Low-Volume Intense Exercise Elicits Post-exercise Hypotension and Subsequent Hypervolemia, Irrespective of Which Limbs Are Exercised

Interactive effects of exercise intensity and recovery posture on postexercise hypotension

Postexercise reduction in blood pressure, termed postexercise hypotension (PEH), is relevant for both acute and chronic health reasons and potentially for peripheral cardiovascular adaptations. We investigated the interactive effects of exercise intensity and recovery postures (seated, supine, and standing) on PEH. Thirteen normotensive men underwent a V̇o2max test on a cycle ergometer and five exhaustive constant load trials to determine critical power (CP) and the gas exchange threshold (GET). Subsequently, work-matched exercise trials were performed at two discrete exercise intensities (10% > CP and 10% < GET), with 1 h of recovery in each of the three postures. For both exercise intensities, standing posture resulted in a more substantial PEH (all P < 0.01). For both standing and seated recovery postures, the higher exercise intensity led to larger reductions in systolic [standing: -33 (11) vs. -21 (8) mmHg; seated: -34 (32) vs. -17 (37) mmHg, P < 0.01], diastolic [standing: -18 (7) vs. -8 (5) mmHg; seated: -10 (10) vs. -1 (4) mmHg, P < 0.01], and mean arterial pressures [-13 (8) vs. -2 (4) mmHg, P < 0.01], whereas in the supine recovery posture, the reduction in diastolic [-9 (9) vs. -4 (3) mmHg, P = 0.08) and mean arterial pressures [-7 (5) vs. -3 (4) mmHg, P = 0.06] was not consistently affected by prior exercise intensity. PEH is more pronounced during recovery from exercise performed above CP versus below GET. However, the effect of exercise intensity on PEH is largely abolished when recovery is performed in the supine posture.NEW & NOTEWORTHY The magnitude of postexercise hypotension is greater following the intensity above the critical power in a standing position.

The effects of high-intensity exercise training and detraining with and without active recovery on postexercise hypotension in young men

Whether high-intensity exercise training and detraining combined with skeletal muscle pump (MP) could alter the magnitude of postexercise hypotension has not been investigated. We therefore sought to determine whether the combination of MP (unloaded back-pedaling) with 4 weeks of high-intensity exercise training and detraining could alter the magnitude of postexercise hypotension. Fourteen healthy men underwent 4 weeks of high-intensity exercise training (5 consecutive days per week for 15 min per session at 40% of the difference between the gas exchange threshold and maximal oxygen uptake [i.e., Δ40%]) followed by detraining for 4 weeks. Assessments were conducted at Pre-training (Pre), Post-training (Post) and after Detraining with (MP) and without MP (Con). The exercise test in the Pre, Post and the Detraining consisted of 15 min exercise at Δ40% followed by 1 h of recovery. At all time-points, the postexercise reduction in mean arterial pressure (MAP) was reduced in MP compared to Con (all p < 0.01). Four weeks of high-intensity exercise training resulted in a reduction in the magnitude of postexercise hypotension (i.e., the change in MAP from baseline was mitigated) across both trials (All p < 0.01) when compared to Pre and Detraining. Following Detraining, the reduction of MAP from baseline was reduced compared to Pre, but was not different from Post. We conclude that high-intensity exercise training combined with skeletal MP reduces the magnitude of postexercise hypotension, and this effect is partially retained for 4 weeks following the complete cessation of high-intensity exercise training.

Acute and adaptive cardiovascular and metabolic effects of passive heat therapy or high-intensity interval training in patients with severe lower-limb osteoarthritis

