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Sprint interval training (SIT) protocols involving brief (≤15 s) work bouts improve aerobic and anaerobic performance, highlighting peak speed generation as a potentially important adaptive stimulus. To determine the physiological and psychological effects of reducing the SIT work bout duration, while maintaining total exercise and recovery time, 43 healthy males (n = 27) and females (n = 16) trained for 4 weeks (3 times/week) using one of the following running SIT protocols: (i) 30:240 (n = 11; 4–6 × 30-s bouts, 4 min rest); (ii) 15:120 (n = 11; 8–12 × 15-s bouts, 2 min rest); (iii) 5:40 (n = 12; 24–36 × 5-s bouts, 40 s rest); or (iv) served as a nonexercising control (n = 9). Protocols were matched for total work (2–3 min) and rest (16–24 min) durations, as well as the work-to-rest ratio (1:8 s). Pre- and post-training measures included a graded maximal oxygen consumption test, a 5-km time trial, and a 30-s maximal sprint test. Self-efficacy, enjoyment, and intentions were assessed following the last training session. Training improved maximal oxygen consumption (5.5%; P = 0.006) and time-trial performance (5.2%; P = 0.039), with a main effect of time for peak speed (1.7%; P = 0.042), time to peak speed (25%; P < 0.001), and body fat percentage (1.4%; P < 0.001) that appeared to be driven by the training. There were no group effects for self-efficacy (P = 0.926), enjoyment (P = 0.249), or intentions to perform SIT 3 (P = 0.533) or 5 (P = 0.951) times/week. This study effectively demonstrated that the repeated generation of peak speed during brief SIT work bouts sufficiently stimulates adaptive mechanisms promoting increases in aerobic and anaerobic capacity.

Adaptations to sprint interval training (SIT) are observed with brief (≤15-s) work bouts highlighting peak power generation as an important metabolic stimulus. This study examined the effects of manipulating SIT work bout and recovery period duration on energy expenditure (EE) during and postexercise, as well as postexercise fat oxidation rates. Nine active males completed a resting control session (CTRL) and 3 SIT sessions in randomized order: (i) 30:240 (4 × 30-s bouts, 240-s recovery); (ii) 15:120 (8 × 15-s bouts, 120-s recovery); (3) 5:40 (24 × 5-s bouts, 40-s recovery). Protocols were matched for the total duration of work (2 min) and recovery (16 min), as well as the work-to-recovery ratio (1:8 s). EE and fat oxidation rates were derived from gas exchange measured before, during, and for 3 h postexercise. All protocols increased EE versus CTRL (P < 0.001). Exercise EE was greater (P < 0.001) with 5:40 (209 kcal) versus both 15:120 (163 kcal) and 30:240 (138 kcal), while 15:120 was also greater (P < 0.001) than 30:240. Postexercise EE was greater (P = 0.014) with 15:120 (313 kcal) versus 5:40 (294 kcal), though both were similar (P > 0.077) to 30:240 (309 kcal). Postexercise fat oxidation was similar (P = 0.650) after 15:120 (0.104 g·min−1) and 30:240 (0.116 g·min−1) and both were greater (P < 0.030) than 5:40 (0.072 g·min−1) and CTRL (0.049 g·min−1). In conclusion, shorter SIT work bouts that target peak power generation increase exercise EE without compromising postexercise EE, though longer bouts promote greater postexercise fat utilization.

Sprint-interval training (SIT) is a viable method to improve health and fitness. However, researchers have questioned the utility of SIT because of its strenuous nature. The current study aimed to determine if manipulating the sprint and recovery duration, while maintaining the 1:8 work to rest ratio, could uncover a more favourable SIT protocol. Nine healthy active males (age, 23.3 ± 3.0 years; body mass index, 22.4 ± 2.2 kg·m−2; maximal oxygen consumption, 48.9 ± 5.3 mL·kg−1·min−1) participated in 3 experimental running SIT sessions: (i) 30:240 (4 × 30-s efforts, 240-s recovery), (ii) 15:120 (8 × 15-s efforts, 120-s recovery), (iii) 5:40 (24 × 5-s efforts, 40-s recovery), and (iv) a final behavioural choice follow-up session. Affect, intentions, task self-efficacy, enjoyment, and preference were evaluated. Midway through exercise, affect became more positive for 5:40 compared with 30:240 (p < 0.05) and postexercise affect was greater for both 5:40 (p = 0.014) and 15:120 (p = 0.015) compared with 30:240. Participants expressed greater intentions to perform 5:40 3 and 5 times/week compared with 15:120 and 30:240 (p < 0.05). Participants felt more confident in their ability to perform 5:40 (p = 0.001) and 15:120 (p = 0.008) compared with 30:240. The 5:40 session was also rated as more enjoyable than 15:120 (p = 0.025) and 30:240 (p = 0.026). All participants preferred the 5:40 protocol. These data suggest that shorter sprints with more repetitions are perceived as more enjoyable and lead to greater intentions to engage in SIT.

