Characterising sex-related differences in lower- and higher-threshold motor unit behaviour through high-density surface electromyography.

Emerging questions in neuromuscular physiology revolve around whether males and females share similar neural control in diverse tasks across a broad range of intensities. In order to explore these features, high-density electromyography was used to record the myoelectrical activity of biceps brachii during trapezoidal isometric contractions at 35% and 70% of maximal voluntary force (MVF) on 11 male and 13 female participants. Identified motor units were then classified as lower-threshold (recruited at ≤30%MVF) and higher-threshold (recruited at >30%MVF). The discharge rate, interspike interval variability, recruitment and derecruitment thresholds, and estimates of neural drive to motor neurons were assessed. Female lower-threshold motor units showed higher neural drive (P < 0.001), accompanied by higher discharge rate at recruitment (P = 0.006), plateau (P = 0.001) and derecruitment (P = 0.001). On the other hand, male higher-threshold motor units showed greater neural drive (P = 0.04), accompanied by higher discharge rate at recruitment (P = 0.005), plateau (P = 0.04) and derecruitment (P = 0.01). Motor unit discharge rate normalised by the recruitment threshold was significantly higher in female lower-threshold motor units (P < 0.001), while no differences were observed in higher-threshold motor units. Recruitment and derecruitment thresholds are higher in males across all intensities (P < 0.01). However, males and females have similar activation and deactivation strategies, as evidenced by similar recruitment-to-derecruitment ratios (P > 0.05). This study encompasses a broad intensity range to analyse motor unit sex-related differences, highlighting higher neural drive and discharge rates in female lower-threshold motor units, elevated recruitment and derecruitment thresholds in males, and convergences in activation and deactivation strategies. HIGHLIGHTS: What is the central question of the study? Do male and female motor units behave similarly in low- and high-intensity contractions? What is the main finding and its importance? Female motor units show higher discharge rates in low-intensity tasks and lower discharge rates in high-intensity tasks, with no differences in recruitment behaviour. A broader inter-spike interval variability was also observed in females. These findings underline that there are sex-specific differences concern the firing strategies based on task intensity.

Exercise Prescription for Postprandial Glycemic Management.

The detrimental impacts of postprandial hyperglycemia on health are a critical concern, and exercise is recognized a pivotal tool in enhancing glycemic control after a meal. However, current exercise recommendations for managing postprandial glucose levels remain fairly broad and require deeper clarification. This review examines the existing literature aiming to offer a comprehensive guide for exercise prescription to optimize postprandial glycemic management. Specifically, it considers various exercise parameters (i.e., exercise timing, type, intensity, volume, pattern) for crafting exercise prescriptions. Findings predominantly indicate that moderate-intensity exercise initiated shortly after meals may substantially improve glucose response to a meal in healthy individuals and those with type 2 diabetes. Moreover, incorporating short activity breaks throughout the exercise session may provide additional benefits for reducing glucose response.

Muscular Adaptations to Concurrent Resistance Training and High-Intensity Interval Training in Adults with Type 2 Diabetes: A Pilot Study.

This pilot study aimed to compare the effects of eight weeks of concurrent resistance training (RT) and high-intensity interval training (HIIT) vs. RT alone on muscle performance, mass and quality in adults with type 2 diabetes (T2DM). Twelve T2DM adults were randomly allocated to the RT + HIIT (n = 5) or RT (n = 7) group. Before and after training, maximal oxygen uptake (VO2max), muscle strength and power were evaluated by calorimetry, dynamometry and one-repetition maximum (1RM) test. Quadriceps muscle volume was determined by MRI, and muscle quality was estimated. After RT, VO2max (+12%), knee muscle power (+20%), quadriceps muscle volume (+5.9%) and quality (leg extension, +65.4%; leg step-up, +223%) and 1RM at leg extension (+66.4%), leg step-up (+267%), lat pulldown (+60.9%) and chest press (+61.2%) significantly increased. The RT + HIIT group improved on VO2max (+27%), muscle volume (+6%), muscle power (+9%) and 1RM at lat pulldown (+47%). No other differences were detected. Among groups, changes in muscle quality at leg step-up and leg extension and VO2max were significantly different. The combination of RT and HIIT effectively improves muscle function and size and increases cardiorespiratory fitness in adults with T2DM. However, HIIT combined with RT may interfere with the development of muscle quality.

