Neuromuscular and perceptual responses during repeated cycling sprints-usefulness of a "hypoxic to normoxic" recovery approach.

PURPOSE: We investigated the consequence of varying hypoxia severity during an initial set of repeated cycling sprints on performance, neuromuscular fatigability, and exercise-related sensations during a subsequent set of repeated sprints in normoxia. METHODS: Nine active males performed ten 4-s sprints (recovery = 30 s) at sea level (SL; FiO2 ~ 0.21), moderate (MH; FiO2 ~ 0.17) or severe normobaric hypoxia (SH; FiO2 ~ 0.13). This was followed, after 8 min of passive recovery, by five 4-s sprints (recovery = 30 s) in normoxia. RESULTS: Mean power decrement during Sprint 10 was exacerbated in SH compared to SL and MH (- 34 ± 12%, - 22 ± 13%, - 25 ± 14%, respectively, p < 0.05). Sprint performance during Sprint 11 recovered to that of Sprint 1 in all conditions (p = 0.267). All exercise-related sensations at Sprint 11 recovered significantly compared to Sprint 1, with no difference for Set 2 (p > 0.05). Ratings of overall perceived discomfort, difficulty breathing, and limb discomfort were exacerbated during Set 1 in SH versus SL (p < 0.05). Compared to SL, the averaged MPO value for Set 2 was 5.5 ± 3.0% (p = 0.003) lower in SH. Maximal voluntary force and twitch torque decreased similarly in all conditions immediately after Set 1 (p < 0.05), without further alterations after Set 2. Peripheral and cortical voluntary activation values did not change (p > 0.05). CONCLUSION: Exercise-related sensations, rather than neuromuscular function integrity, may play a pivotal role in influencing performance of repeated sprints and its recovery.

Exercise-related sensations contribute to decrease power during repeated cycle sprints with limited influence on neural drive.

PURPOSES: We manipulated the inspired oxygen fraction (FiO2) to examine the effects of physiological perturbations on exercise-related sensations and the neural drive of the quadriceps during repeated, brief, maximal cycle sprints. METHODS: Nine active males completed a repeated sprint cycle protocol (10 × 4-s maximal sprints with 30 s of passive recovery) in normoxia (NM; FiO2 0.21) and severe normobaric hypoxia (HY; FiO2 0.13). Peak power, quadriceps Root Mean Squared electromyography (RMS EMG), physiological (heart rate, arterial oxygen saturation, blood lactate concentration) and perceptual responses were recorded. RESULTS: The 10 sprints in HY were associated with lower arterial oxygen saturation values compared to NM [80.7 ± 0.9 vs. 95.6 ± 0.6%; P < 0.001; effect size (ES) = 0.98], higher blood lactate values (11.9 ± 0.4 vs. 9.9 ± 0.9 mmol L-1; P = 0.05; ES = 0.36), and greater exercise-related sensations (~36%; P < 0.001; ES > 0.47). Mean power for sprints 1-10 were lower (-13 ± 3%; P = 0.001; ES = 0.79), and sprint decrement was more pronounced in HY compared to NM (21.4 ± 3.7 vs. 13.2 ± 2.7%; P = 0.003). There was a 17% decrease in RMS EMG activity from the first to the last sprint (P < 0.001; ES = 0.65), independent of condition (P = 0.597; ES = 0.04). CONCLUSIONS: Despite severe hypoxia exacerbating both physiological and perceptual perturbations, the performance decrement observed during the repeated sprint protocol did not coincide with an accentuated decline in RMS EMG activity. These data suggest that higher-than-normal exercise-related sensations or perceptions coincide with fatigue during repeated sprinting, independent of changes in neural drive, when the task characteristics are known beforehand.

Voluntary muscle and motor cortical activation during progressive exercise and passively induced hyperthermia.

This study examined whether central fatigue was exacerbated by an increase in muscle contractile speed caused by passive hyperthermia (PaH) and whether exercise-induced hyperthermia (ExH) combined with related peripheral fatigue influenced this response. The ExH was induced by cycling at 60% of maximal oxygen uptake in 38°C conditions and the PaH by sitting in a 48°C climate chamber. Ten men performed brief (≈ 5 s) and sustained (30 s) maximal voluntary isometric contractions (MVCs) of the knee extensors at baseline (CON, ∼37.1°C) and during moderate (MOD, ≈ 38.5°C) and severe (SEV, ∼39.5°C) hyperthermia. Motor nerve and transcranial magnetic stimulation were used to assess voluntary muscle and cortical activation level, along with contractile properties. Brief MVC force decreased to a similar extent during SEV-ExH (-8%) and SEV-PaH (-6%; P < 0.05 versus CON). Sustained MVC force also decreased during MOD-ExH (-10%), SEV-ExH (-13%) and SEV-PaH (-7%; P < 0.01 versus CON). Motor nerve and cortical activation were reduced on reaching MOD (≈ 3%) and SEV (≈ 5%) ExH and PaH during the brief and sustained MVCs (P < 0.01 versus CON). Peak twitch force decreased on reaching SEV-ExH and SEV-PaH (P < 0.05 versus CON). Following transcranial magnetic stimulation, during the brief and sustained MVCs the peak muscle relaxation rate increased in ExH and PaH (P < 0.01 versus CON). The increase was greatest during the sustained contraction in SEV-PaH (P < 0.01), but this did not exacerbate central fatigue relative to ExH. These results indicate that during fatiguing cycling exercise in the heat, quadriceps peak relaxation rate increases. However, the centrally mediated rate of activation appears sufficient to overcome even the largest increase in muscle relaxation rate, seen during SEV-PaH.

