The Moxy SmO₂ Sensor: What It Is, How It Works, and How It Makes You Faster

If you’ve ever wished you could peek inside your muscles during a ride, the Moxy muscle oxygen (SmO₂) sensor is your window. Instead of guessing how hard “hard” is, SmO₂ shows—in real time—how much oxygen is available in the working muscle. Pair it with your head unit (Garmin/Wahoo), watch it alongside power and heart rate, and you’ll train smarter, recover better, and race with more control.

What is SmO₂ (and THb)?

 

SmO₂ is the percentage of oxygenated hemoglobin in a specific muscle (e.g., vastus lateralis). Think of it as muscle oxygen saturation. It tells you how much oxygen is available in the muscle you’re using while you ride. When you pedal, your muscles need oxygen to make energy. SmO₂ shows whether the oxygen supply is staying full or getting used up. When SmO₂ is high, your muscles have plenty of oxygen, and the effort feels manageable. As the ride gets harder—like on a climb or during intervals—your muscles burn through oxygen faster, and SmO₂ drops. That drop often lines up with the heavy-leg or burning feeling you notice when things get tough. So in simple terms, SmO₂ helps you see how “stressed” your muscles are, not just how hard you’re pushing on the pedals.

THb (total hemoglobin) is about oxygen delivery rather than oxygen use. Hemoglobin is the part of your blood that carries oxygen, and THb shows how much oxygen-carrying blood is present in the muscle. If SmO₂ tells you how full the oxygen tank is, THb tells you how big the fuel line is. When THb rises, more blood reaches the muscle, which usually means your body is trying to deliver more oxygen to meet demand. When it stays flat or drops, blood flow may be limited or already maxed out. For beginners, the key idea is that THb helps explain why SmO₂ changes—whether the issue is using oxygen faster or not delivering enough of it.

Together, SmO₂ and THb give a clearer picture of what’s happening inside your legs than power or heart rate alone. Power tells you what you’re doing, heart rate tells you how your body is responding overall, and SmO₂/THb shows what’s happening right at the muscle. Ever wonder why two rides at the same power can feel totally different? These metrics help explain that. Over time, as you train, you’ll often see SmO₂ drop more slowly and recover faster, which is a great sign your muscles are getting more efficient. Pretty neat way to see your fitness improving.

Why cyclists love it

1. 1. Dial in true endurance
      Keep SmO₂ steady in the aerobic sweet spot (typically ~55–65%). If it drifts downward at “endurance” power, you’re creeping too hard.

1. 1. Find your threshold without a lab
      During a ramp or long step test, watch where SmO₂ takes a sharper downward turn and stops fully recovering between steps. That inflection often lines up with functional threshold.

1. 1. Pace climbs and time trials
      Hold power and keep SmO₂ from crashing early. If SmO₂ nosedives, back off slightly to avoid cooking yourself.

1. 1. Master interval recovery
      Don’t start the next rep on a clock—start when SmO₂ rebounds (e.g., back to ~60%+). That’s personalized recovery.

1. 1. Track adaptations
      Over weeks, you’ll see: higher SmO₂ at the same power, faster post-interval rebound, and less “sag” on long climbs. That’s fitness in pixels.

How to set it up (quickly)

• Place the sensor on your vastus lateralis (outside quad), lower-to-mid-thigh.
• Pair it to your head unit as a Muscle Oxygen sensor.
• Record at least Power, HR, SmO₂, THb (if available), and HRV.
• Warm up 10–15 min so the signal stabilizes.

How to use SmO₂ in training

1) Aerobic Base & EnduranceRide with SmO₂ ~40-55% and stable.

• • If it keeps trending down at a fixed power, reduce power or increase cadence.

• • Use long steady rides or low-tempo blocks; check that SmO₂ recovers well during easy spins.

2) Tempo / Sweet Spot

• • Expect SmO₂ 25-40%.

• • Watch for a slow drift down across long efforts—if it never stops dropping, the work is too hard/long for today.

3) Threshold

• • SmO₂ 20-30% with small changes in power causing noticeable SmO₂ swings.

• • Between repeats, wait until SmO₂ rises to ~60-75% before the next rep.

4) VO₂ / Anaerobic

• • SmO₂ can fall <20%.

• • Use short reps (e.g., 2–4 min or 30/30s) and full recovery guided by SmO₂ rebound, not a fixed timer.

Interpreting tHb

• Rising/slightly higher THb at low–moderate work suggests good local blood flow.
• Flattening or dropping THb at very high intensities may indicate mechanical occlusion (lots of force), limiting delivery—SmO₂ falls faster.
• Over time, you want more stable THb at given workloads and quicker SmO₂ recovery between reps.

A simple starter workout (SmO₂-guided)

Tempo Blocks, SmO₂-steady

• 20–30 min warm-up until SmO₂ stabilizes.
• 3×12 min tempo where SmO₂ sits ~30-40%. If it slips below ~30% and won’t recover mid-block, ease power 5–10 W.
• Recover until SmO₂ ≥ 60%, then start the next block (not before).
• Cool down easy until SmO₂ returns to 65–70%+.

Progression: Add time per block first, then a touch of power, always holding the same SmO₂ target.

Common pitfalls (and quick fixes)

• Chasing power while SmO₂ free-falls → Back off a few watts; cadence up; give it 60–90 s and reassess.
• Expecting identical SmO₂ day-to-day → Temperature, hydration, carb intake, and placement matter. Use trends, not single points.
• Starting reps on a strict clock → Let SmO₂ recovery call the shots; you’ll finish more quality work.
• Bottom line

Power tells you what you’re putting out. SmO₂ tells you what the muscle can support right now. Use both: set the target with power, pace, and recover with SmO₂. You’ll spend more time in the right physiology, waste less energy, and arrive fresher when it matters.