Central Drive to the Paretic Ankle Plantarflexors Affects the Relationship Between Propulsion and Walking Speed After Stroke

Abstract

Background and purpose: The ankle plantarflexor muscles are the primary generators of propulsion during walking. Impaired paretic plantarflexion is a key contributor to interlimb propulsion asymmetry after stroke. Poststroke muscle weakness may be the result of a reduced force-generating capacity, reduced central drive, or a combination of these impairments. This study sought to elucidate the relationship between the neuromuscular function of the paretic plantarflexor muscles and propulsion deficits across individuals with different walking speeds.

Methods: For 40 individuals poststroke, we used instrumented gait analysis and dynamometry coupled with supramaximal electrostimulation to study the interplay between limb kinematics, the neuromuscular function of the paretic plantarflexors (ie, strength capacity and central drive), propulsion, and walking speed.

Results: The strength capacity of the paretic plantarflexors was not independently related to paretic propulsion. Reduced central drive to the paretic plantarflexors independently contributed to paretic propulsion deficits. An interaction between walking speed and plantarflexor central drive was observed. Individuals with slower speeds and lower paretic plantarflexor central drive presented with the largest propulsion impairments. Some study participants with low paretic plantarflexor central drive presented with similarly fast speeds as those with near-normal central drive by leveraging a compensatory reliance on nonparetic propulsion. The final model accounted for 86% of the variance in paretic propulsion (R = 0.86, F = 33.10, P < 0.001).

Discussion and conclusions: Individuals poststroke have latent paretic plantarflexion strength that they are not able to voluntarily access. The magnitude of central drive deficit is a strong indicator of propulsion impairment in both slow and fast walkers.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A298).

Fig. 1.

(A) Propulsion is produced when an ankle plantarflexion moment (M_PF) is generated when the limb is oriented behind the body (ϴ). (B) Combining isometric strength testing with muscle electrostimulation allows assessment of both a muscle’s maximum voluntary strength and strength capacity. The ratio of these force measurements is used to compute Central Drive.

Fig. 2.

(A) Maximum plantarflexion force produced voluntarily and with a superimposed burst of electrical stimulation for the paretic and nonparetic plantarflexors of 40 people poststroke. (B) Deficits in central drive to the paretic and nonparetic plantarflexors, computed as the ratio of maximum voluntary strength and strength capacity. Error bars are the standard error. * indicates a between-limb difference.

Fig. 3.

Relationship between paretic limb propulsion and walking speed is moderated by the central drive to the paretic plantarflexors. Simple slopes are calculated using ±1 standard deviation of the moderator variables—walking speed and central drive to the paretic plantarflexors.

Fig. 4.

(A) Paretic limb propulsion and (B) interlimb propulsion asymmetry ratio for four N=10 subgroups created by stratifying participants based on their walking speed and central drive to the paretic plantarflexors. Error bars are the standard error.