Women over 40 are being urged to change the way they exercise – but the scientific basis for those recommendations is far from settled, with critics warning that sweeping advice may do more harm than good.
The push for sex-specific exercise guidance has been led by Dr Stacy Sims, a Stanford-affiliated sports scientist and author of the mantra “women are not small men”. In podcasts and social media posts, Sims argues that from around the age of 40 – a proxy for perimenopause, when reproductive hormones begin to fluctuate unpredictably – women should abandon moderate cardio in favour of heavy lifting and “polarised” cardio: either sprint interval training or gentle walking, with nothing in between. Those who continue with traditional cardio, she told the Mel Robbins podcast, risk becoming “skinny fat” – lacking quality muscle, carrying fatty tissue within the muscle, and with bones “like chalk”. Sims does not give an upper age limit but outlines different protocols for women in their sixties and beyond.
Her claims have attracted a devoted following, and she has been compared to Dr Chris van Tulleken in her ability to change the conversation single-handedly. Yet her stance remains divisive within sports science, and the debate has exposed a deeper problem: the lack of reliable evidence on how women’s bodies respond to exercise.
The research vacuum
A 2023 editorial in the British Medical Journal highlighted a range of studies showing that women are under-represented in exercise research, creating “distinct knowledge gaps” in areas including sport performance, cardiovascular health, musculoskeletal health, postpartum physiology and lactation research. Separate work from the University of Melbourne found that sports psychology research is disproportionately focused on men, while another paper noted that only 6–9% of reputable sports science studies look exclusively at female athletes. The gap is real, and it has created a vacuum that female-specific fitness influencers – including Sims – have rushed to fill.
Cycle-syncing, the idea that women should tailor workouts around their menstrual cycle, is one example: notionally appealing, but with limited scientific backing to date. Sims, who holds an MSc and PhD and has published dozens of papers, is far more credentialed than many influencers. As she told the Huberman Lab podcast: “When we look right now at [fitness trends], all of that data is really drawn from men and just generalised to women, which is a huge disservice.” Her recommendations are seductive in their certainty. Yet strong claims require strong evidence, and not everyone is convinced her studies meet that bar.
The evidence on cardio and strength
“It really feels good that someone is finally standing up for us, bringing attention to inequalities,” says Laurel Beversdorf, a strength and conditioning coach who co-hosts the podcast Movement Logic with physical therapist Sarah Court. “But it’s actually the same misogynistic playbook that we’ve seen across many decades. Let’s problematise women’s bodies. Let’s fragilise women. Let’s make it all about their hormones.” She adds: “She’s saying men can do a much wider variety of things and benefit, but for women she’s taking exercise options off the table.”
Beversdorf and Court argue that scaring women off moderate-intensity cardio is “potentially extremely harmful”, especially because sprint interval training is not accessible to beginners. “Moderate-intensity cardio has some of the most consistent and robust evidence behind it,” they say. “It delivers some of the most reliable health benefits.” A major 2022 study that tracked more than 100,000 adults over 30 years found that those who engaged in high levels of moderate physical activity were up to 38% less likely to die from heart disease.
The UK government’s physical activity guidelines for adults aged 19–64 advise 150 minutes of moderate-intensity activity per week (or 75 minutes of vigorous activity), plus at least two sessions of strengthening activities that work all major muscle groups. Heavy lifting qualifies, but so do yoga, Pilates, gardening and carrying children. According to the 2021 Health Survey for England, 59% of women met the aerobic guidelines, while only 29% met the strength target. The respective figures for men were 70% and 36%. Sims has said her recommendations are not designed for sedentary women – but when so many women are not meeting even basic guidelines, some experts question the wisdom of narrow, prescriptive advice.
For those already hitting the targets and wanting to improve, the principle of progressive overload – gradually increasing intensity, volume, frequency or duration – remains the bedrock of training. As for strength training, Sims maintains that women over 40 should train exclusively with heavy weights (one to six reps). However, Elizabeth Davies, a fitness coach who trains women at all life stages and author of the forthcoming book Training for Your Old Lady Body, notes that the evidence supports other approaches. “The research is very clear that we can build strength and muscle with lighter weights, so long as we work close enough to the point of failure,” she says.
A question of messaging
Sims dismisses her critics as part of “a very nasty and personal misinformation campaign against me”. She insists her recommendations come from randomised controlled trials on sex differences in ageing. “Everything I talk about is to make women stronger, both physically and mentally,” she says. “To not follow male data blindly. Yes, a lot is the same, but there are also differences, and they are critical, especially as we age.”
Yet critics warn that ultra-specific guidance can itself become a barrier. “There are lots of loud voices on social media giving very specific guidance on how women must train at different life stages,” says Davies. “I think it’s our responsibility not to overcomplicate movement by creating arbitrary rules for women or for certain life stages, unless there is an evidential basis to do this.”
Davies advocates for autoregulation – adjusting training based on how you feel each day. “For most of the women I’ve worked with, the challenge isn’t motivation or discipline, but capacity,” she says. “When you’re running on broken sleep, programmes that rely on ‘no excuses’ or ignoring fatigue can backfire, increasing injury risk or simply making exercise feel like another thing you’re failing at.”
Beversdorf and Court believe that if women over 40 do need special messaging, it is not about physiology but about countering the cultural messages they grew up with – a culture that prized thinness and warned women against lifting weights. “The message that you need to start weight training in your 40s is not incorrect,” says Court. “But it’s not to say that you should be doing cardio up to that point and then switch to weights. Cardio and weights are appropriate for women across their lifespan. The benefits of strength training are a new message to us, which is why it’s been emphasised so much to this age group.”
Some women do find Sims’ approach transformative. A friend of the author, 39, started strength training on Sims’ advice and called it life-changing: “Having spent my 20s and 30s trying all types of exercise, it always felt like it was with the aim of being small. For the first time in my life, I feel like that’s no longer the purpose.”
There is widespread agreement that the era of gendering exercise – strength training for men, cardio and tiny pink dumbbells for women – is fading. But the “women are not small men” mantra also feels restrictive to some. As Court puts it: “We’re used to things being overcomplicated for us as women. We have a ten-step face skincare routine we’re supposed to do every night. But it doesn’t need to be that way. Chase the performance goals that you have, or meet the minimum guidelines. Either way, you’re doing better than a lot of people.”