Chronic Functional Adaptations Induced by the Application of Transcranial Direct Current Stimulation Combined with Exercise Programs: A Systematic Review of Randomized Controlled Trials.

The present systematic review aimed to determine the chronic effects of the combination of transcranial direct current stimulation (tDCS) and exercise on motor function and performance outcomes. We performed a systematic literature review in the databases MEDLINE and Web of Science. Only randomized control trials that measured the chronic effect of combining exercise (comprising gross motor tasks) with tDCS during at least five sessions and measured any type of motor function or performance outcome were included. A total of 22 interventions met the inclusion criteria. Only outcomes related to motor function or performance were collected. Studies were divided into three groups: (a) healthy population (n = 4), (b) neurological disorder population (n = 14), and (c) musculoskeletal disorder population (n = 4). The studies exhibited considerable variability in terms of tDCS protocols, exercise programs, and outcome measures. Chronic use of tDCS in combination with strength training does not enhance motor function in healthy adults. In neurological disorders, the results suggest no additive effect if the exercise program includes the movements pretending to be improved (i.e., tested). However, although evidence is scarce, tDCS may enhance exercise-induced adaptations in musculoskeletal conditions characterized by pain as a limiting factor of motor function.

Analysis of Kinematic and Muscular Fatigue in Long-Distance Swimmers.

Muscle fatigue is a complex phenomenon that is influenced by the type of activity performed and often manifests as a decline in motor performance (mechanical failure). The purpose of our study was to investigate the compensatory strategies used to mitigate mechanical failure. A cohort of 21 swimmers underwent a front-crawl swimming task, which required the consistent maintenance of a constant speed for the maximum duration. The evaluation included three phases: non-fatigue, pre-mechanical failure, and mechanical failure. We quantified key kinematic metrics, including velocity, distance travelled, stroke frequency, stroke length, and stroke index. In addition, electromyographic (EMG) metrics, including the Root-Mean-Square amplitude and Mean Frequency of the EMG power spectrum, were obtained for 12 muscles to examine the electrical manifestations of muscle fatigue. Between the first and second phases, the athletes covered a distance of 919.38 ± 147.29 m at an average speed of 1.57 ± 0.08 m/s with an average muscle fatigue level of 12%. Almost all evaluated muscles showed a significant increase (p < 0.001) in their EMG activity, except for the latissimus dorsi, which showed a 17% reduction (ES 0.906, p < 0.001) during the push phase of the stroke cycle. Kinematic parameters showed a 6% decrease in stroke length (ES 0.948, p < 0.001), which was counteracted by a 7% increase in stroke frequency (ES -0.931, p < 0.001). Notably, the stroke index also decreased by 6% (ES 0.965, p < 0.001). In the third phase, characterised by the loss of the ability to maintain the predetermined rhythm, both EMG and kinematic parameters showed reductions compared to the previous two phases. Swimmers employed common compensatory strategies for coping with fatigue; however, the ability to maintain a predetermined motor output proved to be limited at certain levels of fatigue and loss of swimming efficiency (Protocol ID: NCT06069440).

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions.

BACKGROUND: Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions. METHODS: We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18-30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively. RESULTS: Males were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males. CONCLUSION: This increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.

Differences in time to task failure and fatigability between children and young adults: A systematic review and meta-analysis.

The transition from childhood to adulthood is characterized by many physiological processes impacting exercise performance. Performance fatigability and time to task failure are commonly used to capture exercise performance. This review aimed to determine the differences in fatigability and TTF between youth (including both children and adolescents) and young adults, and to evaluate the influence of exercise modalities (i.e., exercise duration and type of exercise) on these differences. Medline, SPORTDiscus and Cochrane Library were searched. Thirty-four studies were included. The meta-analyses revealed that both children (SMD -1.15; p < 0.001) and adolescents (SMD -1.26; p = 0.022) were less fatigable than adults. Additional analysis revealed that children were less fatigable during dynamic exercises (SMD -1.58; p < 0.001) with no differences during isometric ones (SMD -0.46; p = 0.22). Children (SMD 0.89; p = 0.018) but not adolescents (SMD 0.75; p = 0.090) had longer TTF than adults. Additional analyses revealed 1) that children had longer TTF for isometric (SMD 1.25; p < 0.001) but not dynamic exercises (SMD -0.27; p = 0.83), and 2) that TTF differences between children and adults were larger for short- (SMD 1.46; p = 0.028) than long-duration exercises (SMD 0.20; p = 0.64). Children have higher endurance and are less fatigable than adults. These differences are influenced by the exercise modality, suggesting distinct physiological functioning during exercise between children and adults. The low number of studies comparing these outcomes between adolescents versus children and adults prevents robust conclusions and warrants further investigations in adolescent individuals.