Ventilation and perceived exertion are sensitive to changes in exercise tolerance: arm+leg cycling vs. leg cycling.

Purpose: Growing evidence suggests that respiratory frequency (f R) is a marker of physical effort and a variable sensitive to changes in exercise tolerance. The comparison between arm+leg cycling (Arm+leg) and leg cycling (Leg) has the potential to further test this notion because a greater exercise tolerance is expected in the Arm+leg modality. We systematically compared Arm+leg vs. Leg using different performance tests. Methods: Twelve males underwent six performance tests in separate, randomized visits. Three tests were performed in each of the two exercise modalities, i.e. an incremental test and two time-to-exhaustion (TTE) tests performed at 90% or 75% of the peak power output reached in the Leg incremental test (PPOLeg). Exercise tolerance, perceived exertion, and cardiorespiratory variables were recorded during all the tests. Results: A greater exercise tolerance (p < 0.001) was found for Arm+leg in the incremental test (337 ± 32 W vs. 292 ± 28 W), in the TTE test at 90% of PPOLeg (638 ± 154 s vs. 307 ± 67 s), and in the TTE test at 75% of PPOLeg (1,675 ± 525 s vs. 880 ± 363 s). Unlike V˙O2 and heart rate, both f R and minute ventilation were lower (p < 0.003) at isotime in all the Arm+leg tests vs. Leg tests. Furthermore, a lower perceived exertion was observed in the Arm+leg tests, especially during the TTE tests (p < 0.001). Conclusion: Minute ventilation, f R and perceived exertion are sensitive to the improvements in exercise tolerance observed when comparing Arm+leg vs. Leg, unlike V˙O2 and heart rate.

Adjustments in the motor unit discharge behavior following neuromuscular electrical stimulation compared to voluntary contractions.

Introduction: The application of neuromuscular electrical stimulation superimposed on voluntary muscle contractions (NMES+) has demonstrated a considerable potential to enhance or restore muscle function in both healthy and individuals with neurological or orthopedic disorders. Improvements in muscle strength and power have been commonly associated with specific neural adaptations. In this study, we investigated changes in the discharge characteristics of the tibialis anterior motor units, following three acute exercises consisting of NMES+, passive NMES and voluntary isometric contractions alone. Methods: Seventeen young participants participated in the study. High-density surface electromyography was used to record myoelectric activity in the tibialis anterior muscle during trapezoidal force trajectories involving isometric contractions of ankle dorsi flexors with target forces set at 35, 50% and 70% of maximal voluntary isometric contraction (MVIC). From decomposition of the electromyographic signal, motor unit discharge rate, recruitment and derecruitment thresholds were extracted and the input-output gain of the motoneuron pool was estimated. Results: Global discharge rate increased following the isometric condition compared to baseline at 35% MVIC while it increased after all experimental conditions at 50% MVIC target force. Interestingly, at 70% MVIC target force, only NMES + led to greater discharge rate compared to baseline. Recruitment threshold decreased after the isometric condition, although only at 50% MVIC. Input-output gain of the motoneurons of the tibialis anterior muscle was unaltered after the experimental conditions. Discussion: These results indicated that acute exercise involving NMES + induces an increase in motor unit discharge rate, particularly when higher forces are required. This reflects an enhanced neural drive to the muscle and might be strongly related to the distinctive motor fiber recruitment characterizing NMES+.

The Effects of Postprandial Walking on the Glucose Response after Meals with Different Characteristics.

We evaluated the effect of postprandial walking on the post-meal glycemic response after meals with different characteristics. Twenty-one healthy young volunteers participated in one of two randomized repeated measures studies. Study 1 (10 participants) assessed the effects of 30 min of brisk walking after meals with different carbohydrate (CHO) content (0.75 or 1.5 g of CHO per kg/body weight). Study 2 (11 participants) evaluated the effects of 30 min of brisk walking after consuming a mixed meal or a CHO drink matched for absolute CHO content (75 g). Postprandial brisk walking substantially reduced (p < 0.009) the glucose peak in both studies, with no significant differences across conditions. When evaluating the glycemic response throughout the two hours post-meal, postprandial walking was more effective after consuming a lower CHO content (Study 1), and similarly effective after a mixed meal or a CHO drink (Study 2), although higher glucose values were observed when consuming the CHO drink. Our findings show that a 30 min postprandial brisk walking session improves the glycemic response after meals with different CHO content and macronutrient composition, with implications for postprandial exercise prescription in daily life scenarios.

