News

A wearable device tracked over 42,000 menstrual cycles. Here’s what researchers found

Npj Digital Medicine | May 2026

A woman has roughly 450 menstrual cycles in a lifetime, yet no one had charted day by day how her vital signs change across them. Alexander Gonzalez, Johanna O’Day, Jennifer Hicks and colleagues at Stanford and WHOOP are filling that gap by drawing on 1.2 million days of wrist-tracker data from more than 2,500 women, covering over 42,000 cycles. Women sleeping under about 7.3 hours a night, or bouncing between six and eight, had more irregular cycles. When it comes to sleep loss and its impact on biometric signals, though, losing sleep costs the same no matter where in the cycle it occurred: before a period, during it, or after. Hicks’ advice in the meantime is simpler than any score: sleep enough, and sleep at consistent hours.

Learn more: Wu Tsai Human Performance Alliance news story, Stylist, Stanford Report

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Training in the heat may make it harder for female athletes to absorb iron

Physiological Reports | June 2026

About 35 percent of female athletes are iron deficient, compared with 10 percent of males. Part of the answer lies in a hormone called hepcidin: when the liver senses inflammation, it releases hepcidin, which blocks iron from crossing the gut into the bloodstream for a while. Kathryn Lucernoni, Christopher Minson, and colleagues asked whether training in the heat increases that iron block. Thirteen trained female runners ran at 70 percent of VO2max twice, once at 35°C and once at 11°C. Core temperature climbed twice as high in the heat, and three hours later hepcidin was roughly double the level after the cool run. The authors say the results point to season and environment as factors worth weighing in how athletes manage iron.

At one university, college athletes got hurt most during the first few weeks of the season

Scientific Reports | July 2026

College athletes get hurt most often at the start of the season, not at the end of it. Dani Gonzalez, Samuel Ward, and colleagues at UC San Diego pulled eight years of injury records from 23 of the university’s teams, and analyzed the 4,900+ injuries to identify trends. They found injuries were most frequent in the opening weeks and fell about 4 percent with each week that followed. Every additional hour of weekly training came with an 8 percent higher injury rate. Gender mattered less than expected. The authors suggest that easing athletes into pre-season workloads may help prevent injuries.

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Sprinting and the Nordic hamstring exercise stress the hamstring in different ways

Journal of Biomechanics | May 2026

The Nordic hamstring curl is the go-to exercise for preventing hamstring strains, but new research suggests athletes may also need sprinting exercises to minimize injury risk. Kristen Steudel, Scott Delp, and colleagues built computer models of 14 athletes, each sprinting on a treadmill and also doing Nordic curls. The goal? To estimate how far the biceps femoris – the hamstring muscle that tears most often – was stretched during each exercise. Both exercises made the muscle work about equally hard to slow the body down, but challenged the muscle in different ways. The Nordic exercise builds the muscle’s ability to handle load; sprinting prepares it for the speeds where strains actually occur.

Learn more:  Wu Tsai Human Performance Alliance news story

Image credit: Julie Muccini. Experimental setup for assessing biomechanical measures during the Nordic Hamstring Exercise.

Clinical markers show promise for assessing REDs risk but a common hormone test falls short

Scandinavian Journal of Medicine & Science in Sports | July 2026

Relative Energy Deficiency in Sport (REDs) is when athletes take in less energy than their training demands over a long stretch, and it can affect bones, hormones, immunity, and the heart. There’s no single test for it, so Ida Heikura, Trent Stellingwerff, and colleagues tested 213 national- and world-level athletes to see whether routine clinical measurements could accurately categorize the risk and severity of REDs. They compared against categories defined by the International Olympic Committee’s assessment tool CAT2 (green, yellow, orange, and red). Most measurements were able to separate the most severe (red) from the very low or no risk (green) categories. However, there were not enough athletes in the more severe categories (orange, red) to judge whether the markers work as clinically validated stand-alone indicators. The team also tested a hormone-based assessment that’s often used in the field, which treats T3 and testosterone levels in the bottom quarter of the normal range as an early warning sign. It didn’t hold up: no single cutoff reliably identified athletes at risk. The authors call their findings exploratory and point toward assessments built around the individual athlete.

When you train may be as important as how hard you train

Journal of Applied Physiology | June 2026

Every muscle cell carries a molecular clock running on a roughly 24-hour cycle that supports the natural peaking of endurance later in the waking hours. Alliance agility project awardee Karyn Esser and colleagues at the University of Florida asked whether training at an off-peak hour can shift that molecular clock to match. They put mice on six weeks of treadmill running, with one group training early in its active phase and one later. Early-trained mice started with 83% less endurance than the late-trained group, but caught up within three weeks despite less total training, likely tied to a rise in COX IV, a mitochondrial energy protein, suggesting circadian adaptation to training time. Esser’s takeaway: train at the time you intend to compete.

Learn more:  Wu Tsai Human Performance Alliance news story

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A computer model reveals how different types of training reshape muscle mitochondria in distinct ways

 The Journal of Physiology | August 2026

Mitochondria physically reshape themselves in response to exercise, splitting apart under stress and merging back together during recovery (fission-fusion). Ali Khalilimeybodi, Lingxia Qiao, Simon Schenk, Padmini Rangamani, and colleagues built a computational model to map how sprint, endurance, and resistance training each drive that process differently. Endurance training produced the most sustained remodeling, sprinting drove sharp but brief changes, and resistance the least. The model, validated at 80% accuracy against independent studies, also predicts that pairing an endurance base with one or two weekly HIIT sessions could maximize mitochondrial adaptation.

Why a torn Achilles may heal so poorly

Journal of Orthopaedic Research | May 2026

A standard explanation for why the Achilles tendon heals slowly and incompletely after rupture has been its poor blood supply. A study in rats from Jared Forer, Nick Willett and colleagues points to a second problem: lymphatic fluid that fails to drain out of the tissue. Using glowing dyes, the team found that in ruptured tendons, lymphatic drainage sped up at first, then slowed below healthy levels by eight weeks, even though weight-bearing, pain, and stiffness had all returned to normal. The researchers suspect getting fluid moving again, perhaps through the right rehabilitation exercises, might speed tendon healing.

Learn more:  Wu Tsai Human Performance Alliance news story

Courtesy University of Oregon

Delivering healing signals in the right order matters as much as using the right signals

Biomacromolecules & Journal of Controlled Release | 2025–2026

Some regenerative therapies that succeed in the lab underperform in the clinic, and timing may be why. When tissue is injured, the body releases chemical signals called growth factors in a carefully timed sequence, and delivering them in the wrong order could undermine an otherwise promising treatment. Marian Hettiaratchi, Justin Svendsen, and colleagues engineered affibodies, proteins about one-tenth the size of an antibody, that grip specific growth factors and release them on a programmable delay, from minutes to days. Experiments leveraged this technology to stagger delivery of three growth factors involved in blood vessel repair and found it produced longer, more branched vessel networks than releasing all three at once.

Learn more: UO Knight Campus news story