Seeing Effort: Assessing Coaches' Prediction of the Number of Repetitions in Reserve Before Task-Failure.

BACKGROUND: A key role of resistance training (RT) coaches is to personalize programs based on their trainees' abilities and goals. Specifically, coaches often assess how many repetitions in reserve (RIR) their trainees have until task-failure. Coaches can then modify the number of repetitions assigned per set accordingly. However, coaches' ability to predict the number of RIR is unknown. METHODS: Certified RT coaches (n = 259) were randomly assigned to watch a video of one of eight trainees. The trainees performed two sets of barbell squats and preacher biceps-curls, using 70% or 80% of their 1RM, to task-failure. The coaches predicted trainees' RIR at 33%, 66%, and 90% of the total number of repetitions the trainees completed in each set. We fitted a linear mixed model with various predictors to the prediction errors as the outcomes (i.e., signed and unsigned values of the predicted minus actual repetitions to task-failure). RESULTS: The overall average number of repetitions completed by the trainees was 13.9. The average absolute errors were 4.8, 2.0, and 1.2 repetitions for the 33%, 66%, and 90% time-points, respectively. The absolute prediction error increased for the biceps-curl compared to the squat (1.43, 95% CI [1.13, 1.74]), but decreased for heavier loads (- 1.17, 95% CI [- 2.16, - 0.19]), and in the second set of each exercise (- 1.20, 95% CI [- 1.38, - 1.02]). Surprisingly, coaches' years of experience had a negligible effect on the absolute error (- 0.020, 95% CI [- 0.039, - 0.0007]). Finally, coaches underpredicted the RIR at early time-points but reverted to slight overprediction at later time-points. CONCLUSIONS: Prior coaching experience seems to play a minor role in RIR predictions. However, even short-term exposures to new trainees performing different exercises can substantially improve coaches' RIR predictions.

Muscle oxygenation and time to task failure of submaximal holding and pulling isometric muscle actions and influence of intermittent voluntary muscle twitches.

BACKGROUND: Isometric muscle actions can be performed either by initiating the action, e.g., pulling on an immovable resistance (PIMA), or by reacting to an external load, e.g., holding a weight (HIMA). In the present study, it was mainly examined if these modalities could be differentiated by oxygenation variables as well as by time to task failure (TTF). Furthermore, it was analyzed if variables are changed by intermittent voluntary muscle twitches during weight holding (Twitch). It was assumed that twitches during a weight holding task change the character of the isometric muscle action from reacting (≙ HIMA) to acting (≙ PIMA). METHODS: Twelve subjects (two drop outs) randomly performed two tasks (HIMA vs. PIMA or HIMA vs. Twitch, n = 5 each) with the elbow flexors at 60% of maximal torque maintained until muscle failure with each arm. Local capillary venous oxygen saturation (SvO2) and relative hemoglobin amount (rHb) were measured by light spectrometry. RESULTS: Within subjects, no significant differences were found between tasks regarding the behavior of SvO2 and rHb, the slope and extent of deoxygenation (max. SvO2 decrease), SvO2 level at global rHb minimum, and time to SvO2 steady states. The TTF was significantly longer during Twitch and PIMA (incl. Twitch) compared to HIMA (p = 0.043 and 0.047, respectively). There was no substantial correlation between TTF and maximal deoxygenation independently of the task (r = - 0.13). CONCLUSIONS: HIMA and PIMA seem to have a similar microvascular oxygen and blood supply. The supply might be sufficient, which is expressed by homeostatic steady states of SvO2 in all trials and increases in rHb in most of the trials. Intermittent voluntary muscle twitches might not serve as a further support but extend the TTF. A changed neuromuscular control is discussed as possible explanation.