Exercise is painful and difficult to perform for patients with severe lower-limb osteoarthritis; consequently, reduced physical activity contributes to increased cardiometabolic disease risk. The aim of this study was to characterize the acute and adaptive cardiovascular and metabolic effects of two low or no impact therapies in patients with severe lower-limb osteoarthritis: passive heat therapy (Heat) and high-intensity interval training (HIIT) utilizing primarily the unaffected limbs, compared to a control intervention of home-based exercise (Home). Participants completed up to 12 weeks of either Heat (20-30 min immersed in 40°C water followed by ~15-min light resistance exercise), HIIT (6-8 × 60-s intervals on a cross-trainer or arm ergometer at ~90-100% peak V ̇ $$ \dot{V} $$ O₂ ) or Home (~15-min light resistance exercise); all 3 sessions/week. Reductions in systolic (12 & 10 mm Hg), diastolic (7 & 4 mm Hg), and mean arterial (8 & 6 mm Hg) blood pressure (BP) were observed following one bout of Heat or HIIT exposure, lasting for the duration of the 20-min monitoring period. Across the interventions (i.e., 12 weeks), resting systolic BP and diastolic BP decreased with Heat (-9 & -4 mm Hg; p < 0.001) and HIIT (-7 & -3 mm Hg; p ≤ 0.011), but not Home (0 & 0 mm Hg; p ≥ 0.785). The systolic and diastolic BP responses to an acute exposure of Heat or HIIT in the first intervention session were moderately correlated with adaptive responses across the intervention (r ≥ 0.54, p ≤ 0.005). Neither intervention improved indices of glycemic control (p = 0.310). In summary, both Heat and HIIT induced potent immediate and adaptive hypotensive effects, and the acute response was moderately predictive of the long-term response.

Bout duration in high-intensity interval exercise modifies hematologic, metabolic and antioxidant responses

Objective

This study compared hematologic, metabolic and antioxidant responses between three high-intensity interval exercise (HIIE) trials of different bout duration and a continuous exercise trial (CON), all with equal average intensity, total work, and duration.

Methods

Eleven healthy young males performed four trials involving 20 min of cycling, either continuously (49% of power at VO₂max, PPO), or intermittently with 48 10-s bouts (HIIE10), 16 30-s bouts (HIIE30) or 8 60-s bouts (HIIE60) at 100% PPO, with a 1:1.5 work-to-recovery ratio at 15% PPO. Venous blood was obtained before, immediately after, and 1 h post-exercise to evaluate hematologic, metabolic and antioxidant responses. Blood lactate concentration was measured in capillary blood during exercise, while urine lactate was measured before and 1 h post-exercise.

Results

Post-exercise leukocyte count (mean ± SD; 9.7 ± 2.8 k μL^-1), uric acid concentration (0.35 ± 0.10 mmol L^-1), glucose concentration (6.56 ± 1.44 mmol L^-1), and plasma volume change (-13.5 ± 4.4%) were greater in HIIE60 compared to all other trials (p < 0.05). One-hour post-exercise, lymphocytes decreased below pre-exercise values in all HIIE trials, and uric acid increased in the HIIE60 trial (p < 0.05). Urine lactate concentration 1 h post-exercise increased compared to pre-exercise only in HIIE60 (19-fold, p < 0.001), and this was related with the higher blood lactate concentration during exercise in that trial.

Conclusions

These findings highlight the importance of bout duration, given that shorter bouts of HIIE (30 s or 10 s) induce lower blood cell perturbations, metabolic stress, and antioxidant responses compared to the commonly used 1-min bouts, despite equal total work, duration, and work-to-recovery ratio.

Cardiac autonomic disturbance following sprint-interval exercise in untrained young males: Does exercise volume matter?

Objectives

This study examined the influences of the volume of all-out sprint-interval exercise (SIE) on acute post-exercise heart rate variability (HRV) recovery.

Methods

HRV recovery following a session of (i) 2 × 30-s SIE (SIE₂), (ii) 4 × 30-s SIE (SIE₄), and (iii) non-exercising control (CON) were compared in 15 untrained young males. Time domain [standard deviation of normal-to-normal intervals, root mean square of successive R-R differences] and frequency domain [low frequency (0.04–0.14 Hz), high frequency (0.15–0.40 Hz)] measures of HRV were assessed every 20 min for 140 min after the exercise, and every hour during the first 4 h of actual sleep time at immediate night. All trials were scheduled at 19:00.

