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Alannah K.A. McKay, Peter Peeling, David B. Pyne, Nicolin Tee, Marijke Welveart, Ida A. Heikura, Avish P. Sharma, Jamie Whitfield, Megan L. Ross, Rachel P.L. van Swelm, Coby M. Laarakkers, and Louise M. Burke

This study implemented a 2-week high carbohydrate (CHO) diet intended to maximize CHO oxidation rates and examined the iron-regulatory response to a 26-km race walking effort. Twenty international-level, male race walkers were assigned to either a novel high CHO diet (MAX = 10 g/kg body mass CHO daily) inclusive of gut-training strategies, or a moderate CHO control diet (CON = 6 g/kg body mass CHO daily) for a 2-week training period. The athletes completed a 26-km race walking test protocol before and after the dietary intervention. Venous blood samples were collected pre-, post-, and 3 hr postexercise and measured for serum ferritin, interleukin-6, and hepcidin-25 concentrations. Similar decreases in serum ferritin (17–23%) occurred postintervention in MAX and CON. At the baseline, CON had a greater postexercise increase in interleukin-6 levels after 26 km of walking (20.1-fold, 95% CI [9.2, 35.7]) compared with MAX (10.2-fold, 95% CI [3.7, 18.7]). A similar finding was evident for hepcidin levels 3 hr postexercise (CON = 10.8-fold, 95% CI [4.8, 21.2]; MAX = 8.8-fold, 95% CI [3.9, 16.4]). Postintervention, there were no substantial differences in the interleukin-6 response (CON = 13.6-fold, 95% CI [9.2, 20.5]; MAX = 11.2-fold, 95% CI [6.5, 21.3]) or hepcidin levels (CON = 7.1-fold, 95% CI [2.1, 15.4]; MAX = 6.3-fold, 95% CI [1.8, 14.6]) between the dietary groups. Higher resting serum ferritin (p = .004) and hotter trial ambient temperatures (p = .014) were associated with greater hepcidin levels 3 hr postexercise. Very high CHO diets employed by endurance athletes to increase CHO oxidation have little impact on iron regulation in elite athletes. It appears that variations in serum ferritin concentration and ambient temperature, rather than dietary CHO, are associated with increased hepcidin concentrations 3 hr postexercise.

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Ed Maunder, Deborah K. Dulson, and David M. Shaw

Purpose: Considerable interindividual heterogeneity has been observed in endurance performance responses following induction of a ketogenic diet (KD). It is plausible that a physiological stress response in the period following the dramatic dietary shift associated with transition to a KD may explain this heterogeneity. Methods: In a randomized, crossover study design, 8 trained male runners completed an incremental exercise test and ran to exhaustion at 70%VO2max before and after a 31-day rigorously controlled habitual diet or KD intervention, and recorded heart rate variability (root mean square of the sum of successive differences in R–R intervals [rMSSD]) upon waking each morning along with the recovery–stress questionnaire for athletes each week. Data were analyzed using linear mixed models. Results: A significant reduction in rMSSD was observed in the KD (−9.77 [4.03] ms, P = .02), along with an increase in day-to-day variability in rMSSD (2.1% [1.0%], P = .03). The reduction in rMSSD in the KD for the subgroup of individuals exhibiting impaired exercise capacity following induction of the KD approached significance (Δ −22 [15] ms, P = .06, N = 4); whereas no effect was observed in those who exhibited unchanged exercise capacity (Δ 5 [18] ms, P = .61, N = 4). No main effects were observed for recovery–stress questionnaire for athletes. Conclusions: Our data suggest those working with endurance athletes transitioning onto a KD may consider using noninvasive, inexpensive resting heart rate variability measures to gain individual-level insights into the likely short-term effects on exercise capacity.

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Pedro L. Valenzuela, Fernando Rivas, and Guillermo Sánchez-Martínez

Purpose: To describe the effects of COVID-19 lockdown and a subsequent retraining on the training workloads, autonomic responses, and performance of a group of elite athletes. Methods: The training workloads and heart rate variability (assessed through the log-transformed root mean square of successive R–R intervals) of 7 elite badminton players were registered daily during 4 weeks of normal training (baseline), 7 to 10 weeks of lockdown, and 6 to 8 weeks of retraining. Physical performance was assessed at baseline and after each phase by means of a countermovement jump and the estimated squat 1-repetition maximum. Results: A reduction in training workloads was observed in all participants during the lockdown (−63.7%), which was accompanied by a reduced heart rate variability in all but one participant (−2.0%). A significant reduction was also observed for countermovement jump (−6.5%) and 1-repetition maximum performance (−11.5%), which decreased in all but one participant after the lockdown. However, after the retraining phase, all measures returned to similar values to those found at baseline. At the individual level, there were divergent responses, as exemplified by one athlete who attenuated the reduction in training workloads and increased her performance during the lockdown and another one who markedly reduced his workload and performance, and got injured during the retraining phase. Conclusions: Although there seems to be a large interindividual variability, COVID-19 lockdown is likely to impose negative consequences on elite athletes, but these detrimental effects might be avoided by attenuating reductions in training workloads and seem to be overall recovered after 6 to 8 weeks of retraining.

