Merged plantarflexor muscle activity is predictive of poor walking performance in post-stroke hemiparetic subjects

Abstract

Stroke is the leading cause of long-term disability and individuals post-stroke often experience impaired walking ability. The plantarflexor (PF) muscles are critical to walking through their contributions to the ground reaction forces and body segment energetics. Previous studies have shown muscle activity during walking can be grouped into co-excited muscle sets, or modules. Improper co-activation, or merging of modules, is a common impairment in individuals post-stroke. The purpose of this study was to determine the influence of merged PF modules on walking performance in individuals post stroke by examining balance control, body support and propulsion, and walking symmetry. Muscle modules were identified using non-negative matrix factorization to classify subjects as having an independent or merged PF module. The merged group had decreased balance control with a significantly higher frontal plane whole-body angular momentum than both the independent and control groups, while the independent and control groups were not significantly different. The merged group also had higher paretic braking and nonparetic propulsion than both the independent and control groups. These results remained when comparisons were limited to subjects who had the same number of modules, indicating this was not a general effect due to subjects with merged PF having fewer modules. It is likely that a merged PF module is indicative of general PF dysfunction even when some activation occurs at the appropriate time. These results suggest an independent PF module is critical to walking performance, and thus obtaining an independent PF module should be a crucial aim of stroke rehabilitation.

Keywords: Biomechanics; Gait; Muscle modules; Rehabilitation; Synergies.

Fig. 1.

Representative NNMF results, where each row represents one module and shows the weighted contribution of each muscle to that module (left column) and the activation of that module throughout a gait cycle (right column). Results are shown for: A) control subject with four independent modules, and B) subject with a PF module merged with module 1 (purple trace).

Fig. 2.

Group averaged frontal plane A) H and B) H_R ± one standard deviation for the merged, independent and control groups. Results are shown for unitless H (normalized by body mass, leg length, and walking speed) and H not normalized by walking speed (units = m/s, normalized by body mass and leg length). Vertical lines show average paretic and nonparetic heel strikes (PHS, NHS) and toe-offs (PTO, NTO) as indicated on the horizontal axis. ‘*” indicates a significant difference between the H_R of the merged group and the independent and control groups.

Fig. 3.

Group averaged vertical ground reaction forces ± one standard deviation for the paretic leg (solid line) and non-paretic leg (dashed line) for the merged and independent groups (left and right sides of the control group, respectively).

Fig. 4.

Group averaged braking and propulsion ± one standard deviation (normalized by subject weight and walking speed) for the paretic and non-paretic legs of the merged and independent groups (left and right legs for the control group, respectively). ‘*’ indicates significance between the merged group and both other groups.

Fig. 5.

Paretic step length ratio for each subject in the merged, independent and control groups. The horizontal dashed line indicates perfect symmetry.