Stroke is the main cause of adult disability.1 Weakness of the lower limb muscles is one of the main impairments after stroke2,3 and is associated with limited ability to perform activities of daily living4,5 and with reduced gait speed in individuals with stroke.6 Community walking ability is a common goal for individuals with stroke; therefore, interventions aiming at increasing gait speed should be considered. Therefore, it is important to understand the magnitude of the associations between muscle groups of the lower limbs and gait speed.

Mentiplay et al.6 conducted a systematic review to assess the associations between the strength of the lower limb muscles and gait speed in individuals with stroke and reported very weak to strong associations.6 However, the included studies had several limitations.6 The majority of these eligible studies only assessed one muscle group, and few assessed the non-paretic lower limb muscles and maximum gait speed. As the evaluation of various muscle groups may assist in the interpretation of the relative importance of each group to gait speed and the non-paretic muscles also show strength impairment,3 bilateral measures of seven lower limb muscle groups were obtained in the present study. Furthermore, maximum gait speed was evaluated in this study as it is an important requirement for the performance of activities in community-based settings and for ensuring individuals’ safety when crossing the street.7,8

Individuals with stroke are classified according to typical phases of spontaneous motor recovery progress.9 These phases are determined based on the timing of important biological processes of neural repair, which may affect impairment and functional limitations.9 An existing longitudinal study observed significant improvements in gait speed between the evaluations performed at both 1 week (acute phase) and 3 months (sub-acute phase) after stroke.10 Thus, it is important to take the post-stroke phases into account. As none of the studies that were included in a previous review6 took the post-stroke phases into account, the present study assessed only patients with sub-acute stroke.

In addition to strength deficits in the lower limb muscles, patients with stroke also present trunk muscle weakness.11–14 Previous studies have already reported significant associations between weakness of the trunk flexors/extensors and impaired balance and lower independence in walking and transfers, such as getting up from a sitting position.12,15 However, none of the studies investigated the associations between measures of strength of the trunk muscles and gait speed after stroke. A better understanding of these associations could clarify which muscle group(s) should be targeted during strength training after stroke.

The aims of this study were to investigate the associations between measures of strength of the lower limb and trunk muscles and comfortable and maximum gait speeds and to identify the muscle group that would best predict gait speed in individuals with sub-acute stroke. This information may help clinicians identify the muscles to target during strength evaluation for planning strengthening programs aimed at increasing gait speed after stroke.

From a list of approximately 265 individuals, 38 could not be contacted (Fig. 2). Forty-four individuals with sub-acute stroke (24 men) were eligible and participated in the study. Their mean age was 62 years, SD=14, and their mean post-stroke time was 4 months, SD=1. Their mean comfortable gait speed was 0.8m/s, SD=0.38, whereas their maximum gait speed was 1.1m/s, SD=0.54. The clinical and demographic characteristics of the participants are given in Table 1.

Figure 2.

Flow diagram through the study.

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As shown in Table 2, weak to moderate associations were found between measures of strength of the lower limb muscles and both comfortable (0.36≤r≤0.53; p<0.05) and maximum (0.37≤r≤0.59; p<0.05) gait speeds, except for the non-paretic knee flexors and comfortable gait speed (p=0.06). Weak to moderate associations were also found between measures of strength of the trunk muscles and comfortable gait speed (0.39≤r≤0.52; p<0.05) in individuals with sub-acute stroke; there were also moderate associations between measures of strength of the trunk muscles and maximum gait speed (0.50≤r≤0.61; p<0.05) in individuals with sub-acute stroke.

The results of the regression analyses indicated that only the non-paretic ankle dorsiflexors and left lateral trunk flexors were retained in the models (Table 3). The non-paretic dorsiflexors and the left lateral trunk flexors explained 29% (B=0.09, 0.04≤95% confidence interval for B≤0.14, p=0.002) and 42% (B=0.12, 0.07≤95% confidence interval for B≤0.18, p<0.001) of the variance in maximum and comfortable gait speeds, respectively (Table 3).

Weak to moderate associations were found between measures of strength of the lower limb and trunk muscles and both comfortable and maximum gait speeds, except for the non-paretic knee flexors, which did not show any association with comfortable gait speed. However, only the non-paretic dorsiflexors and the left lateral trunk flexors remained as predictors of comfortable and maximum gait speeds, respectively.