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Adam Mallett, Phillip Bellinger, Wim Derave, Katie McGibbon, Eline Lievens, Ben Kennedy, Hal Rice, and Clare Minahan

Purpose: To determine the influence of muscle fiber typology (MFT) on the pacing strategy of elite swimmers competing in the 200-m freestyle event. Method: The top 3 career-best performances from 25 elite 200-m freestyle swimmers were analyzed—12 women (1:58.0 [0:01.3] min:s) and 13 men (1:48.4 [0:02.5]). Muscle carnosine concentration was quantified by proton magnetic resonance spectroscopy in the gastrocnemius and soleus muscles and expressed as a carnosine aggregate z score (CAZ score) relative to an age- and gender-matched nonathlete control group to estimate MFT. Linear regression models were employed to examine the influence of MFT on the percentage of overall race time spent in each 50-m lap. Results: Swimmers with a higher CAZ score spent a greater percentage of race time in lap 3 compared with swimmers with a lower CAZ score (0.1%, 0.0% to 0.2%; mean, 90% confidence interval, P = .02). For every 1% increase in the percentage of race time spent in lap 1, the percentage of race time spent in lap 3 decreased by 0.4% for swimmers with a higher CAZ score (0.2% to −0.5%, P = .00, r = −.51), but not for swimmers with a lower CAZ score (−0.1%, −0.3% to 0.1%, P = .28, r = −.18). The percentage of race time spent in lap 4 decreased by 0.8% for higher-CAZ-score swimmers (−0.5% to −1.0%, P = .00, r = −.66) and by 0.9% for lower-CAZ-score swimmers (−0.6% to −1.3%, P = .00, r = −.65) when lap 1 percentage increased by 1%. Conclusion: MFT may influence the pacing strategy of swimmers in the 200-m freestyle event, which provides an avenue for maximizing individualized pacing strategies of elite swimmers.

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Filippo Dolci, Andrew E. Kilding, Tania Spiteri, Paola Chivers, Ben Piggott, Andrew Maiorana, and Nicolas H. Hart

Purpose: To investigate the acute effect of repeated-sprint activity (RSA) on change-of-direction economy (assessed using shuttle running economy [SRE]) in soccer players and explore neuromuscular and cardiorespiratory characteristics that may modulate this effect. Methods: Eleven young elite male soccer players (18.5 [1.4] y old) were tested on 2 different days during a 2-week period in their preseason. On day 1, lower-body stiffness, power and force were assessed via countermovement jumps, followed by an incremental treadmill test to exhaustion to measure maximal aerobic capacity. On day 2, 2 SRE tests were performed before and after a repeated-sprint protocol with heart rate, minute ventilation, and blood lactate measured. Results: Pooled group analysis indicated no significant changes for SRE following RSA due to variability in individual responses, with a potentiation or impairment effect of up to 4.5% evident across soccer players. The SRE responses to RSA were significantly and largely correlated to players’ lower-body stiffness (r = .670; P = .024), and moderately (but not significantly) correlated to players’ force production (r = −.455; P = .237) and blood lactate after RSA (r = .327; P = .326). Conclusions: In summary, SRE response to RSA in elite male soccer players appears to be highly individual. Higher lower-body stiffness appears as a relevant physical contributor to preserve or improve SRE following RSA.

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Mehdi Kordi, Len Parker Simpson, Kevin Thomas, Stuart Goodall, Tom Maden-Wilkinson, Campbell Menzies, and Glyn Howatson

Purpose: To assess the association between the W′ and measures of neuromuscular function relating to the capacity of skeletal muscle to produce force in a group of elite cyclists. Methods: Twenty-two athletes specializing in a range of disciplines and competing internationally volunteered to participate. Athletes completed assessments of maximum voluntary torque (MVT), voluntary activation, and isometric maximum voluntary contraction to measure rate of torque development (RTD). This was followed by assessment of peak power output (PPO) and 3-, 5-, and 12-minute time trials to determine critical power. Pearson correlation was used to examine associations with critical power and W′. Goodness of fit was calculated, and significant relationships were included in a linear stepwise regression model. Results: Significant positive relationships were evident between W′ and MVT (r = .82), PPO (r = .70), and RTD at 200 milliseconds (r = .59) but not with RTD at 50 milliseconds and voluntary activation. Correlations were also observed between critical power and RTD at 200 milliseconds and MVT (r = .54 and r = .51, respectively) but not with PPO, voluntary activation, or RTD at 50 milliseconds. The regression analysis found that 87% of the variability in W′ (F 1,18 = 68.75; P < .001) was explained by 2 variables: MVT (81%) and PPO (6%). Conclusions: It is likely that muscle size and strength, as opposed to neural factors, contribute meaningfully to W′. These data can be used to establish training methods to enhance W′ to improve cycling performance in well-trained athletes.