We investigated whether sprint interval training (SIT) was a time-efficient exercise strategy to improve insulin sensitivity and other indices of cardiometabolic health to the same extent as ...traditional moderate-intensity continuous training (MICT). SIT involved 1 minute of intense exercise within a 10-minute time commitment, whereas MICT involved 50 minutes of continuous exercise per session.
Sedentary men (27±8y; BMI = 26±6kg/m2) performed three weekly sessions of SIT (n = 9) or MICT (n = 10) for 12 weeks or served as non-training controls (n = 6). SIT involved 3x20-second 'all-out' cycle sprints (~500W) interspersed with 2 minutes of cycling at 50W, whereas MICT involved 45 minutes of continuous cycling at ~70% maximal heart rate (~110W). Both protocols involved a 2-minute warm-up and 3-minute cool-down at 50W.
Peak oxygen uptake increased after training by 19% in both groups (SIT: 32±7 to 38±8; MICT: 34±6 to 40±8ml/kg/min; p<0.001 for both). Insulin sensitivity index (CSI), determined by intravenous glucose tolerance tests performed before and 72 hours after training, increased similarly after SIT (4.9±2.5 to 7.5±4.7, p = 0.002) and MICT (5.0±3.3 to 6.7±5.0 x 10-4 min-1 μU/mL-1, p = 0.013) (p<0.05). Skeletal muscle mitochondrial content also increased similarly after SIT and MICT, as primarily reflected by the maximal activity of citrate synthase (CS; P<0.001). The corresponding changes in the control group were small for VO2peak (p = 0.99), CSI (p = 0.63) and CS (p = 0.97).
Twelve weeks of brief intense interval exercise improved indices of cardiometabolic health to the same extent as traditional endurance training in sedentary men, despite a five-fold lower exercise volume and time commitment.
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.
Physiological responses to acute and chronic exercise are mediated by characteristics of the training programme. Training volume is the product of the frequency, intensity, and duration of exercise. The format of the training programme, although often complex, can generally be characterized as moderate‐intensity continuous training (MICT), high‐intensity interval training (HIIT), or sprint interval training (SIT). Characteristics of the training programme influence the magnitude of the skeletal muscle, cardiovascular and integrative adaptations to exercise. In particular, there is strong evidence that exercise intensity mediates mitochondrial adaptations to exercise and improvements in maximum aerobic capacity (V̇O2 max ); however, the influence of exercise intensity is uncertain for specific cardiovascular adaptations, including capillarization, maximum cardiac output and stroke volume, and blood volume.
Key points
A classic unresolved issue in human integrative physiology involves the role of exercise intensity, duration and volume in regulating skeletal muscle adaptations to training.
We employed ...counterweighted single‐leg cycling as a unique within‐subject model to investigate the role of exercise intensity in promoting training‐induced increases in skeletal muscle mitochondrial content.
Six sessions of high‐intensity interval training performed over 2 weeks elicited greater increases in citrate synthase maximal activity and mitochondrial respiration compared to moderate‐intensity continuous training matched for total work and session duration.
These data suggest that exercise intensity, and/or the pattern of contraction, is an important determinant of exercise‐induced skeletal muscle remodelling in humans.
