TIME COURSE OF NEUROMUSCULAR ADAPTATIONS DURING HIGH-INTENSITY INTERVAL TRAINING

High-intensity interval training (HIIT) is a popular mode of exercise that is safe, time efficient, and effective at improving cardiorespiratory function (peak VO2), but its effects on neuromuscular function are unknown. The purpose of the present investigation was to: 1) determine the time-course of neuromuscular adaptations from modified, low-volume HIIT, 2) to examine the influence of HIIT on spinal and corticospinal excitability, 3) to determine the preliminary effectiveness of HIIT to improve occupational and functional task performance. Forty-six healthy, previously untrained men and women (61% Female; age, 24 ± 5 years; BMI, 23.3 ± 3.0) were recruited for this study. All participants were randomly allocated into a bi-weekly, cycle-based HIIT or control group for 6-weeks. Participants underwent pre- and post-intervention assessments of: body composition, ultrasound derived muscle volume and echo intensity (EI), leg extension and leg press isometric strength, muscle activation and %voluntary activation (%VA), corticospinal (active motor threshold) and spinal (H/MMAX) excitability, occupational and functional assessments (stair climb, vertical jump, and balance), peak VO2, and time to exhaustion (TTE). Additional ultrasound and leg press strength measures were assessed on three separate occasions within the intervention period (mid-testing). The intervention group performed 10, 1-min bouts of cycling at 90% peak VO2 at each of the 12 training sessions. Data were analyzed using the intention-to-treat framework, with complimentary complete-case analyses. The results did not reveal significant group*time increases in leg extension or leg press strength (P ≥ 0.149), muscle activation (P ≥ 0.281), corticospinal excitability (P ≥ 0.859), H/MMAX -(P ≥ 0.537), or %VA (P ≥ 0.363). Total quadriceps and vastus lateralis, medialis, and intermedius muscle volumes (P ≤ 0.001) increased (+9.2 – 18.5%) following HIIT, but not rectus femoris volume (P = 0.198) or EI (P = 0.273). HIIT also improved balance (P ≤ 0.05), peak VO2 (P < 0.001), TTE (P < 0.002), but not vertical jump and stair climb performance (P ≥ 0.333) or body composition (P ≥ 0.14). Although six-weeks of low-volume, cycle-based HIIT had no significant impact on muscle strength and muscle activation, it improved quadriceps muscle size (after just 3 sessions) and balance performance.Doctor of Philosoph