Are Muscle Oxygen Sensors the Next Great Fitness Wearable?

May 19, 2023

A small Minnesota company believes it has developed the future of fitness tech. Now it has to teach the rest of us how to use it.

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On a typical training ride in Spain's Sierra Nevada, Tokyo Olympics triathlon champion Kristian Blummenfelt might start near Granada, at around 3,000 feet above sea level, and finish as high as 10,000 feet. A key mantra for Norway's world-beating triathlon squad is intensity control—each workout is neither easier nor harder than what the coach prescribes. But the elevation change makes it difficult to dial in the pace. As the air becomes thinner, steadily decreasing oxygen levels mean that heart rate and power output no longer consistently indicate how hard the body is working. Lactate, which requires a small drop of blood, is too unwieldy a measure to keep them on target. So Blummenfelt and his training partners rely on a relatively obscure and unheralded piece of wearable tech, one that the team's sports scientist and Olympic coach, Olav Aleksander Bu, says has become a crucial tool in their training regimen: a muscle-oxygen sensor.

It's no secret that endurance demands ­oxygen. The standard laboratory measure is the VO2 max test, which quantifies how much oxygen you can inhale, diffuse from your lungs into your bloodstream, and then pump to the muscles throughout your body. But the devil is in the details. When a rock climber hanging by her fingers reaches the end of her endurance, she may not even be breathing hard. It's the muscles in her forearms that can't get oxygen quickly enough, even though there's plenty circulating elsewhere in the body. If you stick a ­muscle-oxygen sensor slightly larger than a matchbook on that climber's forearm—something sports scientist Andri Feldmann and his colleagues at the University of Bern, in Switzerland, recently did—you can predict when she’ll fall. Feldmann has also used them with skiers and soccer players. "I think muscle oxygen should replace heart rate as the primary biomarker for athletes," he says.

The technology used to measure muscle oxygen is called near-infrared spectroscopy, or NIRS. By shining light through your skin and measuring what's reflected, NIRS can assess what percentage of hemoglobin and myoglobin molecules in the muscle and tissue underneath are carrying oxygen. If that number is increasing toward 100 percent, it means the oxygen supply exceeds the muscle's demands; if it's drifting down toward zero, demand is outstripping supply. Pedaling as hard as you can for five minutes might get your quads down below 20 percent, and elite athletes can push even lower. (The basic idea is similar to pulse oximeters, but those measure oxygen in your bloodstream rather than in a specific muscle.) "NIRS has been used in exercise ­physiology for decades," says Brad Wilkins, a physiologist at Gonzaga University and a former director at Nike's Sport Research Laboratory. But the NIRS devices were cumbersome and expensive, starting at $15,000, so they seldom left the lab.

That began to change in 2012, when a mechanical engineer in Minnesota named Roger Schmitz started developing a simpler, cheaper NIRS sensor. At first Schmitz figured he could incorporate the technology into a medical device, for conditions like heart failure, but a cardiologist from the University of Minnesota warned him that getting approval from the FDA would be a huge hurdle. "He said, ‘Why don't you make it for athletes? Then you can get it on the market right away,’ " Schmitz recalls. His Moxy sensor debuted in 2013, with an initial price that hovered around $1,000. In the years that followed, a couple of cheaper rivals, made by BSX and Humon, emerged, but both companies have stopped selling muscle-oxygen sensors. The current cost of a Moxy sensor is $800. Whether it's worth the price depends on the answer to a question that Schmitz and others have been debating for almost a decade now: Can muscle-oxygen data really help athletes train and compete better?

The Moxy device quickly attracted a ­community of tinkerers, most notably a physiologist and trainer named Juerg Feldmann—Andri's father—who developed some of the first evaluation protocols for athletes using muscle-oxygen sensors. Red Bull tested the sensor on cyclists as early as 2014, and members of the Canadian national kayak team attached them to their biceps; Schmitz says several Tour de France riders have tried them, too. When Nike launched the Breaking2 project, which culminated in a sub-two-hour marathon attempt in 2017, it used Moxy sensors with athletes, including Olympic marathon champion Eliud Kipchoge.

The Breaking2 team wanted to use muscle oxygen to signal whether the pace necessary to overcome the two-hour-marathon barrier could be maintained. Data published last summer by Wilkins and his former Nike colleagues confirmed that the trend line—whether muscle oxygen is rising, stable, or dropping—reveals a "critical metabolic rate" that separates sustainable from unsustainable efforts, and in the latter case predicts how long you have left before you hit the wall. It's the kind of information you might like displayed on a smartwatch, but interpreting the data in real time is tricky, because other factors such as the length and intensity of a warm-up can influence muscle-oxygen levels. That's a challenge the Moxy team is currently working on, Schmitz says.

Bu and his Norwegian triathletes use lactate and VO2 max lab tests to identify key training intensities, then benchmark them to a given level of muscle oxygen. For example, Blummenfelt's lactate threshold occurs at a muscle-oxygen level of 18 to 19 percent, measured on his quadriceps. Leading up to the Olympics, he was able to sustain that for 70 to 80 minutes; before his Ironman debut in November, where he posted the fastest time ever recorded, he pushed it to around 90 minutes. The muscle-oxygen reading kept his training efforts at the desired level regardless of the effects of altitude, heat, and other environmental factors. "I use it primarily for prescribing intensity," Bu says, "especially when traversing into new climates."

Given the results that Bu's triathletes have posted, it's inevitable that more athletes will experiment with muscle oxygen, perhaps at one of the dozens of Moxy-certified training centers across the country. But even Schmitz warns that users shouldn't expect easy answers from the device. "The human body is complex," he says. "If you convert that to red light, green light, you lose something." Wilkins, too, is bullish but cautious. "We’ve been measuring heart rate for 100 years, but we’ve done a horrible job of teaching people to use it," he points out. The challenge, then, isn't about technology; it's about communication. "I absolutely think that the muscle-oxygen signal is a useful data point that people can apply to their training and performance," Wilkins says. "Now how do we translate that?"