Results

In comparison to CON, both SIE₂ and SIE₄ attenuated the HRV markedly (p < 0.05), while the declined HRV restored progressively during recovery. Although the sprint repetitions of SIE₄ was twice as that of SIE₂, the declined HRV indices at corresponding time points during recovery were not different between the two trials (p > 0.05). Nevertheless, the post-exercise HRV restoration in SIE₂ appeared to be faster than that in SIE₄. Regardless, nocturnal HRV measured within 10 h following the exercise was not different among the SIE and CON trials (p > 0.05).

Conclusion

Such findings suggest that the exercise volume of the SIE protocol may be a factor affecting the rate of removal of the cardiac autonomic disturbance following the exercise. In addition, rest for ∼10 h following either session of the SIE protocol appears to be appropriate for the cardiovascular system to recover.

A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation

NEW FINDINGS

Central question to the study? Are primary indices of heat adaptation (e.g., expansion of plasma volume and reduction in resting core temperature) differentially affected by the three major modes of short-term heat acclimation, i.e., exercise in the heat, hot water immersion and sauna? Main finding and its importance? The three modes elicited typical adaptations expected with short-term heat acclimation, however these were not significantly different between modes. This comparison has not previously been done and highlights that individuals can expect similar adaptation to heat regardless of the mode used.

ABSTRACT

Heat acclimation (HA) can improve heat tolerance and cardiovascular health. The mode of HA potentially impacts the magnitude and time course of adaptations, but almost no comparative data exist. We therefore investigated adaptive responses to three common modes of HA, particularly with respect to plasma volume. Within a crossover repeated-measures design, 13 physically-active participants (5 female) undertook four, 5-d HA regimes (60 min/d) in randomised order, separated by ≥4 wk. Rectal temperature (T_re ) was clamped at neutrality via 36.6C (thermoneutral) water immersion (TWI; i.e., control condition), or raised by 1.5°C via heat stress in 40°C water (HWI), Sauna (55°C, 52% RH), or exercise in humid heat (40°C, 52% RH; ExH). Adaptation magnitude was assessed as the pooled response across days 4 to 6, while kinetics was assessed via the 6-d time series. Plasma volume expansion was similar in all heated conditions but only higher than TWI in ExH (by 4%, p = 0.036). Approximately two thirds of the expansion was attained within the initial 24 h and was moderately related to that present on day 6, regardless of HA mode (r = 0.560-0.887). Expansion was mediated by conservation of both sodium and albumin content, with little evidence for these having differential roles between modes (p = 0.706 and 0.320, respectively). Resting T_re decreased by 0.1-0.3°C in all heated conditions, and SBP decreased by 4 mm Hg, but not differentially between conditions (p≥0.137). In conclusion, HA mode did not substantially affect the magnitude or rate of adaptation in key resting markers of short-term HA. This article is protected by copyright. All rights reserved.

Isometric exercise versus high-intensity interval training for the management of blood pressure: a systematic review and meta-analysis

OBJECTIVE

We aimed to compare the efficacy of isometric exercise training (IET) versus high-intensity interval training (HIIT) in the management of resting blood pressure (BP).

DESIGN

Systematic review and meta-analysis.

DATA SOURCES

PubMed (MEDLINE), the Cochrane library and SPORTDiscus were systematically searched.

ELIGIBILITY CRITERIA

Randomised controlled trials published between 1 January 2000 and 1 September 2020. Research trials reporting the effects of IET or HIIT on resting BP following a short-term intervention (2-12 weeks).

RESULTS

38 studies were analysed (18 IET and 20 HIIT), including 1583 (672 IET and 911 HIIT) participants, of which 612 (268 IET and 344 HIIT) were controls.IET produced significantly greater reductions in resting BP compared with HIIT with systolic, diastolic and mean BP effect sizes of 8.50 mm Hg vs 2.86 mm Hg (Q=17.10, p<0.001), 4.07 mm Hg vs 2.48 mm Hg (Q=4.71, p=0.03) and 6.46 mm Hg vs 3.15 mm Hg (Q=4.21, p=0.04) respectively. However, HIIT reduced resting heart rate significantly more than IET (3.17bpm vs 1.34bpm, Q=7.63, p=0.006).