Heat stress does not exacerbate tennis-induced alterations in physical performance.

OBJECTIVES: To assess the time course of changes in physical performance in response to match-play tennis under heat stress. METHODS: Two matches consisting of 20 min of effective playing time (2×10 min segments) were played in COOL (∼102 min; ∼22°C and 70% relative humidity (RH)) and HOT (∼119 min; ∼36°C and 35% RH) environments. Repeated-sprint ability (3×15 m, 15 s rest), 15 m sprint time with a direction change (180°), vertical jump height (squat and countermovement jumps) and leg stiffness (multirebound jumps) were assessed in 12 competitive male players prematch, midmatch and postmatch, and 24 and 48 h after match completion. RESULTS: During the repeated-sprint ability test, initial (+2.3% and +3.1%) and cumulated sprint (+1.5% and +2.8%) times increased from prematch to midmatch and postmatch, respectively (p<0.001), while the sprint decrement score did not change. Match-play tennis induced a slowing (average of both conditions: +1.1% and +1.3% at midmatch and postmatch time points; p=0.05) of 15 m sprint time with direction change. Compared with prematch, leg stiffness (-6.4% and -6.5%; p<0.001) and squat jump height (-1.5% and -2.4%; p=0.05), but not countermovement jump height (-0.7% and -1.3%; p>0.05), decreased midmatch and postmatch, respectively, regardless of the condition. Complete recovery in all physical performance markers occurred within 24 h. CONCLUSIONS: In tennis, match-related fatigue is characterised by impaired repeated-sprint ability, explosive power and leg stiffness at midmatch and postmatch, with values restored to prematch baseline 24 h into recovery. In addition, physical performance responses (match and recovery kinetics) are identical when competing in cool and hot environments.

Thermal, physiological and perceptual strain mediate alterations in match-play tennis under heat stress.

OBJECTIVES: This study compared the thermal, physiological and perceptual responses associated with match-play tennis in HOT (∼34°C wet-bulb-globe temperature (WBGT)) and COOL (∼19°C WBGT) conditions, along with the accompanying alterations in match characteristics. METHODS: 12 male tennis players undertook two matches for an effective playing time (ie, ball in play) of 20 min, corresponding to ∼119 and ∼102 min of play in HOT and COOL conditions, respectively. Rectal and skin temperatures, heart rate, subjective ratings of thermal comfort, thermal sensation and perceived exertion were recorded, along with match characteristics. RESULTS: End-match rectal temperature increased to a greater extent in the HOT (∼39.4°C) compared with the COOL (∼38.7°C) condition (p<0.05). Thigh skin temperature was higher throughout the HOT match (p<0.001). Heart rate, thermal comfort, thermal sensation and perceived exertion were also higher during the HOT match (p<0.001). Total playing time was longer in the HOT compared with the COOL match (p<0.05). Point duration (∼7.1 s) was similar between conditions, while the time between points was ∼10 s longer in the HOT relative to the COOL match (p<0.05). This led to a ∼3.4% lower effective playing percentage in the heat (p<0.05). Although several thermal, physiological and perceptual variables were individually correlated to the adjustments in time between points and effective playing percentage, thermal sensation was the only predictor variable associated with both adjustments (p<0.005). CONCLUSIONS: These adjustments in match-play tennis characteristics under severe heat stress appear to represent a behavioural strategy adopted to minimise or offset the sensation of environmental conditions being rated as difficult.

Coping with heat stress during match-play tennis: does an individualised hydration regimen enhance performance and recovery?

