4/10 Improved Vo2 Max
VO2 max is your body’s max capacity to transport and use oxygen during exercise. It is a great measure of physical fitness.
Generally speaking, the higher your VO2 max, the better your fitness level. A higher VO2 max also means that you can exercise at greater intensities for longer periods of time.
The good news is that doing HIIT will result in significant improvements in VO2 max 
This improvement can be achieved whether you are a beginner exerciser or an advanced athlete.
I have keen runners (not joggers) that were worried that if they swapped their daily miles for Smart HIIT with me their speed would drop off – they have found they are able to run less often – saving their joints – and still improve performance in terms of overall speed and distance but sprint finish in races.
They also find that they feel motivated and fresh when they do run! Result.
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Reference Study 
J Strength Cond Res. 2012 Jan;26(1):138-45. doi: 10.1519/JSC.0b013e318218dd77.
Effect of high-intensity interval training on cardiovascular function, VO2max, and muscular force.
Astorino TA, Allen RP, Roberson DW, Jurancich M.
Department of Kinesiology, California State University, San Marcos, California, USA. firstname.lastname@example.org
The purpose of this study was to examine the effects of short-term high-intensity interval training (HIIT) on cardiovascular function, cardiorespiratory fitness, and muscular force. Active, young (age and body fat = 25.3 ± 4.5 years and 14.3 ± 6.4%) men and women (N = 20) of a similar age, physical activity, and maximal oxygen uptake (VO2max) completed 6 sessions of HIIT consisting of repeated Wingate tests over a 2- to 3-week period. Subjects completed 4 Wingate tests on days 1 and 2, 5 on days 3 and 4, and 6 on days 5 and 6. A control group of 9 men and women (age and body fat = 22.8 ± 2.8 years and 15.2 ± 6.9%) completed all testing but did not perform HIIT. Changes in resting blood pressure (BP) and heart rate (HR), VO2max, body composition, oxygen (O2) pulse, peak, mean, and minimum power output, fatigue index, and voluntary force production of the knee flexors and extensors were examined pretraining and posttraining. Results showed significant (p < 0.05) improvements in VO2max, O2 pulse, and Wingate-derived power output with HIIT. The magnitude of improvement in VO2max was related to baseline VO2max (r = -0.44, p = 0.05) and fatigue index (r = 0.50, p 0.05) in resting BP, HR, or force production was revealed. Data show that HIIT significantly enhanced VO2max and O2 pulse and power output in active men and women.
PMID: 22201691 [PubMed – indexed for MEDLINE]