The Effect of Increasing Blood Flow Restriction Pressure When the Contractions Are Already Occlusive.

CONTEXT: Blood flow restricted exercise involves the use of external pressure to enhance fatigue and augment exercise adaptations. The mechanisms by which blood flow restricted exercise limits muscular endurance are not well understood. OBJECTIVE: To determine how increasing blood flow restriction pressure impacts local muscular endurance, discomfort, and force steadiness when the contractions are already occlusive. DESIGN: Within-participant, repeated-measures crossover design. SETTING: University laboratory. PATIENTS: A total of 22 individuals (13 males and 9 females). INTERVENTION: Individuals performed a contraction at 30% of maximal isometric elbow flexion force for as long as possible. One arm completed the contraction with 100% of arterial occlusion pressure applied, while the other arm had 150% of arterial occlusion pressure applied. At the end of the protocol, individuals were asked to rate their perceived discomfort. MAIN OUTCOME MEASURES: Time to task failure, discomfort, and force steadiness. RESULTS: Individuals had a longer time to task failure when performing the 100% arterial occlusion condition compared with the 150% arterial occlusion pressure condition (time to task failure = 82.4 vs 70.8 s; Bayes factors = 5.77). There were no differences in discomfort between the 100% and 150% conditions (median discomfort = 5.5 vs 6; Bayes factors = 0.375) nor were there differences in force steadiness (SD of force output 3.16 vs 3.31 N; Bayes factors = 0.282). CONCLUSION: The results of the present study suggest that, even when contractions are already occlusive, increasing the restriction pressure reduces local muscle endurance but does not impact discomfort or force steadiness. This provides an indication that mechanisms other than the direct alteration of blood flow are contributing to the increased fatigue with added restrictive pressure. Future studies are needed to examine neural mechanisms that may explain this finding.

Echo intensity is weakly associated with muscular strength and endurance in young, healthy adults.

The purpose of this study was to evaluate the relationships between echo intensity (EI) and muscular strength (maximal voluntary isometric contraction [MVIC]) and endurance (time-to task-failure [TTF]) in young adults. Ultrasonography was used to examine the dominant vastus lateralis of 15 young men and 15 young women. The participants completed an MVIC to assess muscular strength and sustained a bilateral, leg extension at 45% MVIC to assess muscular endurance. There was a significant (r = -0.354, p = 0.028) correlation between EI and normalized MVIC force for the combined sample (n = 30). There was also a significant (r = -0.520, p = 0.002) correlation between EI and absolute MVIC force for the combined sample. The correlation between EI and TTE was significant for the men (r = -0.449, p = 0.047) and the combined sample (r = -0.477, p = 0.004). Overall, muscle quality as assessed by EI does not appear to be strongly related to muscular strength, but there may be a modest correlation with muscular endurance.

Men Exhibit Greater Pain Pressure Thresholds and Times to Task Failure but Not Performance Fatigability Following Self-Paced Exercise.

The purpose of the current study was to determine if, and to what extent, sex differences in performance fatigability after a sustained, bilateral leg extension, anchored to a moderate rating of perceived exertion (RPE), could be attributed to muscle size, muscular strength, or pain pressure threshold (PPT) in young, healthy adults. Thirty adults (men: n = 15, women: n = 15) volunteered to complete a sustained leg extension task anchored to RPE = 5 (10-point OMNI scale) as well as pretest and posttest maximal voluntary isometric contraction (MVIC) trials. The fatigue-induced decline in MVIC force was defined as performance fatigability. We used muscle cross-sectional area (mCSA) to quantify muscle size and a dolorimeter to assess PPT. The sustained task induced fatigue such that both men and women exhibited significant (p < 0.05) decreases in MVIC force from pretest to posttest (M = 113.3, SD =24.2 kg vs. M = 98.3, SD = 23.1 kg and M = 73.1, SD =14.5 kg vs. M = 64.1, SD = 16.2 kg, respectively), with no significant sex differences in performance fatigability (grand M = 12.6, SD =10.6%). Men, however, exhibited significantly (p < 0.05) longer time to task failure (TTF) than women (M = 166.1, SD =83.0 seconds vs. M = 94.6, SD =41.7) as well as greater PPT (M = 5.9, SD = 2.2 kg vs. M = 3.4, SD =1.1 kg). The only significant predictor of performance fatigability was PPT. In conclusion, differences in PPT, at least in part, mediate variations in TTF during self-paced exercise anchored to a specific RPE and resulting in performance fatigability.