Walking Attenuates Postprandial Glycemic Response: What Else Can We Do without Leaving Home or the Office?

We evaluated the effects of different exercise types suitable for a home/work setting on the postprandial glucose response. Twenty-three healthy, active, young individuals performed one of two studies (12 in Study 1 and 11 in Study 2), with four randomized protocols each. After a meal high in carbohydrate content (1 g of carbohydrate per kg of body weight), in Study 1, participants performed 30 min of either walking (WALK), bench stepping exercise (STEP) or isometric wall squat (SQUAT); in Study 2, participants performed 30 min of either walking (WALK), neuromuscular electrical stimulation alone (P_NMES) or superimposed on voluntary muscle contraction (VC_NMES). In both studies, participants performed a prolonged sitting condition (CON) that was compared to the exercise sessions. In Study 1, WALK and STEP significantly reduced the glucose peak compared to CON (p < 0.011). In Study 2, the peak was significantly reduced in WALK compared to CON, P_NMES and VC_NMES (p < 0.011) and in VC_NMES compared to CON and P_NMES (p < 0.011). A significant reduction of 3 h glucose iAUC was found for WALK and VC_NMES compared to CON and P_NMES (p < 0.033). In conclusion, WALK is the most effective strategy for improving the postprandial glycemic response. However, STEP and VC_NMES can also be used for reducing postprandial glycemia.

Quercetin Modulates IGF-I and IGF-II Levels After Eccentric Exercise-Induced Muscle-Damage: A Placebo-Controlled Study.

Background: Prolonged or unaccustomed eccentric exercise may cause muscle damage and depending from its extent, this event negatively affects physical performance. Objectives: The aim of the present investigation was to evaluate, in humans, the effect of the flavonoid quercetin on circulating levels of the anabolic insulin-like growth factor 1 (IGF-I) and insulin-like growth factor 2 (IGF-II), produced during the recovery period after an eccentric-induced muscle damage (EIMD). Methods: A randomized, double-blind, crossover study has been performed; twelve young men ingested quercetin (1 g/day) or placebo for 14 days and then underwent an eccentric-induced muscle damaging protocol. Blood samples were collected, and cell damage markers [creatine kinase (CK), lactate dehydrogenase (LDH) and myoglobin (Mb)], the inflammatory responsive interleukin 6 (IL-6), IGF-I and IGF-II levels were evaluated before the exercise and at different recovery times from 24 hours to 7 days after EIMD. Results: We found that, in placebo treatment the increase in IGF-I (72 h) preceded IGF-II increase (7 d). After Q supplementation there was a more marked increase in IGF-I levels and notably, the IGF-II peak was found earlier, compared to placebo, at the same time of IGF-I (72 h). Quercetin significantly reduced plasma markers of cell damage [CK (p<0.005), LDH (p<0.001) and Mb (p<0.05)] and the interleukin 6 level [IL-6 (p<0.05)] during recovery period following EIMD compared to placebo. Conclusions: Our data are encouraging about the use of quercetin as dietary supplementation strategy to adopt in order to mitigate and promote a faster recovery after eccentric exercise as suggested by the increase in plasma levels of the anabolic factors IGF-I and IGF-II.

The Effect of Different Postprandial Exercise Types on Glucose Response to Breakfast in Individuals with Type 2 Diabetes.

Postprandial exercise represents an important tool for improving the glycemic response to a meal. This study evaluates the effects of the combination and sequence of different exercise types on the postprandial glycemic response in patients with type 2 diabetes. In this repeated-measures crossover study, eight patients with type 2 diabetes performed five experimental conditions in a randomized order: (i) uninterrupted sitting (CON); (ii) 30 min of moderate intensity aerobic exercise (walking) (A); (iii) 30 min of combined aerobic and resistance exercise (AR); (iv) 30 min of combined resistance and aerobic exercise (RA); and (v) 15 min of resistance exercise (R). All the exercise sessions started 30 min after the beginning of a standardized breakfast. All the exercise conditions showed a significant attenuation of the post-meal glycemic excursion (P < 0.003) and the glucose incremental area under the curve at 0-120 min (P < 0.028) and 0-180 min (P < 0.048) compared with CON. A greater reduction in the glycemic peak was observed in A and AR compared to RA (P < 0.02). All the exercise types improved the post-meal glycemic response in patients with type 2 diabetes, with greater benefits when walking was performed alone or before resistance exercise.

