High Intensity Aerobic Interval Training Stimulates Muscular Hypertrophy in Untrained Subjects

• Project Type: Class
• Directed Project Contributors: C. Losey, D. Thrush, A. Malinowski, M. Piacentini, E. Hayes; S. Gearhart, J. Norton, J. Schick, K. Snyder Supported by The Women’s Giving Circle

Aerobic exercise (AE) is typically associated with cardiovascular system and metabolic muscle adaptations, but there is potential for changes in muscle size and function as well. Evidence suggests that AE can improve or maintain muscle cross-sectional area (CSA) and/or muscle function in elderly populations, but this has not been observed in young populations. The lack of response in young subjects may be due to an insufficient stimulus to promote muscle growth. High intensity interval training (HIIT) while running may provide a stronger force of contraction than typical long duration AE, which could be the key stimulus for muscular growth, yet this has not been previously examined.

PURPOSE: To examine the impact of 10 weeks of HIIT on skeletal muscle CSA and function in a young adult population.

• Aerobic activity is typically associated with cardiovascular system and metabolic muscle adaptation, but there is potential for skeletal muscle adaptation as well.
• Evidence suggests that aerobic exercise can improve or even maintain muscle CSA and/or muscle power in elderly populations2,3,4,6, but this has not been noted in young adult populations.
• High intensity aerobic interval training (HIIT) utilized while running may provide a force of contraction strong enough to be a key stimulus for muscular growth, yet this has not been previously examined.
• PURPOSE: To examine the impact of 10 weeks of HIIT on skeletal muscle CSA and function in a young adult population.

Muscle CSA of the right vastus lateralis (VL) and leg extension speed was assessed before and after 10 weeks of 3x/week HIIT in 12 college-aged subjects. The HIIT protocol consisted of 4 intervals of 4 minutes running at 95% max heart rate (MHR) followed by 4 minutes of recovery at 70% MHR. VL CSA was determined by manual planimetry from panoramic ultrasound imaging and compared to a control group (n=5). Leg extension speed at 40% one repetition maximum was measured with a custom made electronic timing system that utilized an infrared photogate to start and stop a timer as the weight stack ascended 20 cm on the leg extension machine. RESULTS: VL CSA significantly increased by an average of 9% (P<0.05), while the control group CSA remained unchanged (-2%). Leg extension time increased by an average of 0.09 seconds (0.09s pre vs. 0.18s post) in the HIIT group (P<0.05).

• HIIT Group: n=12; Control: n=5
• 25 training sessions evenly spread through a 10 week intervention (Figure 2)
• Subjects randomly assigned to two groups:
  • Group 1: HIIT – 3x/wk – 4 min. at 90-95% MHR, 4 min. recovery at 70% MHR — repeated 4x
  • Group 2: HIIT training + 2 continuous 30 min. runs at 60-80% MHR alternated with HIIT days
• No significant difference between groups; combined for analysis.
  • Mann-Whitney U Test to determine differences
• VL CSA at 1/3 the distance from the patella to the hip was assessed via panoramic ultrasound imaging following 30 minutes of supine rest.
• Ultrasound images were traced 5 times by manual planimetry using Image J software (NIH) with a coefficient of variance ≤1.0 maintained. (Figure 1)
• Leg extension speed @ 40% 1RM was timed with a custom photogate on a knee extension machine. (Figure 3)

• There was a statistically significant increase of 9% in VL CSA measured in the HIIT group. This increase in muscle CSA is comparable to gains provided by resistance training.1 (Figure 4)
• Leg extension speed of subjects significantly decreased, which is consistent with the VL single muscle fiber response to run training.5
• The two male subjects in the study showed a differing response to the HIIT protocol with respect to the leg extension speed measurements. Further research could be conducted to investigate gender differences in response to HIIT.
• HIIT training could be highly beneficial from a health standpoint, promoting both metabolic and muscular adaptations.
• Together, the results suggest that HIIT may alter skeletal muscle size and/or function in untrained young adult subjects more than previously believed.

To our knowledge, this is the first data showing improvement in whole muscle size in young people with aerobic type training. The decrease in whole muscle power is consistent with decreases in single muscle fiber power in response to run training previously noted in the literature. Together, these results suggest that HIIT AE may alter skeletal muscle size and/or function in untrained young adult subjects more than previously believed.

1. DeFreitas 2011 EJAP. 2Harber 2009 AJP. 3Konopka 2010 JOG. 4Kuta 1970 JAP. 5Luden 2011 SJMSS. 6Schwartz 1991 MCE.