The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion.

We tested the hypothesis that during whole body exercise, the balance between muscle O2 supply and metabolic demand may elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion. Seventeen active, healthy volunteers (12 males, 5 females; 32 ± 2 yr) participated in two distinct protocols. Study 1 (n = 7) consisted of constant work rate cycling in the moderate, heavy, and severe exercise intensity domains with concurrent measures of pulmonary V̇o2 and local %SmO2 [via near-infrared spectroscopy (NIRS)] on quadriceps and forearm sites. Average %SmO2 at both sites displayed a domain-dependent response (P < 0.05). A negative %SmO2 slope was evident during severe-domain exercise but was positive during exercise below critical power (CP) at both muscle sites. In study 2 (n = 10), quadriceps and forearm site %SmO2 was measured during three continuous running trials to exhaustion and three intermittent intensity (ratio = 60 s severe: 30 s lower intensity) trials to exhaustion. Intensity-dependent negative %SmO2 slopes were observed for all trials (P < 0.05) and predicted zero slope at critical velocity. %SmO2 accurately predicted depletion and repletion of %D' balance on a second-by-second basis (R2 = 0.99, P < 0.05; both sites). Time to exhaustion predictions during continuous and intermittent exercise were either not different or better with %SmO2 [standard error of the estimate (SEE) < 20.52 s for quad, <44.03 s for forearm] versus running velocity (SEE < 65.76 s). Muscle O2 balance provides a dynamic physiological delineation between sustainable and unsustainable exercise (consistent with a "critical metabolic rate") and predicts real-time depletion and repletion of finite work capacity and time to exhaustion.NEW & NOTEWORTHY Dynamic muscle O2 saturation discriminates boundaries between exercise intensity domains, exposes a critical metabolic rate as the highest rate of steady state O2 supply and demand, describes time series depletion and repletion for work above critical power, and predicts time to exhaustion during severe domain whole body exercise. These results highlight the matching of O2 supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

Effects of prior-task failure on arithmetic performance: A study in young and older adults.

Effects of prior-task failure (i.e., decreased performance on a target task following failure on a prior task) were tested in young and older adults. Young and older participants (N=120) accomplished a computational estimation task (i.e., providing the best estimates to arithmetic problems) before and after accomplishing a dot comparison task in a control or in a failure condition. Both groups decreased their performance on the target computational estimation following failure on the prior dot comparison task. Also, prior-task failure led young and older adults to select the better strategy less often and to use the easier strategy more often. Our findings show, for the first time, impaired performance after experiencing failure in both young and older adults. We discuss implications of these findings for further our understanding of effects of task transitions (i.e., prior-task success and failure) on cognitive performance.

Prior exercise impairs subsequent performance in an intensity- and duration-dependent manner.

Prior constant-load exercise performed for 30-min at or above maximal lactate steady state (MLSSp) significantly impairs subsequent time-to-task failure (TTF) compared with TTF performed without prior exercise. We tested the hypothesis that TTF would decrease in relation to the intensity and the duration of prior exercise compared with a baseline TTF trial. Eleven individuals (6 males, 5 females, aged 28 ± 8 yrs) completed the following tests on a cycle ergometer (randomly assigned after MLSSp was determined): (i) a ramp-incremental test; (ii) a baseline TTF trial performed at 80% of peak power (TTFb); (iii) five 30-min constant-PO rides at 5% below lactate threshold (LT-5%), halfway between LT and MLSSp (Delta50), 5% below MLSSp (MLSS-5%), MLSSp, and 5% above MLSSp (MLSS+5%); and (iv) 15- and 45-min rides at MLSSp (MLSS15 and MLSS45, respectively). Each condition was immediately followed by a TTF trial at 80% of peak power. Compared with TTFb (330 ± 52 s), there was 8.0 ± 24.1, 23.6 ± 20.2, 41.0 ± 14.8, 52.2 ± 18.9, and 75.4 ± 7.4% reduction in TTF following LT-5%, Delta50, MLSS-5%, MLSSp, and MLSS+5%, respectively. Following MLSS15 and MLSS45 there were 29.0 ± 20.1 and 69.4 ± 19.6% reductions in TTF, respectively (P < 0.05). It is concluded that TTF is reduced following prior exercise of varying duration at MLSSp and at submaximal intensities below MLSS. Novelty: Prior constant-PO exercise, performed at intensities below MLSSp, reduces subsequent TTF performance. Subsequent TTF performance is reduced in a linear fashion following an increase in the duration of constant-PO exercise at MLSSp.