Effects of exercise before and/or after a mixed lunch on postprandial metabolic responses in healthy male individuals.

PURPOSE: Exercise plays an important role in preventing and treating postprandial dysmetabolism. We investigated the postprandial metabolic responses to a standard lunch when a session of aerobic exercise is performed in the early postprandial phase or divided between the pre- and postprandial period. METHODS: Nine healthy volunteers consumed a standardised mixed lunch and rested for the following 3 h (Con) or performed 40 min of cycling at 65% V̇O2max after lunch (CPPEx), or two 20-min sessions, one before (SplitEx1) and the other after lunch (SplitEx2), at the same intensity. RESULTS: At 1-h post-lunch, a significant reduction (P < 0.001) in glycaemia was observed for CPPEx (- 25 ± 10%) and SplitEx (- 34 ± 7%) compared to Con. Yet, a post-exercise rebound lessened the exercise effect on the glycaemic area under the curve (AUC) at 2 and 3 h. At 1 h, a significant reduction (P < 0.009) in plasma insulin (SplitEx - 53 ± 31%; CCPEx - 48 ± 20%) and C-peptide (SplitEx - 57 ± 20%; CCPEx - 47 ± 24%) was observed compared to Con. Glucose-dependent insulinotropic polypeptide (GIP) increased after the meal, without differences between conditions. Compared with SplitEx1, cortisol response was attenuated during SplitEx2 and CPPEx. At 3 hours, triglyceride AUC was significantly higher (P = 0.039) in SplitEx compared to Con (+ 19 ± 8%). CONCLUSION: Forty minutes of postprandial exercise or 20 min of pre- and postprandial exercise are both effective at attenuating the glycaemic and insulinaemic response to a mixed lunch, while a higher lipaemia was found in the pre- and postprandrial exercise condition.

Effects of Different Exercise Strategies to Improve Postprandial Glycemia in Healthy Individuals.

PURPOSE: We systematically investigated the effects of different exercise strategies on postprandial glycemia. METHODS: Six randomized repeated-measures crossover studies were performed. Study 1 compared the effects of 60 min of brisk walking started at 30, 60, or 90 min after breakfast on postbreakfast and postlunch glycemic responses. Study 2 investigated the effects of 30 min of different exercise types (aerobic vs resistance vs combined). Study 3 compared the effects of 30 min of different aerobic exercise types (walking vs cycling vs elliptical). Study 4 evaluated the effects of 30 min of brisk walking performed 45 min before or 15 and 30 min after breakfast. Study 5 compared 30 with 45 min of postprandial brisk walking. Study 6 compared the effects of a total of 30 min brisk walking exercise fragmented in bouts of 15, 5, or 2.5 min performed every 15 min. RESULTS: Postprandial but not preprandial exercise improved glycemic response (studies 1 and 4). The glycemic peak was attenuated only when exercise started 15 min after the meal (study 4). A similar reduction of the postprandial glycemic response was observed with different exercise types (studies 2 and 3). Thirty and 45 min of brisk walking provided a similar reduction of the postprandial glucose response (study 5). When performing activity breaks, 10 and 20 min of cumulative exercise were sufficient to attenuate postprandial glycemia in the first hour postmeal (study 6). CONCLUSION: Our findings provide insight into how to choose timing, type, duration, and modality for postprandial exercise prescription in healthy individuals.

The effect of pedalling cadence on respiratory frequency: passive vs. active exercise of different intensities.

PURPOSE: Pedalling cadence influences respiratory frequency (fR) during exercise, with group III/IV muscle afferents possibly mediating its effect. However, it is unclear how exercise intensity affects the link between cadence and fR. We aimed to test the hypothesis that the effect of cadence on fR is moderated by exercise intensity, with interest in the underlying mechanisms. METHODS: Ten male cyclists performed a preliminary ramp incremental test and three sinusoidal experimental tests on separate visits. The experimental tests consisted of 16 min of sinusoidal variations in cadence between 115 and 55 rpm (sinusoidal period of 4 min) performed during passive exercise (PE), moderate exercise (ME) and heavy exercise (HE). The amplitude (A) and phase lag (φ) of the dependent variables were calculated. RESULTS: During PE, fR changed in proportion to variations in cadence (r = 0.85, P < 0.001; A = 3.9 ± 1.4 breaths·min-1; φ = - 5.3 ± 13.9 degrees). Conversely, the effect of cadence on fR was reduced during ME (r = 0.73, P < 0.001; A = 2.6 ± 1.3 breaths·min-1; φ = - 25.4 ± 26.3 degrees) and even more reduced during HE (r = 0.26, P < 0.001; A = 1.8 ± 1.0 breaths·min-1; φ = - 70.1 ± 44.5 degrees). No entrainment was found in any of the sinusoidal tests. CONCLUSION: The effect of pedalling cadence on fR is moderated by exercise intensity-it decreases with the increase in work rate-and seems to be mediated primarily by group III/IV muscle afferents, at least during passive exercise.

