Characterization of performance fatigability during a self-paced exercise

Acute neuromuscular, cardiovascular, and muscle oxygenation responses to low-intensity aerobic interval exercises with blood flow restriction

We investigated the influence of short- and long-interval cycling exercise with blood flow restriction (BFR) on neuromuscular fatigue, shear stress and muscle oxygenation, potent stimuli to BFR-training adaptations. During separate sessions, eight individuals performed short- (24 × 60 s/30 s; SI) or long-interval (12 × 120 s/60 s; LI) trials on a cycle ergometer, matched for total work. One leg exercised with (BFR-leg) and the other without (CTRL-leg) BFR. Quadriceps fatigue was quantified using pre- to post-interval changes in maximal voluntary contraction (MVC), potentiated twitch force (QT) and voluntary activation (VA). Shear rate was measured by Doppler ultrasound at cuff release post-intervals. Vastus lateralis tissue oxygenation was measured by near-infrared spectroscopy during exercise. Following the initial interval, significant (P < 0.05) declines in MVC and QT were found in both SI and LI, which were more pronounced in the BFR-leg, and accounted for approximately two-thirds of the total reduction at exercise termination. In the BFR-leg, reductions in MVC (-28 ± 15%), QT (-42 ± 17%), and VA (-15 ± 17%) were maximal at exercise termination and persisted up to 8 min post-exercise. Exercise-induced muscle deoxygenation was greater (P < 0.001) in the BFR-leg than CTRL-leg and perceived pain was more in LI than SI (P < 0.014). Cuff release triggered a significant (P < 0.001) shear rate increase which was consistent across trials. Exercise-induced neuromuscular fatigue in the BFR-leg exceeded that in the CTRL-leg and was predominantly of peripheral origin. BFR also resulted in diminished muscle oxygenation and elevated shear stress. Finally, short-interval trials resulted in comparable neuromuscular and haemodynamic responses with reduced perceived pain compared to long-intervals.

Trehalose Improved 20-min Cycling Time-Trial Performance After 100-min Cycling in Amateur Cyclists

Carbohydrate (CHO) supplementation during endurance exercise can improve performance. However, it is unclear whether low glycemic index (GI) CHO leads to differential ergogenic and metabolic effects compared with a standard high GI CHO. This study investigated the ergogenic and metabolic effects of CHO supplementation with distinct GIs, namely, (a) trehalose (30 g/hr), (b) isomaltulose (30 g/hr), (c) maltodextrin (60 g/hr), and (d) placebo (water). In this double-blind, crossover, counterbalanced, placebo-controlled study, 13 male cyclists cycled a total of 100 min at varied exercise intensity (i.e., 10-min stages at 1.5, 2.0, and 2.5 W/kg; repeated three times plus two 5-min stages at 1.0 W/kg before and after the protocol), followed by a 20-min time trial on four separated occasions. Blood glucose and lactate (every 20 min), heart rate, and ratings of perceived exertion were collected throughout, and muscle biopsies were taken before and immediately after exercise. The results showed that trehalose improved time-trial performance compared with placebo (total work done 302 ± 39 vs. 287 ± 48 kJ; p = .01), with no other differences between sessions (all p ≥ .07). Throughout the 100-min protocol, blood glucose was higher with maltodextrin compared with the other supplements at all time points (all p < .05). Heart rate, ratings of perceived exertion, muscle glycogen content, blood glucose, and lactate were not different between conditions when considering the 20-min time trial (all p > .05). Trehalose supplementation throughout endurance exercise improved cycling performance and appears to be an appropriate CHO source for exercise tasks up to 2 hr. No ergogenic superiority between the different types of CHO was established.

The Characteristics of Endurance Events with a Variable Pacing Profile-Time to Embrace the Concept of "Intermittent Endurance Events"?

A variable pacing profile is common in different endurance events. In these races, several factors, such as changes in elevation or race dynamics, lead participants to perform numerous surges in intensity. These surges are so frequent that certain events, such as cross-country (XC) skiing, mountain biking (MTB), triathlon, and road cycling, have been termed "intermittent endurance events". The characteristics of these surges vary depending on the sport: MTB and triathlon require athletes to perform numerous short (<10 s) bouts; XC skiing require periods of short- and moderate-(30 s to 2 min) duration efforts, while road cycling is comprised of a mix of short-, moderate-, and long-duration (>2 min) bouts. These bouts occur at intensities above the maximal metabolic steady state (MMSS), with many efforts performed at intensities above the athletes' maximal aerobic power or speed (MAP/MAS) (i.e., supramaximal intensities). Given the factors that influence the requirement to perform surges in these events, athletes must be prepared to always engage in a race with a highly stochastic pace. The aim of this review is to characterize the variable pacing profile seen in endurance events and to discuss how the performance of multiple maximal and supramaximal surges in intensity can affect how athletes fatigue during a race and influence training strategies that can lead to success in these races.

