficial for gait rehabilitation post-stroke because it will overcome the limited gait adaptation of stroke individuals during regular split-belt walking (Malone and Bastian, 2014). In addition, it will target paretic propulsion forces, which directly contribute to gait asymmetry (Bowden et al., 2006; Balasubramanian et al., 2007) and can be modulated by stroke individuals when required by the task (Kesar et al., 2011, 2014; Reisman et al., 2013; Awad et al., 2014; Hsiao et al., 2015, 2016). Of note, previous research indicates that overground walking post-stroke is most improved following decline, rather than incline, interventions due to greater similarity between the flat and decline environments (Carda et al., 2013). Thus, the augmented adaptation in the incline environment may not translate to overground walking. Therefore, the efficacy of our protocol as an intervention will depend on future work assessing its generalization to level walking. Lastly, we find that incline split-belt walking leads to faster adaptation. This could be exploited to increase the slower adaptation rate of older populations (Sombric et al., 2017) who will be the primary recipients of gait rehabilitation. In sum, interventions that increase propulsion demands may be a viable rehabilitation strategy, but future work is necessary to determine the efficacy of incline split-belt walking over other forms of training.