One-week quercetin intervention modifies motor unit recruitment patterns before and during resistance exercise in older adults: A randomized controlled trial.

We investigated the effects of one-week quercetin ingestion on motor unit (MU) behavior and muscle contractile properties before, during, and after a single session of resistance exercise in older adults. Twenty-four older adults were divided into two groups: those receiving quercetin glycosides (QUE) or placebo (PLA), and they performed a single session of resistance exercise. MU behavior before and during resistance exercise and electrically elicited contraction before and after resistance exercise were measured (Day 1), and the same measurements were conducted again after 7 days of placebo or quercetin glycoside ingestion (Day 8). The MU recruitment threshold (RT) was decreased (p < 0.001, 25.6 ± 10.1 to 23.6 ± 9.5 %MVC) and the exerted force normalized by the MU firing rate (FR) was increased (p = 0.003, 1.13 ± 0.24 to 1.18 ± 0.22 %MVC/pps) from Days 1 to 8, respectively, in QUE but not PLA (p = 0.263, 22.6 ± 11.9 to 21.9 ± 11.6 %MVC; p = 0.713, 1.09 ± 0.20 to 1.10 ± 0.19 %MVC/pps, respectively). On Day 1, a significant correlation between MURT and%change in MUFR from the first to last contractions during the resistance exercise was observed in both groups (QUE: p = 0.009, rs = 0.308; PLA: p < 0.001, rs = 0.403). On Day 8 %change in MUFR was negatively correlated with MURT in QUE (p = 0.044, rs = -0.251), but there was no significant correlation in PLA (p = 0.844). There was no difference in electrically elicited contraction before and after the resistance exercise between QUE and PLA (p < 0.05). These results suggest that one-week quercetin ingestion in older adults lowered MURT and led to greater fatigue in MU with higher RT than with lower RT during resistance training.

Sympathetic Neural Control at Rest and During the Cold Pressor Test in Patients With Heart Failure With Preserved Ejection Fraction.

BACKGROUND: We tested the hypothesis that patients with heart failure with preserved ejection fraction (HFpEF) would have greater muscle sympathetic nerve activity (MSNA) at rest and sympathetic reactivity during a cold pressor test compared with non-heart failure controls. Further, given the importance of the baroreflex modulation of MSNA in the control of blood pressure (BP), we hypothesized that patients with HFpEF would exhibit a reduced sympathetic baroreflex sensitivity. METHODS: Twenty-eight patients with HFpEF and 44 matched controls (mean±SD: 71±8 versus 70±7 years; 9 men/19 women versus 16 men/28 women) were studied. BP, heart rate, and MSNA (microneurography) were measured during 6 to 10 minutes of supine rest and the 2-minute cold pressor test. Spontaneous sympathetic baroreflex sensitivity was assessed during supine rest. RESULTS: Patients with HFpEF had higher resting MSNA burst frequency (39±14 versus 31±12 bursts/min; P=0.020) and lower sympathetic baroreflex sensitivity (-2.83±0.76 versus -3.57±1.19 bursts/100 heartbeats/mm Hg; P=0.019) than controls, but burst incidence was not different between groups (56±19 versus 50±20 bursts/100 heartbeats; P=0.179). During the cold pressor test, increases in MSNA indices did not differ between groups (P=0.135-0.998), but patients had a smaller increase in diastolic BP (Δ4±6 versus Δ14±11 mm Hg; P<0.001) compared with controls. CONCLUSIONS: Despite augmented resting MSNA burst frequency, burst incidence was not significantly different between groups, and sympathetic baroreflex sensitivity was reduced in patients with HFpEF. Furthermore, patients had preserved sympathetic reactivity but attenuated diastolic BP responses during the cold pressor test. These data suggest that, during physiological stress, sympathetic reactivity is intact, but the peripheral pathway for sympathetic vasoconstriction may be impaired in HFpEF.

Time-of-day effects on motor unit firing and muscle contractile properties in humans.

