Train.Red develops and manufactures wearable muscle oxygen sensors and systems that track your muscles in real time, using near-infrared spectroscopy (NIRS) to measure muscle oxygen saturation (SmO2% or TSI%) and hemoglobin concentration. Most training tools already cover speed, workload, and heart rate. Train.Red covers the part they miss: what's actually happening inside the muscle. Train.Red is an independent company, founded by a core team with a background at Artinis Medical Systems, the Dutch NIRS manufacturer behind the current golden standard in the field. That's where the founding team's NIRS expertise comes from; it packed that scientific grounding into 20 grams of wearable tech, no lab required. Measuring oxygen in the muscle is only the starting point, though. Train.Red's coaching layer turns that data into features any athlete can use, no science degree needed: when a muscle is fatiguing, how long it needs to recover, whether an interval was too hard, too easy, or right on target. Muscle oxygen is where Train.Red started, not where it ends. The company's broader focus is human performance as a whole, physical and cognitive, which puts Train.Red across a wider field than muscle monitoring alone: sports labs, physical performance, wellness, and health. The belief behind it stays the same regardless of which system is doing the measuring: everyone should be able to benefit from their own body's potential, and use that data to actually perform better.

FYER vs. FYER PRO vs. PLUS: what's the difference?

Train.Red's three muscle oxygen monitors, FYER 2.0, FYER PRO, and PLUS 2.0, are built on two different NIRS designs. FYER 2.0 and FYER PRO use Train.Red's own line sensor technology. PLUS 2.0 uses the gold-standard NIRS technology found in clinical and research-grade systems. A line sensor is a row of tiny light receivers rather than one fixed detector. It works something like a camera's shutter speed: if the muscle is reflecting a lot of light, the sensor shortens how long the receivers stay on; if the signal is weak, it lengthens that exposure window and adjusts the transmitter's output to match. That constant self-calibration is what keeps the reading accurate as muscle tissue, skin tone, and blood flow vary from person to person and session to session. PLUS 2.0's gold-standard sensor takes a different route to accuracy: it's calibrated in-house against phantoms, physical models built to simulate human tissue and known oxygen levels, the same calibration method used in clinical NIRS research. FYER 2.0 weighs 20 grams, is IPX7 waterproof, and samples SmO2 at 10Hz. That's enough resolution to track fatigue trends clearly through a session. Battery life runs about 20 hours per charge. It reports SmO2% directly, without the full hemoglobin breakdown. FYER PRO shares the same lightweight, waterproof body as the FYER 2.0. It samples up to 100Hz, though, and adds O2Hb, HHb, tHb, and HbDiff to the readout alongside SmO2%. PLUS 2.0 uses six measurement channels instead of one. That lets it read across a wider tissue depth, or cover several muscle sites at once, which suits fuller metabolic profiling. It's 26 grams, IPX4 sweat-resistant, and runs about 12 hours per charge. All three feed into the same free app and Muscle States coaching layer over BLE/ANT+. Riders, runners, HYROX and CrossFit athletes, and strength coaches all read the same underlying signal through the same interface, whichever sensor they're wearing. Every sensor exposes its full raw data too. But raw data was never meant to be the finish line: the app exists to turn 'SmO2 dropped to 42%' into 'back off pace now, before you feel it.'

How does a muscle oxygen sensor actually measure SmO2?

It shines near-infrared light through the skin into the muscle and reads how much of that light bounces back. Oxygenated and deoxygenated hemoglobin absorb near-infrared light at slightly different wavelengths. By measuring the reflected light, the sensor works out the ratio between the two. That ratio is SmO2, expressed as a percentage. The measurement happens continuously and non-invasively, right at the muscle. That means it captures what's happening in that specific tissue as it happens, not an estimate inferred from somewhere else in the body. Train.Red applies this principle, refined over decades of NIRS research, in a package built for training rather than a lab bench.

SmO2 vs. heart rate: why does muscle oxygen matter more for intervals?

Heart rate tells you how hard your heart is working. It doesn't tell you how your muscle is actually doing. It's a whole-body number, it lags behind what's really happening, and it shifts with caffeine, heat, sleep, or stress, none of which has much to do with your quads at that specific moment. SmO2 skips all of that. It shows, muscle by muscle, whether oxygen supply is keeping up with oxygen demand right now. An athlete can catch local fatigue building before it shows up in pace or power output, identify their actual physiological limiter instead of a stand-in for it, and set work and rest intervals around what the muscle needs.