CONCLUSION

While both modes are efficacious, IET appears to be the superior mode of exercise in the management of resting BP. However, HIIT may achieve wider physiological benefits, with greater reductions in resting heart rate.

Functional High-Intensity Interval Training Lowers Body Mass and Improves Coordination, Strength, Muscular Endurance, and Aerobic Endurance of Inmates in a German Prison

This study examined the effects of circuit-like functional high-intensity interval training (HIIT) on body composition and motor performance of inmates in an open German prison. The group of inmates (n=11) consisted of predominantly overweight males [average body-mass-index (BMI)=31.2]. They performed 6weeks of training including 3 sessions per week. The 6-week training program was framed by a pre-test and a post-test that assessed anthropometry and motor performance. On average, the inmates participated in 91.9% of all training sessions. The intervention significantly lowered body mass (p=0.007) and BMI (p=0.006). Fat mass and fat-free mass did not change significantly from pre-test to post-test. The times in 20m sprint did not change. The performance in lateral jumping from side-to-side (p=0.024), standing long jump (p=0.001), and 30–15 Intermittent Fitness Test (p<0.001) improved significantly. The greatest improvements were observed in the number of sit-ups (p<0.001) and push-ups (p<0.001). These findings suggest that (functional) HIIT is a practical and effective training method in the context of a prison.

Post-exercise Hypotension Following a Single Bout of High Intensity Interval Exercise vs. a Single Bout of Moderate Intensity Continuous Exercise in Adults With or Without Hypertension: A Systematic Review and Meta-Analysis of Randomized Clinical Trials

Background: Post-exercise hypotension (PEH) is an important tool in the daily management of patients with hypertension. Varying the exercise parameters is likely to change the blood pressure (BP) response following a bout of exercise. In recent years, high-intensity interval exercise (HIIE) has gained significant popularity in exercise-based prevention and rehabilitation of clinical populations. Yet, to date, it is not known whether a single session of HIIE maximizes PEH more than a bout of moderate-intensity continuous exercise (MICE).

Objective: To compare the effect of HIIE vs. MICE on PEH by means of a systematic review and meta-analysis.

Methods: A systematic search in the electronic databases MEDLINE, Embase, and SPORTDiscus was conducted from the earliest date available until February 24, 2020. Randomized clinical trials comparing the transient effect of a single bout of HIIE to MICE on office and/or ambulatory BP in humans (≥18 years) were included. Data were pooled using random effects models with summary data reported as weighted means and 95% confidence interval (CIs).

Results: Data from 14 trials were included, involving 18 comparisons between HIIE and MICE and 276 (193 males) participants. The immediate effects, measured as office BP at 30- and 60-min post-exercise, was similar for a bout of HIIE and MICE (p > 0.05 for systolic and diastolic BP). However, HIIE elicited a more pronounced BP reduction than MICE [(−5.3 mmHg (−7.3 to −3.3)/ −1.63 mmHg (−3.00 to −0.26)] during the subsequent hours of ambulatory daytime monitoring. No differences were observed for ambulatory nighttime BP (p > 0.05).

Conclusion: HIIE promoted a larger PEH than MICE on ambulatory daytime BP. However, the number of studies was low, patients were mostly young to middle-aged individuals, and only a few studies included patients with hypertension. Therefore, there is a need for studies that involve older individuals with hypertension and use ambulatory BP monitoring to confirm HIIE's superiority as a safe BP lowering intervention in today's clinical practice.

Systematic Review Registration: PROSPERO (registration number: CRD42020171640).

Acute Effect of Interval vs. Continuous Exercise on Blood Pressure: Systematic Review and Meta-Analysis

Fundamento

O exercício aeróbio contínuo (EC) é uma das principais recomendações não farmacológicas para prevenção e tratamento da hipertensão arterial sistêmica. O EC é seguro e eficaz para reduzir a pressão arterial cronicamente, assim como nas primeiras horas após sua realização, fenômeno conhecido por hipotensão pós-exercício (HPE). O exercício intervalado (EI) também gera HPE.

Objetivo

Essa revisão sistemática e metanálise buscou comparar a magnitude da HPE entre o EC e EI em adultos.

