Mobility impairments are one of the most commonly reported symptoms in persons with Multiple Sclerosis (PwMS) (Zwibel, 2009; Martin et al., 2006). Among PwMS, 70 % reported difficulties with walking as the most challenging symptom (LaRocca, 2011). Specifically, a reduction in gait speed is one of the notable gait impairments in PwMS (Comber et al., 2017; Cameron and Nilsagard, 2018). These mobility impairments, incorporating gait deficits, may increase fall risk in PwMS, as well as threaten independence and reduce quality of life (Coote et al., 2020; Motl et al., 2011). Forward walking speed is a reliable and responsive measure of functional mobility (Fritz and Lusardi, 2009) and strongly correlates with functional mobility measures in PwMS (Sosnoff et al., 2011). Furthermore, reductions in walking speed have been associated with disability and balance, thereby reducing overall functional independence (Nogueira et al., 2013).

The ability to increase walking speed on demand is critical for navigating our dynamic everyday environments, such as crossing the street, rushing when late, or hurrying to catch a bus or train. The walking speed reserve (WSR) reflects the capacity to increase walking speed on demand and is calculated as the difference between maximal walking speed (MWS) and preferred walking speed (PWS) (Middleton et al., 2016a; Middleton et al., 2016b). A lower WSR indicates a decreased ability to modulate walking speed, implying that the individual typically walks close to their maximal capacity. Similarly, a lower WSR may also be interpreted as an unwillingness to modulate walking speed, perhaps due to the concern of falling at a higher walking speed. Previous studies have highlighted lower WSR in a range of populations, including polio (Klein et al., 2008), stroke survivors (Middleton et al., 2017), and older adults (Middleton et al., 2016a; Callisaya et al., 2017). In PwMS, WSR has been significantly correlated with commonly used clinical walking and balance assessments but did not provide additional insights compared to PWS or MWS in assessing mobility or fall status (Kalron et al., 2017). Therefore, it is plausible that the WSR may provide a sensitive outcome measure in the assessment of functional mobility and screening of fall risk in PwMS.

While many everyday tasks involve movements in the forward direction, movements in other planes are essential to successfully navigate our environment. Specifically, backward walking (BW) is an often-overlooked task that is common in many activities of daily living. For example, everyday scenarios such as opening doors or backing up to sit down all require efficient movements in the backward direction. Recent work has demonstrated that BW is a more cognitively demanding and challenging task compared to FW (Wajda et al., 2013), requiring higher attentional and proprioceptive demands (Hackney and Earhart, 2009; Fritz et al., 2013). PwMS also experience more deficits when walking backward compared to forwards, and previous research has shown that BW is a more sensitive predictor of fall status (Edwards et al., 2020). Moreover, impaired compensatory stepping in the backward direction has been shown sensitive in detecting postural dysfunction in PwMS (Peterson et al., 2016). Many falls occur in the backward direction (Yang et al., 2020); therefore, it is plausible that a compromised ability to modulate speed, particularly in the backward direction, may reduce one's ability to overcome environmental challenges and demands, perhaps increasing fall risk and leading to activity curtailment.

The majority of literature examining WSR has predominantly focused on speed modulation in the forward direction exclusively (Middleton et al., 2016a; Middleton et al., 2016b; Kalron et al., 2017). One previous study examined walking speed reserve in the backward direction (BW-WSR) and demonstrated that the BW-WSR was not useful in discriminating between healthy older adults and healthy older adults at high fall risk (Yada et al., 2021). As both speed modulation and BW are essential to successfully navigate everyday life, examining BW-WSR could provide a unique biomarker for ambulation disability and mobility status. However, in PwMS, previous assessments of WSR have focused on forward walking (FW) only. Therefore, it is plausible that BW-WSR may provide a sensitive clinical assessment of mobility and fall status in PwMS. However, no studies have examined the potential clinical utility of BW-WSR in PwMS.

Therefore, the primary purpose of this study is to investigate the utility of BW-WSR in PwMS. Specifically, we will 1) examine the relationship between BW-WSR and demographics, clinical characteristics (symptom duration, disease severity), self-report measures including the Modified Fatigue Impact Scale (MFIS), and the 12-item MS Walking Scale (MSWS-12) as well as other commonly used clinical measurements of functional mobility such as the timed up-and-go (TUG), 2) compare the relationship between cognitive processing domains and both FW-WSR and BW-WSR, 3) assess the relationship between BW-WSR with prospective falls at 3 months and 6 months.