OBJECTIVES: To determine whether an individualised hydration regimen reduces thermal, physiological and perceptual strain during match-play tennis in the heat, and minimises alterations in neuromuscular function and physical performance postmatch and into recovery. METHODS: 10 men undertook two matches for an effective playing time (ball in play) of 20 min (∼113 min) in ∼37°C and ∼33% RH conditions. Participants consumed fluids ad libitum during the first match (HOT) and followed a hydration regimen (HYD) in the second match based on undertaking play euhydrated, standardising sodium intake and minimising body mass losses. RESULTS: HYD improved prematch urine specific gravity (1.013±0.006 vs 1.021±0.009 g/mL; p<0.05). Body mass losses (∼0.3%), fluid intake (∼2 L/h) and sweat rates (∼1.6 L/h) were similar between conditions. Core temperature was higher during the first 10 min of effective play in HOT (p<0.05), but increased similarly (∼39.3°C) on match completion. Heart rate was higher (∼11 bpm) throughout HOT (p<0.001). Thermal sensation was higher during the first 7.5 min of effective play in HOT (p<0.05). Postmatch knee extensor and plantar flexor strength losses, along with reductions in 15 m sprint time and repeated-sprint ability (p<0.05), were similar in both conditions, and were restored within 24 h. CONCLUSIONS: Both the hydration regimen and ad libitum fluid consumption allowed for minimal body mass losses (<1%). However, undertaking match-play in a euhydrated state attenuated thermal, physiological and perceptual strain. Maximal voluntary strength in the lower limbs and repeated-sprint ability deteriorated similarly in both conditions, but were restored within 24 h.

Peripheral fatigue is not critically regulated during maximal, intermittent, dynamic leg extensions.

Central motor drive to active muscles is believed to be reduced during numerous exercise tasks to prevent excessive peripheral fatigue development. The purpose of the present study was to use hypoxia to exacerbate physiological perturbations during a novel, intermittent exercise task and to explore the time-course and interplay between central and peripheral neuromuscular adjustments. On separate days, 14 healthy men performed four sets of 6 × 5 maximal-intensity, isokinetic leg extensions (1 repetition lasting ∼7 s) at 300°/s (15 and 100 s of passive rest between repetitions and sets, respectively) under normoxia (NM, fraction of inspired O2 0.21), moderate (MH, 0.14), and severe normobaric hypoxia (SH, 0.10). Neuromuscular assessments of the knee extensors were conducted before and immediately after each set. There was an interaction between time and condition on the mean peak torque produced during each set (P < 0.05). RMS/M-wave activity of the rectus femoris decreased across the four sets of exercise, but there was no difference between conditions (8.3 ± 5.1% all conditions compounded, P > 0.05). Potentiated twitch torque decreased post set 1 in all conditions (all P < 0.05) with greater reductions following each set in SH compared with NM but not MH (end-exercise reductions 41.3 ± 3.0% vs. 28.0 ± 3.2%, P < 0.05 and 32.1 ± 3.3%, P > 0.05). In conclusion, severe hypoxia exacerbates both peripheral fatigue development and performance decrements during maximal, intermittent, dynamic leg extensions. In contrast to observations with other exercise modes, during exercise involving a single muscle group the attenuation of central motor drive does not appear to independently regulate the development of peripheral muscle fatigue.

The role of sense of effort on self-selected cycling power output.

PURPOSE: We explored the effects of the sense of effort and accompanying perceptions of peripheral discomfort on self-selected cycle power output under two different inspired O2 fractions. METHODS: On separate days, eight trained males cycled for 5 min at a constant subjective effort (sense of effort of '3' on a modified Borg CR10 scale), immediately followed by five 4-s progressive submaximal (sense of effort of "4, 5, 6, 7, and 8"; 40 s between bouts) and two 4-s maximal (sense of effort of "10"; 3 min between bouts) bouts under normoxia (NM: fraction of inspired O2 [FiO2] 0.21) and hypoxia (HY: [FiO2] 0.13). Physiological (Heart Rate, arterial oxygen saturation (SpO2) and quadriceps Root Mean Square (RMS) electromyographical activity) and perceptual responses (overall peripheral discomfort, difficulty breathing and limb discomfort) were recorded. RESULTS: Power output and normalized quadriceps RMS activity were not different between conditions during any exercise bout (p > 0.05) and remained unchanged across time during the constant-effort cycling. SpO2 was lower, while heart rate and ratings of perceived difficulty breathing were higher under HY, compared to NM, at all time points (p < 0.05). During the constant-effort cycling, heart rate, overall perceived discomfort, difficulty breathing and limb discomfort increased with time (all p < 0.05). All variables (except SpO2) increased along with sense of effort during the brief progressive cycling bouts (all p < 0.05). During the two maximal cycling bouts, ratings of overall peripheral discomfort displayed an interaction between time and condition with ratings higher in the second bout under HY vs. NM conditions. CONCLUSION: During self-selected, constant-effort and brief progressive, sub-maximal, and maximal cycling bouts, mechanical work is regulated in parallel to the sense of effort, independently from peripheral sensations of discomfort.