Quercetin Supplementation Improves Neuromuscular Function Recovery from Muscle Damage.

This study was aimed at investigating whether quercetin (Q) may improve the recovery of neuromuscular function and biochemical parameters in the 7 days following an eccentric exercise-induced muscle damage (EEIMD). Sixteen men (25.9 ± 3.3 y) ingested Q (1000 mg/day) or placebo (PLA) for 14 days following a double-blind crossover study design. A neuromuscular (NM) test was performed pre-post, 24 h, 48 h, 72 h, 96 h and 7 days after an intense eccentric exercise. The force-velocity relationship of the elbow flexor muscles and their maximal voluntary isometric contraction (MVIC) were recorded simultaneously to the electromyographic signals (EMG). Pain, joint angle, arm circumference, plasma creatine kinase (CK) and lactate-dehydrogenase (LDH) were also assessed. The results showed that Q supplementation significantly attenuated the strength loss compared to PLA. During the recovery, force-velocity relationship and mean fibers conduction velocity (MFCV) persisted significantly less when participants consumed PLA rather than Q, especially at the highest angular velocities (p < 0.02). A greater increase in biomarkers of damage was also evident in PLA with respect to Q. Q supplementation for 14 days seems able to ameliorate the recovery of eccentric exercise-induced weakness, neuromuscular function impairment and biochemical parameters increase probably due to its strong anti-inflammatory and antioxidant action.

Reproducibility of muscle fibre conduction velocity during linearly increasing force contractions.

Muscle fibre conduction velocity (MFCV) is a basic physiological parameter biophysically related to the diameter of muscle fibres and properties of the sarcolemma. The aim of this study was to assess the intersession reproducibility of the relation between voluntary force and estimates of average muscle fibre conduction velocity (MFCV) from multichannel high-density surface electromyographic recordings (HDsEMG). Ten healthy men performed six linearly increasing isometric ankle dorsiflexions on two separate experimental sessions, 4 weeks apart. Each session involved the recordings of voluntary force during maximal isometric (MViF) and submaximal ramp contractions at 35-50-70% of MViF. Concurrently, the HDsEMG activity was detected from the tibialis anterior muscle and MFCV estimates were derived in 250-ms epochs. Absolute and relative reproducibility of MFCV initial value (intercept) and rate of change (regression slope) as a function of force were assessed by within-subject coefficient of correlation (CVw) and with intraclass correlation coefficient (ICC). MFCV was positively correlated with voluntary force (R2 = 0.75 ± 0.12) in all individuals and test conditions (P < 0.001). Average CVw for MFCV intercept and slope were of 2.6 ± 2.0% and 11.9 ± 3.2% and ICC values of 0.96 and 0.94, respectively. Overall, MFCV regression coefficients showed a high degree of intersession reproducibility in both absolute and relative terms. These results may have important practical implications in the tracking of training-induced neuromuscular changes and/or in the monitoring of the progress of neuromuscular disorders when a full sEMG signal decomposition is problematic or not possible.

Respiratory Frequency as a Marker of Physical Effort During High-Intensity Interval Training in Soccer Players.

