HIIT may be failing you for two big reasons. Spoiler alert, here they are:

1. You aren’t actually hormonally in a place you’ll benefit.
2. You are doing them outside a sweet spot that is optimal for results

So I’ll quickly address #1 and have a deeper discussion on #2 and what actually is happening during HIIT and why it has the potential to be so good, if you’re ready!

Your Glucose Metabolism to Know Why HIIT May Be Failing You

The classic form of “all out” HIIT is the Wingate test. After about 3 to 5 minutes of warm-up the subject cycles for 30 seconds at maximum effort against a standardized resistance. Typically four to six Wingate tests are performed separated by 4 minutes of rest, for a total of 2 to 3 minutes of maximal exercise spread over 15 to 30 minutes. This “all out” cycle ergometer form of HIT is also referred to as sprint interval training (SIT).

In intense exercise (>80% VO2max), unlike at lesser intensities, glucose is the exclusive muscle fuel. Catecholamine levels rise markedly, causing glucose production to rise seven- to eightfold while glucose utilization is only increased three- to fourfold. In people without diabetes there is a small blood glucose increase during intense exercise that increases further immediately at exhaustion and persists for up to 1 hour. Plasma insulin levels rise, correcting the glucose level and restoring muscle glycogen. This physiological response would be absent in type 1 diabetics.

Your Aerobic Endurance to Know Why HIIT May Be Failing You

HIIT is effective in improving aerobic endurance. In one study six “all out” SIT sessions over 2 weeks improved the mean cycle endurance time to fatigue while cycling at approximately 80% of pretraining VO2max by 100% (from 26 to 51 minutes).

This required a total high-intensity exercise time of only 15 minutes with a total training time commitment of approximately 2.5 hours. In another study, a less intense version of HIIT (6–10 cycling bouts of 30 seconds each at 125% of the power at VO2max with 2 minutes recovery) produced a similar improvement in VO2max after 4 weeks of training, as was seen in the more intense SIT group (three to five “all out” 30-second cycling bouts with 4 minutes of recovery).  The less intense HIT required only half the intensity but double the repetitions of the SIT, and may be more practical for the nonathlete.

After high-intensity exercise, insulin sensitivity is typically increased, meaning the body may require less insulin to utilize glucose effectively, which could lead to a slight rise in insulin levels during recovery as the body replenishes glycogen stores.

The lower the conditioning level the more insulin is likely to be increased.

The longer the activity level, the more insulin is likely to be secreted after exercise.

If adequate recovery does not occur between intervals there may be a greater elevation in stress hormones.

So either … keep the intensity high and the duration extremely short, or make this a longer session with up to 4 minutes between all-out bursts still with a total time of 20 minutes of interval rounds, adding warm up and cool down making it a 30 minute session.

References:

Adams OP. The impact of brief high-intensity exercise on blood glucose levels. Diabetes Metab Syndr Obes. 2013;6:113-122 https://doi.org/10.2147/DMSO.S29222

Erik A. Richter, Lykke Sylow, Mark Hargreaves; Interactions between insulin and exercise. Biochem J 12 November 2021; 478 (21): 3827–3846. doi: https://doi.org/10.1042/BCJ20210185

Resources:

Flipping50 Membership:https://www.flippingfifty.com/cafe

Sleep Yourself Skinny:https://www.flippingfifty.com/sleep-yourself-skinny

Protein Products:https://www.flippingfifty.com/protein

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How to Exercise with High or Low Cortisol in Menopause:https://www.flippingfifty.com/high-or-low-cortisol-in-menopause