Intrinsic factors related to neuromuscular function are time-of-day dependent, but diurnal rhythms in neural and muscular components of the human neuromuscular system remain unclear. The present study aimed to investigate the time-of-day effects on neural excitability and muscle contractile properties by assessing the firing properties of tracked motor units and electrically evoked twitch muscle contraction. In 15 young adults (22.9 ± 4.7 yr), neuromuscular function was measured in the morning (10:00), at noon (13:30), in the evening (17:00), and at night (20:30). Four measurements were completed within 24 h. The measurements consisted of maximal voluntary contraction (MVC) strength of knee extension, recording of high-density surface electromyography (HDsEMG) from the vastus lateralis during ramp-up contraction to 50% of MVC, and evoked twitch torque of knee extensors by electrical stimulation. Recorded HDsEMG signals were decomposed to individual motor unit firing behaviors and the same motor units were tracked among the times of day, and recruitment thresholds and firing rates were calculated. The number of detected and tracked motor units was 127. Motor unit firing rates significantly increased from morning to noon, evening, and night (P < 0.01), but there were no significant differences in recruitment thresholds among the times of day (P > 0.05). Also, there were no significant effects of time of day on evoked twitch torque (P > 0.05). Changes in the motor unit firing rate and evoked twitch torque were not significantly correlated (P > 0.05). These findings suggest that neural excitability may be affected by the time of day, but it did not accompany changes in peripheral contractile properties in a diurnal manner.NEW & NOTEWORTHY We investigated the variations of tracked motor unit firing properties and electrically evoked twitch contraction during the day within 24 h. The variation of motor unit firing rate was observed, and tracked motor unit firing rate increased at noon, in the evening, and at night compared with that in the morning. The variation in motor unit firing rate was independent of changes in twitch contraction. Motor unit firing rate may be affected by diurnal rhythms.

Association between physical fitness tests and neuromuscular properties.

PURPOSE: While various fitness tests have been developed to assess physical performances, it is unclear how these tests are affected by differences, such as, in morphological and neural factors. This study was aimed to investigate associations between individual differences in physical fitness tests and neuromuscular properties. METHODS: One hundred and thirty-three young adults participated in various general physical fitness tests and neuromuscular measurements. The appendicular skeletal muscle mass (ASM) was estimated by bioelectrical impedance analysis. Echo intensity (EI) was evaluated from the vastus lateralis. During submaximal knee extension force, high-density surface electromyography of the vastus lateralis was recorded and individual motor unit firings were detected. Y-intercept (i-MU) and slope (s-MU) from the regression line between the recruitment threshold and motor unit firing rate were calculated. RESULTS: Stepwise multiple regression analyses revealed that knee extension strength could be explained (adjusted R2 = 0.712) by ASM (ß = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R2 = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R2 = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R2 = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R2 = 0.631). On the other hand, i-MU and s-MU could be explained by five-sec stepping and counter-movement jump, respectively, but the coefficients of determination were low (adjusted R2 = 0.100 and 0.045). CONCLUSION: Generally developed physical fitness tests were mainly explained by morphological factors, but were weakly affected by neural factors involved in performance.

Can Neuromuscular Electrical Stimulation Enhance the Effect of Sprint Interval Training?

The aim of this study was to determine the effects of subtetanic neuromuscular electrical stimulation combined with voluntary exercise between repeated Wingate tests on sprint exercise performance and blood lactate accumulation during sprint interval training. Fifteen healthy young males volunteered. After 1-min baseline, participants underwent the Wingate test twice. They performed a 4-min intervention between tests: neuromuscular electrical stimulation with free-weight cycling or voluntary cycling alone [43.6 (8.0) watts], which matched oxygen consumption with neuromuscular electrical stimulation with free-weight cycling. The blood lactate concentration was assessed at the end of the baseline, at 3-min intervention, and on recovery at 1, 3, 5, and 10 min after the second Wingate test. Peak and mean blood lactate concentration during recovery were significantly greater with neuromuscular electrical stimulation with free-weight cycling than voluntary cycling alone (P>0.036 and P=0.011, respectively). Peak power, mean power, and rate of decline (fatigue index) were not significantly different between conditions in both Wingate tests (condition/interaction all P>0.300, partial η2<0.1). Subtetanic neuromuscular electrical stimulation combined with voluntary exercise indicated similar exercise performance and fatigue levels during Wingate tests, but enhanced blood lactate accumulation compared to oxygen consumption-matched voluntary cycling during sprint interval training.