Métodos

Realizou-se uma revisão sistemática de estudos publicados em revistas indexadas nas bases PubMed, Web of Knowledge, Scopus e CENTRAL até março de 2020 que compararam a magnitude da HPE entre o EC versus EI. Foi definida HPE entre 45 e 60 minutos pós-exercício. As diferenças entre grupos sobre a pressão arterial foram analisadas por meio do modelo de efeito aleatório. Os dados foram reportados como diferença média ponderada (WMD) e 95% de intervalo de confiança (IC). Valor p menor que 0,05 foi considerado estatisticamente significativo. A escala TESTEX (0 a 15) foi usada para verificação da qualidade metodológica dos estudos.

Resultados

O EI apresentou HPE de maior magnitude sobre a pressão arterial sistólica (WMD: -2,93 mmHg [IC95%: -4,96, -0,90], p = 0,005, I² = 50%) e pressão arterial diastólica (WMD: -1,73 mmHg [IC95%: -2,94, -0,51], p = 0,005, I² = 0%) quando comparado ao EC (12 estudos; 196 participantes). A pontuação dos estudos na escala TEXTEX variou entre 10 e 11 pontos.

Conclusões

O EI gerou HPE de maior magnitude quando comparado ao EC entre 45 e 60 minutos pós-exercício. A ausência de dados sobre eventos adversos durante o EI e EC nos estudos impede comparações sobre a segurança dessas estratégias. (Arq Bras Cardiol. 2020; 115(1):5-14)

Heat-induced hypervolemia: Does the mode of acclimation matter and what are the implications for performance at Tokyo 2020?

Tokyo 2020 will likely be the most heat stressful Olympics to date, so preparation to mitigate the effects of humid heat will be essential for performance in several of the 33 sports. One key consideration is heat acclimation (HA); the repeated exposure to heat to elicit physiological and psychophysical adaptations that improve tolerance and exercise performance in the heat. Heat can be imposed in various ways, including exercise in the heat, hot water immersion, or passive exposure to hot air (e.g., sauna). The physical requirements of each sport will determine the impact that the heat has on performance, and the adaptations required from HA to mitigate these effects. This review focuses on one key adaptation, plasma volume expansion (PVE), and how the mode of HA may affect the kinetics of adaptation. PVE constitutes a primary HA-mediated adaptation and contributes to functional adaptations (e.g., lower heart rate and increased heat loss capacity), which may be particularly important in athletes of "sub-elite" cardiorespiratory fitness (e.g., team sports), alongside athletes of prolonged endurance events. This review: i) highlights the ability of exercise in the heat, hot-water immersion, and passive hot air to expand PV, providing the first quantitative assessment of the efficacy of different heating modes; ii) discusses how this may apply to athletes at Tokyo 2020; and iii) provides recommendations regarding the protocol of HA and the prospect for achieving PVE (and the related outcomes).

Heat and Dehydration Additively Enhance Cardiovascular Outcomes following Orthostatically-Stressful Calisthenics Exercise

