You’re Still Performing—But Your Body Is Breaking Down: Dr. Stacy Sims Reveals the Warning Signs Women Miss
You can still crush the workout. Hit your numbers. Set a PR. Look incredibly fit. And underneath it all, your body may already be struggling.
That's one of the most important ideas that came out of our latest conversation with Dr. Stacy Sims on the Crackin' Backs Podcast. The warning signs don't necessarily begin with a dramatic injury or sudden drop in performance. They may appear much earlier as subtle changes in sleep, menstrual cycle, appetite, mood, resting heart rate, body temperature, energy, or recovery. Sometimes it's even simpler than that: a workout you've done dozens of times suddenly feels unusually difficult.
The problem is that many women have never been taught how to interpret those signals. Instead, we've created a culture that celebrates pushing through them.
You're tired? Train anyway.
You're sore? Tough it out.
Your watch says you're recovered? Follow the program.
But Sims challenges us to ask a much smarter question: Is your body actually adapting to your training—or are you getting better at ignoring the warning signs?
Dr. Stacy Sims Wants Women to Understand Their Own Physiology
Stacy T. Sims, MSc, PhD, is an internationally recognized exercise physiologist and nutrition scientist whose work has focused on female physiology, training, nutrition, recovery, and performance. She's perhaps best known for the phrase “Women are not small men,” a simple statement that helped challenge decades of exercise and nutrition recommendations based largely on research conducted on men.
But when Sims returned to Crackin' Backs, we didn't want to simply revisit why women and men are different.
We wanted to take that conversation somewhere more practical.
If female physiology matters, how can a woman use that information tomorrow morning? How does she decide whether today is the day to push hard, modify the workout, or recover? How does she know whether she's tired because she lacks motivation or because she's under-fueled, under-recovered, iron deficient, hormonally stressed, or simply carrying too much total stress?
Sims' answer isn't another complicated algorithm. In many ways, it's the opposite. Stop looking for one perfect number and start paying attention to patterns.
A female athlete can still be performing at a high level while becoming increasingly under-recovered. Her body may begin whispering long before it starts screaming. Her menstrual cycle changes. Sleep becomes fragmented. Her resting heart rate is different from normal. She becomes unusually irritable or hits a wall every afternoon. Her appetite disappears even as training volume increases. She wakes up exhausted despite supposedly sleeping eight hours. Or the workout that normally feels easy suddenly feels brutally difficult.
Sims experienced this herself. After several stressful days and a business trip, she went out on a familiar hike and described feeling as though she were “trying to walk through mud.” Her fitness hadn't suddenly disappeared. What had changed was the accumulation of stress she was carrying.
That's an important distinction, because our fitness culture tends to have one response when something feels harder: push harder. But sometimes the more intelligent question is: Why does this feel harder today?
5 Key Insights From This Episode
1. Your Menstrual Cycle May Be Telling You More Than You Realize
Forget “cycle syncing” for a moment. Sims believes women should first understand something much more fundamental: know your normal. A menstrual cycle isn't simply something that happens once a month; changes in its normal pattern can provide information about how the body is handling accumulated physiological stress.
And training is only one source of that stress. The body doesn't neatly separate a hard workout from a work deadline, poor sleep, emotional stress, travel, inadequate nutrition, or problems at home. It all enters the same system. Sims explains that excessive stress can affect hormonal signaling involved in ovulation and the menstrual cycle, meaning a significant change in cycle length or bleeding pattern may deserve attention rather than simply being dismissed as an inconvenience.
That doesn't mean panicking every time a cycle varies. It means establishing a baseline and recognizing when your baseline changes. Your normal matters more than somebody else's perfect number.
2. Sometimes “Low Motivation” Is Actually Low Fuel
Now imagine a woman who's training harder but doesn't feel particularly hungry. She's tired all the time, her sleep isn't great, she's becoming irritable, and workouts are beginning to feel harder. She looks at herself and thinks, I need to get more disciplined.
Sims wants women to consider another possibility: maybe you're not eating enough.
Low energy availability occurs when the energy coming into the body isn't sufficient to support the demands of exercise along with normal physiological function. The problem is that as training volume increases, Sims explains that appetite and thirst cues can sometimes become disrupted. The athlete who actually needs more fuel may not necessarily feel dramatically hungrier.
That's where the cycle can become self-perpetuating. You train more, eat too little, recover poorly, sleep poorly, wake up tired, add caffeine, force yourself through another workout, and repeat the process. Eventually what looks like a motivation problem may actually be a recovery problem.
Sims even discusses how inadequate fueling can contribute to nighttime awakenings in some athletes and why an appropriate protein-rich evening snack may sometimes support overnight fueling and sleep.
The point isn't that every woman should start eating before bed. It's that recovery problems sometimes begin at the dinner table rather than in the training room.
3. Your Smartwatch Shouldn't Know Your Body Better Than You Do
This may be one of the most relevant parts of the conversation because we've never had more information about ourselves. HRV, sleep scores, readiness, recovery, resting heart rate, steps, calories, training load, glucose and temperature can all be measured, scored, and delivered to us before our feet even hit the floor in the morning.
That information can be incredibly useful. But Sims sees a growing problem when the number becomes more important than the person.
Different wearables can calculate recovery differently, and Sims argues that some algorithms don't adequately represent the nuances of female physiology. A woman can feel great while her device tells her to recover, or feel terrible while her score says she's ready to train.
Her solution is surprisingly low-tech. Sometimes she asks women to put the wearable away temporarily and relearn what hunger, fatigue, thirst, sleep, energy, and recovery actually feel like.