Differences in acute neuromuscular response after single session of resistance exercise between young and older adults.

AIMS: The purpose of this study was to investigate differences in the acute response after resistance exercise between young and older adults. METHODS: Seventeen young and 18 older adults performed a single session of resistance exercise, consisting of 3 sets of 10 isometric knee extensions. Maximal voluntary contraction (MVC), motor unit (MU) activity of the vastus lateralis, and electrically elicited torque of the knee extensor were measured before and after the resistance exercise. RESULTS: Although both groups showed the same degree of decline in MVC (young: -15.2 ± 14.3 %, older: -16.4 ± 7.9 %, p = 0.839), electrically elicited torque markedly decreased in the young group (young: -21.5 ± 7.7 %, older: -14.3 ± 9.5 %, p < 0.001), and the decrease in the MU firing rate was greater in the older group (young: -26.1 ± 24.1 %, older: -44.7 ± 24.5 %, p < 0.001). Changes in the MU firing rate following the exercise were correlated with the MU recruitment threshold in the older group (p < 0.001, rs = 0.457), but not young group (p = 0.960). DISCUSSION: These results showed that young adults exhibited a greater acute response in the peripheral component, whereas older adults showed a greater acute response in the central component of the neuromuscular system, and the acute response in MUs with a high recruitment threshold following resistance exercise was smaller than in those with a low recruitment threshold in older adults. These findings may partly explain why there are different chronic adaptations to resistance training between young and older adults.

Impact of maternal obesity on resting muscle sympathetic nerve activity during uncomplicated pregnancy: a longitudinal assessment.

Maternal obesity increases the risk of adverse pregnancy outcomes. The mechanisms that contribute to this elevated risk are unclear but may be related to greater activity of the sympathetic nervous system, which is associated with hypertensive disorders of pregnancy. We hypothesized that resting muscle sympathetic nerve activity (MSNA) would be greater in women with obesity during pregnancy when compared with normal-weight women. Blood pressure, heart rate, and MSNA were recorded during 5 min of supine rest in 14 normal-weight women [body mass index (BMI) 22.1 ± 2.1 (SD) kg/m2] and 14 women with obesity (BMI 33.9 ± 3.5 kg/m2) during (early and late) pregnancy and postpartum. All women had uncomplicated pregnancies. Resting MSNA burst frequency was not different between groups during early (normal weight 17 ± 10 vs. obesity 22 ± 15 bursts/min, P = 0.35) but was significantly greater in the obesity group during late pregnancy (23 ± 13 vs. 35 ± 15 bursts/min, P = 0.031) and not different postpartum (10 ± 6 vs. 9 ± 7 bursts/min, P = 0.74). These findings were also apparent when comparing burst incidence and total activity. Although still within the normotensive range, systolic blood pressure was greater in the obesity group across all time points (P = 0.002). Diastolic blood pressure was lower during pregnancy compared with postpartum (P < 0.001) and not different between groups (P = 0.488). Heart rate increased throughout pregnancy in both groups (P < 0.001). Our findings suggest that maternal obesity is associated with greater increases in sympathetic activity even during uncomplicated pregnancy. Future research is needed to determine if this is linked with an increased risk of adverse outcomes or is required to maintain homeostasis in pregnancy.NEW & NOTEWORTHY The impact of maternal obesity on resting muscle sympathetic nerve activity was examined during (early and late) and after uncomplicated pregnancy. Resting muscle sympathetic nerve activity is not different during early pregnancy or postpartum but is significantly elevated in women with obesity during late pregnancy when compared with normal-weight women. Future research is needed to determine if this is linked with an increased risk of adverse outcomes or is required to maintain homeostasis in pregnancy.