Exercise and exogenous heat each stimulate multiple adaptations, but their roles are not well delineated, and that of the related stressor, dehydration, is largely unknown. While severe and prolonged hypohydration potentially “silences” the long-term heat acclimated phenotype, mild and transient dehydration may enhance cardiovascular and fluid-regulatory adaptations. We tested the hypothesis that exogenous heat stress and dehydration additively potentiate acute (24 h) cardiovascular and hematological outcomes following exercise. In a randomized crossover study, 10 physically-active volunteers (mean ± SD: 173 ± 11 cm; 72.1 ± 11.5 kg; 24 ± 3 year; 6 females) completed three trials of 90-min orthostatically-stressful calisthenics, in: (i) temperate conditions (22°C, 50% rh, no airflow; CON); (ii) heat (40°C, 60% rh) whilst euhydrated (HEAT), and (iii) heat with dehydration (no fluid ~16 h before and during exercise; HEAT+DEHY). Using linear mixed effects model analyses, core temperature (T_CORE) rose 0.7°C more in HEAT than CON (95% CL: [0.5, 0.9]; p < 0.001), and another 0.4°C in HEAT+DEHY ([0.2, 0.5]; p < 0.001, vs. HEAT). Skin temperature also rose 1.2°C more in HEAT than CON ([0.6, 1.8]; p < 0.001), and similarly to HEAT+DEHY (p = 0.922 vs. HEAT). Peak heart rate was 40 b·min⁻¹ higher in HEAT than in CON ([28, 51]; p < 0.001), and another 15 b·min⁻¹ higher in HEAT+DEHY ([3, 27]; p = 0.011, vs. HEAT). Mean arterial pressure at 24-h recovery was not consistently below baseline after CON or HEAT (p ≥ 0.452), but was reduced 4 ± 1 mm Hg after HEAT+DEHY ([0, 8]; p = 0.020 vs. baseline). Plasma volume at 24 h after exercise increased in all trials; the 7% increase in HEAT was not reliably more than in CON (5%; p = 0.335), but was an additional 4% larger after HEAT+DEHY ([1, 8]; p = 0.005 vs. HEAT). Pooled-trial correlational analysis showed the rise in T_CORE predicted the hypotension (r = −0.4) and plasma volume expansion (r = 0.6) at 24 h, with more hypotension reflecting more plasma expansion (r = −0.5). In conclusion, transient dehydration with heat potentiates short-term (24-h) hematological (hypervolemic) and cardiovascular (hypotensive) outcomes following calisthenics.

Acute high-intensity interval running increases markers of gastrointestinal damage and permeability but not gastrointestinal symptoms

The purpose of this study was to investigate the effects of high-intensity interval running on markers of gastrointestinal (GI) damage and permeability alongside subjective symptoms of GI discomfort. Eleven male runners completed an acute bout of high-intensity interval training (HIIT) (eighteen 400-m runs at 120% maximal oxygen uptake) where markers of GI permeability, intestinal damage, and GI discomfort symptoms were assessed and compared with resting conditions. Compared with rest, HIIT significantly increased serum lactulose/rhamnose ratio (0.051 ± 0.016 vs. 0.031 ± 0.021, p = 0.0047; 95% confidence interval (CI) = 0.006 to 0.036) and sucrose concentrations (0.388 ± 0.217 vs. 0.137 ± 0.148 mg·L^-1; p < 0.001; 95% CI = 0.152 to 0.350). In contrast, urinary lactulose/rhamnose (0.032 ± 0.005 vs. 0.030 ± 0.005; p = 0.3; 95% CI = -0.012 to 0.009) or sucrose concentrations (0.169% ± 0.168% vs. 0.123% ± 0.120%; p = 0.54; 95% CI = -0.199 to 0.108) did not differ between HIIT and resting conditions. Plasma intestinal-fatty acid binding protein (I-FABP) was significantly increased (p < 0.001) during and in the recovery period from HIIT whereas no changes were observed during rest. Mild symptoms of GI discomfort were reported immediately and at 24 h post-HIIT, although these symptoms did not correlate to GI permeability or I-FABP. In conclusion, acute HIIT increased GI permeability and intestinal I-FABP release, although these do not correlate with symptoms of GI discomfort. Furthermore, by using serum sampling, we provide data showing that it is possible to detect changes in intestinal permeability that is not observed using urinary sampling over a shorter time-period.

Functional High-Intensity Circuit Training Improves Body Composition, Peak Oxygen Uptake, Strength, and Alters Certain Dimensions of Quality of Life in Overweight Women