We employed counterweighted single‐leg cycling as a unique model to investigate the role of exercise intensity in human skeletal muscle remodelling. Ten young active men performed unilateral graded‐exercise tests to measure single‐leg V̇O2, peak and peak power (Wpeak). Each leg was randomly assigned to complete six sessions of high‐intensity interval training (HIIT) 4 × (5 min at 65% Wpeak and 2.5 min at 20% Wpeak) or moderate‐intensity continuous training (MICT) (30 min at 50% Wpeak), which were performed 10 min apart on each day, in an alternating order. The work performed per session was matched for MICT (143 ± 8.4 kJ) and HIIT (144 ± 8.5 kJ, P > 0.05). Post‐training, citrate synthase (CS) maximal activity (10.2 ± 0.8 vs. 8.4 ± 0.9 mmol kg protein−1 min−1) and mass‐specific pmol O2•(s•mg wet weight)−1 oxidative phosphorylation capacities (complex I: 23.4 ± 3.2 vs. 17.1 ± 2.8; complexes I and II: 58.2 ± 7.5 vs. 42.2 ± 5.3) were greater in HIIT relative to MICT (interaction effects, P < 0.05); however, mitochondrial function i.e. pmol O2•(s•CS maximal activity)−1 measured under various conditions was unaffected by training (P > 0.05). In whole muscle, the protein content of COXIV (24%), NDUFA9 (11%) and mitofusin 2 (MFN2) (16%) increased similarly across groups (training effects, P < 0.05). Cytochrome c oxidase subunit IV (COXIV) and NADH:ubiquinone oxidoreductase subunit A9 (NDUFA9) were more abundant in type I than type II fibres (P < 0.05) but training did not increase the content of COXIV, NDUFA9 or MFN2 in either fibre type (P > 0.05). Single‐leg V̇O2, peak was also unaffected by training (P > 0.05). In summary, single‐leg cycling performed in an interval compared to a continuous manner elicited superior mitochondrial adaptations in human skeletal muscle despite equal total work.
Key points
A classic unresolved issue in human integrative physiology involves the role of exercise intensity, duration and volume in regulating skeletal muscle adaptations to training.
We employed counterweighted single‐leg cycling as a unique within‐subject model to investigate the role of exercise intensity in promoting training‐induced increases in skeletal muscle mitochondrial content.
Six sessions of high‐intensity interval training performed over 2 weeks elicited greater increases in citrate synthase maximal activity and mitochondrial respiration compared to moderate‐intensity continuous training matched for total work and session duration.
These data suggest that exercise intensity, and/or the pattern of contraction, is an important determinant of exercise‐induced skeletal muscle remodelling in humans.
Brief, intense interval training describes a style of exercise characterized by short bouts of strenuous effort interspersed with recovery periods. The method increases whole body maximal oxygen ...uptake (V̇o
), but the underlying physiological basis is unclear. V̇o
represents the functional limit of the integrative oxygen cascade, which refers to the physiological steps involved in oxygen transport and utilization from atmospheric air to mitochondrial metabolism. There is insufficient evidence to definitively state which steps in the oxygen cascade are responsible for the improvement in V̇o
after brief, intense interval training. Studies typically focus on specific physiological variables that are often characterized as "central" or "peripheral" based in part on their location in the body. Recent work suggests that training for ≥6 wk improves V̇o
in part by increasing maximal cardiac output and expanding blood volume, responses that are expected to augment central oxygen delivery. Other responses to brief, intense interval training, including increased capillary and mitochondrial density, may contribute to increases in V̇o
via enhanced skeletal muscle oxygen extraction and/or increased muscle diffusing capacity. This is especially evident after relatively short-term training and despite no change in central oxygen delivery factors. Mechanistic investigations, particularly employing contemporary technologies, are needed to advance our understanding of the early time course of the V̇o
response to brief, intense interval training and the extent to which changes in specific oxygen cascade processes compare with traditional endurance training.
Direct sampling of human skeletal muscle using the needle biopsy technique can facilitate insight into the biochemical and histological responses resulting from changes in exercise or feeding. ...However, the muscle biopsy procedure is invasive, and analyses are often expensive, which places pragmatic restraints on sample sizes. The unilateral exercise model can serve to increase statistical power and reduce the time and cost of a study. With this approach, 2 limbs of a participant are randomized to 1 of 2 treatments that can be applied almost concurrently or sequentially depending on the nature of the intervention. Similar to a typical repeated measures design, comparisons are made within participants, which increases statistical power by reducing the amount of between-person variability. A washout period is often unnecessary, reducing the time needed to complete the experiment and the influence of potential confounding variables such as habitual diet, activity, and sleep. Variations of the unilateral exercise model have been employed to investigate the influence of exercise, diet, and the interaction between the 2, on a wide range of variables including mitochondrial content, capillary density, and skeletal muscle hypertrophy. Like any model, unilateral exercise has some limitations: it cannot be used to study variables that potentially transfer across limbs, and it is generally limited to exercises that can be performed in pairs of treatments. Where appropriate, however, the unilateral exercise model can yield robust, well-controlled investigations of skeletal muscle responses to a wide range of interventions and conditions including exercise, dietary manipulation, and disuse or immobilization.