An analysis of the perceived causes leading to task-failure in resistance-exercises.

BACKGROUND: While reaching task-failure in resistance-exercises is a topic that attracts scientific and applied interest, the underlying perceived reasons leading to task-failure remain underexplored. Here, we examined the reasons subjects attribute to task-failure as they performed resistance-exercises using different loads. METHODS: Twenty-two resistance-trained subjects (11-females) completed one Repetition-Maximum (RM) tests in the barbell squat and bench-press. Then, in the next two counterbalanced sessions, subjects performed two sets to task-failure in both exercises, using either 70% or 83% of 1RM. Approximately 30 seconds after set-completion, subjects verbally reported the reasons they perceived to have caused them to reach task-failure. Their answers were recorded, transcribed, and thematically analyzed. The differences between the frequencies of the identified categories were then tested using a mixed logistic regression model. RESULTS: The most commonly reported reason was muscle fatigue (54%, p < 0.001), mostly of the target muscles involved in each exercise. However, remote muscles involved to a lesser extent in each exercise were also reported. Approximately half of the remaining reasons included general fatigue (26%), pain (12%), cardiovascular strain (11%), and negative affect (10%), with the latter three reported more often in the squat (p = 0.022). CONCLUSIONS: In contrast to our expectations, task-failure was perceived to be caused by a range of limiting factors other than fatigue of the target muscles. It now remains to be established whether different perceived limiting factors of resistance-exercises lead to different adaptations, such as muscular strength and hypertrophy.

Sex differences in fatigability after ischemic preconditioning of non-exercising limbs.

BACKGROUND: Ischemic preconditioning (IPC) is suggested to decrease fatigability in some individuals but not others. Sex differences in response to IPC may account for this variability and few studies systematically investigated the effects of IPC in men and women. The goal of this study was to determine if time to task failure, perception of pain, and neuromuscular mechanisms of fatigability were altered by IPC in men and women. METHODS: Ten women (29 ± 5 years old) and 10 men (28 ± 6 years old) performed isometric contractions with the plantar flexor muscles of the dominant leg at 20% of maximal voluntary contraction until task failure. We used a repeated measures design where each individual performed 3 randomized and counterbalanced test sessions: (A) IPC session, cuff inflation and deflation (5 min each repeated 3 times) performed before the exercise by inflating cuffs to the non-dominant leg and arm; (B) sham session, cuffs were inflated for a short period (1 min); and (C) control session, no cuffs were involved. RESULTS: Compared with control, IPC increased time to task failure in men (mean difference, 5 min; confidence interval (CI) of mean difference, 2.2; 7.8 min; P = 0.01) but not women (mean difference, - 0.6 min; CI of mean difference, - 3.5; 2.4 min; P = 0.51). In men, but not women, the IPC-induced increase in time to task failure was associated with lower response to pressure pain (r = - 0.79). IPC further exposed sex differences in arterial pressure during fatiguing contractions (session × sex: P < 0.05). Voluntary activation, estimated with the twitch interpolation technique, and presynaptic inhibition of leg Ia afferents were not altered after IPC for men and women. The tested variables were not altered with sham. CONCLUSIONS: The ergogenic effect of IPC on time to task failure was observed only in men and it was associated with reductions in the perception of pain. This pilot data suggest the previously reported inter-individual variability in exercise-induced fatigability after IPC could be a consequence of the sex and individual response to pain.

Task failure during sustained low-intensity contraction is not associated with a critical amount of central fatigue.