Try that tomorrow morning. Before looking at your watch, ask yourself how you actually slept. How do your muscles feel? Are you unusually heavy or sore? Are you thirsty? Did you wake naturally or fight the alarm? How is your energy?
Then look at the data.
If the numbers agree with your body, great. If they don't, that's not a failure of the technology or of you. It's information worth investigating.
4. The Best Workout Isn't Always the Hardest Workout
We've created a strange relationship with exercise. If you didn't sweat enough, it wasn't a workout. If you didn't suffer, you didn't push hard enough. If you leave the gym feeling good, maybe you should have done more.
Sims challenges that mentality with a remarkably simple strategy. When you wake up, rate yourself from one to ten physically and then mentally. Maybe physically you're a six but mentally you're a two. Don't immediately cancel the workout, but don't blindly follow the plan either. Start moving. Warm up. Do a few sets and reassess.
Maybe your body wakes up and suddenly you're an eight physically and a six mentally. Great—train. But if you're still flat, exhausted, heavy, and mentally drained, Sims suggests changing the purpose of the session. Make it a technique day. Work on mobility. Reduce the load. Practice skills. You can maintain the habit without forcing the stress.
That is an important mindset shift. The purpose of training isn't simply to survive today's workout. It's to create tomorrow's adaptation. Sometimes the smartest athlete in the room is the one who knows when not to push.
5. Strength Is About Much More Than the Gym
Our conversation eventually moved from adult women to girls, and this may be where Sims' message becomes even more important.
Puberty changes the female body. Hips widen, limbs lengthen, biomechanics change, and coordination can temporarily feel different. A girl who once felt naturally athletic may suddenly feel awkward in her own body. Without the right guidance, she may interpret that change as I'm not athletic anymore or I'm not good enough to play.
Sims believes strength training can help rewrite that story. She's not simply talking about lifting weights to build muscle. She's talking about developing the posterior chain, balance, multidirectional movement, agility, power, and the ability to run, jump, land, decelerate, and control a changing body. Those abilities can support athletic performance and body confidence while also contributing to bone development and injury resilience.
And that foundation doesn't stop mattering when competitive sports end. Strength becomes an investment in what a woman will still be able to do decades later. Muscle, bone, balance, mobility, independence, and healthy aging are all part of the same conversation.
Sims also makes an important point about variety. Strength training doesn't mean abandoning Pilates, yoga, running, martial arts, hiking, or the sport you love. Each provides something different. The goal isn't to find one “perfect” form of exercise and do it forever. The goal is to build a body capable of continuing to do the things that give your life meaning.
Why This Conversation Matters Now
We're living through the age of health optimization. We track sleep, glucose, heart rate, steps, recovery, calories, temperature, and training. We follow influencers telling us when to eat, when to fast, when to exercise, what supplements to take, and what our morning routine should look like. Now we can ask artificial intelligence what our symptoms mean and receive an answer in seconds.
Yet despite having access to more health information than any generation before us, Sims raises an uncomfortable possibility: What if all of this information is making some of us worse at listening to ourselves?
Technology isn't the enemy. Data isn't the enemy. Hard training isn't the enemy. The problem begins when we remove what Sims calls the “human in the loop.”
That's especially important because female physiology isn't static across a lifetime. A teenage girl navigating puberty isn't physiologically identical to a 30-year-old athlete. Pregnancy changes the equation. Perimenopause changes it again. Postmenopause brings another set of considerations. Sims argues that training, nutrition, sleep, and recovery should acknowledge these transitions rather than treating a woman's physiology as one straight line from adolescence through older age.
Maybe that's the larger lesson here. Stop blindly following somebody else's perfect program and start learning to understand the body you're actually living in.
What You’ll Learn
There was far more in our conversation with Dr. Stacy Sims than we could possibly cover in one article. In the full episode, we explore why heart rate variability (HRV) may not always mean what you think it means and why Sims says changes in HRV can sometimes raise questions about iron status in female athletes. We dig deeper into low energy availability, under-fueling, sleep disruption, body temperature, appetite, hydration, menstrual changes, stress, and recovery—and the subtle signs that may appear while an athlete is still performing well.
We also talk about when to push through a workout and when doing so may actually work against adaptation. Sims explains why she sometimes tells women to take off their wearable and relearn what hunger, fatigue, energy, and recovery actually feel like. From there, the conversation expands into puberty, strength training, female biomechanics, posterior-chain development, cross-training, ACL injury risk, and why so many girls are leaving sports at precisely the time movement could have such a profound impact on their future health.
And then the conversation takes an unexpected turn into AI and women's health—and Sims doesn't hold back. Her concern isn't simply about technology. It's about what happens when algorithms, wearables, social media, and generic health advice become more trusted than physiology itself.
Which brings us back to where this conversation began.
Tomorrow morning, before you look at your watch, check your recovery score, or automatically follow the workout somebody else wrote for you, ask yourself a few simple questions. How did I sleep? Is my resting heart rate different? Am I fighting the alarm? Do my muscles feel unusually heavy? Am I thirsty? Am I properly fueled? And when I start moving, do I begin to feel better—or worse?
Your body may already be answering the most important question:
Do I push, modify, or recover today?
Learning to recognize those signals isn't weakness. It may be one of the most sophisticated performance tools you have.
Because sometimes the body whispers long before performance falls apart.
The question is whether you're listening.
For the full story and unfiltered conversation, listen/watch the Crackin’ Backs Podcast.