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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., (Copyright © 2024 Elsevier Inc. All rights reserved.)

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 R 2  = 0.712) by ASM (β = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R 2  = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R 2  = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R 2  = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R 2  = 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 R 2  = 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., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The aim of the present study was to investigate the acute effect of caffeine or quercetin ingestion on motor unit firing patterns and muscle contractile properties before and after resistance exercise. High-density surface electromyography (HDs-EMG) during submaximal contractions and electrically elicited torque in knee extensor muscles were measured before (PRE) and 60 min after (POST1) ingestion of caffeine, quercetin glycosides, or placebo, and after resistance exercise (POST2) in ten young males. The Convolution Kernel Compensation technique was used to identify individual motor units of the vastus lateralis muscle for the recorded HDs-EMG. Ingestion of caffeine or quercetin induced significantly greater decreases in recruitment thresholds (RTs) from PRE to POST1 compared with placebo (placebo: 94.8 ± 9.7%, caffeine: 84.5 ± 16.2%, quercetin: 91.9 ± 36.7%), and there were significant negative correlations between the change in RTs (POST1-PRE) and RT at PRE for caffeine (rs = - 0.448, p < 0.001) and quercetin (rs =  - 0.415, p = 0.003), but not placebo (rs =  - 0.109, p = 0.440). Significant positive correlations between the change in firing rates (POST2-POST1) and RT at PRE were noted with placebo (rs = 0.380, p = 0.005) and quercetin (rs = 0.382, p = 0.007), but not caffeine (rs = 0.069, p = 0.606). No significant differences were observed in electrically elicited torque among the three conditions. These results suggest that caffeine or quercetin ingestion alters motor unit firing patterns after resistance exercise in different threshold-dependent manners in males., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Background: Insufficient rigidity of the foot owing to its ligaments and muscles can decrease the attenuation of the ground reaction force during landing. Therefore, dysfunction of the ankle invertors may increase the proximal joint load during landing., Research Question: What are the effects of the fatigued ankle invertors on workload in the lower extremity joints during single-leg landing?, Methods: Twenty-seven young adults (13 men and 14 women) performed landing trials in the forward and medial directions before and after exercise-induced fatigue of the ankle invertors. The exercise consisted of repeated concentric and eccentric ankle inversions until the maximum torque was below 80% of the baseline value. Negative joint workload during the landing tasks was calculated for the hip, knee, and ankle in the sagittal and frontal planes. Additionally, lower extremity work (the sum of the work of the hip, knee, and ankle) was calculated., Results: Invertor fatiguing exercise resulted in a significant increase in negative joint work in the frontal and sagittal plane hip and the frontal plane knee during medial landing, whereas no significant change in negative joint work was observed during forward landing., Significance: These findings suggested that ankle invertor dysfunction may induce a high load on the proximal joints and have direction-specific effects., Competing Interests: Declaration of Competing Interest The authors declare that they have no competing financial interests or personal relationships that may have influenced the work reported in this study., (Copyright © 2024 Elsevier B.V. All rights reserved.)

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.

Muscular dysfunctions involving a decline in muscle strength are often induced by loss of muscle mass in older adults. Understanding neural activation in older adults in addition to muscular characteristics may be important to prevent such age-related dysfunctions. This study aimed to investigate the difference in motor unit firing patterns between community-dwelling older individuals with normal and low skeletal muscle mass. Sixty-six older adults (62-90 years) performed muscle strength and function tests. On conducting high-density surface electromyography of the vastus lateralis, individual motor unit firing properties were assessed. Individual motor units were divided into three different recruitment threshold groups and their firing rates were compared. The skeletal muscle quantity and quality were assessed using bioimpedance methods and ultrasound images. They were divided into two groups according to sarcopenia criteria: a normal group (n = 39) and presarcopenia group with low skeletal muscle mass but normal physical functions (n = 21). Skeletal muscle mass and muscle thickness were greater and echo intensity was lower in the normal group than presarcopenia group. Motor units in normal older adults fired at different rates with a hierarchy depending on their recruitment threshold, observed as a normal phenomenon. However, motor units in the presarcopenia group fired without showing the hierarchical pattern. The results suggest that older adults with low skeletal muscle mass exhibited an abnormal neural input pattern, in addition to declines in muscle quantity and quality., Competing Interests: Declaration of Competing Interest None, (Copyright © 2023 Elsevier B.V. All rights reserved.)

Alaska pollack protein (APP), has been reported as a protein source that can enhance muscle hypertrophy more than other protein sources in animal studies. This study aimed to examine the effects of APP ingestion on muscle quantity and quality in young adults. Fifty-five young college students were assigned to two groups: APP and placebo (whey protein: WP) groups, and instructed to ingest 4.5 g of each protein in addition to daily meals, and to maintain their usual daily physical activities for 3 mo. Twenty-one and 23 students completed the intervention and were analyzed in APP and WP groups, respectively. The maximum knee extension torque significantly increased in both groups during the intervention. The motor unit discharge rate, which is an indicator of activation, for a given force level significantly decreased in both groups during the intervention, but its decrease in the APP group was significantly greater than in the WP group. Echo intensity of the vastus lateralis evaluated by ultrasound images significantly decreased in both groups. The muscle thickness and skeletal muscle mass did not change. Small amount of additional APP intake induces greater effects on neural activation than WP, suggesting the greater neural economy of generation of force.