Near-infrared spectroscopy (NIRS) provides a simple and reliable measure of skeletal muscle oxidative capacity; however, its relationship to aerobic fitness and sex are unclear. We hypothesized that ...NIRS-derived oxidative capacity in the vastus lateralis (VL) and medial gastrocnemius (MG) would be correlated with indices of aerobic fitness and independent of sex. Twenty-six participants (13 males, 13 females) performed ramp- and step-incremental tests to volitional exhaustion on separate days to establish maximal oxygen uptake (V̇o
), peak power output (PPO), lactate threshold (LT), gas exchange threshold (GET), respiratory compensation point (RCP), and maximal fat oxidation (MFO). Data were normalized to lean body mass to account for sex-based differences in body composition. Exercise tests were preceded by duplicate measurements of NIRS-derived oxidative capacity on the VL and MG muscles (i.e., repeated arterial occlusions following a brief set of muscle contractions). Skeletal muscle oxidative capacity for the VL (means ± SD: 21.9 ± 4.6 s) and MG (22.5 ± 6.1 s) were similar but unrelated (
= 0.03,
= 0.39). Skeletal muscle oxidative capacity for the VL, but not the MG (
> 0.05 for all variables), was significantly correlated with V̇o
(
= 0.24;
= 0.01), PPO (
= 0.23;
= 0.01), LT (
= 0.23;
= 0.01), GET (
= 0.23;
= 0.01), and RCP (
= 0.27;
= 0.006). MFO was not correlated with VL or MG skeletal muscle oxidative capacity (
> 0.05). Females (54.9 ± 4.5 mL·kg LBM
·min
) and males (56.0 ± 6.2 mL·kg LBM
·min
), matched for V̇o
(
= 0.62), had similar NIRS-derived oxidative capacities for VL (20.7 ± 4.4 vs. 23.2 ± 4.6 s;
= 0.18) and MG (24.4 ± 6.8 vs. 20.5 ± 4.8 s;
= 0.10). Overall, NIRS-derived skeletal muscle oxidative capacity in VL is indicative of aerobic fitness and independent of sex in humans.
Near-infrared spectroscopy (NIRS) can be used to measure skeletal muscle oxidative capacity. Here, we demonstrated that NIRS-derived skeletal muscle oxidative capacity of the vastus lateralis was independent of sex, reliable across and within days, and correlated with maximal and submaximal indices of aerobic fitness, including maximal oxygen uptake, lactate threshold, and respiratory compensation point. These findings highlight the utility of NIRS for investigating skeletal muscle oxidative capacity in females and males.
Introduction
Sprint interval training (SIT), characterized by brief bouts of ‘supramaximal’ exercise interspersed with recovery periods, increases peak oxygen uptake (
V
˙
O
2
peak
) despite a low ...total exercise volume. Per the Fick principle, increased
V
˙
O
2
peak
is attributable to increased peak cardiac output (
Q
˙
peak
) and/or peak arterio-venous oxygen difference (a-vO
2diff
). There are limited and equivocal data regarding the physiological basis for SIT-induced increases in
V
˙
O
2
peak
, with most studies lasting ≤ 6 weeks.
Purpose
To determine the effect of 12 weeks of SIT on
Q
˙
peak
, measured using inert gas rebreathing, and the relationship between changes in
Q
˙
peak
and
V
˙
O
2
peak
.
Methods
15 healthy untrained adults 6 males, 9 females; 21 ± 2 y (mean ± SD) performed 28 ± 3 training sessions. Each session involved a 2-min warm-up at 50 W, 3 × 20-s ‘all-out’ cycling bouts (581 ± 221 W) interspersed with 2-min of recovery, and a 3-min cool-down at 50 W.
