You’re Not Doing HIIT And That’s Why It’s Not Working

Sims explains that in reproductive years, women can generally do what they want, but the undercurrent is that they have less fast-twitch muscle. If you only train in the 10–12 rep range, you'll get some strength and hypertrophy, but you'll miss the power and speed adaptations that come from lower rep, explosive work. She advocates for a periodized approach where you have times of power emphasis—lower reps, heavier weights, plyometrics—alternating with hypertrophy phases and dedicated deloading. This approach contrasts with the steady state of many commercial gym classes and ensures that the innate muscle fiber composition is optimally challenged. She ties this into the need for structure, not just calorie burn, and notes that without this, women will not achieve the same power gains.

Sims confronts the widespread concern that high-intensity interval training spikes cortisol, especially in perimenopausal women. She clarifies that the problem isn't true HIIT, but the pervasive moderate-intensity classes that leave people 'completely slammed.' That moderate grind can perpetuate baseline cortisol elevation without the beneficial hormonal cascade. By polarizing intensity to true max intervals and complete recovery, the hormonal response shifts: growth hormone and testosterone rise post-exercise, which then drops cortisol. In perimenopause, baseline cortisol is already high, but this protocol can lower the new baseline over time, as evidenced by reductions in resting heart rate. She stresses that as women age, the recovery between intervals must be even more disciplined to maintain the quality of each effort, ensuring the stimulus for adaptation remains. This is a clear departure from the standard boot camp model and requires intentional programming.

Sims explains that while the menstrual cycle is often discussed as a uniform 28-day pattern, reality is far more variable both within and between women. Around their period, some women feel amazing two days before and then crash when bleeding starts; others are unaffected. This variability means blanket protocols don't work. Moreover, the incidence of anovulatory cycles is increasing, and many women don't realize they're not ovulating because they still get a bleed—it's just lighter and shorter. Therefore, tracking only the bleed phase can be misleading. She emphasizes that understanding your personal patterns through consistent tracking allows you to 'dial in' training, knowing when to push hard and when to back off, so that hormones work with you rather than against you.

Sims explains that from fetal development, female fetuses have a survival advantage under stress, and after birth, differences in muscle morphology (fiber type) are evident. As children grow, divergence appears in cognition, reaction time, strength, and overall physical fitness—independent of pubertal hormones. She emphasizes that these are inherent sex differences, down to the molecular level, and then hormones are an overlay. Importantly, women spend more than half their lives (pre-puberty and post-menopause) in a non-hormonal state, so focusing only on the menstrual cycle misses foundational physiology. The practical implication is that training must account for these underlying differences—like fewer fast-twitch fibers—regardless of life stage.

Sims states that the disconnect in the high-intensity conversation arises because most people are doing 'high-intensity' workouts that are actually moderate intensity, like 45-minute boot camp classes. That kind of stimulus can perpetuate elevated cortisol because it lacks true maximal efforts and insufficient recovery. In perimenopause, the body is already stressed from hormonal fluctuations, so incorrect high intensity can compound the problem. However, when intervals are strictly at 80% or more of maximum effort, interspersed with recoveries down to about 50% max, each repeated interval is strong, driving a top-end stress that provokes change. The post-exercise response releases growth hormone and testosterone, which subsequently drops cortisol, leading to a reduction in baseline cortisol over time. As fitness improves, resting heart rate comes down, reflecting this adaptation. Therefore, the key is polarization—not avoiding intensity, but doing it properly.

In the reproductive years, women can do these classes and may see some body composition changes because they create a calorie deficit and some strength stimulus, but it's far from optimal. The issue is that these workouts keep participants in a moderate intensity band without allowing true maximal efforts or full recovery, so the hormonal and neuromuscular adaptations that come from true high-intensity intervals or periodized strength work are not achieved. For perimenopausal women, this insufficient stimulus can be particularly detrimental because it may raise cortisol without offsetting benefits. She contrasts these classes with a properly periodized program that includes power phases and true HIIT sessions, which yield far better strength, power, and metabolic adaptations.