Analyzing motor unit (MU) activities of peroneus muscles may reveal the causes of force control deficits of ankle eversion. This study aimed to examine peroneus muscles' MU discharge characteristics and associations between force fluctuation and variability of the neural drive in healthy participants. Thirty-one healthy males participated in this study. MU activities were identified from high-density surface electromyography of peroneus muscles during ankle eversion at 15 and 30% of maximal voluntary contraction (MVC). Participants increased the contraction level until reaching the target and held it for 15 s. The central 10 s of the hold phase were used for analysis. A cumulative spike train (CST) was calculated using MU firings. Variabilities of the force and CST are represented by the coefficient of variation (CoV). Spearman's rank correlation coefficient was used to assess the association between CoV of force and CoV of CST. For 15 and 30 % MVC trials, CoV of force was 1.86 ± 1.59 and 1.57 ± 1.26%, and CoV of CST was 5.01 ± 3.24 and 4.51 ± 2.78%, respectively. The correlation was significant at 15% (rho = 0.27, p < 0.001) and 30% (rho = 0.32, p < 0.001) MVC. Our findings suggest that in peroneus muscles, force fluctuation weakly to moderately correlates with neural drive variability., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

This study aimed to investigate the motor unit firing property immediately after concentric or eccentric contraction exercise. Eighteen healthy men performed repetitive maximal isokinetic knee extension exercises with only concentric or eccentric contraction until they exerted less than 80% of the baseline strength. Before and after the fatiguing exercise, high-density surface electromyography of the vastus lateralis was recorded during submaximal ramp-up isometric contraction and individual motor units were identified. Only motor units that could be tracked before and after exercise were analyzed. Muscle cross-sectional area of the vastus lateralis was measured using ultrasound, and electrically evoked torque was recorded before and after the exercise. Sixty-five and fifty-three motor units were analyzed before and after the concentric and eccentric contractions, respectively. The results showed that motor units with moderate to high recruitment thresholds significantly decreased recruitment thresholds under both conditions, and the motor unit discharge rates significantly increased after concentric contraction compared to eccentric contraction. A greater muscle cross-sectional area was observed with concentric contraction. The evoked torque was significantly decreased under both conditions, but no difference between the conditions. These results suggest that fatiguing exercise with concentric contraction contributes to greater neural input to muscles and metabolic responses than eccentric contraction., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Purpose: This study was aimed at determining the relationship between ankle plantar flexor force steadiness and postural control during single leg standing on stable and unstable platforms., Methods: For the thirty-three healthy participants, force steadiness, at target torques of 5%, 20%, and 50% of the maximum voluntary torque (MVT) of the ankle plantar flexors, was measured. Force steadiness was calculated as the coefficient of variation of force. Single leg standing on stable and unstable platforms was performed using the BIODEX Balance System SD. The standard deviation of the anteroposterior center of pressure (COP) displacements was measured as the index for postural control. During both measurements, muscle activities of the soleus were collected using surface electromyography., Results: On the stable platform, the COP fluctuation significantly correlated with force steadiness at 5% of MVT (r = 0.512, p = 0.002). On the unstable platform, the COP fluctuation significantly correlated with force steadiness at 20% of MVT (r = 0.458, p = 0.007). However, the extent of muscle activity observed for a single leg standing on both stable and unstable platforms was significantly greater than the muscle activity observed while performing force steadiness tasks at 5% and 20% of MVT, respectively., Conclusion: Postural stability during single leg standing on stable and unstable platforms may be related to one's ability to maintain constant torque at 5% and 20% of MVT regardless of the muscle activity. These results suggest that the required abilities to control muscle force differ depending on the postural control tasks.

The aim of the study was to investigate the effect of static stretching (SS) with different rest intervals on muscle stiffness. Fifteen healthy males participated in the study. Four bouts of thirty-second SS for the gastrocnemii were performed at the maximal dorsiflexion using dynamometer with two different rest intervals between stretches, namely 0 s (R0) and 30 s (R30). Each participant underwent both stretching protocols at least 48 h apart in a random order. Between each bout of SS, the ankle was moved to 20°-plantar-flexion in 3 s, held for each rest interval time, and then returned to the stretching position in 3 s. The shear elastic modulus of the medial gastrocnemius was measured before (PRE) and immediately after (POST) four bouts of SS to assess muscle stiffness of the medial gastrocnemius. Two-way repeated measures analysis of variance (protocol × time) indicated a significant interaction effect on the shear elastic modulus. The shear elastic modulus significantly decreased after SS in both protocols [R0, PRE: 11.5 ± 3.3 kPa, POST: 10.0 ± 2.6 kPa, amount of change: 1.6 ± 0.9 kPa (13.0 ± 5.2%); R30, PRE: 11.0 ± 2.8 kPa, POST: 10.2 ± 2.1 kPa, amount of change: 0.8 ± 1.3 kPa (6.0 ± 10.4%)]. Furthermore, the SS with 0-s rest interval induced greater decrease in shear elastic modulus when compared to SS with 30-s rest interval (p = 0.023). Thus, when performing SS to decrease muscle stiffness, rest intervals between stretches should be minimized., (Copyright © 2019 Elsevier Ltd. All rights reserved.)