The blood serum metabolome profile after different phases of a 4-km cycling time trial: Secondary analysis of a randomized controlled trial

It has been assumed that exercise intensity variation throughout a cycling time trial (TT) occurs in alignment of various metabolic changes to prevent premature task failure. However, this assumption is based on target metabolite responses, which limits our understanding of the complex interconnection of metabolic responses during exercise. The current study characterized the metabolomic profile, an untargeted metabolic analysis, after specific phases of a cycling 4-km TT. Eleven male cyclists performed three separated TTs in a crossover counterbalanced design, which were interrupted at the end of the fast-start (FS, 600 ± 205 m), even-pace (EP, 3600 ± 190 m), or end-spurt (ES, 4000 m) phases. Blood samples were taken before any exercise and 5 min after exercise cessation, and the metabolomic profile characterization was performed using Nuclear Magnetic Resonance metabolomics. Power output (PO) was also continually recorded. There were higher PO values during the FS and ES compared to the EP (all p < 0.05), which were accompanied by distinct metabolomic profiles. FS showed high metabolite expression in TCA cycle and its related pathways (e.g., glutamate, citric acid, and valine metabolism); whereas, the EP elicited changes associated with antioxidant effects and oxygen delivery adjustment. Finally, ES was related to pathways involved in NAD turnover and serotonin metabolism. These findings suggest that the specific phases of a cycling TT are accompanied by distinct metabolomic profiles, providing novel insights regarding the relevance of specific metabolic pathways on the process of exercise intensity regulation.

Use of subject-specific models to detect fatigue-related changes in running biomechanics: a random forest approach

Running biomechanics are affected by fatiguing or prolonged runs. However, no evidence to date has conclusively linked this effect to running-related injury (RRI) development or performance implications. Previous investigations using subject-specific models in running have demonstrated higher accuracy than group-based models, however, this has been infrequently applied to fatigue. In this study, two experiments were conducted to determine whether subject-specific models outperformed group-based models to classify running biomechanics during non-fatigued and fatigued conditions. In the first experiment, 16 participants performed four treadmill runs at or around the maximal lactate steady state. In the second experiment, nine participants performed five prolonged runs using commercial wearable devices. For each experiment, two segments were extracted from each trial from early and late in the run. For each participant, a random forest model was applied with a leave-one-run-out cross-validation to classify between the early (non-fatigued) and late (fatigued) segments. Additionally, group-based classifiers with a leave-one-subject-out cross validation were constructed. For experiment 1, mean classification accuracies for the single-subject and group-based classifiers were 68.2 ± 8.2% and 57.0 ± 8.9%, respectively. For experiment 2, mean classification accuracies for the single-subject and group-based classifiers were 68.9 ± 17.1% and 61.5 ± 11.7%, respectively. Variable importance rankings were consistent within participants, but these rankings differed from each participant to those of the group. Although the classification accuracies were relatively low, these findings highlight the advantage of subject-specific classifiers to detect changes in running biomechanics with fatigue and indicate the potential of using big data and wearable technology approaches in future research to determine possible connections between biomechanics and RRI.

Sex differences in fatigability and recovery following a 5 km running time trial in recreationally active adults

