Multiple sclerosis (MS) is a central neurological condition that affects an estimated 2.8 million people worldwide (Federation, 2020). Symptoms vary and may include, but are not limited to, deficits in strength, sensation, fine motor function, coordination, vision, fatigue, cognition, muscle tone, and depression (Fox et al., 2015). The course of central nervous system demyelination and the subsequent symptoms and functional limitations are variable and unique to each PwMS.

One major impairment that affects mobility is limb weakness, which is reported in 70% of PwMS (Hoang et al., 2014). Limb weakness is more likely to occur in the lower limbs (LL) because lesions throughout the central nervous system cause greater nerve dysfunction over the longest pathways (Compston, 2005; Hoang et al., 2014; Stevens et al., 2013). LL weakness is associated with impairments in gait (Mañago et al., 2020, 2018; Ramari et al., 2018; Yang et al., 2019), balance (Citaker et al., 2013), LL functional capacity (Ozkul et al., 2022; Ramari et al., 2020), increased fall risk (Cameron and Nilsagard, 2018), and greater disability (Portilla-Cueto et al., 2020). Gait abnormalities including decreased speed, cadence, and step length lead to difficulty walking, which is reported by 41% of PwMS (Broekmans et al., 2013; Cameron and Wagner, 2011; Davies et al., 2017; Larocca, 2011; Thoumie et al., 2005; Wagner et al., 2014). Of those PwMS, 70% said difficulty walking was “the most challenging aspect of their MS”, and 75% said it disrupts their daily lives (Larocca, 2011). Poor LL strength causing increased risk of fall comes with a likelihood that PwMS will have a reduced quality of life (QOL) (Coote et al., 2013; Mazumder et al., 2014), less independence (Stevens and Lee, 2018), and a greater risk of injury (Gianni et al., 2014). PwMS believe that muscle power is one of their most significant losses of function, and muscle weakness affects most activities of daily living and reduces QOL (Multiple Sclerosis Rehabilitation 2013; Heesen et al., 2008; Motl and McAuley, 2009; Thoumie et al., 2005).

Currently, there is limited literature specifying normative strength values of the major LL muscle groups in PwMS, with only reference values existing for the knee extensors (Portilla-Cueto et al., 2020). Compared to healthy controls, PwMS fatigued quicker in the hip flexors, knee flexors and extensors, and ankle dorsiflexors and plantarflexors; and had lower peak torque, muscular strength, and power (Armstrong et al., 1983; Checchia, 1993; Chung et al., 2008; Davies et al., 2017; Jorgensen et al., 2017; Lambert et al., 2001; Newsome et al., 2011; Wagner et al., 2014). This has been shown to be from reduced neural activation and motor unit recruitment, reduced muscle metabolic response, and increased atrophy of the muscles from disuse (Stevens et al., 2013). Numerous studies have compared LL strength of PwMS to healthy controls (Armstrong et al., 1983; Checchia, 1993; Chung et al., 2008; Davies et al., 2015; Jorgensen et al., 2017; Lambert et al., 2001; Scott et al., 2011; Wagner et al., 2014; Wens et al., 2014); however, very few studies have measured muscle strength in the LL (Portilla-Cueto et al., 2020), with none measuring strength in all major LL joints (hip, knee, and ankle) in a large number of PwMS objectively. Additionally, while normative strength data for major muscle groups exist for healthy children and adults, no studies have provided regression-based normative strength data for the LL in PwMS, but rather only discrete normative values for the knee extensor (Portilla-Cueto et al., 2020). While discrete norms, which are stratified by one or more demographic variables, require a larger sample size to adequately represent the individuals in each subgroup, regression-based norms can utilize smaller sample sizes as the demographics are treated continuously (Marrie et al., 2020; Oosterhuis et al., 2016). Furthermore, this approach can be more precise, as instability can arise from discrete norms when an individual moves from one subgroup to another, but the raw performance remains the same.

Moreover, a normative dataset for LL strength is necessary to determine which LL weaknesses are prevalent in PwMS, design future studies to address the specific problems associated with weakness, create objective goals, develop and implement individualized patient rehabilitation plans to improve strength and functional capacity, and evaluate rehabilitation treatments (Cagnie et al., 2007; Gross and Brugnolotti, 1992; Harbo et al., 2012; Wiggin et al., 2006). Therefore, this study aimed to develop gender-specific regression-based normative prediction equations, with 95% confidence intervals, for maximal isometric peak torque of major LL muscles (hip flexors, extensors, and abductors, knee extensors and flexors, and ankle plantarflexors and dorsiflexors) in PwMS. A secondary aim was to characterize the prevalence of LL weakness in PwMS, defined as ≥ 2 SD below values reported for healthy individuals.