Reduced oxygen desaturation in the vastus lateralis of chronic stroke survivors during graded muscle contractions.

BACKGROUND: Few studies have examined changes in skeletal muscle physiology post-stroke. This study examined changes in tissue oxygen saturation (StO2) of the vastus lateralis (VL) muscle of stroke survivors and age-matched control participants during maximal and submaximal isometric contractions of the knee extensor muscles. OBJECTIVES: We hypothesized that tissue oxygen desaturation (ΔStO2) during knee extensor muscle contractions would be less in the VL in the paretic vs. the non-paretic and control legs. METHODS: Ten chronic stroke survivors (>6 months post-stroke) with lower extremity muscle weakness and 10 age-matched controls completed this prospective cohort study. Maximum voluntary contractions (MVCs) of the knee extensor muscles were assessed with a Biodex dynamometer and StO2 of the VL was measured using near-infrared spectroscopy. RESULTS: In the paretic leg of the stroke survivors little change in StO2 of the VL was observed during an MVC (ΔStO2 = -1.7 ± 1.8%) compared to the non-paretic (ΔStO2 = -5.1 ± 6.1%; p < 0.05) and control legs (ΔStO2 = -14.4 ± 8.8%; p < 0.05 vs. paretic and non-paretic leg). These differences remained when normalizing for strength differences between the legs. Compared to controls, both the paretic and non-paretic VL showed pronounced reductions in ΔStO2 during ramp and hold contractions equal to 20%, 40%, or 60% of the MVC (p < 0.05 vs. controls at all load levels). CONCLUSIONS: These results indicate that oxygen desaturation in response to isometric muscle contractions is impaired in both the paretic and non-paretic leg muscle of stroke survivors compared to age-matched controls, and these differences are independent of differences in muscle strength.

Stroke increases ischemia-related decreases in motor unit discharge rates.

Following stroke, hyperexcitable sensory pathways, such as the group III/IV afferents that are sensitive to ischemia, may inhibit paretic motor neurons during exercise. We quantified the effects of whole leg ischemia on paretic vastus lateralis motor unit firing rates during submaximal isometric contractions. Ten chronic stroke survivors (>1 yr poststroke) and 10 controls participated. During conditions of whole leg occlusion, the discharge timings of motor units were identified from decomposition of high-density surface electromyography signals during repeated submaximal knee extensor contractions. Quadriceps resting twitch responses and near-infrared spectroscopy measurements of oxygen saturation as an indirect measure of blood flow were made. There was a greater decrease in paretic motor unit discharge rates during the occlusion compared with the controls (average decrease for stroke and controls, 12.3 ± 10.0% and 0.1 ± 12.4%, respectively; P < 0.001). The motor unit recruitment thresholds did not change with the occlusion (stroke: without occlusion, 11.68 ± 5.83%MVC vs. with occlusion, 11.11 ± 5.26%MVC; control: 11.87 ± 5.63 vs. 11.28 ± 5.29%MVC). Resting twitch amplitudes declined similarly for both groups in response to whole leg occlusion (stroke: 29.16 ± 6.88 vs. 25.75 ± 6.78 Nm; control: 38.80 ± 13.23 vs 30.14 ± 9.64 Nm). Controls had a greater exponential decline (lower time constant) in oxygen saturation compared with the stroke group (stroke time constant, 22.90 ± 10.26 min vs. control time constant, 5.46 ± 4.09 min; P < 0.001). Ischemia of the muscle resulted in greater neural inhibition of paretic motor units compared with controls and may contribute to deficient muscle activation poststroke. NEW & NOTEWORTHY Hyperexcitable inhibitory sensory pathways sensitive to ischemia may play a role in deficient motor unit activation post stroke. Using high-density surface electromyography recordings to detect motor unit firing instances, we show that ischemia of the exercising muscle results in greater inhibition of paretic motor unit firing rates compared with controls. These findings are impactful to neurophysiologists and clinicians because they implicate a novel mechanism of force-generating impairment poststroke that likely exacerbates baseline weakness.

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

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.

Impaired Hyperemic Response to Exercise Post Stroke.

Individuals with chronic stroke have reduced perfusion of the paretic lower limb at rest; however, the hyperemic response to graded muscle contractions in this patient population has not been examined. This study quantified blood flow to the paretic and non-paretic lower limbs of subjects with chronic stroke after submaximal contractions of the knee extensor muscles and correlated those measures with limb function and activity. Ten subjects with chronic stroke and ten controls had blood flow through the superficial femoral artery quantified with ultrasonography before and immediately after 10 second contractions of the knee extensor muscles at 20, 40, 60, and 80% of the maximal voluntary contraction (MVC) of the test limb. Blood flow to the paretic and non-paretic limb of stroke subjects was significantly reduced at all load levels compared to control subjects even after normalization to lean muscle mass. Of variables measured, increased blood flow after an 80% MVC was the single best predictor of paretic limb strength, the symmetry of strength between the paretic and non-paretic limbs, coordination of the paretic limb, and physical activity. The impaired hemodynamic response to high intensity contractions was a better predictor of lower limb function than resting perfusion measures. Stroke-dependent weakness and atrophy of the paretic limb do not explain the reduced hyperemic response to muscle contraction alone as the response is similarly reduced in the non-paretic limb when compared to controls. These data may suggest a role for perfusion therapies to optimize rehabilitation post stroke.