Fatigue-related mechanisms induced by low-intensity prolonged contraction in lower limb muscles are currently unknown. This study investigated central fatigue kinetics in the knee extensors during a low-intensity sustained isometric contraction. Eleven subjects sustained a 10% maximal voluntary contraction (MVC) until task failure (TF) with neuromuscular evaluation every 3 minutes. Testing encompassed transcranial magnetic stimulation to evaluate maximal voluntary activation (VATMS ), motor evoked potential (MEP), and silent period (SP), and peripheral nerve stimulation to assess M-wave. Rating of perceived exertion (RPE) was also recorded. MVC progressively decreased up to 50% of the time to TF (ie, 50%TTF ) and then plateaued, reaching ~50% at TF (P < .001). VATMS progressively decreased up to 90%TTF and then plateaued, the decrease reaching ~20% at TF (P < .001). SP was lengthened early (ie, from 20%TTF ) during the exercise and then plateaued (P < .01). No changes were reported for MEP evoked during MVC (P = .87), while MEP evoked during submaximal contractions decreased early (ie, from 20%TTF ) during the exercise and then plateaued (P < .01). RPE increased linearly during the exercise to be almost maximal at TF. M-waves were not altered (P = .88). These findings confirm that TF is due to the subjects reaching their maximal perceived effort rather than any particular central event or neuromuscular limitations since MVC at TF was far from 10% of its original value. It is suggested that strategies minimizing RPE (eg, motivational self-talk) should be employed to enhance endurance performance.

Menstrual Cycle Effects on Exercise-Induced Fatigability.

Estrogen and progesterone have distinct concentrations across the menstrual cycle, each one promoting several physiological alterations other than preparing the uterus for pregnancy. Whether these physiological alterations can influence motor output during a fatiguing contraction is the goal of this review, with an emphasis on the obtained effect sizes. Studies on this topic frequently attempt to report if there is a statistically significant difference in fatigability between the follicular and luteal phases of the menstrual cycle. Although the significant difference (the P-value) can inform the probability of the event, it does not indicate the magnitude of it. We also investigated whether the type of task performed (e.g., isometric vs. dynamic) can further influence the magnitude by which exercise-induced fatigue changes with fluctuations in the concentration of ovarian hormones. We retrieved experimental studies in eumenorrheic women published between 1975 and 2019. The initial search yielded 921 studies, and after manual refinement, 46 experimental studies that reported metrics of motor output in both the follicular and luteal phases of the menstrual cycle were included. From these retrieved studies, 15 showed a statistical difference between the luteal and follicular phases (seven showing less fatigability during the luteal phase and eight during the follicular phase). The effect size was not consistent across studies and with a large range (-6.77; 1.61, favoring the luteal and follicular phase, respectively). The inconsistencies across studies may be a consequence of the differences in the limb used during the fatiguing contraction (upper vs. lower extremity), the type of contraction (isometric vs. dynamic), the muscle mass engaged (single limb vs. full body), and the techniques used to define the menstrual cycle phase (e.g., serum concentration vs. reported day of menses). Further studies are required to determine the effects of a regular menstrual cycle phase on the exercise-induced fatigability.

Separate and combined effects of local and systemic hypoxia in resistance exercise.

PURPOSES: This study quantified performance, physiological, and perceptual responses during resistance exercise to task failure with blood flow restriction (BFR), in systemic hypoxia, and with these stimuli combined. METHODS: Fourteen young men were tested for 1-repetition maximum (1RM) in the barbell biceps curl and lying triceps extension exercises. On separate visits, subjects performed exercise trials (4 sets to failure at 70% 1RM with 90 s between sets) in six separate randomized conditions, i.e., in normoxia or hypoxia (fraction of inspired oxygen = 20.9% and 12.9%, respectively) combined with three different levels of BFR (0%, 45%, or 60% of resting arterial occlusion pressure). Muscle activation and oxygenation were monitored via surface electromyography and near-infrared spectroscopy, respectively. Arterial oxygen saturation, heart rate, and perceptual responses were assessed following each set. RESULTS: Compared to set 1, the number of repetitions before failure decreased in sets 2, 3, and 4 for both exercises (all P < 0.001), independently of the condition (P > 0.065). Arterial oxygen saturation was lower with systemic hypoxia (P < 0.001), but not BFR, while heart rate did not differ between conditions (P > 0.341). Muscle oxygenation and activation during exercise trials remained unaffected by the different conditions (all P ≥ 0.206). A significant main effect of time, but not condition, was observed for overall perceived discomfort, difficulty breathing, and limb discomfort (all P < 0.001). CONCLUSION: Local and systemic hypoxic stimuli, or a combination of both, did not modify the fatigue-induced change in performance, trends of muscle activation or oxygenation, nor exercise-related sensations during a multi-set resistance exercise to task failure.

