Ischemic conditioning increases strength and volitional activation of paretic muscle in chronic stroke: a pilot study

Abstract

Ischemic conditioning (IC) on the arm or leg has emerged as an intervention to improve strength and performance in healthy populations, but the effects on neurological populations are unknown. The purpose of this study was to quantify the effects of a single session of IC on knee extensor strength and muscle activation in chronic stroke survivors. Maximal knee extensor torque measurements and surface EMG were quantified in 10 chronic stroke survivors (>1 yr poststroke) with hemiparesis before and after a single session of IC or sham on the paretic leg. IC consisted of 5 min of compression with a proximal thigh cuff (inflation pressure = 225 mmHg for IC or 25 mmHg for sham) followed by 5 min of rest. This was repeated five times. Maximal knee extensor strength, EMG magnitude, and motor unit firing behavior were measured before and immediately after IC or sham. IC increased paretic leg strength by 10.6 ± 8.5 Nm, whereas no difference was observed in the sham group (change in sham = 1.3 ± 2.9 Nm, P = 0.001 IC vs. sham). IC-induced increases in strength were accompanied by a 31 ± 15% increase in the magnitude of muscle EMG during maximal contractions and a 5% decrease in motor unit recruitment thresholds during submaximal contractions. Individuals who had the most asymmetry in strength between their paretic and nonparetic legs had the largest increases in strength ( r2 = 0.54). This study provides evidence that a single session of IC can increase strength through improved muscle activation in chronic stroke survivors. NEW & NOTEWORTHY Present rehabilitation strategies for chronic stroke survivors do not optimally activate paretic muscle, and this limits potential strength gains. Ischemic conditioning of a limb has emerged as an effective strategy to improve muscle performance in healthy individuals but has never been tested in neurological populations. In this study, we show that ischemic conditioning on the paretic leg of chronic stroke survivors can increase leg strength and muscle activation while reducing motor unit recruitment thresholds.

Keywords: electromyography; ischemic conditioning; muscle strength; stroke rehabilitation.

Figures

Fig. 1.

A: protocol summary of the ischemic conditioning (IC) protocol. Subjects performed a series of isometric maximum voluntary contractions (MVC) of the knee extensor muscles, followed by a submaximal contraction equal to 40% of their maximum using a Biodex dynamometer. After the initial contractions were completed, the subjects moved to a bed, where the IC protocol was performed. The subjects laid in the supine position, and a blood pressure cuff was placed around the proximal thigh of paretic leg and inflated to either 225 (IC condition) or 25 mmHg (sham condition) for 5 min. After 5 min of inflation, the cuff was deflated for 5 min, and this was repeated for 5 cycles. Following the IC or sham protocol, subjects were placed back in the Biodex dynamometer, and knee extensor MVCs and submaximal contractions were repeated. B and C: representative torque traces of an MVC from a single subject before and after the IC and sham conditions, respectively, are shown. Note the increase in MVC magnitude for the IC condition.

Fig. 2.

Individual knee extensor maximum voluntary contraction (MVC) responses of the paretic leg before and after either ischemic conditioning (IC) or sham treatment. A and B: individuals in the IC group demonstrated an increase in knee extensor MVC following IC (P < 0.05; 2-way repeated-measures ANOVA; A), and no difference following sham treatment (P > 0.05; B). C: on average, individuals in the IC group demonstrated a 16.1 ± 14.5% relative increase in knee extensor strength following IC (P < 0.05). *P < 0.05, pre vs. post or IC vs. sham. ●, Individual measures.

Fig. 3.

Changes in knee extensor strength following IC as a function of leg impairment. A: there was a strong correlation between asymmetry in MVC magnitude between the paretic and nonparetic leg and %change in MVC in response to IC. Subjects who showed a greater degree of asymmetry in knee extensor strength between their paretic and nonparetic legs showed a greater improvement in paretic leg strength following IC (r2 = 0.55, P = 0.014). B: subjects who had the lowest Lower Extremity Fugl Meyer Score also had the largest improvements in knee extensor strength following IC (r2 = 0.61; P = 0.008). C: there was a moderate correlation between self-selected walking speed and gains in strength following IC, whereby subjects who walked the slowest tended to have the largest increases in strength (r2 = 0.33, P = 0.08). ●, Individual measures.

Fig. 4.

Changes in vastus lateralis electromyography (EMG) measurements that accompanied IC-induced increases in knee extensor torque. A: average root mean square of the EMG signal during MVCs was increased following IC (P = 0.01; paired t-test). B: a single-subject spatial activation map of the change in the RMS of the EMG across the EMG array pre- to post-IC during the MVCs. Coloring reflects the degree of change, where red indicates the largest increases and blue indicates decreases in the root mean square (RMS) of the EMG. C: modified entropy increased following IC (P = 0.02; paired t-test). This indicates increased homogeneity in the potential distribution across the array. D: there was an IC-induced decrease in the average coefficient of variation of the EMG signal from each channel in the array (P = 0.02; t-test). *P < 0.05, pre- vs. post-IC.

Fig. 5.

Motor unit firing behavior and recruitment during the submaximal ramp and hold task. A: single-subject raster plot of incidences of action potentials superimposed on the torque generated during the ramp and hold pre- (left) and post-IC (right). Each row is a separate motor unit matched between time points. B: average force recruitment thresholds decreased following IC reflecting increased excitability of the motoneuron pools (P < 0.01; paired t-test).