ABSTRACTFemales demonstrate greater fatigue resistance compared to males in tasks ranging from single-limb contractions to whole-body exercise, including running. Many of the studies investigating sex differences in fatigability following running, however, occur after long duration, low-intensity tasks and it is unknown whether there is a sex difference in fatigability following high-intensity running. This study compared fatigability and recovery following a 5 km running time trial in young males and females. Sixteen recreationally active participants (8 males, 8 females, age: 23 ± 4 years) completed a familiarisation and experimental trial. Knee-extensor maximal voluntary contractions (MVCs) were performed before and up to 30 min after a 5 km time trial on a treadmill. Heart rate and rating of perceived exertion (RPE) were recorded after every kilometre during the time trial. Although not significantly different, males completed the 5 km time trial 15% faster than females (p = 0.095). Heart rate (p = 0.843) and RPE (p = 0.784) were similar between the sexes during the trial. Prior to running, males had larger MVCs (p = 0.014). The relative decrease in MVC force was less in females than males immediately post-exercise (-4.6 ± 2.4% vs. -15.1 ± 3.0%, p < 0.001) and at 10-minutes post-exercise (p = 0.018). At 20- and 30-minutes recovery, however, relative MVC force was not different between the sexes (p ≥ 0.129). These data demonstrate that females experienced less fatigability of the knee extensors than males following a high-intensity 5 km running time trial. The findings highlight the need to understand responses to exercise in both sexes and have implications for recovery from training and exercise prescription.Highlights Data regarding sex differences in fatigability following high-intensity running is relatively sparse.Therefore, this study quantified the decrease in knee-extensor maximum voluntary contraction force (MVC) following a 5-km self-paced running time trial.Despite similar heart rates and ratings of perceived exertion, the percentage decrease in MVC was three times greater in males compared to females.Relative MVCs remained greater in females compared to males until 20 min post-exercise.

Effects of caffeine on central and peripheral fatigue following closed- and open-loop cycling exercises

We examined whether endurance performance and neuromuscular fatigue would be affected by caffeine ingestion during closed- and open-loop exercises. Nine cyclists performed a closed-loop (4,000-m cycling time trial) and an open-loop exercise (work rate fixed at mean power of the closed-loop trial) 60 min after ingesting caffeine (CAF, 5 mg/kg) or placebo (PLA, cellulose). Central and peripheral fatigue was quantified via pre- to post-exercise decrease in quadriceps voluntary activation and potentiated twitch force, respectively. Test sensitivity for detecting caffeine-induced improvements in exercise performance was calculated as the mean change in time divided by the error of measurement. Caffeine ingestion reduced the time of the closed-loop trial (PLA: 375.1±14.5 s vs CAF: 368.2±14.9 s, P=0.024) and increased exercise tolerance during the open-loop trial (PLA: 418.2±99.5 s vs CAF: 552.5±106.5 s, P=0.001), with similar calculated sensitivity indices (1.5, 90%CI: 0.7-2.9 vs 2.8, 90%CI: 1.9-5.1). The reduction in voluntary activation was more pronounced (P=0.019) in open- (-6.8±8.3%) than in closed-loop exercises (-1.9±4.4%), but there was no difference between open- and closed-loop exercises for the potentiated twitch force reduction (-25.6±12.8 vs -26.6±12.0%, P>0.05). Caffeine had no effect on central and peripheral fatigue development in either mode of exercise. In conclusion, caffeine improved endurance performance in both modes of exercise without influence on post-exercise central and peripheral fatigue, with the open-loop exercise imposing a greater challenge to central fatigue tolerance.

Concurrent Evolution of Biomechanical and Physiological Parameters With Running-Induced Acute Fatigue

Understanding the influence of running-induced acute fatigue on the homeostasis of the body is essential to mitigate the adverse effects and optimize positive adaptations to training. Fatigue is a multifactorial phenomenon, which influences biomechanical, physiological, and psychological facets. This work aimed to assess the evolution of these three facets with acute fatigue during a half-marathon. 13 recreational runners were equipped with one inertial measurement unit (IMU) on each foot, one combined global navigation satellite system-IMU-electrocardiogram sensor on the chest, and an Android smartphone equipped with an audio recording application. Spatio-temporal parameters for the running gait, along with the heart rate, its variability and complexity were computed using validated algorithms. Perceived fatigability was assessed using the rating-of-fatigue (ROF) scale at every 10 min of the race. The data was split into eight equal segments, corresponding to at least one ROF value per segment, and only level running parts were retained for analysis. During the race, contact time, duty factor, and trunk anteroposterior acceleration increased, and the foot strike angle and vertical stiffness decreased significantly. Heart rate showed a progressive increase, while the metrics for heart rate variability and complexity decreased during the race. The biomechanical parameters showed a significant alteration even with a small change in perceived fatigue, whereas the heart rate dynamics altered at higher changes. When divided into two groups, the slower runners presented a higher change in heart rate dynamics throughout the race than the faster runners; they both showed similar trends for the gait parameters. When tested for linear and non-linear correlations, heart rate had the highest association with biomechanical parameters, while the trunk anteroposterior acceleration had the lowest association with heart rate dynamics. These results indicate the ability of faster runners to better judge their physiological limits and hint toward a higher sensitivity of perceived fatigue to neuromuscular changes in the running gait. This study highlights measurable influences of acute fatigue, which can be studied only through concurrent measurement of biomechanical, physiological, and psychological facets of running in real-world conditions.