Results
Measurements performed before and after training showed that 12 weeks of SIT increased
Q
˙
peak
(17.0 ± 3.7 vs 18.1 ± 4.6 L/min,
p
= 0.01, partial
η
2
= 0.28) and
V
˙
O
2
peak
(2.63 ± 0.78 vs 3.18 ± 1.1 L/min,
p
< 0.01, partial
η
2
= 0.58). The changes in these two variables were correlated (
r
2
= 0.46,
p
< 0.01). Calculated peak a-vO
2diff
also increased after training (154 ± 22 vs 174 ± 23 ml O
2
/L;
p
< 0.01) and was correlated with the change in
V
˙
O
2
peak
(
r
2
= 0.33,
p
= 0.03). Exploratory analyses revealed an interaction (
p
< 0.01) such that
Q
˙
peak
increased in male (+ 10%,
p
< 0.01) but not female participants (+ 0.6%,
p
= 0.96), suggesting potential sex-specific differences.
Conclusion
Twelve weeks of SIT increased
Q
˙
peak
by 6% in previously untrained participants and the change was correlated with the larger 21% increase in
V
˙
O
2
peak
.
Oral iron supplementation is the first-line treatment for addressing iron deficiency, a concern particularly relevant to women who are susceptible to sub-optimal iron levels. Nevertheless, the impact ...of iron supplementation on the gut microbiota of middle-aged women remains unclear. To investigate the association between iron supplementation and the gut microbiota, healthy females aged 40-65 years (
= 56, BMI = 23 ± 2.6 kg/m
) were retrospectively analyzed from the Alberta's Tomorrow Project. Fecal samples along with various lifestyle, diet, and health questionnaires were obtained. The gut microbiota was assessed by 16S rRNA sequencing. Individuals were matched by age and BMI and classified as either taking no iron supplement, a low-dose iron supplement (6-10 mg iron/day), or high-dose iron (>100 mg/day). Compositional and functional analyses of microbiome data in relation to iron supplementation were investigated using various bioinformatics tools. Results revealed that iron supplementation had a dose-dependent effect on microbial communities. Elevated iron intake (>100 mg) was associated with an augmentation of Proteobacteria and a reduction in various taxa, including
,
,
,
,
, and
. Metagenomic prediction further suggested the upregulation of iron acquisition and siderophore biosynthesis following high iron intake. In conclusion, adequate iron levels are essential for the overall health and wellbeing of women through their various life stages. Our findings offer insights into the complex relationships between iron supplementation and the gut microbiota in middle-aged women and underscore the significance of iron dosage in maintaining optimal gut health.
Single-leg cycling is a commonly used intervention in exercise physiology that has applications in exercise training and rehabilitation. The addition of a counterweight to the contralateral pedal ...helps single-leg cycling mimic cycling patterns of double-leg cycling. To date, no research has tested (a) the influence of a wide range of counterweight masses on a person's cycling biomechanics and (b) the optimal counterweight mass to emulate double-leg cycling. The purpose of this study was to determine the effects of varying counterweights on the kinematics (joint angles) and kinetics (joint moments, work) of cycling using a 3D analysis. Twelve participants cycled at 50W or 100W with different counterweight masses (0 to 30 lbs, 2.5 lbs increments), while we analyzed the pedal force data, joint angles, joint moments, and joint power of the lower limb using 3D motion capture and 3D instrumented pedals to create participant-specific musculoskeletal models. The results showed that no single-leg cycling condition truly emulated double-leg cycling with respect to all measured variables, namely pedal forces (p less than or equal to 0.05), joint angles (p less than or equal to 0.05), joint moments(p less than or equal to 0.05), and joint powers (p less than or equal to 0.05), but higher counterweights resulted in single-leg cycling that was statistically similar (p > 0.05), but descriptively, asymptotically approached the biomechanics of double-leg cycling. We suggest that a 20-lb counterweight is a conservative estimate of the counterweight required for using single-leg cycling in exercise physiology studies, but further modifications are needed to the cycle ergometer for the biomechanics of single-leg cycling to match those of double-leg cycling.
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