Most sprint interval training (SIT) research involves cycling as the mode of exercise and whether running SIT elicits a similar excess postexercise oxygen consumption (EPOC) response to cycling SIT is unknown. As running is a more whole-body–natured exercise, the potential EPOC response could be greater when using a running session compared with a cycling session. The purpose of the current study was to determine the acute effects of a running versus cycling SIT session on EPOC and whether potential sex differences exist. Sixteen healthy recreationally active individuals (8 males and 8 females) had their gas exchange measured over ∼2.5 h under 3 experimental sessions: (i) a cycle SIT session, (ii) a run SIT session, and (iii) a control (CTRL; no exercise) session. Diet was controlled. During exercise, both SIT modes increased oxygen consumption (cycle: male, 1.967 ± 0.343; female, 1.739 ± 0.296 L·min−1; run: male, 2.169 ± 0.369; female, 1.791 ± 0.481 L·min−1) versus CTRL (male, 0.425 ± 0.065 L·min−1; female, 0.357 ± 0.067; P < 0.001), but not compared with each other (P = 0.234). In the first hour postexercise, oxygen consumption was still increased following both run (male, 0.590 ± 0.065; female, 0.449 ± 0.084) and cycle SIT (male, 0.556 ± 0.069; female, 0.481 ± 0.110 L·min−1) versus CTRL and oxygen consumption was maintained through the second hour postexercise (CTRL: male, 0.410 ± 0.048; female, 0.332 ± 0.062; cycle: male, 0.430 ± 0.047; female, 0.395 ± 0.087; run: male, 0.463 ± 0.051; female, 0.374 ± 0.087 L·min−1). The total EPOC was not significantly different between modes of exercise or males and females (P > 0.05). Our data demonstrate that the mode of exercise during SIT (cycling or running) is not important to O2 consumption and that males and females respond similarly.

Vitamin D status positively associates with skeletal muscle mass and function in adolescents. The C-3 alpha epimer of 25-hydroxyvitamin D3 (3-epi-25(OH)D3) is high in infants, yet the potential impacts of 25-hydroxyvitamin D3 (25(OH)D3) and 3-epi-25(OH)D3 on skeletal muscle development are largely unexplored. The objective of this study was (i) to explore how the concentrations of 25(OH)D3 and 3-epi-25(OH)D3 track with body composition (lean mass (LM) and fat mass (FM)) and (ii) to determine the association between 25(OH)D3 and 3-epi-25(OH)D3 in infancy. Healthy breastfed infants (n = 132) were followed from 1 to 12 months of age as part of a vitamin D dose–response study (NCT00381914). Anthropometry and diet were assessed. Body composition was measured with dual-energy X-ray absorptiometry. Plasma 25(OH)D3 and 3-epi-25(OH)D3 concentrations were evaluated using liquid chromatography tandem mass spectrometry. Plasma 25(OH)D3 and 3-epi-25(OH)D3 increased from 1 to 3 months of age and decreased thereafter (p < 0.05). Infants with 25(OH)D3 concentrations above 75 nmol/L did not have a higher LM (g or %; p > 0.273) than those below this cutoff. LM was not associated with 25(OH)D3, whereas LM% was positively associated with 25(OH)D3 (β = 0.03; CI: 0.01 to 0.06; p = 0.006), while accounting for sex, weight-for-age Z-score, protein and fat intake, and age. For FM, the variables accounting for a significant amount of the variation were plasma 25(OH)D3 concentration (β = −2.38; CI: −4.35, −0.41; p = 0.019), weight-for-age Z-score, protein and fat intake, and time. In healthy infants, higher vitamin D status associates with leaner body composition, though the effect is smaller in magnitude relative to growth.

Whole-body vibration (WBV) training uses a vertically oscillating platform and reports suggest that this perturbation elicits reflexive muscle contractions that augment muscle activity and contribute to increased strength. No WBV study has measured both upper- and lower-body muscle activation. The purpose of this study was to determine the optimal WBV stimulus (frequency × amplitude) to increase electromyography (EMG) in upper- and lower-body muscles for three distinctive unloaded actions: isometric semi-squat, dynamic leg squats, and static and dynamic bilateral bicep curls. Surface EMG was measured for the vastus lateralis (VL), biceps femoris (BF), biceps brachii (BB), and triceps brachii (TB) in 10 recreationally active male university students (24.4 ± 2.0 years; mean ± SD) when WBV was administered at 2 and 4 mm and at 25, 30, 35, 40, and 45 Hz. EMG changes are reported as the difference between WBV and no WBV EMG root mean square expressed as a percentage of maximum voluntary exertion (%MVE). In static semi-squat, WBV increased muscle activity 2.9%–6.7% in the VL and 0.8%–1.2% in the BF. During dynamic squatting, WBV increased muscle activity in the VL by 3.7%–8.7% and in the BF by 0.4%–2.0%. In a static biceps curl, WBV had no effect on BB EMG, but did increase TB activity 0.3%–0.7%. During dynamic biceps curls, WBV increased BB EMG activity by 0.6%–0.8% and TB activity by 0.2%–1.0%. The higher WBV amplitude (4 mm) and frequencies (35, 40, 45 Hz) resulted in the greatest increases in EMG activity.