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.

Distinct pacing profiles result in similar perceptual responses and neuromuscular fatigue development: Why different "roads" finish at the same line?

ABSTRACTThe current study analysed the effect of distinct pacing profiles (i.e. U, J, and inverted J) in the perceptual responses and neuromuscular fatigue (NMF) development following a 4-km cycling time trial (TT). Twenty-one cyclists with similar training status were allocated into three different groups based on their pacing profile spontaneously adopted during TT. Rating of perceived exertion (RPE), oxygen uptake (⩒O₂) and heart rate (HR) were continuously recorded. NMF was assessed by using isometric maximal voluntary contractions (IMVC), while the central [i.e. voluntary activation (VA)] and peripheral fatigue of knee extensors [i.e. peak torque of potentiated twitches (TwPt)] were evaluated using electrically evoked contractions performed pre and 2 min after the TT. TT performance was not different amongst pacing profiles (U = 377 ± 20 s; J = 392 ± 23 s; J-i = 381 ± 20 s) (all P > 0.05). RPE, ⩒O₂ and HR increased similarly throughout the TT regardless the pacing strategy (all P > 0.05). Similarly, IMVC (U = -9.9 ± 8.8; J = -9.6 ± 4.5%; J-i = -13.8 ± 11.3%), VA (U = -2.3 ± 1.7%; J = -5.4 ± 2.2%; J-i = -6.4 ± 4.5%) and TwPt (U = -32.5 ± 12.0%; J = -29.5 ± 8.0%; J-i = -33.6 ± 13.6%) were similar amongst pacing profiles (all P > 0.05). Therefore, endurance athletes with similar training status showed the same perceived responses and NMF development regardless the pacing profile spontaneously adopted. It was suggested that these responses occurred in order to preserve a similar rate of change in systemic responses (i.e. RPE, ⩒O₂ and HR) and NMF development, ultimately resulting in same TT performance.Highlights Different pacing profiles resulted in the same performance in a 4-km cycling time trial.The similar performance might be due to achievement of the same sensory tolerance limit.There was no difference for perceptual, metabolic and neuromuscular fatigue responses.

Effects of induced local ischemia during a 4-km cycling time trial on neuromuscular fatigue development

The present study analyzed the effects of local ischemia during endurance exercise on neuromuscular fatigue (NMF). Nine cyclists performed, in a counterbalanced order, two separate 4-km cycling time trials (TT) with (ISCH) or without (CONTR) induced local ischemia. NMF was characterized by using isometric maximal voluntary contractions (IMVC), whereas central [voluntary activation (VA)] and peripheral fatigue [peak torque of potentiated twitch (TwPt)] of knee extensors were evaluated using electrically evoked contractions performed before (PRE) and 1 min after (POST) the TT. Electromyographic activity (EMG), power output (PO), oxygen uptake (V̇o₂), and rating of perceived exertion (RPE) were also recorded. The decrease in IMVC (-15 ± 9% vs. -10 ± 8%, P = 0.66), VA (-4 ± 3% vs. -3 ± 3%, P = 0.46), and TwPt (-16 ± 7% vs. -19 ± 14%, P = 0.67) was similar in ISCH and CONTR. Endurance performance was drastically reduced in ISCH condition (512 ± 29 s) compared with CONTR (386 ± 17 s) (P < 0.001), which was accompanied by lower EMG, PO, and V̇o₂ responses (all P < 0.05). RPE was greater in ISCH compared with CONTR (P < 0.05), but the rate of change was similar throughout the TT (8.19 ± 2.59 vs. 7.81 ± 2.01 RPE.% of total time^-1, P > 0.05). These results indicate that similar end-exercise NMF levels were accompanied by impaired endurance performance in ISCH compared with CONTR. These novel findings suggest that the local reduced oxygen availability affected the afferent feedback signals to the central nervous system, ultimately increasing perceived effort and reducing muscle activity and exercise intensity to avoid surpassing a sensory tolerance limit before the finish line.

Dynamic Changes of Performance Fatigability and Muscular O2 Saturation in a 4-km Cycling Time Trial

PURPOSE

The current study characterized the performance fatigability etiology, immediately after exercise cessation, and its relation to the dynamic changes in muscle O2 saturation (SmO2) at different TT phases.