Impaired regulation post-stroke of motor unit firing behavior during volitional relaxation of knee extensor torque assessed using high density surface EMG decomposition.

The purpose of this study was to use high density surface EMG recordings to quantify stroke-related abnormalities in motor unit firing behavior during repeated sub-maximal knee extensor contractions. A high density surface EMG system (sEMG) was used to record and extract single motor unit firing behavior in the vastus lateralis muscle of 6 individuals with chronic stroke and 8 controls during repeated sub-maximal isometric knee extension contractions. Paretic motor unit firing rates were increased with subsequent contractions (6.19±0.35 pps vs 7.89±0.66 pps, P <; 0.05) during task phases of torque decline as compared to controls (6.95±0.40 pps vs 6.68±0.41 pps). In addition, corresponding rates of torque decline were decreased for the paretic leg as compared to the non-paretic leg. These results suggest that regulation of declining forces may be impaired post stroke due to prolonged firing of paretic motor units.

Developing generalists for Kentucky.

Since 1985, nearly half of the graduates of the University of Kentucky College of Medicine have chosen generalist careers, even though its students received almost no outpatient ambulatory training in primary care before 1990. This study determined the factors influencing the choice of generalist specialties in the absence of ambulatory training experience. A questionnaire was mailed to the 516 graduates of the classes of 1964 through 1989 who had entered a generalist specialty. A three-way ANOVA with one repeated measure was used to determine whether there were statistically significant differences in the responses of practitioners in the three generalist specialties (family practice, general internal medicine, or general pediatrics). Sufficiently complete responses were received from 187 graduates (116 family practitioners, 40 general pediatricians, and 31 general internists). Many of the physicians who had spent formative years in rural areas were practicing in rural communities. Many respondents had already decided upon a generalist career before entering medical school. Clerkships in internal medicine and pediatrics were an important influence, as was mentor role modeling. For pediatricians, an elective ambulatory care experience was also important. Educational experiences exert meaningful influences on students interested in a generalist career. Formal ambulatory care training experiences, while not critical for the selection of a generalist career, may heighten or confirm interest. Efforts that encourage students from rural communities to enter medical school appear to produce rural physicians.

Factors influencing medical students' choice of academic medicine as a career.

The purpose of this study was to identify the characteristics of physicians who chose academic medicine as a career. A questionnaire was sent to all graduates of the University of Kentucky College of Medicine who held full-time positions in academic medical centers (n = 143). Ninety graduates (63%) returned usable questionnaires. Most of the physicians grew up in urban areas. Seventy-seven percent of the graduates entered academic medicine directly from their residency or fellowship programs. The most important factors cited by respondents as influencing a career choice of academic medicine were an interest in teaching and a belief that their personality and skills suited them to an academic environment. An interest in doing research was not a very important factor. Respondents also indicated why they chose their particular specialty. The two most important factors were the content of the specialty and intellectual stimulation. Most of these physicians (64%) were very satisfied with their careers in academic medicine.

Demographic, educational, and psychosocial factors influencing the choices of primary care and academic medical careers.

PURPOSE: To identify demographic, psychosocial, and educational variables that differentiate physicians who have chosen careers in primary care from those who have chosen academic medicine. METHOD: Questionnaires were distributed in the spring of 1992 to 704 physicians (546 in primary care practices and 143 in academic medicine) who had graduated from the University of Kentucky College of Medicine, 1964-1991. Mann-Whitney U tests and analyses of variance were used for statistical comparisons. RESULTS: A total of 336 physicians responded: 246 in primary care and 90 in academic medicine. The primary care physicians tended to come from smaller cities than did the academic physicians (p < .0001). The primary care physicians also had made their career choices earlier than did the academic physicians (p < .0001). For the academic physicians, long-term participation in research, intellectual stimulation, content of specialty, and influence of a mentor or role model were significantly more important factors than they were for the primary care physicians, for whom length of training, direct patient contact, and threats of malpractice suits were significantly more important. CONCLUSION: The results corroborate the findings of previous studies that suggest that career-choice factors are influenced by admission procedures and curricular structures. The number of graduates choosing careers in either primary care or academic medicine may be increased by increasing their experiences in those fields. Medical schools may be able to use demographic, psychosocial, and curricular factors to fulfill their particular primary mandates, whether they be producing physicians in primary care or in academic medicine.