The effects of circuit-like functional high-intensity training (Circuit_HIIT) alone or in combination with high-volume low-intensity exercise (Circuit_combined) on selected cardio-respiratory and metabolic parameters, body composition, functional strength and the quality of life of overweight women were compared. In this single-center, two-armed randomized, controlled study, overweight women performed 9-weeks (3 sessions·wk⁻¹) of either Circuit_HIIT (n = 11), or Circuit_combined (n = 8). Peak oxygen uptake and perception of physical pain were increased to a greater extent (p < 0.05) by Circuit_HIIT, whereas Circuit_combined improved perception of general health more (p < 0.05). Both interventions lowered body mass, body-mass-index, waist-to-hip ratio, fat mass, and enhanced fat-free mass; decreased ratings of perceived exertion during submaximal treadmill running; improved the numbers of push-ups, burpees, one-legged squats, and 30-s skipping performed, as well as the height of counter-movement jumps; and improved physical and social functioning, role of physical limitations, vitality, role of emotional limitations, and mental health to a similar extent (all p < 0.05). Either forms of these multi-stimulating, circuit-like, multiple-joint training can be employed to improve body composition, selected variables of functional strength, and certain dimensions of quality of life in overweight women. However, Circuit_HIIT improves peak oxygen uptake to a greater extent, but with more perception of pain, whereas Circuit_combined results in better perception of general health.

Similar metabolic response to lower- versus upper-body interval exercise or endurance exercise

PURPOSE

To compare energy use and substrate partitioning arising from repeated lower- versus upper-body sprints, or endurance exercise, across a 24-h period.

METHODS

Twelve untrained males (24±4 y) completed three trials in randomized order: (1) repeated sprints (five 30-s Wingate, 4.5-min recovery) on a cycle ergometer (SIT_Legs); (2) 50-min continuous cycling at 65% V̇O₂max (END); (3) repeated sprints on an arm-crank ergometer (SIT_Arms). Respiratory gas exchange was assessed before and during exercise, and at eight points across 22h of recovery.

RESULTS

Metabolic rate was elevated to greater extent in the first 8h after SIT_Legs than SIT_Arms (by 0.8±1.1kJ/min, p=0.03), and tended to be greater than END (by 0.7±1.3kJ/min, p=0.08). Total 24-h energy use (exercise+recovery) was equivalent between SIT_Legs and END (p = 0.55), and SIT_Legs and SIT_Arms (p=0.13), but 24-h fat use was higher with SIT_Legs than END (by 26±38g, p=0.04) and SIT_Arms (by 27±43g, p=0.05), whereas carbohydrate use was higher with SIT_Arms than SIT_Legs (by 32±51g, p=0.05). Plasma volume-corrected post-exercise and fasting glucose and lipid concentrations were unchanged.

CONCLUSION

Despite much lower energy use during five sprints than 50-min continuous exercise, 24-h energy use was not reliably different. However, (i) fat metabolism was greater after sprints, and (ii) carbohydrate metabolism was greater in the hours after sprints with arms than legs, while 24-h energy usage was comparable. Thus, sprints using arms or legs may be an important adjunct exercise mode for metabolic health.

Physiological adaptations to interval training and the role of exercise intensity

Interval exercise typically involves repeated bouts of relatively intense exercise interspersed by short periods of recovery. A common classification scheme subdivides this method into high-intensity interval training (HIIT; 'near maximal' efforts) and sprint interval training (SIT; 'supramaximal' efforts). Both forms of interval training induce the classic physiological adaptations characteristic of moderate-intensity continuous training (MICT) such as increased aerobic capacity (V̇O2 max ) and mitochondrial content. This brief review considers the role of exercise intensity in mediating physiological adaptations to training, with a focus on the capacity for aerobic energy metabolism. With respect to skeletal muscle adaptations, cellular stress and the resultant metabolic signals for mitochondrial biogenesis depend largely on exercise intensity, with limited work suggesting that increases in mitochondrial content are superior after HIIT compared to MICT, at least when matched-work comparisons are made within the same individual. It is well established that SIT increases mitochondrial content to a similar extent to MICT despite a reduced exercise volume. At the whole-body level, V̇O2 max is generally increased more by HIIT than MICT for a given training volume, whereas SIT and MICT similarly improve V̇O2 max despite differences in training volume. There is less evidence available regarding the role of exercise intensity in mediating changes in skeletal muscle capillary density, maximum stroke volume and cardiac output, and blood volume. Furthermore, the interactions between intensity and duration and frequency have not been thoroughly explored. While interval training is clearly a potent stimulus for physiological remodelling in humans, the integrative response to this type of exercise warrants further attention, especially in comparison to traditional endurance training.