PURPOSE: Variables currently used in soccer training monitoring fail to represent the physiological demand of the player during movements like accelerations, decelerations, and directional changes performed at high intensity. We tested the hypothesis that respiratory frequency (fR) is a marker of physical effort during soccer-related high-intensity exercise. METHODS: A total of 12 male soccer players performed a preliminary intermittent incremental test and 2 shuttle-run high-intensity interval training (HIIT) protocols, in separate visits. The 2 HIIT protocols consisted of 12 repetitions over 9 minutes and differed in the work-to-recovery ratio (15:30 vs 30:15 s). Work rate was self-paced by participants to achieve the longest possible total distance in each HIIT protocol. RESULTS: Work-phase average metabolic power was higher (P < .001) in the 15:30-second protocol (31.7 [3.0] W·kg-1) compared with the 30:15-second protocol (22.8 [2.0] W·kg-1). Unlike heart rate and oxygen uptake, fR showed a fast response to the work-recovery alternation during both HIIT protocols, resembling changes in metabolic power even at supramaximal intensities. Large correlations (P < .001) were observed between fR and rating of perceived exertion during both 15:30-second (r = .87) and 30:15-second protocols (r = .85). CONCLUSIONS: Our findings suggest that fR is a good marker of physical effort during shuttle-run HIIT in soccer players. These findings have implications for monitoring training in soccer and other team sports.

Strength Training Increases Conduction Velocity of High-Threshold Motor Units.

PURPOSE: Motor unit conduction velocity (MUCV) represents the propagation velocity of action potentials along the muscle fibers innervated by individual motor neurons and indirectly reflects the electrophysiological properties of the sarcolemma. In this study, we investigated the effect of a 4-wk strength training intervention on the peripheral properties (MUCV and motor unit action potential amplitude, RMSMU) of populations of longitudinally tracked motor units (MU). METHODS: The adjustments exhibited by 12 individuals who participated in the training (INT) were compared with 12 controls (CON). Strength training involved ballistic (4 × 10) and sustained (3 × 10) isometric ankle dorsiflexions. Measurement sessions involved the recordings of maximal voluntary isometric force and submaximal isometric ramp contractions, whereas high-density surface EMG was recorded from the tibialis anterior. High-density surface EMG signals were decomposed into individual MU discharge timings, and MU was tracked across the intervention. RESULTS: Maximal voluntary isometric force (+14.1%, P = 0.003) and average MUCV (+3.0%, P = 0.028) increased in the INT group, whereas normalized MU recruitment threshold (RT) decreased (-14.9%, P = 0.001). The slope (rate of change) of the regression between MUCV and MU RT increased only in the INT group (+32.6%, P = 0.028), indicating a progressive greater increase in MUCV for higher-threshold MU. The intercept (initial value) of MUCV did not change after the intervention (P = 0.568). The association between RMSMU and MU RT was not altered by the training. CONCLUSION: The increase in the rate of change in MUCV as a function of MU RT, but not the initial value of MUCV, suggests that short-term strength training elicits specific adaptations in the electrophysiological properties of the muscle fiber membrane in high-threshold MU.

The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding.

KEY POINTS: Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles. There are few data, however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions. The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment-threshold force of motor units and a similar input-output gain of the motor neurons. The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties. ABSTRACT: The strength of a muscle typically begins to increase after only a few sessions of strength training. This increase is usually attributed to changes in the neural drive to muscle as a result of adaptations at the cortical or spinal level. We investigated the change in the discharge characteristics of large populations of longitudinally tracked motor units in tibialis anterior before and after 4 weeks of strength training the ankle-dorsiflexor muscles with isometric contractions. The adaptations exhibited by 14 individuals were compared with 14 control subjects. High-density electromyogram grids with 128 electrodes recorded the myoelectric activity during isometric ramp contractions to the target forces of 35%, 50% and 70% of maximal voluntary force. The motor unit recruitment and derecruitment thresholds, discharge rate, interspike intervals and estimates of synaptic inputs to motor neurons were assessed. The normalized recruitment-threshold forces of the motor units were decreased after strength training (P < 0.05). Moreover, discharge rate increased by 3.3 ± 2.5 pps (average across subjects and motor units) during the plateau phase of the submaximal isometric contractions (P < 0.001). Discharge rates at recruitment and derecruitment were not modified by training (P < 0.05). The association between force and motor unit discharge rate during the ramp-phase of the contractions was also not altered by training (P < 0.05). These results demonstrate for the first time that the increase in muscle force after 4 weeks of strength training is the result of an increase in motor neuron output from the spinal cord to the muscle.

The Effects of Quercetin Supplementation on Eccentric Exercise-Induced Muscle Damage.