METHODS

Twelve males performed three separated TT of different distances, in a crossover counterbalanced design, until the end of the fast-start (FS, 827 ± 135 m), even-pace (EP, 3590 ± 66 m), or end-spurt (ES, 4000 m) TT phases. Performance fatigability was characterized by using isometric maximal voluntary contractions (IMVC), whereas the maximal voluntary activation (VA) and contractile function of knee extensors (e.g., peak torque of potentiated twitches [TwPt]) were evaluated using electrically evoked contractions performed before and immediately after each exercise bouts. SmO2, power output (PO), and EMG were also recorded.

RESULTS

Immediately after the FS phase, there were lower values for IMVC (-23%), VA (-8%), and TwPt (-43%) (all P < 0.001), but no further changes were measured after EP (IMVC, -28%; VA, -8%; TwPt, -38%). After the ES phase, IMVC (-34%) and TwPt (-59%) further decreased compared with the previous phases (P < 0.05). There were lower SmO2 and higher EMG/PO values during FS and ES compared with EP phase.

CONCLUSION

FS and EP phases had similar performance fatigability etiology, but ES showed further impairments in contractile function. This later finding might be due to the abrupt changes in SmO2 and EMG/PO because of the high exercise intensity during the ES, which elicited maximal decline in contractile function at the finish line.

Fatigue development and perceived response during self-paced endurance exercise: state-of-the-art review

Performance in self-paced endurance exercises results from continuous fatigue symptom management. While it is suggested that perceived responses and neuromuscular fatigue development may determine variations in exercise intensity, it is uncertain how these fatigue components interact throughout the task. To address the fatigue development in self-paced endurance exercises, the following topics were addressed in the present review: (1) fatigue development during constant-load vs. self-paced endurance exercises; (2) central and peripheral fatigue and perceived exertion interconnections throughout the self-paced endurance exercises; and (3) future directions and recommendations. Based on the available literature, it is suggested (1) the work rate variations during a self-paced endurance exercise result in transitions between exercise intensity domains, directly impacting the end-exercise central and peripheral fatigue level when compared to constant-load exercise mode; (2) central and peripheral fatigue, as well as perceived exertion response contribute to exercise intensity regulation at the different stages of the trial. It seems that while neuromuscular fatigue development might be relevant at beginning of the trial, the perceived exertion might interfere in the remaining parts to achieve maximal values only at the finish line; (3) future studies should focus on the mechanisms underpinning fatigue components interactions throughout the task and its influence on exercise intensity variations.

Neuromuscular responses to fatiguing locomotor exercise

Over the last two decades, an abundance of research has explored the impact of fatiguing locomotor exercise on the neuromuscular system. Neurostimulation techniques have been implemented prior to and following locomotor exercise tasks of a wide variety of intensities, durations, and modes. These techniques have allowed for the assessment of alterations occurring within the central nervous system and the muscle, while techniques such as transcranial magnetic stimulation and spinal electrical stimulation have permitted further segmentalization of locomotor exercise-induced changes along the motor pathway. To this end, the present review provides a comprehensive synopsis of the literature pertaining to neuromuscular responses to locomotor exercise. Sections of the review were divided to discuss neuromuscular responses to maximal, severe, heavy and moderate intensity, high-intensity intermittent exercise, and differences in neuromuscular responses between exercise modalities. During maximal and severe intensity exercise, alterations in neuromuscular function reside primarily within the muscle. Although post-exercise reductions in voluntary activation following maximal and severe intensity exercise are generally modest, several studies have observed alterations occurring at the cortical and/or spinal level. During prolonged heavy and moderate intensity exercise, impairments in contractile function are attenuated with respect to severe intensity exercise, but are still widely observed. While reductions in voluntary activation are greater during heavy and moderate intensity exercise, the specific alterations occurring within the central nervous system remain unclear. Further work utilizing stimulation techniques during exercise and integrating new and emerging techniques such as high-density electromyography is warranted to provide further insight into neuromuscular responses to locomotor exercise.