Muscles transform metabolic energy into mechanical energy to generate force and produce accelerations and decelerations of body segments or of the entire body.2 Gait speed is the rate of linear forward motion of the body; thus, theoretically, muscle strength may be related to gait speed. As muscle strength and gait speed are reduced after stroke,2,30 it is important to understand the associations between these variables. A review6 that aimed at identifying studies that had investigated the associations between the strength of the lower limb muscles and gait speed in individuals with stroke found significant very weak to strong associations between the strength of the paretic and non-paretic (0.11≤r≤0.83) lower limb muscles and comfortable and maximum gait speed. The results of most of the studies included by Mentiplay et al.6 in individuals after stroke (although not classified according to post-stroke phases) are in line with the findings of this study in individuals with sub-acute stroke, which found significant weak to moderate associations between the strength of the lower limb muscles and comfortable gait speed (0.36≤r≤0.53). However, according to Mentiplay et al.,6 very few studies have investigated the associations between the strength of the lower limb muscles and maximum gait speed in individuals with stroke. Of the 21 studies included in their review, only seven (33.3%) and four (19.1%) assessed the associations between the strength of the paretic and non-paretic lower limbs, respectively, and maximum gait speed in individuals after stroke. However, only the paretic and non-paretic knee extensors and the paretic ankle plantar flexors were assessed.6 The present study assessed the associations between 14 lower limb muscle groups and maximum gait speed in individuals with sub-acute stroke and found significant weak to moderate associations (0.37≤r≤0.59). Therefore, most lower limb muscle groups contribute to comfortable and maximum gait speed in individuals with sub-acute stroke.

The trunk muscles have two main functions during gait: to ensure stability and enable movement.11 The activity of the trunk muscles during walking in healthy individuals, measured by surface electromyography, increases at higher speeds.11,31 This is probably due to the increased demand for stabilization and the higher need for compensation of the rotational movements.11 However, to our knowledge, no studies have investigated the associations between the strength of the trunk muscles and gait speed in individuals with stroke. This present study found weak to moderate associations between the strength of the trunk muscles and comfortable gait speed (0.39≤r≤0.52) and moderate associations between that of the trunk muscles and maximum gait speed (0.50≤r≤0.61) in individuals with sub-acute stroke. These results emphasize the importance of considering the trunk muscles when assessing gait speed in individuals with stroke. As the impairment of the trunk muscles after stroke may be related to gait speed, these muscles should be considered during evaluations for planning strengthening interventions in individuals with sub-acute stroke.

Previous studies that performed regression analyses to identify the muscle groups that would predict gait speed after stroke only assessed comfortable gait speed in individuals in the chronic phase.6,32–35 Among all the lower limb muscles that remained significant in the regression analyses, the paretic ankle dorsiflexors appeared with greater frequency, and explained 30–49% of the variance in comfortable gait speed.32–34 This could be explained by the fact that weakness of the dorsiflexors prevents foot clearance during gait, resulting in decreased step length and a series of compensations to avoid foot drop,34 which, in turn, may lead to reduced gait speed.34 These results are not in agreement with the findings of the present study, which found that the non-paretic dorsiflexors predicted comfortable gait speed in individuals with sub-acute stroke. The differences could be explained by the fact that none of the studies that performed regression analyses in individuals with stroke assessed individuals in the sub-acute phase. As mentioned before, muscle strength impairment is more pronounced during the early post-stroke phases,2 as spontaneous motor recovery has not stabilized.7 Another possible hypothesis for the divergence in findings is that the mean comfortable gait speed of the participants in the present study (0.8m/s) was higher than that in other studies (between 0.5 and 0.75m/s). This finding highlights the importance of evaluating, and possibly strengthening, the non-paretic muscles after stroke.

None of the studies that performed regression analyses in individuals with stroke investigated the associations between the strength of the trunk muscles and maximum gait speed, which limits the comparisons of the results. A trunk kinematic deviation, often observed during post-stroke gait, is characterized by a large lateral displacement toward the non-paretic side.13,36 It is believed that this augmentation in lateral displacement is due to the weakness of the hip abductors, as trunk lateral displacement in healthy individuals is eccentrically controlled by the hip abductors.36 Thus, a possible explanation for the fact that the left lateral trunk flexors predicted maximum gait speed in this study is that this muscle group attempts to compensate for the weakness of the hip abductors and control lateral trunk displacement. Because the majority of the participants in this study had left hemiparesis, it was expected that there would be a large lateral displacement toward the right side and that the left lateral trunk flexors would be required for control. In addition, the weakness of the left lateral trunk flexors could be associated with hip hiking during gait for foot clearance due to the lack of flexion of the hip, knee, and ankle joints.13 To walk at maximum speeds, foot clearance may be more important; thus, hip hiking could be a compensatory strategy adopted by the participants. To confirm these hypotheses, futures studies should measure trunk kinematics.

The absence of studies that included individuals with sub-acute stroke and also evaluated the trunk muscles limits a deeper discussion regarding the present findings. However, the differences between the present and previous findings in individuals with chronic stroke emphasize the importance of addressing specific phases of spontaneous motor recovery in individuals with stroke.7 A better understanding of the associations between the strength of various muscle groups and gait speed may help clinicians identify the muscle group to target during muscle strength evaluation and possibly during muscle-strengthening interventions aimed at increasing gait speed in individuals with sub-acute stroke.

Significant and positive moderate associations were found between the strength of the lower limb and trunk muscles and comfortable and maximum gait speeds in individuals with sub-acute stroke. However, only the non-paretic dorsiflexors and left lateral trunk flexors explained the variance in comfortable and maximum gait speeds, respectively.

Financial support provided by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Pró-reitoria de Pesquisa da Universidade Federal de Minas Gerais (PRPq/UFMG).

The authors report no conflicts of interest.