The aim of the present investigation was to test the hypothesis that quercetin (Q) may prevent the strength loss and neuromuscular impairment associated with eccentric exercise-induced muscle damage (EEIMD). Twelve young men (26.1 ± 3.1 years) ingested either Q (1000 mg/day) or placebo (PLA) for 14 days using a randomized, double-blind, crossover study design. Participants completed a comprehensive neuromuscular (NM) evaluation before, during and after an eccentric protocol able to induce a severe muscle damage (10 sets of 10 maximal lengthening contractions). The NM evaluation comprised maximal voluntary isometric contraction (MVIC) and force⁻velocity relationship assessments with simultaneous recording of electromyographic signals (EMG) from the elbow flexor muscles. Soreness, resting arm angle, arm circumference, plasma creatine kinase (CK) and lactate dehydrogenase (LDH) were also assessed. Q supplementation significantly increased the isometric strength recorded during MVIC compared to baseline (+4.7%, p < 0.05). Moreover, the torque and muscle fiber conduction velocity (MFCV) decay recorded during the eccentric exercise was significant lower in Q compared to PLA. Immediately after the EEIMD, isometric strength, the force⁻velocity relationship and MFCV were significantly lower when participants were given PLA rather than Q. Fourteen days of Q supplementation seems able to attenuate the severity of muscle weakness caused by eccentric-induced myofibrillar disruption and sarcolemmal action potential propagation impairment.

Respiratory frequency and tidal volume during exercise: differential control and unbalanced interdependence.

Differentiating between respiratory frequency (fR ) and tidal volume (VT ) may improve our understanding of exercise hyperpnoea because fR and VT seem to be regulated by different inputs. We designed a series of exercise manipulations to improve our understanding of how fR and VT are regulated during exercise. Twelve cyclists performed an incremental test and three randomized experimental sessions in separate visits. In two of the three experimental visits, participants performed a moderate-intensity sinusoidal test followed, after recovery, by a moderate-to-severe-intensity sinusoidal test. These two visits differed in the period of the sinusoid (2 min vs. 8 min). In the third experimental visit, participants performed a trapezoidal test where the workload was self-paced in order to match a predefined trapezoidal template of rating of perceived exertion (RPE). The results collectively reveal that fR changes more with RPE than with workload, gas exchange, VT or the amount of muscle activation. However, fR dissociates from RPE during moderate exercise. Both VT and minute ventilation ( V˙E ) showed a similar time course and a large correlation with V˙CO2 in all the tests. Nevertheless, V˙CO2 was associated more with V˙E than with VT because VT seems to adjust continuously on the basis of fR levels to match V˙E with V˙CO2 . The present findings provide novel insight into the differential control of fR and VT - and their unbalanced interdependence - during exercise. The emerging conceptual framework is expected to guide future research on the mechanisms underlying the long-debated issue of exercise hyperpnoea.

Variability of estimates of muscle fiber conduction velocity and surface EMG amplitude across subjects and processing intervals.

The force developed by the human neuromuscular system can change very rapidly (15-50 ms). When processing EMG signals for inferring neural control strategies, it is therefore necessary to extract estimates from short time intervals. In this study, we investigate the relation between joint torque and estimates of average muscle fibre conduction velocity (MFCV) and amplitude (RMS) from surface EMG signals, when varying the duration of the processing interval. Moreover, we assessed the inter-subject variability in RMS and MFCV estimates. Ten healthy subjects performed isometric linearly increasing ankle dorsiflexion contractions up to 70% MVC at a rate of 5%MVCs-1. High-density EMG signals were recorded from the tibialis anterior muscle and MFCV and RMS were estimated in eight time-intervals ranging from 15 to 2000 ms. MFCV and RMS were significantly correlated with force in all subjects and when using all time-intervals (MFCV = 0.77 ±â€¯0.07, RMS = 0.79 ±â€¯0.06 (R2), Pearson-P < 0.01). The variability around the regression line for both MFCV and RMS estimates significantly increased when using intervals <100 ms (P < 0.001). However, the slope of the regression between EMG variables and force did not change with the duration of the interval (P < 0.001). Moreover, MFCV showed a substantially smaller variability across subjects in its relation to force than RMS [average coefficient of variation of regression slopes across all time intervals, 24.48 ±â€¯1.51 (%), whilst for the RMS it was 56.65 ±â€¯0.69 (%)]. These results indicate that estimates of MFCV and RMS as a function of joint torque are unbiased with respect to processing interval duration. Moreover, they reveal that estimates of MFCV are more consistent across subjects than EMG amplitude.