Short-Term Effects of Anodal Transcranial Direct Current Stimulation on Endurance and Maximal Force Production. A Systematic Review and Meta-Analysis.

The purpose of the present systematic review and meta-analysis was to explore the effects of transcranial direct current stimulation (tDCS) on endurance (i.e., time to task failure (TTF)) and maximal voluntary contraction (MVC). Furthermore, we aimed to analyze whether the duration of stimulation, the brain region targeted for stimulation, and the task performed could also influence motor performance. We performed a systematic literature review in the databases MEDLINE and Web of Science. The short-term effects of anodal tDCS and sham stimulation (placebo) were considered as experimental and control conditions, respectively. A total of 31 interventions were included (MVC = 13; TTF = 18). Analysis of the strength-related tDCS studies showed small improvements in the MVC (SMD = 0.19; 95% CI = -0.02, 0.41; p = 0.08). However, the results of the endurance-related interventions indicated a moderate effect on TTF performance (SMD = 0.26; 95% CI = 0.07, 0.45; p = 0.008). Furthermore, the sub-analysis showed that anodal tDCS over M1 and stimulation durations longer than 10 min produced the best results in terms of TTF performance enhancement. Additionally, the effects of anodal tDCS were larger during full body exercises (i.e., cycling) when compared to uniarticular tasks. In conclusion, the current meta-analysis indicated that anodal tDCS leads to small and moderate effects on MVC and TTF, respectively.

Rate of force development to evaluate the neuromuscular fatigue and recovery after an intermittent isometric handgrip task with different blood flow restriction conditions

Aim: To investigate the neuromuscular fatigue and recovery after an intermittent isometric handgrip exercise (IIHE) executed until failure with different blood flow restriction (BFR) conditions (free flow, partial and total vascular restriction). Methods: Thirteen healthy men carried out an IIHE at 45% of maximum voluntary isometric force (MVIF) until failure with total restriction (TR), partial restriction (PR) or free flow (FF). The rate of force development (RFD) was extracted from the MIVF over the time intervals of 0-30, 0-50, 0-100, and 0-200ms and normalized by MVIF [relative RFD (RFDr)]. Results: The RFDr decreased significantly (p<0.01) after the IIHE in all BFR conditions and time intervals studied, remaining lower for five minutes. The medians of the RFDr in FF condition were significantly lower (p=0.01) at 30ms (1.56 %MVIF·s-1) and 50ms (1.70 %MVIF·s-1) when compared to TR at 30ms (2.34 %MVIF·s-1) and 50ms (2.63 %MVIF·s-1) in minute 1 post failure. Conclusions: These results show that, regardless of the blood flow restriction level, there is no RFD recovery five minutes after an exhaustive IIHE. When the task was executed with FF, the reduction of the RFD was greater when compared with the TR condition.(AU)

The Effects of Task Type on Time to Task Failure During Fatigue: A Modified Sørensen Test.

Understanding mechanisms of fatigue of the trunk extensors is important because fatigue is a major factor in predicting incidence of low back pain, but few studies have examined trunk extensor fatigue muscles using differing load types and measured the amplitude and frequency domain of the electromyographic signal to explain these differences. Sixteen healthy participants performed position- and force-matching fatigue tasks in a modified Sørensen test position. Time to task failure was significantly longer during the position-matching task compared to force-matching task (58.3 ± 6.6 min vs. 36.1 ± 5.4 min). This finding is the opposite of that commonly reported for the appendicular muscle, but the mean power frequency shifts and muscle activation patterns of the trunk and hip extensors did not explain this difference. The mean power frequency shifts and muscle activation patterns of the trunk and hip extensors did not explain this difference. The greater time to task failure during the position-matching task may reflect adaptation of the trunk extensor muscles to optimize maintaining specific joint angles more so than specific loads.