Fatigue development and perceived response during self-paced endurance exercise: state-of-the-art review

Performance in self-paced endurance exercises results from continuous fatigue symptom management. While it is suggested that perceived responses and neuromuscular fatigue development may determine variations in exercise intensity, it is uncertain how these fatigue components interact throughout the task. To address the fatigue development in self-paced endurance exercises, the following topics were addressed in the present review: (1) fatigue development during constant-load vs. self-paced endurance exercises; (2) central and peripheral fatigue and perceived exertion interconnections throughout the self-paced endurance exercises; and (3) future directions and recommendations. Based on the available literature, it is suggested (1) the work rate variations during a self-paced endurance exercise result in transitions between exercise intensity domains, directly impacting the end-exercise central and peripheral fatigue level when compared to constant-load exercise mode; (2) central and peripheral fatigue, as well as perceived exertion response contribute to exercise intensity regulation at the different stages of the trial. It seems that while neuromuscular fatigue development might be relevant at beginning of the trial, the perceived exertion might interfere in the remaining parts to achieve maximal values only at the finish line; (3) future studies should focus on the mechanisms underpinning fatigue components interactions throughout the task and its influence on exercise intensity variations.

Caffeine increases peripheral fatigue in low- but not in high-performing cyclists

The influence of cyclists’ performance levels on caffeine-induced increases in neuromuscular fatigue after a 4-km cycling time trial (TT) was investigated. Nineteen cyclists performed a 4-km cycling TT 1 h after ingesting caffeine (5 mg·kg−1) or placebo (cellulose). Changes from baseline to after exercise in voluntary activation (VA) and potentiated 1 Hz force twitch (Qtw,pot) were used as markers of central and peripheral fatigue, respectively. Participants were classified as “high performing” (HP, n = 8) or “low performing” (LP, n = 8) in accordance with their performance in a placebo trial. Compared with placebo, caffeine increased the power, anaerobic mechanical power, and anaerobic work, reducing the time to complete the trial in both groups (p < 0.05). There was a group versus supplement and a group versus supplement versus trial interaction for Qtw,pot, in which the postexercise reduction was greater after caffeine compared with placebo in the LP group (Qtw,pot = −34% ± 17% vs. −21% ± 11%, p = 0.02) but not in the HP group (Qtw,pot = −22% ± 8% vs. −23% ± 10%, p = 0.64). There was no effect of caffeine on VA, but there was a group versus trial interaction with lower postexercise values in the LP group than in the HP group (p = 0.03). Caffeine-induced improvement in 4-km cycling TT performance seems to come at the expense of greater locomotor muscle fatigue in LP but not in HP cyclists.

Novelty Caffeine improves exercise performance at the expense of a greater end-exercise peripheral fatigue in low-performing athletes. Caffeine-induced improvement in exercise performance does not affect end-exercise peripheral fatigue in high-performing athletes. High-performing athletes seem to have augmented tolerance to central fatigue during a high-intensity time trial.

Physiological sex differences affect the integrative response to exercise: acute and chronic implications

NEW FINDINGS

What is the topic of this review? We review sex differences within physiological systems implicated in exercise performance; specifically, how they integrate to determine metabolic thresholds and fatigability. Thereafter, we discuss the implications that these sex differences might have for long-term adaptation to exercise. What advances does it highlight? The review collates evidence from recent physiological studies that have investigated sex as a biological variable, demonstrating that the physiological response to equivalent 'dosages' of exercise is not the same in males and females; thus, highlighting the need to research diversity in physiological responses to interventions.

ABSTRACT

The anatomical and physiological differences between males and females are thought to determine differences in the limits of human performance. The notion of studying sex as a biological variable has recently been emphasized in the biosciences as a vital step in enhancing human health. In this review, we contend that the effects of biological sex on acute and chronic responses must be studied and accounted for when prescribing aerobic exercise, much like any intervention targeting the optimization of physiological function. Emerging evidence suggests that the response of physiological systems to exercise differs between males and females, potentially mediating the beneficial effects in healthy and clinical populations. We highlight evidence that integrative metabolic thresholds during exercise are influenced by phenotypical sex differences throughout many physiological systems. Furthermore, we discuss evidence that female skeletal muscle is more resistant to fatigue elicited by equivalent dosages of high-intensity exercise. How the different acute responses affect the long-term trainability of males and females is considered, with discussion about tailoring exercise to the characteristics of the individual presented within the context of biological sex. Finally, we highlight the influence of endogenous and exogenous sex hormones on physiological responses to exercise in females. Sex is one of many mediating influences on the outcomes of exercise, and with careful experimental designs, physiologists can advance the collective understanding of diversity in physiology and optimize outcomes for both sexes.